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MICROBIOLOGY PAPER I - COMPLETE ANSWERS

(Apurva Shastri - Professional Exam)

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GENERAL MICROBIOLOGY

INTRODUCTION AND BACTERIAL TAXONOMY

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SN 1. Robert Koch - Four Contributions / Koch's Postulates

1. Discovered the causative agents of Anthrax (Bacillus anthracis, 1876), Tuberculosis (M. tuberculosis, 1882), and Cholera (Vibrio cholerae, 1883)
2. Developed Koch's Postulates (1884) - criteria to establish causative link between microorganism and disease
3. Developed solid culture media (used potato slices, then gelatin, then agar - introduced by Angelina Fanny Hesse)
4. Introduced staining techniques using aniline dyes and photography in microbiology

1. The microorganism must be found in all cases of the disease
2. It must be isolated from the diseased host and grown in pure culture
3. The pure culture must cause disease when inoculated into a healthy, susceptible host
4. The microorganism must be re-isolated from the experimentally diseased host and shown to be identical to the original

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SN 2. Eukaryotes and Prokaryotes - Four Differences

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SN 3. Louis Pasteur - Contributions in Microbiology

1. Disproved spontaneous generation (Swan-neck flask experiment, 1859)
2. Germ theory of disease - microorganisms cause infectious diseases
3. Pasteurization - heating milk at 63°C for 30 minutes or 72°C for 15 seconds to kill pathogens
4. Vaccines - developed vaccines for chicken cholera, anthrax (attenuated), and rabies (first human vaccination, 1885)
5. Fermentation - proved it is caused by microorganisms, not spontaneous chemical reaction

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MORPHOLOGY AND PHYSIOLOGY OF BACTERIA

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SN 1. Types of Microscopes / Dark Ground Microscope

1. Light (Bright Field) microscope - routine use, stained specimens
2. Dark Ground microscope - for unstained, living, motile organisms (spirochetes)
3. Phase Contrast microscope - internal structures of living cells
4. Fluorescence microscope - fluorochrome-stained specimens (Ziehl-Neelsen AFB, FITC-labeled antibodies)
5. Electron Microscope - Transmission (TEM) and Scanning (SEM), for ultrastructure and viruses

• Uses a special condenser (cardioid or paraboloid) that directs oblique rays of light at the specimen
• The direct light does not enter the objective; only light scattered/reflected by the specimen enters
• Result: organisms appear bright/white against a dark background
• Uses: Detection of Treponema pallidum (syphilis), Leptospira, spirochetes in fresh specimens

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SN 2. Bacterial Growth Curve (with diagram)

Log (CFU)
    |                   ___________
    |                  /           \
    |                 /             \
    |       _________/               \____________
    |______/
    +-----+--------+------------+-------+---------> Time
     Lag    Log      Stationary   Decline
    Phase  Phase      Phase       Phase

1. Lag Phase: No increase in cell numbers; metabolic activity high; cells adapt, synthesize enzymes
2. Log (Exponential) Phase: Maximum growth rate; cells divide at constant rate (generation time); most susceptible to antibiotics
3. Stationary Phase: Rate of multiplication = rate of death; nutrient depletion, toxic products accumulate; spore formation begins
4. Decline (Death) Phase: Rate of death exceeds multiplication; cell numbers fall logarithmically

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SN 3. Bacterial Spore

• Definition: A highly resistant, dormant structure formed by certain Gram-positive bacteria (endospore)
• Sporulating Bacteria: Bacillus (aerobic), Clostridium (anaerobic)
• Structure: Core (DNA + ribosomes) → Inner membrane → Cortex (peptidoglycan) → Spore coat (keratin-like) → Exosporium
• Position in cell: Central (B. anthracis), subterminal (C. tetani drum-stick), terminal (C. tetani)
• Resistance: Withstands 100°C boiling for hours; killed only by autoclaving (121°C, 15 psi, 15 min) or dry heat (160°C, 1 hour)
• Dipicolinic acid (DPA): Gives heat resistance
• Germination: When favorable conditions return (food, water, warmth)
• Clinical importance: C. tetani, C. perfringens, C. botulinum, B. anthracis

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SN 4. Bacterial Capsule

1. Antiphagocytic - major virulence factor
2. Adhesion to surfaces
3. Protection from dessication
4. Protects from complement-mediated lysis

1. Streptococcus pneumoniae
2. Klebsiella pneumoniae (others: Haemophilus influenzae type b, N. meningitidis)

1. Quellung (Neufeld) Reaction: Capsule swells and becomes visible when mixed with specific anticapsular serum + methylene blue
2. Negative Staining (India Ink/Nigrosin): Capsule appears as clear halo around organism against dark background

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SN 5. Cell Wall of Gram-Positive Organisms / Functions of Cell Wall

• Thick peptidoglycan layer (20-80 nm, multiple layers)
• Peptidoglycan (Murein): Made of alternating N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) units, cross-linked by peptide bridges
• Teichoic acids: Ribitol or glycerol phosphate polymers - provide rigidity, antigenic determinants, attachment to host cells
• Lipoteichoic acids: Anchor to cell membrane; involved in adhesion
• No outer membrane (unlike Gram-negative)

1. Maintains shape of bacterium
2. Provides mechanical protection (withstands osmotic lysis)
3. Antigenicity - site for immune response
4. Selective permeability barrier
5. Target for antibiotics (penicillin inhibits cross-linking of peptidoglycan)
6. Site of action of lysozyme (cleaves NAM-NAG bonds)

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SN 6. Bacterial Flagella

1. Monotrichous (single polar flagellum) - Vibrio cholerae, Pseudomonas aeruginosa
2. Lophotrichous (tuft of flagella at one pole) - Spirillum
3. Amphitrichous (flagella at both poles) - Campylobacter
4. Peritrichous (flagella all around) - E. coli, Salmonella, Proteus

1. Leifson's staining - mordant (tannic acid + basic fuchsin) makes flagella visible under light microscope
2. Electron Microscopy - demonstrates flagella ultrastructure

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LAQ 1. Bacterial Cell Wall - Structure and Function

• Thin peptidoglycan (2-7 nm, single layer) in periplasmic space
• Outer membrane (lipid bilayer): contains:
  • Lipopolysaccharide (LPS): Lipid A (endotoxin - fever, shock) + Core polysaccharide + O-antigen (serotyping)
  • Porins: Allow entry of hydrophilic molecules
  • Lipoprotein (Braun's lipoprotein): Anchors outer membrane to peptidoglycan

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STERILIZATION AND DISINFECTION

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SN 1. Gaseous Disinfectants - Describe with Uses

• Alkylating agent - alkylates amino, carboxyl, hydroxyl, and sulfhydryl groups of proteins and nucleic acids
• Used at 50-60°C (or at room temperature for heat-sensitive items)
• Penetrates plastics, rubber, tubing; used for sterilization
• Uses: Catheters, syringes, heart-lung machine components, space suits
• Disadvantages: Flammable, toxic, carcinogenic; long aeration period needed (24-48 hrs)

• Alkylating agent
• Used as 40% formalin or as formaldehyde gas (from formalin + KMnO4)
• Uses: Fumigation of rooms, sterilization of instruments; inactivation of vaccines
• Disadvantages: Carcinogenic, irritating

• Alkylating agent; more efficient than EO
• Uses: Sterilizing vaccines, plasma, tissue grafts; room disinfection
• Disadvantage: Carcinogenic

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SN 2. Tyndallisation

• On Day 1: Flowing steam at 100°C for 20-30 minutes kills vegetative bacteria (but not spores)
• Interval (overnight at 37°C): Remaining spores germinate into vegetative forms
• On Day 2 and Day 3: Repeat steam treatment kills the newly formed vegetative bacteria
• After 3 cycles, no viable organisms remain

• For sterilizing media that cannot withstand autoclave temperatures (e.g., media containing sugars, gelatin, egg, serum - proteins/sugars would be destroyed at 121°C)
• Examples: Löwenstein-Jensen medium, Serum media, sugar broths

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LAQ 1. Sterilization and Disinfection

• Sterilization: Complete destruction or removal of ALL living microorganisms including spores
• Disinfection: Destruction of most pathogenic microorganisms (not necessarily spores) from inanimate objects
• Antiseptic: Chemical agent applied to living tissue to kill/inhibit microorganisms
• Asepsis: Prevention of entry of microorganisms

1. Heat (most reliable method)
   • Dry heat: Flaming, incineration, hot air oven
   • Moist heat: Pasteurization, boiling, autoclaving, Tyndallisation
2. Radiation: UV (DNA damage), Gamma rays (ionizing)
3. Filtration: Seitz filter (asbestos), Berkefeld filter (diatomaceous earth), membrane filter (cellulose nitrate/acetate)

• Alcohols, aldehydes, halogens, heavy metals, surface-active agents

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1. Flaming/Incineration - inoculating loop, contaminated material
2. Red heat - inoculating wire, platinum loop
3. Hot Air Oven (Pasteur's oven) - 160°C for 1 hour or 180°C for 30 minutes

• Kills by oxidation and protein denaturation
• Used for: Glassware, forceps, scissors, syringes, powders, oils, waxes
• NOT for: Rubber, plastic, media (destroyed at high dry temp)
• Indicator: Browne's tube (green → red), spores of B. subtilis (biological indicator)

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• Principle: Steam under pressure achieves temperatures above 100°C; moist heat at 121°C for 15 minutes at 15 psi (103.4 kPa) denatures and coagulates proteins irreversibly
• Types: Gravity displacement (downward displacement), Prevacuum (high pre-vacuum), Porous load, Flash autoclave
• Applications: Culture media, dressings, surgical instruments, intravenous fluids, rubber gloves
• Operational diagram (schematic):
  • Outer chamber (jacket) → Inner chamber → Safety valve, Pressure gauge, Thermometer → Steam inlet → Air outlet (bottom)
• Items sterilized: Culture media, surgical drapes, gowns, metallic instruments, IV fluids
• Indicator: Bowie-Dick test (chemical), Autoclave tape, Browne's tube (brown → black), Biological indicator: B. stearothermophilus spores

1. Pasteurization (LTLT 63°C/30 min or HTST 72°C/15 sec)
2. Boiling (100°C, 10-30 min - kills vegetative forms, not spores)
3. Autoclaving (121°C, 15 psi, 15 min)
4. Tyndallisation (100°C × 3 days)
5. Inspissation (80-85°C × 3 days - for serum media like LJ medium)

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LAQ 2. Four Chemical Agents for Disinfection / Properties of Ideal Disinfectant

• Mechanism: Denature proteins, disrupt lipid membranes
• Active against: Vegetative bacteria, fungi, some viruses; NOT spores
• Uses: Hand rub, skin disinfection, thermometer disinfection

• Mechanism: Alkylation of amino/carboxyl/hydroxyl groups
• Glutaraldehyde: Active against vegetative bacteria, spores, fungi, viruses; used for endoscopes, surgical instruments
• Formaldehyde: Fumigation, preservative, inactivating vaccines

• Mechanism: Oxidation of sulphydryl groups; disrupts cell membranes
• Chlorine: Water purification (0.5 ppm), disinfection of hospital surfaces
• Iodine (Lugol's iodine, povidone iodine): Skin antiseptic, surgical scrub

• Mechanism: Disrupt cell membrane; denature proteins at high concentrations
• Carbolic acid (phenol): First used by Lister as surgical antiseptic
• Lysol (cresol + soap): For disinfection of floor, excreta
• Rideal-Walker coefficient: Compares disinfecting power to phenol

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1. Wide spectrum of activity (bacteria, fungi, viruses, spores)
2. Active in the presence of organic matter
3. Non-corrosive, non-damaging to instruments and materials
4. Non-toxic to humans and animals
5. Stable during storage
6. Water soluble, easy to prepare
7. Rapid action at room temperature
8. Odourless or pleasant odour
9. Inexpensive and readily available
10. Penetrating ability

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CULTURE MEDIA

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SN 1. Culture Media - Classification with Examples

1. Liquid (Broth): Nutrient broth, Peptone water, Robertson's cooked meat broth
2. Semisolid: Motility media (0.5% agar)
3. Solid: Nutrient agar, Blood agar, MacConkey agar (1.5-2% agar)

1. Simple/Basic media: Nutrient broth, Nutrient agar
2. Complex media: Blood agar, Chocolate agar
3. Synthetic/Defined media: All ingredients known chemically

1. Enriched media: Basic medium + enriching substances for fastidious organisms
   • Examples: Blood agar (fastidious organisms), Chocolate agar (N. gonorrhoeae, H. influenzae), Löwenstein-Jensen (M. tuberculosis)
2. Selective media: Contains substances that inhibit unwanted organisms
   • Examples: MacConkey agar (Gram-negatives only), TCBS agar (Vibrio cholerae), Thayer-Martin medium (Neisseria)
3. Enrichment media: Liquid media that enhance growth of desired organisms
   • Examples: Selenite F broth (Salmonella), Alkaline peptone water (Vibrio cholerae), Tetrathionate broth (Salmonella)
4. Indicator/Differential media: Distinguish organisms by colonial appearance
   • Examples: MacConkey (lactose fermenters - pink vs. NLF - colorless), CLED agar
5. Transport media: Maintain viability without multiplication
   • Examples: Stuart's, Amies, Cary-Blair

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SN 2. Enrichment Media vs. Enriched Media / Solid Culture Media without Agar

• Liquid selective media that favor growth of the pathogen while suppressing others
• Examples: Selenite F broth (for Salmonella), Alkaline peptone water (for Vibrio), Tetrathionate broth

• Basic media supplemented with nutrients (blood, serum, X+V factors) to support fastidious organisms
• Examples: Blood agar (5-10% sheep blood + nutrient agar), Chocolate agar, Löffler's serum slope

• Enrichment media = selective liquid media (suppress competitors)
• Enriched media = non-selective + nutrient-supplemented media (support growth)

1. Löffler's serum slope (coagulated serum - for C. diphtheriae)
2. Löwenstein-Jensen (LJ) medium (coagulated egg + glycerol + asparagine - for M. tuberculosis)

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BACTERIAL GENETICS

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SN 1. Mutational vs. Plasmid-mediated (Transferable) Drug Resistance

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SN 2. Transduction

• Definition: Transfer of bacterial DNA from donor to recipient cell via a bacteriophage
• Types:
  1. Generalized transduction: Any DNA fragment can be transferred; occurs when phage accidentally packages bacterial DNA instead of phage DNA (e.g., P1 phage in E. coli)
  2. Specialized (restricted) transduction: Only specific chromosomal genes adjacent to phage integration site are transferred (e.g., lambda phage transfers gal and bio genes in E. coli)
• Significance: Transfer of toxin genes (diphtheria toxin - beta phage, erythrogenic toxin of S. pyogenes, botulinum toxin)

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SN 3. Conjugation

• Definition: Direct cell-to-cell contact via a sex pilus (F-pilus) between donor (F+) and recipient (F-) bacterium, allowing transfer of plasmid or chromosomal DNA
• Process:
  1. F+ cell forms sex pilus that attaches to F- cell
  2. A conjugation bridge forms
  3. One strand of F plasmid DNA is transferred
  4. Recipient becomes F+
• HFr (High frequency recombination) cell: F factor integrated into chromosome; high rate of chromosomal gene transfer but F factor rarely transferred
• Significance: Major mechanism for spread of antibiotic resistance (R-plasmids) in hospital bacteria (E. coli, Klebsiella, Pseudomonas)

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SN 4. Mutation

• Definition: A heritable change in the nucleotide sequence of DNA
• Types:
  1. Point mutation: Single nucleotide change
     • Missense: Different amino acid
     • Nonsense: Stop codon (premature termination)
     • Silent: Same amino acid (synonymous)
  2. Frame-shift mutation: Insertion or deletion of nucleotides → reading frame altered
  3. Deletion/Insertion
• Spontaneous mutations: Due to errors in DNA replication
• Induced mutations: By mutagens (UV light, nitrous acid, acridine dyes)
• Phenotypic categories: Drug resistance, auxotrophic, colony morphology, virulence

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LAQ 1. Gene Transfer in Bacteria

1. Transformation - uptake of naked DNA from environment
2. Transduction - phage-mediated DNA transfer
3. Conjugation - direct cell-to-cell contact via pilus

• Definition: Uptake of free DNA released from donor cell by a competent recipient cell
• Discovered by: Griffith (1928) in S. pneumoniae (smooth → rough transformation)
• Biochemical proof: Avery, MacLeod, McCarty (1944) proved DNA is the transforming principle
• Process:
  1. Donor cell lyses → releases DNA fragments
  2. Competent recipient cell binds and takes up double-stranded DNA
  3. One strand is degraded; other integrates into chromosome by recombination
• Competence: Natural (S. pneumoniae, H. influenzae, B. subtilis) or artificial (heat shock + CaCl2 - for E. coli in lab)
• Significance: Transfer of antibiotic resistance genes, virulence genes; basis of recombinant DNA technology

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BACTERIOLOGY

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STREPTOCOCCUS

SN 1a. S. pyogenes Non-suppurative Sequelae

1. Acute Rheumatic Fever (ARF):
   • Follows streptococcal pharyngitis (not skin infection)
   • Mechanism: Molecular mimicry - antibodies to streptococcal M protein cross-react with cardiac tissue
   • Features: Migratory polyarthritis, carditis, Sydenham's chorea, subcutaneous nodules, erythema marginatum (Jones criteria)
2. Post-Streptococcal Glomerulonephritis (PSGN):
   • Follows pharyngitis (type 12 M protein) OR skin infection (type 49 M protein)
   • Mechanism: Immune complex deposition in glomerular basement membrane
   • Features: Haematuria, proteinuria, hypertension, oliguria
   • Unlike ARF, PSGN is NOT prevented by penicillin treatment

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SN 1b. S. pneumoniae vs. S. viridans - 8 Differences

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CLOSTRIDIUM

SN 2a. C. botulinum Infection - Pathogenicity, Prevention

• Gram-positive, anaerobic, spore-forming bacillus
• Produces most potent biological toxin (neurotoxin types A-G)

• Toxin is a metalloprotease - cleaves SNARE proteins (SNAP-25, VAMP/synaptobrevin) at neuromuscular junction
• Blocks release of acetylcholine (ACh) → flaccid paralysis
• Foodborne botulism: Ingestion of preformed toxin in improperly canned/preserved food
• Infant botulism: Ingestion of spores (honey) → spores germinate in gut → toxin produced in vivo
• Wound botulism: Spores in wound germinate → toxin produced locally
• Clinical: Descending flaccid paralysis, diplopia, dysphagia, dysphonia → respiratory failure

1. Proper canning - heat canned food at 120°C (pressure cooking destroys spores)
2. Boiling food for 10 minutes before consumption destroys toxin
3. Avoid feeding honey to infants under 1 year
4. Antitoxin (trivalent A, B, E) for treatment/prophylaxis

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SN 2b. Gas Gangrene - Pathogenesis / Laboratory Diagnosis

1. Spores/vegetative forms enter devitalized, ischemic tissue (wound)
2. Low O2 tension → spores germinate → vegetative bacteria multiply
3. Alpha toxin (lecithinase/phospholipase C): Destroys cell membranes → lyses RBCs, WBCs, platelets → gas production (CO2, H2S) from fermentation of sugars
4. Proteolytic enzymes destroy muscle and connective tissue
5. Toxins absorbed → systemic toxemia → hemolysis, renal failure, shock, death
6. Spread is rapid; crepitus (gas in tissue) is characteristic

1. Gram stain of exudate: Large Gram-positive bacilli, absence or paucity of WBCs
2. Culture: Anaerobic culture on Blood agar - double zone haemolysis (alpha-haemolysis inner + beta outer)
3. Nagler Reaction (see below)
4. Biochemical tests: Lecithinase production, stormy fermentation of milk
5. X-ray: Gas in muscle planes
6. Histology: Muscle necrosis without inflammatory infiltrate

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SN 2c. Immunoprophylaxis of Tetanus

1. DTP vaccine (Primary): 3 doses at 6, 10, 14 weeks (EPI schedule); DPT in first year of life
2. Booster: At 18 months (DPT) and 5 years (DT)
3. TT (Tetanus Toxoid): For pregnant women (2 doses TT in pregnancy - prevents neonatal tetanus)
4. Td booster: Every 10 years in adults

• Human Tetanus Immune Globulin (TIG): 250-500 IU IM for unimmunized/uncertain immunization status
• Equine antitoxin (ATS): 1500 IU (if TIG not available) - risk of serum sickness

• Clean wounds, fully immunized: No action
• Tetanus-prone wound, unimmunized: TIG + begin DTP/Td series

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SN 2d. Nagler Reaction

1. Egg yolk agar plate is prepared
2. Half the plate is flooded with C. perfringens antitoxin (anti-alpha toxin)
3. C. perfringens is streaked across both halves
4. Incubate anaerobically at 37°C for 24-48 hrs

• Uninhibited side (without antitoxin): Opalescence/turbidity around colonies (lecithin degraded)
• Inhibited side (with antitoxin): No opalescence (antitoxin neutralizes alpha toxin)

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SALMONELLA

SN 3a. Enteric Fever - Laboratory Diagnosis

• 5-10 mL blood into bile broth (1:10 ratio)
• Subculture onto MacConkey, Blood agar at 24-48 hrs
• Salmonella: Non-lactose fermenting colonies with H2S production

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SN 3b. Widal Test

• Serial dilutions of patient's serum (1:20 to 1:640 or beyond)
• Added to standardized antigen suspensions (TO, TH, AO, AH, BO, BH)
• Incubate at 37°C for 24 hrs → read agglutination

• Baseline titre in endemic area: O ≥1:80, H ≥1:160 is significant
• Fourfold rise in titre in paired sera (2 weeks apart) is diagnostic
• O agglutination (granular/floccular): Active early infection
• H agglutination (large fluffy clumps): Past infection or vaccination
• Vi agglutination: Carrier state

• False positives: Other Salmonella (cross-reactions), malaria, liver disease, immunological disorders
• False negatives: Early treatment, immunosuppression
• Single titre not diagnostic

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SN 3c. Co-agglutination Test (CoA Test) - Role in Diagnosis

• Principle: Based on protein A on S. aureus Cowan I strain binding to Fc portion of IgG antibodies → antibody-coated staphylococci agglutinate when exposed to specific antigen
• Application in Enteric Fever:
  • Antibodies against S. typhi O and H antigens are coated on staphylococci
  • When patient's serum or CSF/urine antigen is added → agglutination = positive
  • Detects antigen in clinical specimens (blood, urine, CSF)
• Advantages: Rapid (2-4 hrs), detects antigen in first week (before antibodies develop), useful in partially treated cases
• Uses in other infections: Meningitis (N. meningitidis, H. influenzae), Pneumococcal disease

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NEISSERIA

SN 4. Non-Gonococcal Urethritis (NGU/NIGU)

1. Chlamydia trachomatis (most common, 40-50%) - serovars D-K
2. Ureaplasma urealyticum
3. Mycoplasma genitalium
4. Trichomonas vaginalis
5. Herpes simplex virus

• Urethral discharge (mucopurulent, less copious than gonorrhoea)
• Dysuria, urethral discomfort
• Often subclinical in women (cervicitis, PID, infertility)

• Gram stain of urethral smear: >5 PMNs/HPF but no Gram-negative intracellular diplococci
• NAAT (PCR) for Chlamydia and Mycoplasma (gold standard)
• Culture of discharge

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STAPHYLOCOCCUS

SN 5a. Staphylococcal Wound Infection - Laboratory Diagnosis

1. Specimen: Wound swab, pus
2. Gram stain: Gram-positive cocci in clusters ("bunches of grapes")
3. Culture on Blood agar: Golden/cream-coloured colonies, beta-haemolysis
4. Mannitol Salt Agar (selective): S. aureus ferments mannitol → yellow colonies; coagulase-negative staphylococci do not
5. Coagulase test: Tube coagulase test (S. aureus = coagulase positive)
6. Catalase test: Positive (differentiates from Streptococcus)
7. Sensitivity testing (antibiogram): Methicillin/oxacillin disk diffusion for MRSA detection

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SN 5b. S. aureus - Four Diseases

1. Skin infections: Furuncle (boil), carbuncle, impetigo, cellulitis
2. Scalded Skin Syndrome (SSSS): Exfoliative toxin (ET-A/ET-B) causes separation of epidermis
3. Toxic Shock Syndrome (TSS): TSST-1 superantigen → massive cytokine release → fever, rash, shock, multi-organ failure
4. Osteomyelitis: Hematogenous spread to metaphysis of long bones

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SN 5c. Staphylococcal Food Poisoning

• Toxin: Preformed heat-stable enterotoxins (A-E, most commonly A)
• Mechanism: Enterotoxin acts as superantigen; also directly stimulates vagal afferents in gut → vomiting
• Source: Contaminated creamy foods (custard, cream puffs, potato salad), nasal carriers
• Incubation period: Short: 1-6 hours (preformed toxin)
• Features: Severe nausea, vomiting, abdominal cramps; NO fever (not invasive), diarrhea; self-limiting (24-48 hrs)
• Diagnosis: Culture of food, patient vomitus; detection of toxin by ELISA; phage typing
• Treatment: Supportive (IV fluids); antibiotics NOT indicated (toxin already formed)

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CORYNEBACTERIUM DIPHTHERIA

SN 6a. Diphtheria - Pathogenicity

• Toxin production: Only lysogenized strains (carrying beta-phage carrying tox gene) produce diphtheria toxin
• Diphtheria Toxin: A-B toxin
  • B fragment: Binds to host cell receptor (heparin-binding EGF receptor)
  • A fragment (active): ADP-ribosylates Elongation Factor 2 (EF-2) → inhibits protein synthesis → cell death
• Local effect: Necrosis of oropharyngeal epithelium → pseudomembrane (grey-white, leathery, firmly adherent; bleeds on removal) - tonsils, pharynx, larynx, trachea
• Systemic effect (toxin spreads via blood):
  • Myocarditis (weeks 2-3) - arrhythmias, heart block
  • Neuropathy: CN palsies (palatal palsy, ocular palsy), peripheral neuropathy (weeks 4-8)
  • Adrenal hemorrhage

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SN 6b. Metachromatic Granules

• Also called Babes-Ernst granules, volutin granules, polar bodies
• Composed of polymetaphosphate (volutin) - energy reserve
• Stain metachromatically - blue when stained with basic dyes like methylene blue (appear red/violet due to color change)
• Position: Concentrated at poles of C. diphtheriae (polar granules)
• Demonstration:
  • Albert's staining: Granules stain green-blue, cytoplasm light green (shows "Chinese letter"/"cuneiform" arrangement of bacilli)
  • Neisser's staining: Granules stain dark brown/black, cytoplasm yellow-brown
• Significance: Characteristic of C. diphtheriae; aids in identification

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SN 6c. Toxigenicity Tests

1. Elek's Gel Precipitation (Immunodiffusion) Test (In vitro):
   • Filter paper strip soaked in antitoxin placed on agar plate
   • C. diphtheriae streaked perpendicular to strip
   • Incubate 48 hrs → white precipitin lines form between toxin (diffusing from bacteria) and antitoxin (diffusing from strip)
   • Lines of identity confirm toxin production
2. Guinea Pig Virulence Test (In vivo):
   • Two guinea pigs: one given diphtheria antitoxin (protected), one not
   • Both injected with bacterial suspension
   • Unprotected animal dies in 4-5 days; protected survives
   • Confirmation: Death of unprotected animal = toxigenic strain
3. PCR for tox gene (most rapid, modern method)

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VIBRIO CHOLERAE

SN 7a. Gardener and Venkataraman's Classification

• O1 strains (cholera-causing):
  • Classical biotype - Ogawa (AB), Inaba (AC), Hikojima (ABC) serotypes
  • El Tor biotype - same serotypes
• Non-O1 strains (Non-agglutinable/NAG vibrios):
  • Do not agglutinate with O1 antiserum
  • Include O139 (Bengal strain) - causes epidemic cholera
  • Other NAG vibrios (sporadic diarrhea)
• Halophilic vibrios (require NaCl for growth) - V. parahaemolyticus, V. alginolyticus

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SN 7b. Classical vs. El Tor Vibrios - Differences

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SN 7c. Laboratory Diagnosis of V. cholerae

1. Hanging drop preparation: Comma-shaped, highly motile ("shooting star" motility); immobilized by specific O1 antiserum (motility inhibition test)
2. Dark ground microscopy: Vibrios visible
3. Gram stain: Gram-negative curved rods, comma-shaped

1. Alkaline peptone water (APW) pH 8.6: Enrichment broth - grow at surface
2. TCBS agar (Thiosulfate Citrate Bile Salt Sucrose): Yellow colonies (sucrose fermenter)
3. MacConkey agar: NLF (pale) colonies
4. Monsur's GTTM agar: Grey translucent colonies

• Oxidase positive, catalase positive
• String test positive (in 0.5% sodium deoxycholate)
• Indole positive

• Slide agglutination with polyvalent O1 antiserum, then with Ogawa and Inaba monospecific sera

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SN 7d. Halophilic Vibrios / Kanagawa Phenomenon

• Two Examples:
  1. Vibrio parahaemolyticus - seafood-associated gastroenteritis; food poisoning
  2. Vibrio alginolyticus - ear and wound infections

• Definition: Beta-haemolysis produced by V. parahaemolyticus on Wagatsuma agar (special blood agar with human blood)
• Due to thermostable direct hemolysin (TDH) - also called Kanagawa haemolysin
• Significance: Kanagawa-positive strains are pathogenic; produce diarrhea via enterotoxin mechanism
• Named after Kanagawa Prefecture in Japan where it was discovered
• Epidemiologically, >95% of clinical isolates are Kanagawa-positive

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CHLAMYDIA TRACHOMATIS

SN 8a. Four Diseases

1. Trachoma (A, B, C)
2. Urogenital infections/NGU (D-K)
3. LGV (L1-L3)
4. Neonatal inclusion conjunctivitis/pneumonitis (D-K)

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SN 8b. Laboratory Diagnosis of Chlamydial Infection

1. NAAT (PCR/TMA) - Gold standard; highest sensitivity/specificity; can be done on urine
2. Cell Culture (McCoy cells, HeLa cells): Intracytoplasmic inclusions (iodine-staining with Lugol's)
3. Direct Fluorescent Antibody (DFA) stain: Monoclonal antibodies against MOMP; elementary bodies stain apple-green
4. ELISA for Chlamydial antigen
5. Serology (MIF test): Useful for LGV and neonatal pneumonitis; fourfold rise in titre
6. Giemsa stain (neonatal conjunctival scraping): Intracytoplasmic inclusions (blue-purple)

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SN 8c. Chlamydia vs. Virus - Differences

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SN 8d. Serotypes of Chlamydia - Enumerate / Infections Caused

• A, B, Ba, C → Trachoma
• D, E, F, G, H, I, J, K → Genital infections (urethritis, cervicitis, PID, epididymitis, neonatal conjunctivitis, infant pneumonia)
• L1, L2, L2a, L3 → Lymphogranuloma Venereum (LGV)

• C. psittaci: Psittacosis (ornithosis) - atypical pneumonia from birds
• C. pneumoniae (TWAR): Community-acquired pneumonia, atherosclerosis association

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SHIGELLA

SN 9a. Shigella Dysentery - Pathogenicity / Laboratory Diagnosis

1. Ingestion of small dose (10-100 organisms) - very low infective dose
2. Organisms resist gastric acid, reach colon
3. Invade M cells overlying Peyer's patches via integrin (type III secretion system - IpaB, IpaC proteins)
4. Spread laterally from cell to cell using actin-based motility (IcsA/VirG protein)
5. Cause apoptosis of macrophages and epithelial cells → mucosal ulceration
6. Shiga toxin (S. dysenteriae type 1): Inhibits protein synthesis (RNA N-glycosidase cleaves 28S rRNA) + cytotoxicity; can cause HUS
7. Result: Mucosal inflammation, ulceration → bloody mucoid stools (dysentery)

1. Specimen: Fresh stool/rectal swab (mucus and blood-stained portion)
2. Direct Gram stain: PMNs in stool
3. Culture: Selenite F broth (enrichment) → MacConkey agar (NLF pale/colorless), DCA, XLD, Hektoen agar
4. Biochemical tests: Oxidase negative, non-motile, urease negative, does not produce H2S or gas from glucose; ferments glucose without gas
5. Serology: Slide agglutination with group-specific (A, B, C, D) and type-specific antisera

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MYCOBACTERIUM TUBERCULOSIS

SN 10. Four Methods of Detection with Principles

1. Ziehl-Neelsen (ZN) staining (Acid-Fast Staining):
   • Principle: Mycobacteria have mycolic acid in cell wall that binds carbol-fuchsin and resists decolorization by acid-alcohol (acid-fast)
   • AFB appear red/pink bacilli against blue background
   • 3+ smear positive = reliable; sensitivity ~60% (requires 5000-10,000 bacilli/mL)
2. Culture (Gold standard):
   • LJ medium (egg-based, inspissated): Slow-growing, buff-colored, rough, raised colonies (cauliflower/breadcrumb) in 4-8 weeks
   • BACTEC MGIT (Liquid broth): Faster (1-3 weeks); detects O2 consumption by fluorescence
   • Identifies M. tuberculosis; allows drug sensitivity testing
3. PCR (NAAT) / GeneXpert MTB/RIF:
   • Principle: Amplifies specific M. tuberculosis DNA (IS6110 insertion element); real-time PCR detects rifampicin resistance (rpoB gene mutations)
   • Rapid (2 hrs for GeneXpert), highly sensitive and specific
   • Detects even in smear-negative cases
4. Mantoux test (Tuberculin Skin Test):
   • Principle: Type IV hypersensitivity (delayed-type); PPD (0.1 mL = 5 TU) injected intradermally
   • Read at 48-72 hrs; induration ≥10 mm = positive (≥5 mm in HIV/immunocompromised)
   • Does NOT distinguish active from latent infection or BCG vaccination

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SN 10b. Tuberculosis - Pathogenesis

1. Primary TB (Ghon's complex):
   • Inhaled droplet nuclei (1-5 microns) reach alveoli
   • Macrophages phagocytose bacilli but cannot kill them (lipoarabinomannan inhibits phagosome-lysosome fusion)
   • Cell-mediated immunity develops in 2-8 weeks → granuloma formation (epithelioid cells, Langhan's giant cells, lymphocytes, central caseation)
   • Ghon focus (subpleural, lower upper/upper lower lobe) + hilar lymph nodes = Ghon complex (primary complex)
   • Heals by fibrosis and calcification (RANKE complex) in most; latency
2. Post-Primary (Reactivation) TB:
   • Waning immunity → reactivation of dormant bacilli (typically in apex of upper lobes - high O2)
   • Extensive caseation, cavitation, fibrosis
   • Symptoms: Cough, hemoptysis, weight loss, night sweats, fever
3. Hematogenous spread: Miliary TB (millet seed shadows on X-ray), TB meningitis, renal TB, skeletal TB

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SPIROCHETES

SN 11a. Leptospira - Laboratory Diagnosis

• Week 1 (Leptospiraemic phase): Blood and CSF
• Week 2+ (Immune phase): Urine
• Late: Urine (for several months)

1. Dark ground microscopy: Direct examination of blood/urine - motile, coiled spirochetes (low sensitivity)
2. Culture (Fletcher's/EMJH medium): Aerobic, 30°C for 6-13 weeks (very slow)
3. MAT (Microscopic Agglutination Test): Gold standard serology; serum + live Leptospira; titre ≥1:100 (or fourfold rise) is diagnostic; species-specific
4. ELISA: IgM ELISA for early diagnosis (detects IgM from day 5-7)
5. PCR: Rapid, sensitive; useful early

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SN 11b. Syphilis - Serological Diagnosis

1. VDRL (Venereal Disease Research Laboratory): Flocculation test; detects reagin (IgG + IgM) against cardiolipin-lecithin antigen; quantitative; becomes negative with treatment (used to monitor treatment)
2. RPR (Rapid Plasma Reagin): Similar to VDRL; uses carbon particles; can be done at room temp (field use)

1. FTA-ABS (Fluorescent Treponemal Antibody Absorption): Detects specific anti-Treponema antibodies after absorption with Reiter's spirochete antigen; remains positive for life (even after treatment)
2. TPHA (Treponema pallidum Haemagglutination Assay): RBCs coated with Treponema antigen; agglutinate in presence of patient antibody; sensitive, specific
3. TPI (Treponema pallidum Immobilization test): Gold standard (historical); immobilizes live T. pallidum in presence of antibody + complement; expensive
4. MHA-TP (Microhaemagglutination): Similar to TPHA

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SN 11c. VDRL - Principle, Applications, Advantages, Limitations

1. Screening for syphilis (all stages)
2. CSF-VDRL for diagnosis of neurosyphilis
3. Monitoring treatment response (titre falls with treatment)
4. Screening antenatal mothers
5. Blood donor screening

1. Simple, cheap, rapid
2. Quantitative - can monitor treatment
3. Reliable for primary and secondary syphilis
4. Useful as blood bank screening test

• Biological false positives in: Borrelia, Lupus (SLE), TB, leprosy, malaria, infectious mononucleosis, viral hepatitis, pregnancy, vaccinations
• Becomes negative in late syphilis (prozone phenomenon if undiluted serum)
• Prozone phenomenon: False negative due to antibody excess in undiluted serum

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SN 11d. Leptospirosis - Pathogenesis / Laboratory Diagnosis

1. Infection via skin abrasions or mucous membranes from contaminated water/soil (animal urine - rats, dogs, cattle)
2. Leptospiraemic phase (Week 1): Bacteremia → fever, myalgia (esp. calves), headache, conjunctival suffusion
3. Immune phase (Week 2+): Antibodies + organ damage:
   • Weil's disease (severe leptospirosis): Jaundice (hepatic damage) + renal failure (ARF) + hemorrhage
   • Meningitis: CSF pleocytosis
   • Uveitis: Late complication

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HAEMOPHILUS

SN 12a. X and V Factors / Satellitism

• Definition: Colonies of Haemophilus grow larger around colonies of S. aureus on blood agar
• Mechanism: S. aureus produces V factor (NAD) and lyses RBCs (releases X factor/haematin)
• Method: Streak test organism (H. influenzae) on blood agar, place disc/streak of S. aureus across the plate; incubate → H. influenzae colonies only grow near S. aureus
• Diagram:

  Blood agar plate:
  [Staphylococcus streak] - - - - - - >
  . . large H. flu . . . . small/no growth
  

• Use: Presumptive identification of H. influenzae

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SN 12b. H. influenzae - Four Lesions

1. Meningitis: H. influenzae type b (Hib) - most common bacterial meningitis in children under 5 (before Hib vaccine)
2. Epiglottitis: Cherry-red swollen epiglottis → airway emergency in children
3. Pneumonia: Lobar or bronchopneumonia
4. Otitis media and Sinusitis: H. influenzae type b and non-typeable strains

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E. COLI

SN 13a. Enterotoxigenic E. coli (ETEC)

• Most common cause of traveler's diarrhea and diarrhea in children in developing countries
• Colonization factors (CFA/I, CFA/II): Fimbriae mediate attachment to small intestinal epithelium
• Toxins:
  • LT (Heat-Labile Toxin): Similar to cholera toxin; activates adenylate cyclase → ↑cAMP → Cl- secretion → watery diarrhea
  • ST (Heat-Stable Toxin): Activates guanylate cyclase → ↑cGMP → inhibits Na+ absorption
• Disease: Profuse watery diarrhea (no blood/mucus); "rice-water" like; self-limiting
• Transmitted via contaminated food and water

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SN 13b. E. coli Types Causing Diarrhea / Laboratory Tests for Each

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STREPTOCOCCUS PNEUMONIAE

SN 14a. Pneumococcal Vaccine

• Contains purified capsular polysaccharides of 23 serotypes
• T-independent immune response; NOT effective in children < 2 years
• For: Adults ≥65 years, high-risk adults (asplenic, immunocompromised)

• Polysaccharide conjugated to carrier protein (CRM197) → T-dependent response
• Effective in infants from 6 weeks of age
• Part of Universal Immunization Programme (India) - 3 doses at 6, 10, 14 weeks + booster

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SN 14b. Morphology and Cultural Characteristics

• Gram-positive lancet-shaped diplococci (flame/candle-flame shape)
• Capsulated (polysaccharide capsule)
• Non-motile, non-spore forming
• On Gram stain of sputum: intracellular diplococci in PMNs

• Grows on enriched media (Blood agar, Chocolate agar)
• Blood agar: Small, mucoid, dome-shaped colonies (draughtsman/checker-coin appearance) surrounded by alpha-haemolysis (greenish discoloration)
• Colonies collapse centrally (due to autolysis by autolysin)
• Optochin sensitive (zone of inhibition ≥14 mm with 5 µg optochin disc)
• Bile soluble
• Growth enhanced by 5% CO2

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ATYPICAL MYCOBACTERIA

SN 15. Runyon's Classification / Two Examples of Each

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MYCOBACTERIUM LEPRAE

SN 16a. Morphology

• Cannot be cultured in vitro (obligate intracellular parasite)
• Acid-fast bacillus (AFB) - weakly acid-fast (decolorized by 5% H2SO4, not 20% H2SO4)
• Appearance: Bacilli arranged in clusters called globi or in parallel bundles ("cigarette bundles")
• Solid-staining bacilli in lesions = viable; granular/fragmented = dead bacilli
• Bacteriological Index (BI): Log scale of AFB seen in slit-skin smear; ranges 1+ to 6+
• Animal model: Nine-banded armadillo (Dasypus novemcinctus), nude mice footpad
• Cannot be grown: Has smallest genome of all Mycobacteria (massive gene decay)

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SN 16b. Tuberculoid vs. Lepromatous Leprosy - Four Differences

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SN 16c. Lepromin Test

• Lepromin = Mitsuda lepromin: Suspension of killed M. leprae in saline (heat-killed, standardized)
• 0.1 mL injected intradermally
• Two readings:
  • Fernandez reaction (48-72 hrs): Early induration = measures sensitization (non-specific, measures prior BCG or M. tuberculosis exposure)
  • Mitsuda reaction (3-4 weeks): Granulomatous nodule = positive; measures ability to mount granulomatous response (cell-mediated immunity)

1. Classification of leprosy (TT positive, LL negative)
2. Prognostic indicator (positive = better prognosis)
3. Epidemiological surveys (assess community immunity)
4. Testing efficacy of vaccines (BCG, other candidate vaccines)

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LAQ 1. Pulmonary Tuberculosis - Laboratory Diagnosis / Pathogenesis

1. GeneXpert MTB/RIF (Xpert): WHO-endorsed; detects M. tuberculosis AND rifampicin resistance within 2 hours; replaces smear microscopy in many settings
2. Line Probe Assay (LPA/Hain GenoType MTBDRplus): Detects resistance to INH (katG, inhA mutations) and rifampicin; rapid
3. CBNAAT (Cartridge-Based NAAT): Same as GeneXpert
4. MODS assay (Microscopic Observation Drug Susceptibility): Liquid culture in 7-14 days
5. Whole Genome Sequencing (WGS): Complete drug resistance profiling; epidemiological tracking
6. IGRA (Interferon-Gamma Release Assay) - QuantiFERON-TB Gold: Detects latent TB; not affected by BCG vaccination (unlike Mantoux); measures IFN-γ response to ESAT-6 and CFP-10 antigens

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LAQ 2. Spirochetes - Classification / Treponemal Tests for Syphilis

1. Dark ground microscopy of chancre exudate (motile T. pallidum)
2. DFA-TP (Direct fluorescent antibody for T. pallidum) - identifies even in oral/rectal lesions
3. PCR (T. pallidum PCR)
4. VDRL/RPR usually positive but in early primary may be negative (window period)
5. FTA-ABS becomes positive earliest (first treponemal test to become positive)

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LAQ 3. Chlamydiae - Classify / Pathogenesis / Complications / Lab Diagnosis

• C. trachomatis (15 serovars A-L3) - see above
• C. pneumoniae (TWAR) - respiratory pathogen
• C. psittaci - psittacosis

1. Elementary body (EB): Extracellular, metabolically inactive, infectious form; attaches to host cell; diameter 300 nm
2. Reticulate body (RB): Intracellular, metabolically active, replicative form; binary fission; diameter 1000 nm
3. RBs condense back to EBs → cell lyses and releases EBs to infect new cells

1. EBs invade conjunctival epithelial cells
2. Intracellular replication → cell death → inflammation
3. Repeated infections → progressive scarring (arlt's line) → trichiasis → corneal scarring → blindness

• PID, salpingitis, tubo-ovarian abscess, ectopic pregnancy, infertility
• Perihepatitis (Fitz-Hugh-Curtis syndrome)
• Reactive arthritis (Reiter's syndrome: urethritis + arthritis + uveitis)
• Neonatal conjunctivitis and pneumonia

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LAQ 4. Enterobacteriaceae - Classify / Enteric Fever - Lab Diagnosis / Pathogenesis

• Gram-negative, facultative anaerobic rods; oxidase negative; glucose fermenting
• Lactose Fermenters: E. coli, Klebsiella, Enterobacter, Citrobacter
• Non-Lactose Fermenters (NLF): Salmonella, Shigella, Proteus, Yersinia

1. Ingestion of S. typhi (≥10⁵ organisms) in contaminated food/water
2. Organisms penetrate ileal mucosa via M cells → taken up by macrophages
3. Travel to mesenteric lymph nodes → thoracic duct → bloodstream (Primary bacteremia - asymptomatic; Day 1-2)
4. Localize in reticuloendothelial system (liver, spleen, bone marrow) → multiply
5. Secondary bacteremia (Symptomatic; Day 7-10): Organisms re-enter blood → fever (step-ladder), headache, relative bradycardia
6. Excretion in bile → re-enter gut → Peyer's patches (re-infection) → ulceration, necrosis → rose spots (week 2), hemorrhage, perforation (week 3)

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LAQ 5. Non-Tuberculous Mycobacteria (NTM) - Classify / Buruli's Ulcer

• Caused by: M. ulcerans (Group III non-chromogen NTM)
• Distribution: Sub-Saharan Africa (Buruli county, Uganda; also Benin, Ghana, Australia)
• Pathogenesis: Produces mycolactone - cytotoxic polyketide exotoxin → extensive subcutaneous necrosis and ulceration with undermined edges, painless (toxin is immunosuppressive and analgesic)
• Features: Painless nodule → ulcerates → large, painless ulcer with undermined edges; typically on limbs
• Lab Diagnosis: AFB staining, PCR for IS2404 insertion sequence, culture (very slow - 6-12 weeks)
• Treatment: Rifampicin + Clarithromycin/Streptomycin × 8 weeks; surgical debridement

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LAQ 6. Vibrio cholerae - Pathogenesis / Lab Diagnosis

1. Ingestion of large inoculum (10⁸ organisms) - killed by gastric acid (less acid = more susceptible: antacids, achlorhydria)
2. Organisms colonize small intestinal mucosa; not invasive
3. Cholera toxin (CT): A-B toxin
   • B subunit (5 copies): Binds GM1 ganglioside receptor on intestinal epithelium
   • A subunit: ADP-ribosylates Gs-alpha protein → permanently activates adenylate cyclase → ↑↑cAMP → massive Cl- secretion + inhibition of Na/Cl absorption
4. Profuse, painless, watery diarrhea - rice-water stools (10-20 L/day) - no blood, no mucus, no inflammation
5. Dehydration → hypovolemic shock → death (if untreated)
6. Toxin-coregulated pilus (TCP): Colonization factor; co-regulated with CT by ToxR protein

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LAQ 7. Corynebacterium diphtheria - Lab Diagnosis

• Albert's stain: Club-shaped bacilli in Chinese letter/cuneiform arrangement; granules stain bluish-green, cytoplasm light green
• Gram stain: Gram-positive pleomorphic rods (variable staining)

• Löffler's serum slope: Rapid growth in 6-8 hrs; metachromatic granules demonstrated by Albert's stain
• Tellurite media (Potassium tellurite - selective):
  • Hoyle's medium: Brown-black colonies with dark haloes
  • McLeod's chocolate-tellurite: Dark grey-black colonies
  • Tellurite inhibits other organisms; C. diphtheriae reduces tellurite to tellurium (black)
• Blood agar: Non-haemolytic colonies

• Gravis (biotype gravis): Large, grey, irregular (daisy-head/breadcrumb colonies)
• Mitis: Small, black, circular, convex, smooth
• Intermedius: Small, flat, grey; intermediate between gravis and mitis

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LAQ 8. Anaerobes - Define / Classify

• Gram-positive spore-forming bacilli: Clostridium spp. (C. tetani, C. perfringens, C. botulinum, C. difficile)
• Gram-positive non-spore-forming bacilli: Actinomyces, Propionibacterium, Bifidobacterium
• Gram-negative bacilli: Bacteroides fragilis, Fusobacterium, Prevotella, Porphyromonas
• Gram-positive cocci: Peptostreptococcus, Peptococcus
• Gram-negative cocci: Veillonella

• Gas gangrene (C. perfringens), Tetanus (C. tetani), Botulism (C. botulinum)
• Antibiotic-associated diarrhea (C. difficile)
• Aspiration pneumonia (mouth anaerobes)
• Intra-abdominal abscess (B. fragilis)

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LAQ 9. Clostridium - Gas Gangrene Pathogenesis / Lab Diagnosis

1. Staphylococcus aureus (most common)
2. Coagulase-negative staphylococci (CoNS) - S. epidermidis (prosthetic device infections)
3. E. coli and other Enterobacteriaceae
4. Pseudomonas aeruginosa
5. Enterococcus faecalis
6. Clostridium perfringens (gas gangrene after contaminated wounds)
7. Bacteroides fragilis (abdominal surgery)
8. Streptococcus pyogenes (Group A)
9. Klebsiella pneumoniae
10. MRSA (methicillin-resistant S. aureus)

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LAQ 10. Neisseria Meningitidis - Meningitis Lab Diagnosis

• CSF: Turbid/cloudy; ↑pressure; ↑cells (PMNs), ↑protein, ↓glucose (< 2.2 mmol/L or <50% serum)

• Gram-negative intracellular diplococci (kidney-bean shaped) within PMNs

• Inoculate CSF directly at bedside (keep warm - organism is fragile)
• Chocolate agar or Thayer-Martin medium at 37°C in 5-10% CO2
• Oxidase-positive, smooth grey colonies

• Rapid antigen detection in CSF, serum, urine
• Detects capsular polysaccharide antigens of groups A, B, C, Y, W135
• Rapid (minutes); useful when Gram stain negative (partially treated cases)

• Most sensitive; useful in partially treated meningitis; detects meningococcal DNA in CSF

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LAQ 11. Neisseria Gonorrhoeae - Morphology / Cultural Characteristics / Pathogenicity / Lab Diagnosis

• Gram-negative diplococci (coffee bean/kidney-bean shape, opposing sides flattened)
• Non-motile, non-spore forming, non-capsulated (exception: freshly isolated strains)
• Pili (fimbriae) - major virulence factor

• Fastidious; requires enriched media (chocolate agar, Thayer-Martin)
• Thayer-Martin medium: Contains vancomycin (Gram+), colistin (Gram-), nystatin (fungi)
• Growth in 5-10% CO2, 35-37°C
• Colonies: Translucent, convex, mucoid; 4 colony types (T1-T4); T1 and T2 are piliated and virulent

• Pili: Attach to non-ciliated columnar epithelial cells; antigen variation (PilE gene)
• Por protein (Protein I/Por A, Por B): Outer membrane protein; inhibits phagolysosome fusion; serotyping
• Opa proteins (Protein II): Enhance attachment and penetration into cells
• IgA protease: Cleaves secretory IgA
• LOS (Lipooligosaccharide): Endotoxin-like activity; damage to fallopian tubes
• Diseases: Urethritis, cervicitis, PID, disseminated gonococcal infection (DGI - arthritis-dermatitis), neonatal ophthalmia

1. Gram stain of urethral discharge: Gram-negative intracellular diplococci in PMNs (sensitivity >95% in symptomatic males; only 50% in females/rectal/pharyngeal)
2. Culture on Thayer-Martin: Oxidase-positive grey colonies; 35-37°C in CO2
3. Biochemical: Oxidase positive; ferments glucose only (not maltose - unlike N. meningitidis)
4. NAAT (PCR/TMA): Gold standard for all sites; can be done on urine; most sensitive

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APPLIED MICROBIOLOGY

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SN 1. Biomedical Wastes - Define / Categories / Disposal / Colour Code

1. Incineration: Yellow bag items (anatomical waste, soiled items) at >800°C (double chamber)
2. Autoclaving: Red bag items before recycling; disinfects
3. Sharps pit/needle destroyer: White bag sharps
4. Deep burial: In rural areas (anatomical/pathological waste)
5. Chemical disinfection: Liquid waste (hypochlorite 0.5-1%)

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SN 2. Hospital-Acquired Infections (HAI) - Define / Two Organisms

1. Staphylococcus aureus (MRSA) - surgical site infections, bacteremia
2. Klebsiella pneumoniae (ESBL/KPC-producing) - UTI, pneumonia, blood stream infection

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SN 3. Segregation of Hospital Waste

• At the point of generation (bedside/OR/lab): 4-colour coded containers as above
• No mixing: Never mix different categories
• Sharps: Never recap needles; deposit directly into white sharp container
• Liquid waste: Discard into drain only after chemical disinfection (sodium hypochlorite)
• Storage: Within hospital: Maximum 48 hours; then transferred to common biomedical waste treatment facility (CBWTF)
• Transport: Labelled bags with biohazard symbol; separate vehicle; records maintained

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LAQ 1. Hospital-Acquired Infections (HAI)

1. UTI (30-40%): Most common; associated with urinary catheter (CAUTI) - E. coli, Klebsiella
2. Surgical Site Infection (SSI 20%): S. aureus, MRSA, E. coli
3. Pneumonia/VAP (15-20%): Ventilator-associated pneumonia - Pseudomonas, Acinetobacter, MRSA, Klebsiella
4. Bloodstream infection (BSI) / CLABSI (15%): CoNS, S. aureus, Candida
5. C. difficile infection: Antibiotic-associated; after broad-spectrum antibiotics

• Invasive devices (catheters, IV lines, ventilators, surgical drains)
• Immunocompromise (steroids, chemotherapy)
• Prolonged antibiotic use
• ICU admission
• Extremes of age

1. Hand hygiene (5 moments - WHO): Most important single intervention
2. Standard precautions (gloves, gowns, masks)
3. Bundle care (VAP bundle, CLABSI bundle)
4. Rational antibiotic use (antibiotic stewardship)
5. Surveillance (active monitoring, infection control committee)
6. Environmental cleaning and disinfection
7. Isolation of infected/colonized patients

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CLINICAL MICROBIOLOGY

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SN 1. Zoonotic Diseases - Four with Causative Agents

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SN 2. Urinary Tract Infection - Four Organisms

1. E. coli (most common: 80% of community UTI)
2. Staphylococcus saprophyticus (young sexually active women)
3. Klebsiella pneumoniae
4. Proteus mirabilis (urease-positive; causes staghorn calculi; alkaline urine)
5. Enterococcus faecalis (hospital-acquired)
6. Pseudomonas aeruginosa (catheter-associated)

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SN 3. PUO - Organisms / Diagnosis of Enteric Fever in 1st Week

• Bacterial: Salmonella typhi (enteric fever), Brucella, Mycobacterium tuberculosis, Infective endocarditis (S. viridans, S. aureus), Abscesses (liver, subphrenic)
• Viral: EBV, CMV, HIV
• Parasitic: Plasmodium (malaria), Toxoplasma, Visceral leishmaniasis (kala azar)
• Fungal: Histoplasma, Cryptococcus (immunocompromised)

1. Blood culture (most important - sensitivity 80-90% in untreated cases)
2. Bone marrow culture (most sensitive - positive even in treated cases)
3. Clot culture (blood clot after serum separated)
4. PCR for S. typhi DNA
5. Widal test: May be negative in week 1 (antibodies not yet produced)

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SN 4. Pyogenic Meningitis - Laboratory Diagnosis

1. Gram stain: Organism identification (N. meningitidis - GNIDc; S. pneumoniae - GPDc; H. influenzae - GNcb)
2. Culture on blood agar + chocolate agar (CO2): Primary isolation
3. Latex agglutination (LAT): Rapid antigen test (30 min); useful in partially treated
4. India ink (Cryptococcus neoformans): If immunocompromised
5. PCR: For all common organisms; most sensitive in treated cases
6. Blood cultures (simultaneously)

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LAQ 1. Lower Respiratory Tract Infection - Bacteria Causing

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LAQ 2. Urinary Tract Infection - Organisms / Lab Diagnosis / Significant Bacteriuria

• In symptomatic women with pyuria: ≥10² CFU/mL is significant
• In catheterized patients: ≥10³ CFU/mL

1. Specimen: Midstream clean-catch urine; catheter specimen; suprapubic aspirate
2. Microscopy (wet film): >10 WBCs/mm³ = pyuria; bacteria seen
3. Dipstick test: Leucocyte esterase + nitrite test (screening)
4. Urine culture and sensitivity:
   • Plating on CLED (cystine lactose electrolyte deficient) agar or MacConkey agar
   • Colony count: Significant ≥10⁵ CFU/mL
   • Identification + antibiotic sensitivity (MIC)
5. Gram stain of centrifuged urine: One or more organisms per OIF = significant

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LAQ 3. Pyrexia of Unknown Origin (PUO)

1. Detailed history: Travel, animal contact, occupation, immune status, previous TB, drug history
2. Physical exam: Lymph nodes, hepatosplenomegaly, heart murmurs, skin rashes, joints
3. Blood: CBC, LFT, RFT, blood cultures (×3 sets, 12 hrs apart before antibiotics), serology (Widal, Brucella, Weil-Felix, Paul-Bunnell), thick and thin smear for malaria
4. Bone marrow culture/biopsy (TB, brucella, kala azar)
5. Imaging: X-ray chest, USG abdomen, CT thorax/abdomen/pelvis
6. Urine culture, UACR
7. Echocardiography (if endocarditis suspected)
8. Specific tests: Mantoux, IGRA (TB); Leishmania serology (kala azar); PCR panels

1. Salmonella typhi (enteric fever)
2. Mycobacterium tuberculosis (TB)
3. Brucella species (brucellosis)
4. Streptococcus viridans/S. aureus (infective endocarditis)

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LAQ 4. Meningitis - Organisms Causing / Pyogenic Meningitis Lab Diagnosis / Meningococcal Meningitis

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LAQ 5. STD - Organisms Causing

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LAQ 6. Bacterial Food Poisoning - Bacteria / Pathogenesis / Lab Diagnosis

• Preformed heat-stable enterotoxin A (SEA) in food
• Acts as superantigen: cross-links MHC class II on APC with TCR on T cells → massive cytokine release
• Also stimulates enteric nervous system → emesis
• No fever, no invasion
• Self-limiting (24 hrs)

1. Implicated food: Culture + staphylococcal count (>10⁵/gram significant); toxin detection (ELISA, RPLA)
2. Patient vomitus/stool: Culture for organism
3. Phage typing of S. aureus isolates from food and nose of food handler
4. For Salmonella: Culture of food, stool; serotyping
5. For Clostridium perfringens: Spore count in food (>10⁵ spores/gram)

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IMMUNOLOGY

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SN 1. Exotoxins and Endotoxins - Four Differences

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SN 2. Modes of Transmission of Infectious Agents

1. Contact transmission:
   • Direct: Person-to-person (STDs, skin infections, blood contact)
   • Indirect: Via contaminated objects (fomites) - stethoscopes, doorknobs
   • Droplet: Large respiratory droplets (>5 microns) - influenza, meningococcal disease (within 1 metre)
2. Airborne (droplet nuclei): Droplets ≤5 microns; remain suspended in air; travel >1 metre - TB, measles, varicella, COVID-19
3. Vehicular transmission: Contaminated food/water (enteric fever, cholera), blood (HIV, HBV), IV fluids
4. Vector-borne:
   • Biological: Pathogen multiplies in vector (malaria in Anopheles mosquito, plague in Plasmodium in Anopheles)
   • Mechanical: Passive carriage on vector's body (housefly → typhoid)
5. Vertical transmission (mother to child): Transplacental (TORCH), birth canal (GBS, HSV, gonorrhoea), breast milk (HIV, HTLV)

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LAQ 1. Microbial Pathogenicity and Virulence / Determinants of Bacterial Virulence

• Pathogenicity: Ability of a microorganism to cause disease in a susceptible host
• Virulence: Degree or intensity of pathogenicity; measured by LD50 (lethal dose for 50% of test animals) or ID50 (infective dose)
• Pathogen: Organism capable of causing disease
• Opportunistic pathogen: Causes disease only in compromised host (Pseudomonas, Candida, PCP)

• Exotoxins: (see Immunology SN 1 above)
• Endotoxins: LPS of Gram-negative cell wall; systemic effects (sepsis)

• Coagulase (S. aureus): Converts fibrinogen to fibrin; protects from phagocytosis
• Hyaluronidase (S. pyogenes, Clostridium): Breaks down hyaluronic acid; "spreading factor"
• Collagenase (C. perfringens): Breaks down collagen
• IgA protease (N. gonorrhoeae, S. pneumoniae, H. influenzae): Cleaves secretory IgA
• Streptokinase (S. pyogenes): Dissolves fibrin clots; aids spread
• Lecithinase (C. perfringens): Alpha toxin; destroys cell membranes

• Anti-phagocytic (S. pneumoniae, H. influenzae, K. pneumoniae, N. meningitidis)
• Protects from complement-mediated lysis

• Allow attachment to mucosal surfaces (E. coli fimbriae, gonococcal pili, S. pyogenes M protein and lipoteichoic acid)
• Without adhesion → organisms swept away by mucociliary clearance/urinary flow

• Type III secretion systems (T3SS): "Molecular syringe" - injects effector proteins into host cell (Salmonella, Shigella, Yersinia)
• Actin-based motility (Listeria - ActA protein; Shigella - IcsA/VirG)

• Pilus antigenic variation in N. gonorrhoeae (pilin gene cassettes)
• Phase variation in flagella (Salmonella H1/H2)
• Avoids immune detection

• Mycobacteria: Inhibit phagosome-lysosome fusion (LAM)
• S. aureus: Protein A binds Fc of IgG (blocks opsonization)
• Leishmania: Survive within macrophage lysosomes

• Polysaccharide matrix protects organisms from antibiotics and immune cells
• Clinically: CoNS on prosthetic devices, Pseudomonas in CF lungs

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UNSPECIFIED TOPICS

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SN 1. Blood Culture

• 5-10 mL blood (adults); 1-3 mL (children); by strict aseptic technique
• Best taken at onset of fever (before antibiotic administration)
• Ideally 3 sets from different sites 15-30 minutes apart

• Brain Heart Infusion (BHI) broth / Tryptic Soy Broth (TSB)
• Robertson's Cooked Meat Broth (for anaerobes)
• Biphasic (Castaneda's bottle): Contains liquid broth + solid agar slope; tilt to subculture without opening
• BACTEC automated system: Fluorescent O2 sensor detects bacterial growth; most rapid (positive in 12-24 hrs)

• Positive: Growth identified + sensitivity testing
• Contaminants: CoNS in single bottle (but significant if multiple bottles or prosthetic valve)

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SN 2. Swarming / Two Bacteria / Two Methods to Inhibit

1. Proteus mirabilis (most classical example) - characteristic bull's-eye/concentric ring pattern
2. Clostridium tetani - delicate swarming on blood agar (like "medusa head" colony)

1. Increasing agar concentration to 3-5% (ordinary agar is 1.5-2%); firmer surface reduces motility
2. Sodium chloride (6% NaCl) in medium - inhibits swarming
3. p-Nitrophenylglycerol (PNPG) or Chloral hydrate - added to media
4. CLED medium (Cystine Lactose Electrolyte Deficient): Lacks electrolytes → inhibits swarming; used for urine culture

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SN 3. Universal Safety Precautions (Universal Precautions)

• Introduced by CDC (1985) in response to HIV epidemic
• Principle: Treat ALL blood and body fluids as potentially infectious regardless of patient status

1. Hand hygiene: Before and after patient contact; after glove removal; 5 moments of hand hygiene (WHO)
2. Personal Protective Equipment (PPE):
   • Gloves: Contact with blood, body fluids, mucous membranes, non-intact skin
   • Gown/apron: Risk of splashing
   • Mask + Eye protection (goggles/face shield): Risk of splashing or aerosol generation
3. Sharps management:
   • Never recap needles by two-hand technique
   • Dispose in puncture-proof sharps containers immediately
   • Use safety-engineered devices (retractable needles)
4. Respiratory hygiene/cough etiquette: Cover mouth, use tissues, hand hygiene
5. Safe injection practices: One needle, one syringe, one patient; never reuse
6. Safe handling of contaminated linen and equipment
7. Environmental cleaning: Regular disinfection of surfaces

• Contact precautions: MRSA, CDI, VRE
• Droplet precautions: Influenza, meningococcal disease
• Airborne precautions: TB, measles, varicella

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• Navy blue section headers (General Micro, Bacteriology, Immunology, etc.)
• Teal sub-section headers for each topic
• Orange question markers for quick scanning
• Color-striped tables (all comparison tables like Exo vs. Endotoxin, Gram+/- cell wall, etc.)
• Bullet points compacted to minimal spacing
• Smaller font with tight leading - all content preserved, no truncation
• 66 pages → 24 pages (63% reduction)

• 24 pages → 6 pages (75% reduction) - all content intact
• 3-column layout with color-coded section headers
• 8 different section colors (Navy, Teal, Green, Purple, Orange, etc.) for instant visual navigation
• Color-striped tables with teal headers for all comparison tables
• Navy header bar + teal accent stripe on every page
• Dark footer listing all major topic areas
• Bullet points, bold key terms, and orange question markers preserved

• 2-column layout - more readable than the 6-page version (larger font, better spacing)
• 10 color-coded section banners - each major topic gets its own color (Navy, Teal, Green, Purple, Orange, Indigo, etc.)
• Teal-header tables with alternating row shading for all comparison tables
• Navy header bar with cyan accent stripe on every page + page number
• Dashed column divider between the two columns
• Dark footer on every page
• All content fully intact - every Q&A from the original 24-page document

• Original PDF: 24 pages → Compressed to 11 pages (54% reduction)
• 2-column layout with comfortable font size (6.5pt) - easy to read
• 10 color-coded section banners (Navy, Teal, Green, Purple, Orange, Indigo, etc.) for instant topic navigation
• Color-striped comparison tables with teal headers
• Navy header with cyan accent stripe + page number on every page
• Dark footer on every page
• All content fully intact - every Q&A preserved

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MICROBIOLOGY PAPER II - COMPLETE ANSWERS

(Apurva Shastri - Professional Exam)

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IMMUNOLOGY

INFECTION

SN 1. Define: Carrier, Contact Carrier, Paradoxical Carrier, Convalescent Carrier

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IMMUNITY

SN 1. Innate Immunity - Mechanisms

• Intact skin (keratin layer) - impermeable to most organisms
• Mucous membranes - trap organisms in mucus
• Mucociliary escalator (respiratory tract) - sweeps organisms upward
• Cough, sneeze reflexes
• Flushing action of urine, tears, saliva

• pH: Gastric acid (pH 2) kills most ingested organisms; acid vaginal pH
• Lysozyme: In tears, saliva, nasal secretions - cleaves NAM-NAG bonds in bacterial cell wall
• Lactoferrin: Binds iron - bacteriostatic (deprives bacteria of iron)
• Defensins: Antimicrobial peptides in neutrophil granules and epithelial cells
• Complement (alternative pathway): Activated directly by bacterial surfaces
• Interferons (IFN-α, IFN-β): Released by virus-infected cells - induce antiviral state in neighboring cells
• Acute phase proteins: CRP, serum amyloid A, mannan-binding lectin (MBL) - opsonize bacteria
• Temperature: Fever inhibits many pathogens; cold areas (skin surface 34°C) restrict some fungi

• Phagocytes:
  • Neutrophils (PMNs) - first responders; phagocytose and kill bacteria via oxidative burst (H2O2, superoxide, hypochlorite) and granule enzymes (myeloperoxidase, elastase, lactoferrin)
  • Macrophages - tissue phagocytes; long-lived; process antigens; release cytokines (TNF, IL-1, IL-6, IL-12)
  • Dendritic cells - bridge innate and adaptive immunity; professional antigen-presenting cells
• Natural Killer (NK) cells: Lymphocytes that kill virus-infected cells and tumor cells without prior sensitization; recognize absence of MHC class I ("missing self")
• Mast cells, Basophils, Eosinophils: Innate effectors in allergy and parasitic infections

• Toll-Like Receptors (TLRs): Recognize PAMPs (Pathogen-Associated Molecular Patterns)
  • TLR4 recognizes LPS (Gram-negative endotoxin)
  • TLR9 recognizes bacterial/viral DNA (CpG motifs)
  • TLR3 recognizes double-stranded RNA (viral)
• NOD receptors, RIG-I: Intracellular innate sensors

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ANTIBODY - IMMUNOGLOBULIN

SN 1. IgG - Structure and Function

• Basic immunoglobulin structure: Two heavy chains (γ-chains) + two light chains (κ or λ), linked by disulfide bonds
• MW: ~150,000 Daltons (7S)
• Domains: Each heavy chain has 1 VH + 3 CH domains (CH1, CH2, CH3); each light chain has VL + CL
• Fab fragment (Fragment antigen-binding): VH + VL + CH1 + CL - contains antigen-binding site
• Fc fragment (Fragment crystallizable): CH2 + CH3 - mediates effector functions (complement activation, FcR binding)
• Hinge region: Flexible region between CH1 and CH2 - allows Fab arm movement
• Papain cleaves at hinge → 2 Fab + 1 Fc
• Pepsin cleaves below hinge → F(ab')2 + pFc'
• Subclasses: IgG1, IgG2, IgG3, IgG4 (differ in hinge length, complement activation)

1. Most abundant serum immunoglobulin (75-80% of total Ig)
2. Opsonization: Fc region binds Fc-γR on phagocytes → enhanced phagocytosis
3. Complement activation (classical pathway) via CH2 domain (IgG1, IgG2, IgG3)
4. Neutralization of toxins and viruses
5. ADCC (Antibody-Dependent Cell-Mediated Cytotoxicity) via NK cells
6. Placental transfer (only Ig that crosses placenta via FcRn) - provides passive immunity to neonate
7. Secondary immune response - main antibody produced in secondary/anamnestic response
8. Long half-life: ~23 days (longest of all Ig)

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SN 2. IgM - Properties / Functions / Structure

• Pentamer: 5 basic immunoglobulin units (each: 2 heavy μ-chains + 2 light chains) joined by J-chain (joining chain) and disulfide bonds
• MW: ~900,000 Daltons (19S)
• 10 antigen-binding sites (Fab fragments) - highest valency
• J-chain: Glycoprotein that polymerizes the pentamer; also found in IgA

1. First antibody produced in primary immune response (earliest marker of recent infection)
2. Largest immunoglobulin (pentameric, 19S, 900 kDa)
3. Confined to intravascular space (does NOT cross placenta or enter tissue fluids easily due to large size)
4. Most efficient complement activator (classical pathway - single molecule can activate C1q)
5. Best agglutinating antibody (10 binding sites - high valency)
6. Half-life: ~5 days (short)
7. Present on B-cell surface as monomer (BCR)
8. Important in early defense against bacteremia

• Primary response antibody (IgM → IgG class switching)
• Agglutination of bacteria, RBCs (blood group antibodies ABO are IgM)
• Complement activation (most efficient)
• Opsonization
• Isohemagglutinins (anti-A, anti-B) are IgM

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SN 3. Define/Classify Antibodies

1. IgG (γ chain) - 4 subclasses
2. IgA (α chain) - 2 subclasses
3. IgM (μ chain)
4. IgD (δ chain)
5. IgE (ε chain)

• Monoclonal (single clone, single specificity) vs. Polyclonal

• Natural (isohemagglutinins) vs. Immune (after exposure)

• Agglutinins, precipitins, opsonins, antitoxins, neutralizing antibodies, complement-fixing antibodies

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SN 4. IgA - Diagram Description

• Dimer: 2 IgA monomers joined by J-chain
• Secretory component (SC): Poly-Ig receptor derived; protects SIgA from proteolytic digestion in mucosal secretions
• Found in: Saliva, tears, colostrum, breast milk, respiratory/GI/GU secretions
• First line of mucosal defense - prevents adherence of pathogens to epithelial cells

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LAQ 1. Antibody - Immunoglobulins / IgG Structure and Function

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ANTIGEN-ANTIBODY REACTIONS

SN 1. Agglutination vs. Precipitation - Differences with Examples

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SN 2. ELISA with Applications

1. Direct ELISA: Antigen coated on plate → enzyme-labeled antibody added directly → substrate → color
2. Indirect ELISA: Antigen on plate → primary antibody from patient → enzyme-labeled secondary antibody → substrate → color (detects patient antibody)
3. Sandwich ELISA: Capture antibody on plate → antigen → detection antibody (labeled) → substrate; most sensitive; for antigen detection
4. Competitive ELISA: Patient antigen competes with labeled antigen for antibody binding; inversely proportional signal

1. Serology: HIV diagnosis (anti-HIV antibody), HBsAg detection, Dengue NS1 antigen, anti-HCV, anti-Toxoplasma
2. Blood bank screening: HIV, HBV, HCV, syphilis in donated blood
3. Drug detection: Therapeutic drug monitoring, drugs of abuse
4. Hormone assays: Pregnancy test (hCG), thyroid hormones, insulin
5. Food testing: Allergens, pesticide residues, food adulteration
6. Research: Cytokine quantification, protein assays

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SN 3. Agglutination Reactions - Enumerate / Widal Test / Tube Agglutination

1. Direct (slide/tube) agglutination
2. Passive (indirect) agglutination - see SN 4
3. Reverse passive agglutination
4. Co-agglutination
5. Inhibition of agglutination (haemagglutination inhibition - HAI)
6. Antiglobulin (Coombs) test

• Principle: Tube agglutination; detects agglutinating antibodies (O and H) against S. typhi in patient serum
• Procedure: Serial doubling dilutions of patient serum (1:20 to 1:640+) mixed with standardized Salmonella antigen suspensions (TO, TH, AO, AH, BO, BH); incubate 37°C/24 hrs; read agglutination
• Interpretation: O titre ≥1:80, H titre ≥1:160 (endemic area); fourfold rise in paired sera = diagnostic
• O agglutination: Granular clumps (active infection); H agglutination: Large fluffy clumps (past infection/vaccination)

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SN 4. Passive Agglutination Tests

• Sheep/human RBCs → Indirect Hemagglutination (IHA)
• Latex beads → Latex Agglutination Test (LAT)
• Bentonite, charcoal, Staphylococcus (protein A) → co-agglutination

1. TPHA/MHA-TP (syphilis) - T. pallidum antigen on sheep RBCs
2. RA Latex test - IgG on latex for Rheumatoid Factor detection
3. Latex agglutination for CSF antigens (meningococcal/pneumococcal meningitis)
4. RPR (Rapid Plasma Reagin) - cardiolipin on carbon particles
5. Wuchereria bancrofti - IHA for filarial antibodies

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LAQ 1. Antigen-Antibody Reactions / Prozone Phenomenon / Agglutination

1. Specificity (key-lock complementarity)
2. Reversibility (non-covalent bonds: hydrogen, hydrophobic, van der Waals, ionic)
3. Proportionality (optimal ratio for visible reaction)
4. Two stages: Primary binding (rapid, invisible) → Secondary effect (visible: precipitation, agglutination)

• When antibody concentration is very high (excess) relative to antigen, each antigen molecule is saturated with antibody - no cross-linking - NO visible agglutination/precipitation
• False-NEGATIVE result despite high antibody titre
• Solution: Dilute the serum (test beyond prozone zone)
• Seen in: Widal test, VDRL (undiluted serum in secondary syphilis), Brucella SAT

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LAQ 2 & 4. Precipitation Reactions - Principle / Applications

• Soluble antigen + antibody → insoluble lattice network → visible precipitate
• Requires optimal antigen-antibody ratio (equivalence zone); excess antibody (prozone) or antigen (postzone) gives no precipitate

1. Ring (interface) test: Antigen layered over antibody in tube; white precipitate ring at interface
2. Ouchterlony (Double Diffusion in Agar): Both antigen and antibody diffuse toward each other in agar; precipitin line forms where they meet at equivalence; demonstrates: identity, partial identity, non-identity
3. Single Radial Immunodiffusion (SRID/Mancini): Antigen diffuses into antibody-containing agar; ring diameter² ∝ antigen concentration; used to quantify Ig levels
4. Immunoelectrophoresis: Serum proteins separated by electrophoresis, then precipitated with antiserum; identifies M-band in myeloma
5. Elek's test (immunodiffusion for toxin): See Paper I answers
6. Counterimmunoelectrophoresis (CIE): Electrophoresis drives antigen and antibody toward each other; faster than Ouchterlony; CSF antigen detection
7. Immunoturbidimetry/Nephelometry: Light scattering by immune complexes; used for quantification of CRP, complement, Ig

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COMPLEMENT SYSTEM

SN 1. Classical Pathway of Complement

1. Recognition: C1q binds Fc of IgM (1 molecule) or 2 IgG molecules → C1r and C1s activated → C1 complex
2. C1s cleaves C4 → C4a (anaphylatoxin) + C4b (binds to surface)
3. C4b + C2 → C1s cleaves C2 → C2a + C2b → C3 convertase (C4b2a)
4. C3 convertase cleaves C3 → C3a (anaphylatoxin, chemotaxis) + C3b (opsonin - binds CR1 on phagocytes; also joins C4b2a)
5. C5 convertase (C4b2a3b) cleaves C5 → C5a (most potent anaphylatoxin + chemotactic) + C5b
6. Terminal Complement (MAC): C5b + C6 + C7 + C8 + C9 (poly-C9) → Membrane Attack Complex (MAC) → pore in bacterial membrane → lysis

1. Lysis of bacteria (MAC)
2. Opsonization (C3b, C4b bind to CR1 on phagocytes → enhanced phagocytosis)
3. Anaphylatoxins (C3a, C4a, C5a) → mast cell degranulation → histamine → vasodilation, increased permeability
4. Chemotaxis (C5a, C3a) → attracts neutrophils and monocytes to infection site
5. Immune complex solubilization (C3b prevents precipitation of immune complexes)
6. Enhancement of adaptive immunity (C3d on antigen binds CR2 on B cells → lowers activation threshold)

SN 2. Complement Cascade and Biological Effects

• Spontaneous hydrolysis of C3 → C3(H2O) → factor B binds → factor D cleaves B → C3 convertase (C3bBb, stabilized by properdin) → amplification loop
• Activated by: LPS, zymosan (fungal), cobra venom factor; does NOT require antibody

• MBL (Mannose-Binding Lectin) binds mannose on bacteria → MASP-1, MASP-2 activated → cleave C4 and C2 (like classical)

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IMMUNE RESPONSE

SN 1. Cell-Mediated Immunity (CMI) - Tests for Detection

1. Tuberculin test (Mantoux test): 0.1 mL PPD intradermally; read induration at 48-72 hrs; tests CMI to M. tuberculosis
2. Lepromin test (Mitsuda): Tests CMI to M. leprae
3. Dinitrochlorobenzene (DNCB) test: Contact sensitization test for general CMI competence
4. Candida/Mumps/Trichophyton skin tests: Recall antigen tests for general CMI

1. Lymphocyte Transformation Test (LTT) / Lymphocyte Blastogenesis: PBMCs + specific antigen → T cells proliferate → measured by ³H-thymidine incorporation or BrdU
2. Leukocyte Migration Inhibition Test (LMIT): T cells sensitized to antigen → release MIF (Migration Inhibitory Factor) → inhibits macrophage migration in capillary tube; positive = CMI present
3. Cytotoxic T Lymphocyte (CTL) assay: CD8+ T cells kill target cells; measured by ⁵¹Cr release
4. Flow cytometry: CD4/CD8 ratio, activation markers (CD25, CD69)
5. ELISPOT: Detects individual cytokine-secreting cells (IFN-γ); basis of QuantiFERON-TB Gold for latent TB
6. Interferon-gamma release assays (IGRA)

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HYPERSENSITIVITY

SN 1. Classify Hypersensitivity / Type III - Describe

• Mechanism: Soluble antigen-antibody complexes (immune complexes) form in antigen excess → not cleared by phagocytes → deposit in vessel walls, glomeruli, synovial membranes, choroid plexus
• Complement activation → C3a, C5a → mast cell degranulation + neutrophil chemotaxis → neutrophils release lysosomal enzymes → tissue damage
• Arthus Reaction (local Type III): Intradermal antigen injection in immunized individual → local immune complex deposition → edema, hemorrhage, necrosis (6-8 hrs)
• Serum sickness (systemic Type III): After injection of foreign serum (heterologous antisera, e.g., antitetanus horse serum) → fever, urticaria, arthralgia, proteinuria (7-10 days)
• Diseases: SLE (anti-dsDNA), Post-streptococcal GN, Rheumatoid arthritis, Hypersensitivity pneumonitis (Farmer's lung, Bird fancier's lung)

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SN 2. Type IV Hypersensitivity

1. Sensitization: First exposure → APCs present antigen to CD4+ Th1 cells → Th1 cells sensitized (clonal expansion + memory)
2. Elicitation (Challenge): Re-exposure → Th1 cells recognize antigen-MHC II on APCs → release cytokines: IFN-γ (activates macrophages), TNF-β (lymphotoxin - cytotoxic), IL-2 (T cell proliferation), MIF (migration inhibitory factor)
3. Effector: Activated macrophages → release proteases, reactive oxygen species → tissue damage; granuloma formation in chronic DTH

1. Tuberculin reaction (Mantoux test) - prototype
2. Contact dermatitis (nickel, formaldehyde, poison ivy - urushiol)
3. Granulomatous hypersensitivity (TB, leprosy, sarcoidosis, schistosomiasis)
4. Transplant rejection (allograft)
5. Graft-versus-host disease

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SN 3. Anaphylaxis

1. Sensitization: Antigen → IgE produced → IgE binds to FcεRI on mast cells and basophils (primed state)
2. Re-exposure: Antigen cross-links 2 IgE molecules on mast cell → immediate degranulation
3. Mediators released: Histamine, prostaglandins, leukotrienes (LTC4, LTD4), PAF, tryptase
4. Effects: Vasodilation, increased vascular permeability → oedema; bronchospasm; urticaria; cardiovascular collapse

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LAQ 1 & 2. Hypersensitivity - Three Types of Immediate Hypersensitivity / Type I

1. Primary exposure: Antigen (allergen) processed by APCs → Th2 cells activated → IL-4, IL-5, IL-13 → B cells class switch to IgE
2. Sensitization: IgE binds to FcεRI on mast cells (skin, mucosa, lungs) and basophils - no symptoms yet
3. Re-exposure: Allergen cross-links adjacent IgE-FcεRI complexes on mast cell → signal transduction → Ca²⁺ influx → degranulation

• Preformed (immediate, 0-30 min): Histamine, heparin, tryptase, chemotactic factors
  • Histamine: H1 → vasodilation, bronchospasm, itch; H2 → gastric acid
• Newly synthesized (late, 4-6 hrs): Prostaglandin D2, Leukotriene C4, D4, E4 (SRS-A - slow-reacting substance of anaphylaxis); PAF
  • LTC4/LTD4: Bronchospasm (1000x more potent than histamine), mucus secretion

• Anaphylaxis (systemic, life-threatening)
• Allergic asthma (bronchospasm)
• Allergic rhinitis/hay fever (sneezing, rhinorrhea)
• Urticaria and angioedema (skin)
• Food allergy (peanuts, milk, eggs)
• Atopic dermatitis (eczema)

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LAQ 3. Hypersensitivity - Classify / Pathogenesis / Type IV

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AUTOIMMUNITY

SN 1. Autoimmune Diseases - Four Features

1. Presence of autoantibodies or autoreactive T cells directed against self-antigens (e.g., anti-dsDNA in SLE, anti-acetylcholine receptor in myasthenia gravis)
2. Association with MHC (HLA) alleles - genetic predisposition (e.g., HLA-DR3/DR4 in Type 1 diabetes, HLA-B27 in ankylosing spondylitis)
3. Female predominance (most autoimmune diseases affect women more - hormonal influence)
4. Chronicity and remission-relapse pattern - autoimmune diseases tend to be lifelong with episodic flares

LAQ 1. Autoimmunity - Define / Mechanisms

1. Release of sequestered antigens: Normally hidden antigens (lens protein, myelin, sperm) released by infection/trauma → treated as foreign → autoimmune response (sympathetic ophthalmia, orchitis)
2. Molecular mimicry: Microbial antigens structurally similar to self-antigens → antibodies/T cells cross-react with self tissues
   • Example: Streptococcal M protein → cross-reacts with cardiac myosin → rheumatic fever
   • Campylobacter jejuni ganglioside mimicry → Guillain-Barré syndrome
3. Polyclonal B cell activation: Some pathogens (EBV, LPS) activate B cells non-specifically → production of autoantibodies
4. Epitope spreading: Initial autoimmune response → tissue damage → release of more self-antigens → activation of additional autoreactive clones → self-perpetuating cycle
5. Loss of peripheral tolerance:
   • Regulatory T cell (Treg) failure: FoxP3+ Tregs suppress autoreactive cells; deficiency → autoimmunity (IPEX syndrome)
   • Defective Fas/FasL apoptosis: Autoreactive cells not deleted (ALPS - autoimmune lymphoproliferative syndrome)
6. Bystander activation: Inflammation near self-tissues activates APCs that present self-antigens → autoreactive T cells activated
7. Altered self-antigens: Drug-modified antigens (e.g., penicillin binding to red cell surface → autoimmune hemolysis), viral modification of self-proteins
8. Superantigen stimulation: Superantigens activate large numbers of T cells non-specifically → some autoreactive

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VIROLOGY

GENERAL PROPERTIES OF VIRUS

SN 1. Egg Inoculation Method for Viral Culture

• 8-14 day old fertile hen's egg used
• Contains 5 sites for inoculation

1. Candle the egg (7-12 days) to identify air sac and embryo
2. Mark and make a small hole over air sac; second hole over CAM
3. Suction air → CAM drops → forms artificial air sac
4. Inoculate 0.1-0.2 mL virus suspension onto CAM through 2nd hole
5. Seal holes with paraffin; incubate at 37°C for 48-72 hrs
6. Observe pock lesions on CAM

• Virus isolation and propagation
• Vaccine production (influenza vaccine - allantoic fluid)
• Virus titration (pock counting)

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SN 2. Methods of Detecting Viral Growth in Cell Cultures

• Most common method; observed by inverted microscope
• Cells show: rounding, swelling, shrinkage, syncytia formation (giant multinucleated cells), cell lysis
• Examples: HSV → rapid CPE (24-48 hrs); Adenovirus → "bunch of grapes" cluster; Respiratory syncytial virus (RSV) → syncytia; CMV → "owl eye" cells

• Viruses with haemagglutinin on infected cell surface adsorb guinea pig RBCs
• Add RBCs to cell monolayer; wash; infected cells hold RBCs
• Used for: Influenza, Parainfluenza, Mumps

• Virus in cell culture supernatant agglutinates RBCs in wells
• Used for: Influenza, Mumps

• Non-cytopathic virus grows in cells → prevents CPE by cytopathic virus added later
• Used for: Rubella (infects cells without CPE; detected by blocking CPE of challenge Echovirus)

• Fluorescent-labeled antibodies react with viral antigens in infected cells
• Direct IF: Fluorescent antibody directly; Indirect IF: Unlabeled antibody + fluorescent anti-antibody
• Used for: Rabies (negri bodies), RSV, CMV, HSV

• Virus replication → cell death → no glucose metabolism → no acid production → medium stays red/alkaline
• Used for: Enteroviruses, polioviruses

• Oncogenic viruses transform cells → loss of contact inhibition → foci of piled-up cells visible

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SN 3. Stages of Viral Multiplication (Replication Cycle)

1. Adsorption (Attachment):
   • Viral surface protein binds to specific host cell receptor
   • Example: HIV gp120 → CD4 + CCR5/CXCR4; Influenza HA → sialic acid; Rabies → acetylcholine receptor
2. Penetration (Entry):
   • Receptor-mediated endocytosis (most viruses)
   • Membrane fusion (enveloped viruses - HIV, Influenza)
   • Direct injection (bacteriophages - only DNA injected)
   • Viropexis (engulfment)
3. Uncoating (Eclipse phase):
   • Viral capsid removed; nucleic acid released into cytoplasm
   • Viral enzymes or lysosomal enzymes responsible
4. Biosynthesis (Replication):
   • DNA viruses (most replicate in nucleus): DNA → mRNA → viral proteins; new DNA synthesized
   • RNA viruses (replicate in cytoplasm): +ssRNA acts as mRNA; -ssRNA requires RNA-dependent RNA polymerase (RdRp); Retroviruses use reverse transcriptase (RNA → DNA → RNA)
5. Assembly (Maturation):
   • Viral components assembled into new virions
   • Capsid proteins surround nucleic acid
6. Release:
   • Lysis (non-enveloped viruses) - cell bursts → virus released
   • Budding (enveloped viruses - HIV, Influenza) - cell membrane wraps around nucleocapsid; cell may survive
   • Exocytosis (some viruses)

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SN 4. Embryonated Hen's Egg - Cross Section / Routes / Uses

• Shell → Shell membrane → Air sac → Chorioallantoic membrane (CAM) → Allantoic cavity (allantoic fluid) → Amniotic cavity (amnion, amniotic fluid) → Embryo → Yolk sac

1. Embryonated hen's egg
2. Cell (tissue) culture - most widely used today
3. Animal inoculation (laboratory animals - mice, guinea pigs, rabbits)

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VIRUS-HOST INTERACTIONS

SN 1. Negri Bodies

• Definition: Intracytoplasmic inclusion bodies found in neurons infected with Rabies virus
• Location: Hippocampal neurons (Ammon's horn/CA1-CA4), Purkinje cells of cerebellum
• Nature: Aggregates of rabies viral nucleocapsids (RNP - ribonucleoprotein) within cytoplasm
• Appearance: Eosinophilic (pink), round to oval, sharply defined, 2-10 µm; contain basophilic inner granules
• Demonstration:
  • Seller's stain (magenta/red Negri bodies against blue-grey neurons) - on brain impression smears
  • H&E stain - eosinophilic intracytoplasmic inclusions
  • Immunofluorescence (most sensitive and specific)
• Diagnostic significance: Presence of Negri bodies in brain = pathognomonic of rabies
• Note: Absent in ~20% of rabies cases; hence immunofluorescence is preferred

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SN 2. Inclusion Bodies

1. Negri bodies - Rabies virus; neurons; eosinophilic (Seller's stain)
2. Bollinger bodies - Fowlpox virus; cytoplasmic; contain Borrel bodies (individual virions)
3. Henderson-Patterson (Molluscum) bodies - Molluscum contagiosum virus; keratinocytes
4. Guarnieri bodies - Vaccinia/Variola virus; cytoplasmic

1. Cowdry type A - HSV, VZV, CMV, Yellow fever; eosinophilic, surrounded by clear halo, marginated chromatin ("owl eye" in CMV)
2. Cowdry type B - Poliovirus, Rabies (older classification); multiple small, no halo
3. Torres bodies - Yellow fever; intranuclear eosinophilic
4. Intranuclear inclusions of Adenovirus - basophilic, fill nucleus ("smudge cells")

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VIRUSES

SN 1. Hepatitis B - Laboratory Diagnosis

1. ELISA for HBsAg, Anti-HBc, HBeAg, Anti-HBs
2. HBV DNA by PCR (quantitative - viral load; qualitative for occult HBV)
3. Liver function tests (ALT, AST elevated)
4. Liver biopsy for histology, fibrosis staging
5. HBsAg by CLIA/RIA (blood bank screening)

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SN 2. Rhabdoviruses - Immunoprophylaxis / Schedule / Non-neural Vaccines

1. HDCV (Human Diploid Cell Vaccine) - MRC-5 cells; gold standard; 1 mL IM
2. PCECV (Purified Chick Embryo Cell Vaccine) - Rabipur; 1 mL IM
3. PVRV (Purified Vero Rabies Vaccine) - Verobrab; 0.5 mL IM

• Wound care (most important): Wash wound with soap and water 15 min + povidone iodine
• RIG (Rabies Immune Globulin): Given on Day 0 only; infiltrate into wound; HRIG 20 IU/kg or ERIG 40 IU/kg
• Vaccine (Essen schedule): Days 0, 3, 7, 14, 28 - 5 doses IM deltoid
• Zagreb (2-1-1) schedule: Days 0(×2 sites), 7, 21 - 4 doses; WHO-approved; saves doses

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SN 3. Influenza Virus and HIV - Diagrams (Described)

• Enveloped, segmented -ssRNA virus (8 segments)
• Outer spikes:
  • Haemagglutinin (HA) - triangular trimer; attaches to sialic acid receptors; fusion protein; vaccine target
  • Neuraminidase (NA) - mushroom-shaped tetramer; cleaves sialic acid → allows release of new virions; target of oseltamivir/zanamivir
• M2 ion channel in envelope (target of amantadine)
• M1 protein (matrix protein) beneath envelope
• Nucleoprotein (NP) around RNA segments
• PB1, PB2, PA proteins (RNA polymerase)

• Enveloped, +ssRNA retrovirus (diploid - 2 copies of RNA)
• Outer envelope: Lipid bilayer + gp120 (SU - surface unit; binds CD4) + gp41 (TM - transmembrane; mediates fusion)
• gp120 + gp41 = gp160 (processed by furin)
• Matrix (MA/p17): Between envelope and core
• Core (capsid, p24): Conical; contains 2 RNA strands + RT + IN + PR
• Enzymes: Reverse transcriptase (p66/p51), Integrase (p32), Protease (p11)
• Genome: gag, pol, env + accessory genes (tat, rev, vif, vpr, vpu, nef)

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SN 4. Type-A Influenza Virus - Antigenic Variations and Significance

• Mechanism: Accumulation of point mutations in the genes for HA and NA due to error-prone RNA polymerase (no proofreading)
• Effect: Gradual, stepwise change in antigenic structure of HA/NA; old antibodies partially effective
• Result: Annual seasonal epidemics (partial immunity in population); new vaccine strain needed each year
• Affects all influenza types (A, B)

• Mechanism: Reassortment of gene segments between human influenza virus and animal (avian/swine) influenza virus in a mixed-infected host (pig as "mixing vessel")
• Effect: Sudden appearance of new HA and/or NA subtype (novel virus); no pre-existing immunity in population
• Result: Pandemic (worldwide epidemic); high morbidity and mortality
• Only affects Type A (has animal reservoirs); 18 HA and 11 NA subtypes known

• Necessitates annual influenza vaccine reformulation (WHO recommends strains twice yearly)
• Basis of pandemics (1918 H1N1 "Spanish flu"; 1957 H2N2; 1968 H3N2; 2009 H1N1 "Swine flu")

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SN 5. Merits and Demerits of Salk's and Sabin's Vaccines

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SN 6. Classify Herpesviridae / Subfamilies / Virus / Infection

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SN 7. HIV - Pathogenesis / Opportunistic Infections / Lab Diagnosis / HIV Testing Strategies

1. HIV gp120 binds CD4 on Th cells + CCR5 (macrophage-tropic, early) or CXCR4 (T-tropic, late) co-receptor
2. gp41 mediates membrane fusion → viral RNA enters cell
3. Reverse transcriptase (error-prone) → proviral DNA → integrase → integrated into host chromosome (provirus) - permanent
4. Provirus latent in resting CD4 cells; activated by NF-κB → viral replication → cell death
5. CD4 count progressively declines (normal 500-1500/µL) → immune deficiency
6. CD4 < 500: Symptomatic HIV; CD4 < 200: AIDS (opportunistic infections)
7. CD8 T cells initially control viremia; exhausted over time

1. Pneumocystis jirovecii Pneumonia (PCP) - CD4 <200; bilateral interstitial pneumonia; "ground-glass" X-ray
2. Cryptococcal meningitis - CD4 <100; Cryptococcus neoformans; diagnosed by India ink + serum/CSF cryptococcal antigen
3. Others: CMV retinitis, MAC infection, Toxoplasma encephalitis, esophageal candidiasis, Kaposi's sarcoma

1. Screening (ELISA/CLIA): Detects anti-HIV antibodies ± p24 antigen (4th generation tests = combo test)
2. Confirmatory: Western blot (bands at gp41, gp120, p24 confirm) or Line Immunoassay (LIA)
3. CD4 count (flow cytometry) - staging, ART initiation, monitoring
4. HIV RNA viral load (RT-PCR/NASBA) - treatment monitoring, diagnosis in neonates
5. HIV p24 antigen - detected in window period before antibodies

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SN 8. Four Oncogenic Viruses

1. HPV (Human Papillomavirus) - types 16, 18 → Cervical cancer, oropharyngeal cancer; E6 (degrades p53), E7 (inactivates Rb)
2. HBV + HCV → Hepatocellular carcinoma (HCC)
3. EBV (Epstein-Barr Virus) → Burkitt's lymphoma (c-myc translocation), Nasopharyngeal carcinoma, Hodgkin's lymphoma, post-transplant lymphoma
4. KSHV/HHV-8 (Kaposi's Sarcoma Herpesvirus) → Kaposi's sarcoma (AIDS-defining)
5. HTLV-1 (Human T-cell Leukemia Virus type 1) → Adult T-cell leukemia/lymphoma (ATL)

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SN 9. Epstein-Barr Virus (EBV)

• Family: Herpesviridae, Gammaherpesvirinae (HHV-4)
• Tropism: B lymphocytes (via CD21/CR2 receptor) and epithelial cells
• Latency: Persists in memory B cells for life

1. Infectious Mononucleosis (glandular fever, "kissing disease"):
   • Fever, pharyngitis (exudative), lymphadenopathy (posterior cervical), splenomegaly, atypical lymphocytes (Downey cells - activated CD8 T cells)
   • Monospot test (heterophile antibody test): Paul-Bunnell test; EBV-infected B cells produce heterophile antibodies that agglutinate sheep/horse RBCs
   • Complications: Splenic rupture, airway obstruction, hemolytic anemia, Guillain-Barré
2. Burkitt's Lymphoma (endemic African type - jaw tumor in children; associated with malaria co-infection; t(8;14) c-myc translocation)
3. Nasopharyngeal Carcinoma (Southern China; EBV in all cases)
4. Hodgkin's Lymphoma (Reed-Sternberg cells contain EBV; mixed cellularity subtype)
5. Post-transplant lymphoproliferative disorder (PTLD)
6. Hairy oral leukoplakia (in HIV patients)

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LAQ 1. HIV - Window Period / Lab Diagnosis / Etiology / Pathogenesis

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LAQ 2. Hepatitis - Classify / Lab Diagnosis / Pathogenesis HBV / Serological Markers / Prophylaxis

1. HBV enters hepatocytes via NTCP receptor (sodium taurocholate cotransporting polypeptide)
2. Viral DNA converted to cccDNA (covalently closed circular DNA) in nucleus - template for all viral RNAs; extremely stable - basis of chronicity
3. HBV is NOT directly cytopathic; liver damage is immune-mediated (CD8+ T cell attack on HBV-infected hepatocytes)
4. Vigorous immune response → viral clearance + liver damage → acute hepatitis → resolution
5. Poor immune response (neonates, immunocompromised) → persistent infection → chronic hepatitis → cirrhosis → HCC (via integration of HBV DNA, HBx protein promoting carcinogenesis)

• HBV Vaccine: Recombinant HBsAg (yeast-derived); 3 doses (0, 1, 6 months); >95% seroprotection
• HBIG (Hepatitis B Immune Globulin): Passive immunization; for needle-stick, newborn of HBsAg+ mother (within 12 hrs + vaccine)
• India EPI Schedule: HBV vaccine at birth (0), 6, 10, 14 weeks (as Pentavalent - DPT-HBV-Hib)

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LAQ 3. Herpes Virus - Classification / VZV / HSV

• Varicella (chickenpox - primary infection):
  • Incubation 14-21 days; fever + generalized pruritic rash
  • Rash: Macule → Papule → Vesicle ("dew drops on rose petal") → Pustule → Crust; crops appear simultaneously (different stages coexist)
  • Highly contagious; respiratory droplets + contact with lesions
• Herpes Zoster (Shingles - reactivation):
  • Painful unilateral dermatomal vesicular rash; follows sensory nerve
  • Post-herpetic neuralgia (PHN) common complication

1. Tzanck smear (scraping from base of vesicle) → multinucleated giant cells (Cowdry A inclusions) - not specific for VZV vs HSV
2. PCR (VZV DNA) - most sensitive and specific; from vesicle fluid
3. DFA (Direct Fluorescent Antibody) with VZV-specific antibody
4. Viral culture (slow; rarely done)
5. Serology: IgM anti-VZV (acute), IgG anti-VZV (immunity)

• HSV-1: Oral herpes (cold sores, herpes labialis); primary herpetic gingivostomatitis; herpes encephalitis (temporal lobe)
• HSV-2: Genital herpes (vesicles on genitalia → ulcers); neonatal herpes; recurrent genital herpes

1. Tzanck smear - multinucleated giant cells, intranuclear inclusions
2. PCR - gold standard (vesicle fluid, CSF for encephalitis)
3. Viral culture (Vero/MRC-5 cells) - CPE in 24-48 hrs
4. DFA with HSV-1/HSV-2 specific antibodies
5. Serology: Type-specific IgG (HSV-1 vs HSV-2) by Western blot/ELISA

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LAQ 4. Polio Viruses - Pathogenicity / Immunoprophylaxis / Lab Diagnosis

• Family: Picornaviridae; Genus: Enterovirus
• Non-enveloped, +ssRNA; 3 serotypes (1, 2, 3); Type 1 most common cause of paralysis
• Stable to acid; killed by heat, chlorine, UV

1. Feco-oral transmission; incubation 7-14 days
2. Infects oropharyngeal and intestinal epithelium → primary replication
3. Spreads to lymph nodes (tonsils, Peyer's patches) → minor viremia
4. Major viremia → CNS (rare, <1% of infections) → anterior horn motor neurons of spinal cord and brainstem
5. Virus destroys motor neurons → asymmetric flaccid paralysis (no sensory loss)
6. Types of infection:
   • Inapparent (90-95%)
   • Abortive polio (minor illness - fever, headache)
   • Non-paralytic (aseptic meningitis)
   • Paralytic (spinal, bulbar, bulbospinal)
7. Post-polio syndrome: Fatigue and new muscle weakness 15-40 years after paralytic polio

1. Virus isolation: Stool (most sensitive - shed for 3-6 weeks); throat swab; CSF (rarely)
   • Inoculate onto RD cells, Vero cells, HEp-2 - CPE (rounding, clumping, lysis)
   • Identify by neutralization with type-specific antisera
2. PCR (RT-PCR for poliovirus VP1 gene) - differentiates wild poliovirus from vaccine-derived (VDPV)
3. CSF: Pleocytosis (lymphocytes), normal glucose, elevated protein (non-specific)
4. Serology: Neutralization test (fourfold rise)

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LAQ 5. Influenza Viruses - Morphology / Antigenic Variations / Pathogenesis / Classification / Antigenic Shift

• Family: Orthomyxoviridae
• Types A, B, C (based on NP and M1 antigens - nucleoprotein and matrix)
• Type A: Animals + humans; pandemics; 18 HA (H1-H18), 11 NA (N1-N11) subtypes
• Type B: Humans only; seasonal epidemics; slower evolution
• Type C: Mild illness; no HA/NA; no epidemics

• Enveloped, -ssRNA, 8 segments (Type A/B); pleomorphic (spherical 80-120 nm or filamentous)
• HA spikes (triangular trimers) and NA spikes (mushroom tetramers) on surface in ratio ~4:1
• M1 matrix protein beneath envelope
• M2 ion channel (Type A only - target of amantadine)
• PB1, PB2, PA (polymerase complex), NP (nucleoprotein), NS1, NS2

1. Respiratory droplet inhalation
2. HA binds sialic acid on respiratory epithelium (α2-6 linkage for human strains; α2-3 for avian)
3. Endocytosis → uncoating → RdRp synthesizes mRNA → viral proteins → assembly
4. NA cleaves sialic acid from mucus → virus reaches epithelium; also cleaves from new virions → release
5. Ciliated epithelial cells destroyed → impaired mucociliary clearance → secondary bacterial pneumonia (S. aureus, S. pneumoniae, H. influenzae)
6. Cytokine storm (severe influenza/H5N1/H1N1) - TNF, IL-6, IL-1 → ARDS

1. Pig infected simultaneously with human influenza (H1N1) and avian influenza (H5N2)
2. Both viruses replicate in same pig cell → 8 gene segments of both viruses mix randomly
3. Novel reassortant virus emerges with new HA (e.g., H5) + human internal genes → pandemic strain
4. No existing population immunity → worldwide spread

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MYCOLOGY

GENERAL ASPECTS

SN 1 & 2. Classify Medically Important Fungi / Morphological Classification

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SUPERFICIAL AND SUBCUTANEOUS MYCOSES

SN 1. Subcutaneous Mycosis

• Infection involving the dermis, subcutaneous tissue, fascia, bone following traumatic implantation of saprophytic fungi from soil/vegetation
• Examples:
  1. Mycetoma (Madura foot) - chronic granulomatous infection; grains in pus
  2. Sporotrichosis - Sporothrix schenckii; "rose thorn" infection; lymphocutaneous form
  3. Chromoblastomycosis - Fonsecaea, Phialophora; cauliflower-like verrucous lesions on limbs
  4. Rhinosporidiosis - Rhinosporidium seeberi; polyps in nose/eye

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SN 2. Mycetoma - Causative Agents / Eumycetoma Lab Diagnosis

• Black grains: Madurella mycetomatis (most common worldwide), Leptosphaeria senegalensis, Exophiala jeanselmei
• White/pale grains: Pseudallescheria boydii (Scedosporium), Acremonium, Aspergillus

• Nocardia brasiliensis, Actinomadura madurae, Streptomyces somaliensis

1. Pus/grain examination: Squeeze grains → wash in saline → KOH preparation → fungal hyphae within grains
2. Grain characteristics: Color, texture, size help identify organism
3. Gram stain/ZN stain: Actinomycetoma - gram-positive filaments; fungal - broad hyphae
4. Culture: Sabouraud's dextrose agar (SDA) at 25-30°C; identify colony + microscopy
5. Histopathology: H&E - grains surrounded by Splendore-Hoeppli material; hyphae within

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SN 3. Dermatophytes - Classify / Lab Diagnosis

1. Trichophyton - infects hair, skin, nails; most species
   • T. rubrum (most common worldwide), T. mentagrophytes, T. tonsurans
2. Microsporum - infects hair and skin (NOT nails)
   • M. canis (from dogs/cats), M. audouinii, M. gypseum
3. Epidermophyton - infects skin and nails (NOT hair)
   • E. floccosum

• Anthropophilic (humans) - T. rubrum, E. floccosum
• Zoophilic (animals) - M. canis, T. verrucosum
• Geophilic (soil) - M. gypseum

1. Specimen: Skin scrapings (from active edge), nail clippings, hair (with roots)
2. Direct microscopy (KOH preparation):
   • 10-20% KOH dissolves keratin → fungal elements visible
   • Skin: branching septate hyphae + arthrospores
   • Hair: Ectothrix (spores outside hair shaft) or Endothrix (spores inside hair shaft)
3. Fluorescence: Calcofluor white stain
4. Wood's lamp: Microsporum species fluoresce green-yellow; Trichophyton usually does NOT
5. Culture on SDA + cycloheximide + chloramphenicol at 25-28°C; 1-3 weeks
   • Macroconidia morphology identifies genus/species

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LAQ 1. Dermatophytes - Classify / Pathogenicity / Lab Diagnosis

• Keratinophilic and keratinolytic (produce keratinases, proteinases)
• Invade stratum corneum (skin), hair shaft, nail plate - do not invade living tissue
• Host reaction: Inflammation mediated by DTH (Type IV) to fungal antigens
• Tinea (ringworm) infections by site:
  • Tinea capitis (scalp), T. corporis (body), T. pedis (athlete's foot), T. cruris (groin/jock itch), T. unguium/onychomycosis (nails), T. barbae (beard)

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SYSTEMIC AND OPPORTUNISTIC MYCOSES

SN 1. Histoplasma capsulatum - Morphology / Growth / Pathogenesis

• Dimorphic fungus
• At 37°C (tissue form/yeast): Small oval yeast cells (2-5 µm) with narrow-based budding; found INTRACELLULARLY in macrophages ("capsulated" = surrounded by halo but NO true capsule)
• At 25°C (environmental/mould): White-brown cottony colonies; tuberculate macroconidia (thick-walled, spiky/thumb-nail projections) - pathognomonic; microconidia (2-5 µm) - infectious form

• SDA at 25°C: Slow-growing (2-4 weeks), white-brown cottony colony
• BHI blood agar at 37°C: Cream yeast colonies
• BHIB (BHI broth) with sheep blood preferred for blood cultures

1. Inhalation of microconidia (bat/bird droppings contaminated soil - Mississippi/Ohio River valleys)
2. Conidia reach alveoli → phagocytosed by macrophages → convert to yeast form (parasitic phase)
3. CMI develops (3-4 weeks) → granuloma formation → calcification → heals (most cases)
4. In immunocompromised (AIDS, CD4 <150): Disseminated histoplasmosis → hepatosplenomegaly, bone marrow involvement, skin lesions, oral ulcers

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SN 2. Opportunistic Mycoses

1. Candida albicans - oral thrush, esophageal candidiasis, vulvovaginitis, systemic candidiasis (catheter-related fungemia)
2. Aspergillus fumigatus - invasive pulmonary aspergillosis, aspergilloma, allergic bronchopulmonary aspergillosis (ABPA)
3. Cryptococcus neoformans - cryptococcal meningitis (CD4 <100 in HIV)
4. Mucor/Rhizopus (Mucormycosis/Zygomycosis) - rhinocerebral, pulmonary, GI, cutaneous; diabetics (DKA), COVID-19 associated mucormycosis (India 2021)
5. Pneumocystis jirovecii - PCP (CD4 <200; bilateral interstitial pneumonia)

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SN 3. Candida albicans

• Morphology: Gram-positive oval budding yeast (3-6 µm) with pseudohyphae and true hyphae; produces chlamydospores (terminal thick-walled spores on corn meal agar)
• Germ tube test: Candida albicans produces germ tubes (short hyphal extensions without constriction) when incubated in serum at 37°C for 2-3 hrs - differentiates C. albicans from other Candida species
• Reynolds-Braude phenomenon = Germ tube test
• Culture on Sabouraud's: White-cream, smooth, yeast-like colonies
• Diseases: Oral thrush, esophageal candidiasis, vulvovaginitis, onychomycosis, systemic/invasive candidiasis (endocarditis, meningitis, UTI in immunocompromised)
• Virulence factors: Adherence (Als proteins), hyphae (tissue invasion), proteinases (Saps), phenotypic switching, biofilm formation

1. KOH + Calcofluor white - pseudohyphae + yeast cells in clinical specimen
2. Gram stain - Gram-positive yeast + pseudohyphae
3. Culture: SDA; germ tube test (C. albicans); CHROMagar Candida (color-differentiation)
4. Serology: Beta-D-glucan (panfungal marker); Candida mannan antigen + anti-mannan
5. Blood culture (BACTEC) for candidemia

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SN 4. Aspergillosis in Humans

1. Allergic Bronchopulmonary Aspergillosis (ABPA): Type I + III hypersensitivity; asthma + eosinophilia + elevated IgE + central bronchiectasis; common in asthmatics + CF patients
2. Aspergilloma (Fungal ball): Aspergillus colonizes pre-existing lung cavity (TB cavity, sarcoidosis) → mass of hyphae + debris; hemoptysis; X-ray: "air crescent sign"
3. Invasive Pulmonary Aspergillosis (IPA): Most serious; in profoundly neutropenic patients (chemotherapy, bone marrow transplant); fever, pleuritic chest pain, hemoptysis; CT: "halo sign" (hemorrhage around fungal nodule)
4. Chronic necrotizing aspergillosis: Slowly progressive; diabetics, COPD, alcoholics

• BAL/sputum: KOH → septate hyphae with acute-angle (45°) branching (Aspergillus)
• Culture: SDA - velvety green colonies with conidiophores (vesicle + phialides + conidia)
• Serum Galactomannan ELISA (Aspergillus cell wall antigen; sensitive in IPA)
• Beta-D-glucan (non-specific for fungi)
• CT chest (halo sign/air crescent sign) + biopsy

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SN 5. Reynolds-Braude Phenomenon

• Same as Germ Tube Test (Reynolds and Braude described it in 1956)
• Principle: Candida albicans produces germ tubes (short, hypha-like extensions without constriction at point of origin) when incubated in human/fetal calf serum at 37°C for 2-3 hours
• Significance: Rapid presumptive identification of C. albicans; C. albicans and C. dubliniensis are germ-tube positive; other Candida species (C. tropicalis, C. glabrata, C. parapsilosis) are negative
• Observation: Under microscope: short, thin, tube-like projections from yeast cells, without constriction

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SN 6. Cryptococcal Meningitis - Laboratory Diagnosis

1. India ink preparation (CSF): Negative staining; cryptococcal capsule appears as clear halo around yeast cells against dark background; "capsule" is characteristic; sensitivity ~60-80% in HIV-associated cryptococcal meningitis
2. Cryptococcal antigen (CrAg) latex agglutination or LFA (lateral flow assay):
   • Detects polysaccharide capsular antigen in CSF and serum
   • Most sensitive test (>95%); can screen serum in HIV patients
   • LFA (fingerstick blood test) used for screening in resource-limited settings
3. Culture on SDA: Mucoid, cream-colored yeast colonies; urease positive
4. Mucicarmine stain / Alcian blue: Stains capsule pink/blue in tissue sections
5. CSF findings: Elevated opening pressure, lymphocytic pleocytosis, elevated protein, low glucose

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SN 7. Fungi causing Opportunistic Infections in HIV / Candida Lab Diagnosis

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LAQ 1. Four Fungi Causing Opportunistic Infections / Lab Diagnosis of Candidiasis

1. Candida albicans
2. Cryptococcus neoformans
3. Aspergillus fumigatus
4. Pneumocystis jirovecii (previously Pneumocystis carinii - formerly classified as protozoan)
5. Mucor/Rhizopus species

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PARASITOLOGY

FLAGELLATES

SN 1. LD Bodies (Leishmania donovani)

• LD Bodies = Leishman-Donovan (LD) bodies = Amastigotes
• Morphology: Small (2-3 µm), oval, intracellular (within macrophages); contain:
  • Large nucleus (stains red with Leishman/Giemsa)
  • Kinetoplast (rod-shaped mitochondrial DNA; stains deeply with Leishman) - diagnostic
  • No free flagellum in this stage
• Location: Within macrophages of reticuloendothelial system - spleen, liver, bone marrow, lymph nodes
• Demonstration:
  1. Splenic aspirate smear (most sensitive, >95%) - Giemsa stain: LD bodies in macrophages
  2. Bone marrow aspirate (safer)
  3. Liver biopsy
  4. Buffy coat of blood
  5. Leishman stain/Giemsa

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SN 2. Acute Giardiasis - Lab Diagnosis / Findings

1. Stool examination (fresh stool):
   • Trophozoites (in liquid/watery stool): Pear-shaped, bilaterally symmetrical, 2 nuclei ("owl-face"), 4 pairs of flagella, 2 median bodies; motility: falling leaf/tumbling
   • Cysts (formed stool): Oval; 4 nuclei; 4 flagella; 2 median bodies; 8-12 µm
2. Concentration methods: Formol-ether sedimentation (for cysts)
3. Duodenal aspirate/biopsy (if stool negative): String test (Enterotest) - capsule swallowed, string into duodenum → aspirate
4. ELISA/RIA for Giardia antigen in stool - most sensitive (~95%)
5. Immunofluorescence (DIF) with monoclonal antibody

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LAQ 1. Leishmania donovani - Life Cycle / Kala Azar - Pathogenicity / Lab Diagnosis

1. Sandfly (vector - Phlebotomus) bites infected human → ingests macrophages with LD bodies (amastigotes)
2. In sandfly midgut: Amastigotes transform into promastigotes (elongated, 15-25 µm, anterior flagellum, extracellular)
3. Promastigotes multiply → migrate to proboscis
4. Sandfly bites human → inoculates promastigotes into skin
5. Promastigotes phagocytosed by macrophages → transform back to amastigotes (LD bodies) → multiply by binary fission → macrophage bursts → infects new macrophages
6. Spread via blood to spleen, liver, bone marrow, lymph nodes

• Massive splenomegaly (most prominent sign), hepatomegaly, lymphadenopathy
• Bone marrow infiltration → pancytopenia (anemia, leucopenia, thrombocytopenia)
• Hypergammaglobulinemia (polyclonal IgG) - non-specific
• Fever (twice daily "double quotidian" fever)
• Progressive wasting, anorexia
• Darkening of skin (Hindi: "Kala Azar" = black fever)
• Post-Kala Azar Dermal Leishmaniasis (PKDL): Skin lesions after treatment (hypopigmented macules, nodules - reservoir of infection)

1. Splenic aspirate (gold standard, >95% sensitivity) - Giemsa stain: LD bodies in macrophages
2. Bone marrow biopsy/aspirate (safer)
3. rK39 dipstick test: Recombinant antigen K39 (kinesin-related protein); detects anti-Leishmania IgG; sensitivity ~90-100%; simple, rapid field test
4. ELISA for anti-Leishmania antibodies
5. PCR (blood, tissue) - most sensitive; not widely available
6. Aldehyde test (Napier's formol gel test): 1 drop of 40% formaldehyde → serum gels in <20 min due to hypergammaglobulinemia; not specific
7. Montenegro test (leishmanin skin test): Intradermal killed promastigotes → DTH reaction; NEGATIVE in active VL (anergic); positive after cure

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SPOROZOA

SN 1. Malaria - Laboratory Diagnosis

1. RDTs (Rapid Diagnostic Tests): Immunochromatographic strips detecting HRP-2 (P. falciparum) or pLDH (all species); point-of-care; 15 min
2. PCR (nested PCR): Most sensitive; species and drug resistance genotyping; research/reference labs
3. QBC (Quantitative Buffy Coat): Acridine orange stains malaria DNA; fluorescence microscopy; centrifuged buffy coat
4. Serology (IFAT, ELISA): Not for acute diagnosis; epidemiological surveys

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SN 2. Plasmodium falciparum - Complications / Lab Diagnosis

1. Cerebral malaria: Sequestration of parasitized RBCs in brain capillaries → coma, convulsions, herniation; mortality ~20%
2. Severe anemia: Hemolysis + bone marrow suppression + RBC removal by spleen
3. Acute Renal Failure (ARF): Tubular necrosis; "blackwater fever" (massive hemolysis + hemoglobinuria → black urine)
4. Pulmonary edema/ARDS
5. Hypoglycemia: Parasite consumes glucose + quinine stimulates insulin
6. Disseminated Intravascular Coagulation (DIC)
7. Hyperparasitemia (>5% parasitemia)
8. Splenic rupture
9. Algid malaria: Circulatory collapse, cold extremities, hypotension

• Cytoadherence: Parasitized RBCs express PfEMP1 (P. falciparum Erythrocyte Membrane Protein 1) on surface → bind ICAM-1, CD36, thrombospondin on endothelium → rosetting + sequestration in microvasculature → organ failure

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LAQ 1. Malignant Tertian Malaria - Life Cycle / Complications / Lab Diagnosis

1. Female Anopheles bites infected human → ingests gametocytes
2. Male gametocyte (microgametocyte) exflagellates → microgametes
3. Fertilization → Ookinete → penetrates midgut wall → Oocyst → Sporogony → Sporozoites
4. Sporozoites migrate to salivary glands

1. Pre-erythrocytic (Exoerythrocytic) schizogony (liver):
   • Mosquito inoculates sporozoites → travel to liver → infect hepatocytes
   • Schizogony: 1 sporozoite → 30,000 merozoites (P. falciparum - 5.5 days)
   • Liver schizonts rupture → merozoites released → enter RBCs
   • NO HYPNOZOITE in P. falciparum (no relapses - only recrudescences)
2. Erythrocytic schizogony:
   • Merozoite → Ring stage (trophozoite) → mature trophozoite → schizont → 16-24 merozoites → RBC ruptures → fever paroxysm (every 48 hrs - tertian)
   • P. falciparum: Mature trophozoites sequester in deep capillaries; only rings + gametocytes in peripheral blood

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LAQ 2. Plasmodium vivax - Morphology / Life Cycle / Lab Diagnosis

• Enlarged, pale (due to Schüffner's dots), irregular-shaped RBCs
• Schüffner's dots (stippling): Eosinophilic granules in infected RBC (represent knob-like projections on RBC membrane)
• Ring stage (early trophozoite): Large ring, occupying 1/3 of RBC; one chromatin dot
• Amoeboid trophozoite: Irregular, pseudopod-like projections; pigment (haemozoin - golden-brown) present
• Schizont: 16-24 merozoites arranged around central pigment (rosette/daisy head)
• Gametocyte: Large, round, fills RBC; microgametocyte (paler, diffuse chromatin); macrogametocyte (compact chromatin)

• Hypnozoites (dormant forms) in liver → RELAPSES months/years later
• Erythrocytic cycle: 48 hrs (Benign Tertian)
• Merozoites preferentially invade young RBCs (reticulocytes) via Duffy blood group antigen (DARC) → Duffy-negative Africans are resistant

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CESTODES (TAPEWORMS)

SN 1. Hydatid Cyst - Causative Agent / Sites / Cross-Section Structure

• Definitive host: Dog (harbors adult tapeworm)
• Intermediate host: Sheep, cattle, humans (accidental)

1. Liver (most common, 65-70%)
2. Lung (20-25%)
3. Bone, brain, kidney (rare)

• Pericyst (Host-derived): Outermost layer; fibrous, avascular host tissue
• Ectocyst (Laminated membrane): Laminated, acellular, white/opalescent - characteristic of Echinococcus
• Endocyst (Germinal/germinal layer): Inner nucleated layer; produces protoscolices (brood capsules + scolices = "hydatid sand"), daughter cysts, and laminated membrane
• Hydatid fluid: Clear, salty; contains scolices, brood capsules, hooks ("hydatid sand")
• Daughter cysts: Secondary cysts within primary cyst

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SN 2. Echinococcus granulosus - Life Cycle

1. Adult tapeworm in small intestine of dog (definitive host); 3-6 mm, 3 proglottids (1 immature, 1 mature, 1 gravid)
2. Gravid proglottids/eggs passed in dog feces → contaminate grass, soil, water
3. Sheep/cattle/humans (intermediate hosts) ingest eggs
4. Egg hatches in duodenum → oncosphere (hexacanth embryo) with 6 hooklets → penetrates intestinal wall → portal vein → liver (first filter)
5. Some pass to lung, brain, bone via circulation
6. Oncosphere → develops into hydatid cyst (slowly over months-years)
7. Dog (definitive host) eats viscera of infected sheep → protoscolices in intestine → develop into adult tapeworms

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SN 3. Taenia saginata vs. Taenia solium - Four Differences

• Fluid-filled bladder (1 cm) with invaginated scolex; found in brain, muscles, eye, subcutaneous tissue
• Neurocysticercosis: Most common cause of acquired epilepsy in developing world; seizures, headache, raised ICP; diagnosed by CT/MRI (calcified cysts) + serology (ELISA)

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LAQ 1. Cestodes - Classify / Cysticercosis - Pathogenesis

1. Source of infection: Ingestion of T. solium eggs (not cysticerci) - via contaminated food/water/hands OR autoinfection (proglottid regurgitated into stomach → oncospheres released)
2. Eggs hatch in intestine → oncospheres penetrate intestinal wall → bloodstream
3. Disseminate to brain (most dangerous), muscles, subcutaneous tissue, eye
4. In tissue → develop into cysticercus (fluid-filled bladder with scolex) over 2-3 months
5. Symptoms appear when cyst dies:
   • Live cyst: Often asymptomatic (immune evasion)
   • Dying cyst: Leaks antigens → intense inflammation, edema, seizures
   • Dead calcified cyst: Epilepsy, headache
6. Neurocysticercosis: Seizures (most common symptom), raised ICP, focal neurological deficits, hydrocephalus (if cysts in ventricles)

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LAQ 2. Taenia solium - Morphology / Life Cycle / Pathogenesis / Lab Diagnosis

• Scolex: Globular, 1 mm; double row of hooks (22-32) on rostellum + 4 suckers ("armed tapeworm")
• Neck: Thin, unsegmented (germinal zone)
• Strobila (body): 800-1000 proglottids; 2-7 m length
• Gravid proglottid: Uterus with 7-12 lateral branches (vs T. saginata 15-30); passed in feces (may be motile)
• Egg: Round, brown, striated shell (embryophore); contains hexacanth embryo (oncosphere) with 6 hooklets; 30-40 µm

1. Intestinal taeniasis: Scolex attaches to jejunum → tapeworm; usually asymptomatic; occasional abdominal pain, weight loss, proglottids in stool
2. Cysticercosis: (See LAQ 1 above)

1. Stool examination: Gravid proglottids (count uterine branches to differentiate T. solium from T. saginata) + eggs (round, striated)
2. Scotch tape swab (anal area - eggs like Enterobius)
3. ELISA/Western blot for anti-taenia antibodies (serology)

1. CT/MRI brain: Cystic lesions (+ "dot sign" = scolex), calcifications, perilesional edema
2. Serology: ELISA (Cysticercus cellulosae antigen) or Enzyme-linked immunoelectrotransfer blot (EITB/Western blot) - most specific (>99%) and sensitive (~94%)
3. Subcutaneous nodule biopsy: Cysticercus on excision + histology (scolex visible)
4. Fundoscopy: Subretinal/vitreous cysticercus

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NEMATODES

SN 1. Strongyloides stercoralis Hyperinfection

• Occurs in immunocompromised hosts (corticosteroids, HTLV-1, AIDS, transplant, hematological malignancies)
• Mechanism: Normal lifecycle has low-level autoinfection (rhabditiform larvae → filariform larvae in gut → penetrate colon wall → back to lungs → swallowed); in hyperinfection, this is massively amplified
• Features:
  • Massive intestinal and pulmonary involvement
  • Larvae carry gut bacteria through intestinal wall → Gram-negative sepsis/meningitis (most fatal complication)
  • Hemorrhagic enteritis, pneumonitis, blood in stool
  • Larva currens (rapidly migrating urticarial skin tracks)
• Diagnosis: Stool examination for larvae (rhabditiform); Baermann technique; serology; larvae in BAL, sputum, skin
• Treatment: Ivermectin (drug of choice); Albendazole

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SN 2. Guinea Worm - Life Cycle (Dracunculus medinensis)

1. Infected water contains Cyclops (water flea/copepod) with L3 larvae
2. Human ingests infected Cyclops in drinking water
3. Cyclops digested in stomach → L3 larvae released → penetrate intestinal wall → body cavity
4. L3 larvae mature → adult worms (male 2-3 cm; female 70-120 cm) in subcutaneous tissue (lower limb, ~12-18 months)
5. Gravid female migrates to skin → blister forms (usually on leg/ankle/foot)
6. On contact with water: Blister ruptures → female worm protrudes → releases millions of L1 larvae into water
7. L1 larvae ingested by Cyclops → develop through L2, L3 stages within 2 weeks
8. Cycle complete

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SN 3. Occult Filariasis

• Vigorous immune (IgE) response against microfilariae → microfilariae rapidly sequestered and destroyed in lungs and other tissues
• Tropical Pulmonary Eosinophilia (TPE):
  • Caused by Wuchereria bancrofti or Brugia malayi
  • Features: Paroxysmal nocturnal cough + wheeze, breathlessness; X-ray: bilateral mottling; markedly elevated eosinophilia + high total IgE + anti-filarial IgE and IgG
  • Microfilariae ABSENT in blood
• Diagnosis: High eosinophilia + high IgE + anti-filarial antibody (IgG4 ELISA) + response to DEC treatment

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SN 4. Wuchereria bancrofti - Morphology / Lab Diagnosis

• Adult worms: Thread-like; reside in lymphatics (inguinal lymph nodes, lymph vessels)
  • Male: 40 mm; Female: 80-100 mm
• Microfilariae (diagnostic stage in blood):
  • Sheathed (sheath stains pink with Giemsa)
  • Nocturnal periodicity (maximum in peripheral blood at night 10 PM - 4 AM) - for sampling
  • No nuclei in tail tip (distinguishes from B. malayi which has 2 distinct nuclei in tail tip)
  • 244-296 µm length

1. Blood film (thick/thin): Collected at 10 PM-2 AM; Giemsa stain; identify microfilariae by sheath + tail nuclear pattern
2. Concentration methods: Knott's concentration technique (blood + formalin, centrifuge); membrane filtration (Millipore)
3. DEC provocation test (Mazzotti test): 50 mg DEC → microfilaraemia increases within 1 hr (can cause fever - not widely used)
4. Serology: Og4C3 ELISA (detects circulating filarial antigen - W. bancrofti specific); ICT card test (immunochromatographic) - detects W. bancrofti antigen; highly sensitive
5. Ultrasonography: "Filarial dance sign" - live adult worms in scrotal lymphatics on USG (pathognomonic)

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SN 5. Ectopic Ascariasis

1. Biliary tract: Worm migrates through ampulla of Vater → bile duct → biliary colic, obstructive jaundice, cholangitis, cholecystitis
2. Pancreatic duct: Pancreatitis
3. Appendix: Appendicitis
4. Liver: Hepatic abscess (rare)
5. Peritoneum: Perforation of intestine → peritonitis
6. Lungs (Löffler's syndrome): Larval migration through lungs → eosinophilia + transient pulmonary infiltrates (not true "ectopic" but aberrant migration)

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LAQ 1. Intestinal Nematodes / Ascaris lumbricoides

1. Ascaris lumbricoides (roundworm)
2. Enterobius vermicularis (pinworm/threadworm)
3. Trichuris trichiura (whipworm)
4. Hookworms: Ancylostoma duodenale and Necator americanus
5. Strongyloides stercoralis

1. Infective stage: Embryonated egg (containing L2 larva); ingested via contaminated food/water/soil
2. Egg hatches in duodenum → L2/L3 larvae released → penetrate intestinal wall → portal vein → liver → heart → lungs (Löffler's syndrome: eosinophilia, transient pulmonary infiltrates, cough)
3. Larvae ascend airways → swallowed → reach small intestine → mature to adult worms
4. Adult worms (female 20-35 cm, male 15-30 cm) in jejunum/ileum → mate
5. Female lays 200,000 eggs/day → passed in feces → embryonate in soil (2-4 weeks) → become infective

• Pulmonary: Löffler's syndrome (larval migration) - eosinophilic pneumonia
• Intestinal: Heavy worm burden → malnutrition, intestinal obstruction (worm bolus - most common surgical emergency in children due to parasites), volvulus
• Ectopic ascariasis (biliary, pancreatic - see SN 5)

1. Stool examination: Fertilized eggs (oval, bile-stained, mammillated cortex; unfertilized eggs are longer, irregular) on direct wet mount or concentration methods
2. Adult worm passage (from mouth, anus)
3. X-ray abdomen: "Bunch of worms" appearance if heavy infection
4. Ultrasound/ERCP: Ectopic ascariasis detection

• Egg (fertilized): Oval, 60×45 µm; outer mammillated (bile-stained albuminoid) coat; inner thin shell; L1 larva inside
• Unfertilized egg: Longer (90 µm), irregular, no larva, disorganized granular mass

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LAQ 2. Enterobius vermicularis - Life Cycle / Pathogenicity / Lab Diagnosis

1. Infective stage: Embryonated egg (with L2 larva)
2. Eggs ingested → hatch in duodenum → larvae migrate to cecum/appendix/colon → adult worms
3. Male dies after copulation; Female (8-13 mm) migrates at night to perianal region → lays eggs (10,000 eggs/female) directly on perianal skin → dies
4. Perianal eggs embryonate rapidly (6-8 hrs) → infective
5. Autoinfection: Child scratches → eggs under fingernails → ingested (or retrograde infection up through anus)
6. Retroinfection: Larvae hatch on perianal skin → migrate back through anus → cecum

• Perianal pruritus (nocturnal itching) - main symptom due to egg-laying female migrating at night
• Rarely: Vulvovaginitis, ectopic appendicitis, eosinophilic peritonitis (perforation)
• Sleep disturbance; secondary bacterial infection from scratching

1. Scotch tape (cellophane tape) swab test (Graham's test): Cellophane tape pressed on perianal region EARLY MORNING (before bathing/defecation) → placed on glass slide → microscopy → Enterobius eggs (oval, flat on one side, D-shaped; contain larva)
2. NIH swab: Wooden paddle smeared with adhesive → perianal swab
3. Stool examination: NOT useful (eggs rarely in stool); occasional worm seen

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LAQ 3. Nematodes - Classify

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LAQ 4 & 5. Hookworm / Ancylostoma duodenale - Life Cycle / Pathogenesis / Lab Diagnosis

• Ancylostoma duodenale (Old World hookworm) - 2 pairs of teeth; south Europe, Asia, Africa
• Necator americanus (New World hookworm) - cutting plates; Americas, Africa, Southeast Asia

• Small (A. duodenale: female 12 mm, male 8 mm), cylindrical, pinkish-white
• Scolex (buccal capsule): 2 pairs of ventral teeth (A. duodenale) vs cutting plates (Necator)
• Copulatory bursa in males (umbrella-like structure at posterior end)
• Egg: Oval, thin-shelled, 60×40 µm; 4-8 cell stage when passed (segmenting); IDENTICAL for both hookworm species

1. Eggs passed in feces → embryonate in warm moist soil (1-2 days) → rhabditiform larvae → filariform larvae (L3) - infective stage in 5-7 days
2. L3 penetrate skin (bare foot/skin contact with soil) → bloodstream → heart → lungs (Löffler's syndrome)
3. Cough up → swallowed → reach small intestine → mature to adult worms (3 weeks)
4. Adults attach to intestinal villi via teeth/plates → suck blood
5. Ancylostoma duodenale can also be ingested (oral infection - transmammary transmission in neonates possible)

1. Skin (entry point): "Ground itch" - pruritic, erythematous papulovesicular rash (usually between toes)
2. Pulmonary: Larval migration → Löffler's syndrome (transient cough, eosinophilia)
3. Intestinal (main effect):
   • Adult worms suck blood: A. duodenale sucks 0.2 mL/worm/day; Necator 0.02 mL
   • Heavy infection → iron deficiency anemia (most important; hypo-chromic microcytic)
   • Hypoproteinemia (protein-losing enteropathy) → hypoalbuminemia → edema
   • Malnutrition, growth retardation in children

1. Stool microscopy (direct wet mount): Eggs (oval, thin-shelled, 4-8 cell stage)
2. Concentration methods: Formol-ether sedimentation; flotation (ZnSO4/saturated salt)
3. Stool culture (Harada-Mori technique): Larvae allowed to hatch on filter paper → rhabditiform then filariform larvae; species identification by larval morphology
4. Blood tests: Eosinophilia (peripheral), microcytic hypochromic anemia; low serum iron, elevated TIBC; hypoalbuminemia
5. Duodenal aspirate (rarely needed)

1. Hookworm (A. duodenale, Necator americanus) - iron deficiency anemia (blood-sucking)
2. Plasmodium species - hemolytic anemia
3. Diphyllobothrium latum - B12 deficiency megaloblastic anemia (worm competes for B12)
4. Leishmania donovani - anemia from hypersplenism + bone marrow suppression

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DIAGNOSTIC PROCEDURES

SN 1. Stool Concentration Methods

1. Zinc Sulfate Flotation (Faust's method):
   • Stool + 33% ZnSO4 (specific gravity 1.18) → centrifuge → cysts/eggs float to surface (coverslip picks up surface film)
   • Best for: Protozoan cysts (Giardia, Entamoeba), hookworm eggs
   • Disadvantage: Distorts thick-shelled eggs (Ascaris fertilized), schistosome eggs, operculated eggs
2. Saturated Salt (NaCl) Flotation:
   • Uses saturated NaCl (SG 1.20); similar principle
   • For hookworm eggs, Enterobius eggs

1. Formol-Ether Concentration (Ritchie's technique) - most widely used:
   • Stool + 10% formalin (fixes) + diethyl ether (removes fat)
   • Centrifuge → cysts/eggs sediment in formalin layer
   • Ether (with fat debris) and formalin layers discarded; sediment examined
   • Best for: ALL parasites (cysts, eggs, larvae); thick-shelled eggs; operculated eggs (Taenia, Fasciola)
   • Advantage: Preserves morphology; kills pathogens
2. Simple sedimentation (gravity): Stool + water; parasites settle; rarely used

• Baermann technique: For Strongyloides larvae (larvae migrate up through warm water)
• Knott's concentration: For microfilariae in blood

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LAQ. Parasites Detected in Blood Smear

1. Plasmodium species (P. falciparum, P. vivax, P. malariae, P. ovale) - ring trophozoites, mature stages, gametocytes
2. Microfilariae (Wuchereria bancrofti, Brugia malayi, Loa loa - for W. bancrofti: nocturnal periodicity; Loa loa: diurnal periodicity)
3. Trypanosoma brucei (African sleeping sickness) - in buffy coat/thick film; trypomastigotes
4. Trypanosoma cruzi (Chagas disease) - trypomastigotes in acute phase
5. Leishmania donovani (Kala Azar) - amastigotes in buffy coat/bone marrow (rarely peripheral blood)
6. Babesia species (babesiosis) - intraerythrocytic ring forms; "Maltese cross/tetrad" forms; no hemozoin

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