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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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