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II MBBS MICROBIOLOGY - MODULE 1

Complete Exam Answers (As per Apurba Sastri's Textbook of Microbiology)

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TOPIC 4: GASTROINTESTINAL AND HEPATOBILIARY SYSTEM

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MI 4.1 - Define and Differentiate: Diarrhea, Dysentery, and Food Poisoning; Enumerate Microbial Agents

DEFINITIONS

• Types: Secretory, osmotic, invasive (inflammatory)

• Types: Bacillary dysentery (caused by Shigella) and Amoebic dysentery (Entamoeba histolytica)

• Incubation period is usually short (1-6 hours for toxin-mediated; 12-48 hours for organism-mediated)

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DIFFERENTIATION TABLE

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MICROBIAL AGENTS

• Bacterial:
  • Vibrio cholerae (secretory, rice-water stools)
  • Enterotoxigenic E. coli - ETEC (traveler's diarrhea)
  • Enteropathogenic E. coli - EPEC (infants)
  • Campylobacter jejuni
  • Salmonella species
• Viral:
  • Rotavirus (most common cause in children <5 years)
  • Norovirus (most common in adults/outbreaks)
  • Astrovirus, Adenovirus (enteric types 40, 41)
• Parasitic:
  • Giardia lamblia (fatty, foul-smelling stools, no blood)
  • Cryptosporidium parvum (immunocompromised)
  • Isospora belli

• Bacillary: Shigella dysenteriae (most severe), S. flexneri, S. sonnei
• Amoebic: Entamoeba histolytica
• Others: Enteroinvasive E. coli (EIEC), Campylobacter jejuni

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MI 4.2 - Epidemiology, Morphology, Pathogenesis, Clinical Features, and Diagnostic Modalities of Agents Causing Diarrhoea

A. VIBRIO CHOLERAE (Bacterial - Most Exam-Important)

• Comma-shaped, Gram-negative rod
• Single polar flagellum - "shooting star" motility
• Biotypes: El Tor (currently prevalent) and Classical
• Serogroups: O1 (El Tor and Classical) and O139

• 7th cholera pandemic caused by El Tor biotype
• Source: contaminated water and food
• Fecal-oral transmission
• Endemic in India, Africa, South-East Asia

• Ingestion -> colonization of small intestine
• CT (Cholera Toxin) = A subunit (enzymatic) + B subunits (binding to GM1 ganglioside)
• A subunit activates adenylate cyclase -> increases cAMP -> massive secretion of Cl-, Na+, water -> profuse watery diarrhea
• No mucosal invasion (no blood/pus in stool)

• Sudden onset profuse watery diarrhea - "rice-water stools" (colorless, fishy odor)
• No fever (afebrile)
• Severe dehydration, hypokalemia, metabolic acidosis
• Sunken eyes, skin turgor lost, "washerwomen's hands"
• Can lead to death within hours if untreated (50% mortality untreated)

• Specimen: Fresh stool (rectal swab or stool in Cary-Blair transport medium)
• Direct microscopy: Vibrios with "shooting star" motility, inhibited by specific antiserum (String test +ve)
• Culture: TCBS agar - Yellow colonies (sucrose fermenter)
• Biochemical: Oxidase +ve, String test (0.5% sodium deoxycholate) positive
• Serology: Vibriocidal antibody (research), agglutination with O1 or O139 antiserum
• Rapid test: Cholera RDT (lateral flow)

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B. ROTAVIRUS (Viral - Most Common Cause of Diarrhea in Children)

• Double-stranded RNA virus, family Reoviridae
• Non-enveloped, wheel-like appearance (Latin: rota = wheel) on EM
• 11 segments of dsRNA
• Outer capsid: VP4 (P antigen) and VP7 (G antigen) - used in vaccine development

• Most common cause of severe diarrhea in children < 5 years globally
• Winter predominance (in temperate climates)
• Fecal-oral and possibly droplet transmission
• Very low infectious dose

• Infects enterocytes of small intestine (tip of villi)
• Destroys villi -> malabsorption
• NSP4 protein acts as viral enterotoxin -> increases intracellular Ca2+ -> secretory diarrhea
• Also decreases brush border enzyme (lactase) activity

• Incubation: 1-3 days
• Fever + vomiting (preceding) then watery diarrhea
• Duration: 5-7 days, self-limiting
• Can cause severe dehydration and death in developing countries

• ELISA for viral antigen in stool (method of choice)
• Electron microscopy: Wheel-shaped particles (research labs)
• RT-PCR (definitive, for G and P typing)
• Latex agglutination (rapid bedside test)

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C. GIARDIA LAMBLIA (Parasitic)

• Trophozoite: Pear-shaped, bilaterally symmetrical, 2 nuclei, 4 pairs of flagella, sucking disc - "Falling leaf motility"
• Cyst: Oval, 4 nuclei, 4 median bodies (infective form)

• Most common intestinal protozoan worldwide
• Waterborne, fecal-oral
• Common in immunodeficiency (especially IgA deficiency)

• Cysts ingested -> excystation in duodenum -> trophozoites attach to mucosa via sucking disc
• Mechanical blockage of villi -> malabsorption of fats, fat-soluble vitamins

• Frothy, foul-smelling, greasy (steatorrhoea) stools - NO blood, NO mucus
• Bloating, flatulence, belching
• Chronic infection leads to malabsorption and weight loss

• Stool examination: Cysts in formed stool, trophozoites in loose stool (Lugol's iodine stain)
• String test (Enterotest): Trophozoites from duodenal secretions
• ELISA/IFA: Antigen detection in stool (most sensitive)
• Duodenal biopsy: When above tests negative

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MI 4.3 - Agents of Food Poisoning: Pathogenesis, Clinical Course, and Laboratory Diagnosis

STAPHYLOCOCCUS AUREUS FOOD POISONING

• Contamination of food (custards, mayonnaise, cream pastries, ice cream)
• S. aureus multiplies and produces heat-stable enterotoxin (Types A-E; Type A most common)
• Toxin activates vagal nerve receptors in gut -> vomiting (central vomiting)
• Also causes diarrhea via secretory mechanism

• Incubation: 1-6 hours (very short - toxin pre-formed)
• Profuse vomiting, nausea, abdominal cramps
• Diarrhea (watery, not bloody)
• Usually NO fever (toxin-mediated)
• Self-limiting: recovers in 24-48 hours

• Culture food and vomitus on Mannitol Salt Agar (MSA) - yellow colonies
• Coagulase test, Protein A, DNase test to confirm S. aureus
• Detection of enterotoxin by ELISA (in food)
• Phage typing for outbreak investigation

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CLOSTRIDIUM PERFRINGENS FOOD POISONING

• Type A strains produce enterotoxin in small intestine (when spores survive reheated meat, germinate and sporulate)
• Enterotoxin acts on tight junctions -> increased permeability -> fluid secretion

• Incubation: 8-16 hours
• Profuse watery diarrhea, crampy abdominal pain
• Vomiting and fever uncommon
• Self-limiting, 24 hours

• Anaerobic culture of food (>10^5 organisms/g)
• Spore count in feces
• ELISA for enterotoxin in stool

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CLOSTRIDIUM BOTULINUM FOOD POISONING

• Preformed neurotoxin (A, B, E types in food-borne botulism) in canned/preserved food
• Toxin absorbed -> bloodstream -> blocks release of acetylcholine at NMJ -> flaccid paralysis

• Incubation: 12-36 hours (can be up to 8 days)
• GI symptoms first (nausea, vomiting)
• Then DESCENDING flaccid paralysis: diplopia, dysphasia, dysarthria -> respiratory muscle paralysis
• Afebrile
• No sensory loss, consciousness intact

• Mouse bioassay (gold standard - serum or food injected into mice)
• ELISA for toxin
• Culture on egg yolk agar (double zone hemolysis with lipase activity)

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MI 4.5 - Acid Peptic Disease (APD) - Helicobacter pylori

ETIOLOGY

• S-shaped or spiral-shaped Gram-negative rod
• Multiple polar sheathed flagella - highly motile (corkscrew motility)
• Produces urease (strongly positive) - key virulence factor

EPIDEMIOLOGY

• Most common chronic bacterial infection worldwide (50-70% of world population)
• Prevalence higher in developing countries (80-90%)
• Transmission: oral-oral (most likely), fecal-oral, iatrogenic (contaminated endoscopes)
• Risk factors: overcrowding, poor sanitation, childhood acquisition

PATHOGENESIS

1. H. pylori colonizes gastric mucosa (antrum > body)
2. Urease hydrolyzes urea -> NH3 -> neutralizes acid around bacteria (protection) + toxic to epithelium
3. VacA (Vacuolating Cytotoxin): Causes vacuolation and apoptosis of gastric epithelial cells
4. CagA (Cytotoxin-Associated Gene A): Injected into host cells via Type IV secretion system -> disrupts cell signaling -> promotes inflammation and carcinogenesis
5. Outer Membrane Proteins (OMP) -> adhesion to mucosa
6. Mucinase, phospholipase -> breakdown of mucous barrier
7. Gastric mucosal damage -> decreased somatostatin -> increased gastrin -> hypersecretion of HCl -> peptic ulcer

• Chronic active gastritis (type B gastritis)
• Duodenal ulcer (>90% associated with H. pylori)
• Gastric ulcer (70-80%)
• Gastric adenocarcinoma (Group I carcinogen - IARC)
• MALT lymphoma

CLINICAL FEATURES

• Burning epigastric pain (worse before meals / at night for DU; after meals for GU)
• Nausea, bloating, belching
• Hematemesis, melena (if ulcer bleeds)
• Perforation (acute abdomen)

LABORATORY DIAGNOSIS

MANAGEMENT

• PPI + Amoxicillin + Clarithromycin

• PPI + Bismuth + Metronidazole + Tetracycline

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MI 4.6 / 4.7 - Viral Hepatitis: Epidemiology, Etiopathogenesis, Clinical Features, Complications

OVERVIEW OF HEPATITIS VIRUSES

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HEPATITIS B VIRUS (HBV) - Most Important for Exam

• 42 nm spherical particle
• Outer envelope: HBsAg (Hepatitis B Surface Antigen)
• Inner nucleocapsid (core): HBcAg, HBeAg, DNA polymerase, partially double-stranded circular DNA

• 350 million chronic carriers worldwide
• Routes: Parenteral (blood, needles), Sexual, Vertical (mother to newborn - most important in high-prevalence areas)
• High-risk groups: Healthcare workers, IVDU, MSM, recipients of blood products, newborns of HBsAg+ mothers

• Liver damage is IMMUNOLOGICALLY mediated (not direct cytopathic effect)
• CD8+ T cells recognize HBcAg/HBeAg on hepatocytes -> attack and lyse infected cells
• Strong immune response -> acute hepatitis / clearance
• Weak immune response -> persistent infection -> chronic hepatitis -> cirrhosis -> HCC
• HBx protein is key for carcinogenesis (transactivates cellular oncogenes)

• Incubation: 45-180 days (average 75 days)
• Pre-icteric phase: Anorexia, nausea, fever, arthralgia, urticaria (serum sickness-like - due to immune complexes)
• Icteric phase: Jaundice, dark urine, pale stool, hepatomegaly
• Post-icteric (convalescent) phase: Recovery

• HBsAg positive for > 6 months
• May progress to cirrhosis and HCC
• Extrahepatic: Membranous glomerulonephritis, polyarteritis nodosa

• Fulminant hepatic failure (rare, 1%)
• Chronic hepatitis -> Cirrhosis -> Portal hypertension
• Hepatocellular Carcinoma (HCC)
• Aplastic anemia

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MI 4.8 / 4.9 - Laboratory Diagnosis of Viral Hepatitis with Emphasis on Viral Markers

HEPATITIS B SEROLOGICAL MARKERS

• Only Anti-HBc IgM is positive during this period.

SEROLOGICAL PATTERNS INTERPRETATION

LABORATORY DIAGNOSIS - COMPLETE APPROACH

• Serum bilirubin (total, direct, indirect) - elevated
• ALT and AST - elevated (ALT > AST in viral hepatitis)
• Alkaline phosphatase - mildly elevated
• PT/INR - prolonged in severe disease
• Serum albumin - low in chronic disease

• ELISA for HBsAg, Anti-HBs, Anti-HBc, HBeAg, Anti-HBe
• ELISA/EIA for Anti-HAV IgM (acute HAV), Anti-HEV IgM (acute HEV)
• Anti-HCV antibody (ELISA/RIBA) - for HCV screening
• HCV RNA (PCR) - to confirm active HCV infection (anti-HCV may be negative early)

• HBV DNA (quantitative PCR) - for viral load, treatment monitoring
• HCV RNA (quantitative PCR) - to confirm viremia and treatment response
• HCV genotyping - 6 genotypes (1-6); important for treatment selection

• "Ground-glass hepatocytes" - pathognomonic of chronic HBV (HBsAg in cytoplasm)
• Councilman bodies (acidophilic bodies) - necrotic hepatocytes
• Rosette formation (hepatocytes)
• Fibrosis staging (Metavir/Ishak scoring)

• HBV vaccine: Recombinant HBsAg (yeast-derived); 0-1-6 months schedule
• Healthcare workers: 3 doses + check anti-HBs titer
• Newborns of HBsAg+ mothers: HBV vaccine + HBIG within 12 hours of birth
• No vaccine for HCV (current efforts ongoing)

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TOPIC 5: MUSCULOSKELETAL, SKIN AND SOFT TISSUE INFECTIONS

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MI 5.1 - Anaerobic Infections: Pathogenesis, Clinical Course, Laboratory Diagnosis

IMPORTANT ANAEROBIC BACTERIA

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GAS GANGRENE (CLOSTRIDIAL MYONECROSIS)

• Wound contamination with spores from soil/feces
• Anaerobic environment (tissue necrosis, ischemia) -> spore germination
• C. perfringens produces alpha toxin (lecithinase/phospholipase C) - destroys cell membranes
• Also produces: collagenase, hyaluronidase, DNase
• Rapid tissue destruction, gas production (CO2 + H2) -> crepitus

• Severe pain at wound site (early and out of proportion to findings)
• Wound: brown/black discoloration, foul-smelling discharge, gas bubbles
• Skin: bronze/brown discoloration, blistering
• Systemic toxemia: high fever, tachycardia, hypotension, shock, jaundice (hemolysis)
• Rapid progression to death if untreated

• Gram stain of wound discharge: Large Gram +ve rods with NO pus cells (neutrophils killed by toxins)
• X-ray: Gas in tissue planes (characteristic)
• Culture: Anaerobic blood agar (24-48 hours) - double zone of hemolysis
• Nagler reaction: Alpha toxin produces lecithinase activity (opaque zone on egg yolk agar - neutralized by specific antitoxin)

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TETANUS

• Spores enter wound -> germinate in anaerobic conditions
• Tetanospasmin (exotoxin): Travels retrogradely along motor neurons -> spinal cord/brain
• Blocks inhibitory neurotransmitters (glycine from Renshaw cells, GABA from interneurons) -> uninhibited motor neuron firing -> spastic paralysis and spasms

• Incubation: 7-21 days
• Trismus (lockjaw) - first sign (masseter spasm)
• Risus sardonicus - sardonic smile (facial muscle spasm)
• Opisthotonus - arching of back
• Generalized tonic spasms triggered by stimuli (sound, light, touch)
• Dysphagia, autonomic instability
• No fever (initially); consciousness preserved

• Primarily CLINICAL diagnosis
• Culture of wound (anaerobic) - confirms organism but not necessary
• Mouse bioassay for toxin (research)

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MI 5.2 - Bone and Joint Infections

OSTEOMYELITIS

• Staphylococcus aureus - most common in all age groups (adults + children)
• Staphylococcus epidermidis - prosthetic joint infections
• Streptococcus pyogenes
• Salmonella - in sickle cell disease patients (classic exam question)
• Pseudomonas aeruginosa - IV drug users, puncture wounds through sneakers
• Mycobacterium tuberculosis - vertebral (Pott's disease)
• Candida - immunocompromised
• Brucella - vertebral osteomyelitis in farmers/veterinarians

• Bacteremia -> seeding of metaphysis (richly vascularized, sluggish flow, no phagocytes)
• Pus formation -> increased pressure -> Involucrum (new bone) and Sequestrum (dead bone)
• Sinus tract formation

• Acute: Fever, localized bone pain, swelling, tenderness, limited movement
• Chronic: Recurrent, draining sinuses, sequestrum

• Blood culture (positive in 50% - most useful)
• Bone biopsy and culture (gold standard)
• ESR, CRP - elevated
• X-ray (changes appear after 10-14 days): Lytic lesions, periosteal reaction
• MRI: Best imaging for early diagnosis
• Radionuclide bone scan (Tc-99m): Early detection

SEPTIC ARTHRITIS

• Staphylococcus aureus (most common, all ages)
• Neisseria gonorrhoeae (most common in sexually active adults)
• Streptococcus pyogenes
• H. influenzae (children < 2 years, pre-vaccine era)

• Synovial fluid aspirate: Cell count (>50,000 WBC/mm3 suggests infection), Gram stain, culture
• Blood culture
• Synovial fluid: Turbid, decreased glucose, elevated protein, elevated WBC (>90% PMNs)

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MI 5.3 / 5.4 / 5.5 - Skin and Soft Tissue Infections

CLASSIFICATION OF CAUSATIVE AGENTS

• Staphylococcus aureus: Folliculitis, furuncle, carbuncle, impetigo (bullous), cellulitis, SSSS
• Streptococcus pyogenes (Group A): Impetigo (non-bullous), erysipelas, cellulitis, necrotizing fasciitis
• Clostridium perfringens: Gas gangrene
• Pseudomonas aeruginosa: Ecthyma gangrenosum (immunocompromised), folliculitis (hot tub)
• Bacillus anthracis: Cutaneous anthrax (malignant pustule)

• Dermatophytes (Tinea infections): Trichophyton, Microsporum, Epidermophyton
• Candida: Cutaneous candidiasis, intertrigo
• Sporothrix schenckii: Sporotrichosis (lymphocutaneous form)
• Chromoblastomycosis (Fonsecaea, Cladosporium)

• Herpes simplex virus (HSV-1, 2): Cold sores, genital herpes
• Varicella Zoster virus: Chickenpox, Herpes Zoster
• Molluscum contagiosum (Poxvirus): Umbilicated papules
• Human Papillomavirus (HPV): Warts (verrucae)

• Sarcoptes scabiei: Scabies (intensely pruritic burrows)
• Leishmania: Cutaneous leishmaniasis (Oriental sore)

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MYCOBACTERIAL INFECTIONS (MI 5.5)

• Lupus vulgaris (most common form) - apple jelly nodules
• Scrofuloderma (TB of lymph nodes draining to skin)
• TB verrucosa cutis
• Miliary TB

• Acid-fast bacilli (AFB) in Ziehl-Neelsen stain
• Non-cultivable in vitro
• Grows in armadillo footpad / mouse footpad (experimental)

• TT (Tuberculoid): Strong CMI, few bacilli (paucibacillary), hypopigmented anesthetic patches with raised edges
• LL (Lepromatous): Weak CMI, many bacilli (multibacillary), leonine facies, madarosis, glove-stocking anesthesia
• BT, BB, BL (Borderline forms)

• Obligate intracellular parasite of Schwann cells and macrophages
• Invasion via PGL-1 (phenolic glycolipid 1)
• Nerve damage -> anesthesia, trophic ulcers, deformity

• Slit skin smear: From earlobes, nasal mucosa, active skin lesions; ZN stain; Bacterial Index (BI) graded 0-6+
• Histopathology: Skin biopsy; TT shows epithelioid granuloma; LL shows foamy (Virchow) cells laden with AFB
• Lepromin test (Mitsuda reaction): Not diagnostic, only indicates immune status (positive in TT, negative in LL)

• Press the skin (to exsanguinate), make a shallow incision, scrape, smear on glass slide
• Stain with Ziehl-Neelsen - AFB appear red on blue background

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TOPIC 6: CENTRAL NERVOUS SYSTEM INFECTIONS

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MI 6.1 - Meningitis

CAUSATIVE AGENTS

• Enteroviruses (most common: Echovirus, Coxsackievirus)
• Herpes simplex virus (HSV-2)
• Mumps virus
• HIV (acute seroconversion)
• Lymphocytic choriomeningitis virus (LCMV)

• Cryptococcus neoformans - most common (HIV/immunocompromised)
• India ink preparation: Capsulated yeast cells in CSF (classic)
• Latex agglutination for cryptococcal antigen in CSF (gold standard)

• Naegleria fowleri - Primary Amoebic Meningoencephalitis (PAM) - swimming in warm freshwater
• Acanthamoeba - Granulomatous Amoebic Encephalitis (GAE) - immunocompromised

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CLINICAL FEATURES

• Classic triad: Fever + Headache + Neck stiffness (nuchal rigidity)
• Kernig's sign: Unable to extend knee when hip is flexed at 90°
• Brudzinski's sign: Passive neck flexion causes involuntary knee flexion
• Photophobia, phonophobia
• Vomiting (projectile)
• Altered sensorium, seizures (severe cases)
• Petechial rash (meningococcal)

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LABORATORY DIAGNOSIS

• Gram -ve diplococci (intracellular) = N. meningitidis
• Gram +ve diplococci = S. pneumoniae
• Gram -ve coccobacilli = H. influenzae
• India ink = Cryptococcus
• AFB stain = M. tuberculosis

• Blood culture (always collect before antibiotics)
• CT scan before LP (if papilledema / focal signs - to rule out raised ICP)
• Antigen detection (latex agglutination): For Meningococcus, Pneumococcus, H. influenzae, Cryptococcus
• PCR (most sensitive) for HSV, enteroviruses, bacterial DNA

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MI 6.2 - Encephalitis

CAUSATIVE AGENTS

• Listeria (brainstem - rhombencephalitis)
• TB (tuberculoma)
• Neurosyphilis (Treponema pallidum)

• Toxoplasma gondii (immunocompromised, HIV - ring-enhancing lesions on MRI)
• Plasmodium falciparum - Cerebral malaria

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CLINICAL FEATURES

• Fever, headache, altered consciousness (encephalopathy)
• Behavioral changes, confusion, disorientation
• Seizures (focal or generalized)
• Focal neurological deficits (hemiplegia, cranial nerve palsies)
• Coma in severe cases
• HSV encephalitis: Personality change, temporal lobe features (olfactory hallucinations, memory changes)

LABORATORY DIAGNOSIS

• Lymphocytic pleocytosis (10-500 cells), normal or slightly elevated protein, normal glucose
• RBCs in CSF = hemorrhagic necrosis (suggests HSV)

• PCR of CSF: Gold standard for HSV, JEV, Enterovirus
• HSV PCR of CSF: Sensitivity 96%, Specificity 99% - replaces brain biopsy
• Serology (IgM capture ELISA):
  • JEV IgM in CSF: Diagnostic (appears by 4th day)
  • JEV IgM in serum (less specific due to cross-reactivity)
• MRI Brain: Method of choice for imaging (shows temporal lobe changes in HSV; thalamic changes in JEV)
• EEG: Periodic lateralizing epileptiform discharges (PLEDs) in HSV encephalitis
• Brain biopsy (rarely needed now with PCR availability)
• Rabies: Negri bodies in cerebellar Purkinje cells (histopathology), DFA on brain tissue (gold standard), skin biopsy (nape of neck) DFA during life

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TOPIC 8: GENITOURINARY AND SEXUALLY TRANSMITTED INFECTIONS

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MI 8.1 - Genitourinary Infections

URINARY TRACT INFECTIONS (UTI)

• Ascending route most common (E. coli from periurethral flora)
• Women more prone (shorter urethra, proximity to anus)
• Risk factors: Catheterization, DM, urinary obstruction, pregnancy, sexual activity (honeymoon cystitis)

• Mid-stream clean catch urine (MSU) for culture
• Significant bacteriuria: ≥10^5 CFU/mL (symptomatic - lower count significant)
• Urine microscopy: >5 WBC/HPF = pyuria
• Culture on CLED agar (Cystine Lactose Electrolyte Deficient) - inhibits Proteus swarming
• Blood agar, MacConkey agar for identification
• Antibiotic sensitivity by Kirby-Bauer disc diffusion

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MI 8.2 - Sexually Transmitted Infections (STIs)

COMMON STIs

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GONORRHEA (Neisseria gonorrhoeae)

• Male: Urethritis (purulent discharge, dysuria), epididymo-orchitis
• Female: Cervicitis (often asymptomatic), PID (pelvic inflammatory disease), Fitz-Hugh-Curtis syndrome
• Disseminated gonococcal infection (DGI): Septic arthritis + skin lesions
• Neonates: Ophthalmia neonatorum (vertical transmission during delivery)

• Gram stain of urethral discharge: Gram -ve intracellular diplococci in PMNs (diagnostic in males: 95% sensitivity)
• Culture on Thayer-Martin (Modified) medium / NYC medium: (chocolate agar with antibiotics - Vancomycin, Colistin, Nystatin, Trimethoprim - VCNT); 5-10% CO2; 48 hours; small grey colonies
• Biochemical: Oxidase +ve, ferments glucose only (not maltose - differentiates from N. meningitidis)
• NAAT (Nucleic Acid Amplification Tests) - PCR: Method of choice (most sensitive, can use urine sample, no culture needed)

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SYPHILIS (Treponema pallidum)

• Painless indurated ulcer = Chancre (at site of inoculation: genitalia, lips, anus)
• Painless inguinal lymphadenopathy
• Heals spontaneously in 3-6 weeks

• Appears 6-8 weeks after primary
• Maculopapular rash involving palms and soles (classic)
• Condyloma lata (moist wart-like lesions)
• Mucous patches
• Generalized lymphadenopathy, fever, malaise

• Gumma (granulomatous lesions in skin, bone, liver)
• Cardiovascular: Aortic aneurysm (ascending), aortic regurgitation
• Neurosyphilis: Tabes dorsalis (posterior column damage), General Paresis of Insane (GPI), Argyll Robertson pupil

• VDRL (Venereal Disease Research Laboratory): Flocculation test using cardiolipin antigen; detects reagin antibody; can be done on CSF also; false positives common (SLE, malaria, TB, leprosy)
• RPR (Rapid Plasma Reagin): Same principle; can be done without microscope

• FTA-ABS (Fluorescent Treponemal Antibody Absorption): Most sensitive; gold standard
• TPHA (Treponema pallidum Haemagglutination Assay): Specific; remains positive for life (not for treatment monitoring)
• TPPA (T. pallidum Particle Agglutination)

• Darkfield microscopy of chancre exudate: Motile spirochetes (only in primary/secondary)
• RPR/VDRL titer: Used to monitor treatment response (4-fold decline = successful treatment)
• PCR (most sensitive for tissue specimens)

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CHLAMYDIA TRACHOMATIS (STI)

• Obligate intracellular organism
• Two forms: Elementary Body (EB - infective, extracellular) and Reticulate Body (RB - replicating, intracellular)
• Cannot grow on routine media; grows in cell culture (McCoy cells / HeLa cells)

• D-K: Urethritis, cervicitis, PID, epididymitis, lymphogranuloma venereum (L1-3), inclusion conjunctivitis, trachoma (A-C)
• L1-L3: LGV (Lymphogranuloma Venereum) - painless genital ulcer -> painful inguinal bubo ("groove sign") -> rectal stricture

• NAAT (PCR): Gold standard for genital C. trachomatis (urine or genital swab)
• Cell culture (McCoy cells) - gold standard but impractical
• DFA (Direct Fluorescent Antibody) - for rapid diagnosis
• Giemsa stain of conjunctival scrapings: Inclusion bodies (Halberstaedter-Prowazek bodies) in cytoplasm
• ELISA for antigen detection

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PREVENTION OF STIs

1. Abstinence / Mutual monogamy
2. Condom use (barrier protection)
3. Vaccines: HPV vaccine (Gardasil: types 6,11,16,18; Cervarix: 16,18); HBV vaccine
4. Syndromic management (WHO recommended in resource-limited settings)
5. Contact tracing and treatment of sexual partners
6. Screening of pregnant women: VDRL, HIV, HBsAg (PPTCT program)
7. Antibiotic prophylaxis for neonates: Silver nitrate / Erythromycin eye drops (ophthalmia neonatorum prevention)
8. HIV PEP / PrEP

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Note on Exam Writing Style (Apurba Sastri approach):

• Always start with definition and classification of the causative agents
• Use structured tables for morphology, epidemiology comparisons
• In pathogenesis, describe toxins/virulence factors by name with their mechanism
• Lab diagnosis should follow the order: Specimen -> Microscopy -> Culture (medium name) -> Biochemical -> Serology -> Molecular
• Always mention the gold standard test explicitly
• For viral diseases, include serological marker interpretation tables
• End answers with prevention/treatment if asked in the competency

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How to Draw Vibrio cholerae in Examination (Apurba Sastri Style)

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DIAGRAM 1: MORPHOLOGY OF V. cholerae

1. Draw a comma-shaped / curved rod (like the letter "C" or a comma)
2. Attach a single long polar flagellum at one end (monotrichous)
3. Label:
   • Comma-shaped curved rod
   • Single polar sheathed flagellum
   • Gram-negative cell wall (outer membrane)
   • "Shooting star motility" (write as a note beside the flagellum)

• Size: 1.5-3 µm x 0.5 µm
• Gram: Negative (pink on Gram stain)

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DIAGRAM 2: CHOLERA TOXIN MECHANISM (Most Important for Exam)

V. cholerae
     |
     | (produces)
     ↓
CHOLERA TOXIN
[A subunit + B5 subunits (pentamer)]
     |
     | B subunits bind to
     ↓
GM1 GANGLIOSIDE on enterocyte membrane
     |
     | A subunit enters cell
     ↓
Activates ADENYLATE CYCLASE
     |
     ↓
ATP → ↑↑ cAMP
     |
     ↓
Activates Protein Kinase A
     |
     ↓
↑ Cl- secretion / ↓ Na+ absorption
     |
     ↓
MASSIVE WATER SECRETION
     |
     ↓
RICE-WATER STOOLS (profuse watery diarrhea)

• B subunits (5 of them, pentameric ring)
• A subunit (enzymatic - ADP-ribosylation of Gs alpha)
• GM1 ganglioside receptor
• Adenylate cyclase (AC)
• cAMP (increased)
• Cl- channel (CFTR)
• Arrow showing water following into lumen

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DIAGRAM 3: TCBS AGAR COLONIES (Culture Diagram)

• Draw yellow colonies on green/blue agar = V. cholerae (sucrose fermenter)
• Write: Thiosulphate Citrate Bile Salt Sucrose (TCBS) agar
• Compare: V. parahaemolyticus = blue/green colonies (non-sucrose fermenter)

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DIAGRAM 4: STRING TEST

• Draw a loop picking a colony
• Show it forming a long mucoid string when pulled (>5 mm)
• Label: 0.5% Sodium deoxycholate solution
• String test positive = V. cholerae

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KEY LABELS TO NEVER FORGET IN EXAM

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Exam Tip (Apurba Sastri): In a 5-mark answer, always draw the cholera toxin mechanism diagram - it fetches the most marks. In a 2-mark answer, draw just the morphology with flagellum. Always label every part of your diagram - unlabeled diagrams get zero credit.

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How to Draw E. coli in Examination (Apurba Sastri Style)

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DIAGRAM 1: MORPHOLOGY

1. Draw a straight rod (rectangular with rounded ends) - unlike Vibrio which is curved
2. Draw multiple flagella all around the cell body (peritrichous = all around, not just at poles)
3. Draw short fimbriae/pili projecting all over the surface (shorter and more numerous than flagella)
4. Show a thin capsule (K antigen) as a halo around the cell
5. Optionally draw one longer sex pilus (F pilus)

• Straight Gram-negative rod (1-3 µm x 0.4-0.7 µm)
• Peritrichous flagella (H antigen)
• Type 1 fimbriae / common pili (adhesion to uroepithelium)
• Capsule (K antigen)
• Outer membrane (LPS - O antigen)
• Sex pilus / F pilus (conjugation)

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DIAGRAM 2: ANTIGENIC STRUCTURE (Most Exam-Important)

         K antigen (Capsule) - outermost
              ↕
         O antigen (LPS - Somatic) - on outer membrane
              ↕
       Cell body (contains cytoplasm, nucleoid)
              ↕
         H antigen (Flagella) - on flagella

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DIAGRAM 3: GRAM-NEGATIVE CELL WALL (Cross-section)

OUTSIDE (lumen/environment)
         |
  ┌──────────────────┐
  │    O antigen      │  ← polysaccharide chains
  │  (LPS outer part) │
  ├──────────────────┤
  │   Outer Membrane  │  ← lipid bilayer (porin proteins)
  ├──────────────────┤
  │  Periplasmic      │
  │     Space         │  ← thin peptidoglycan layer (1-3 layers)
  ├──────────────────┤
  │   Inner (Cyto-    │
  │  plasmic) Membrane│
  └──────────────────┘
         |
  CYTOPLASM (nucleoid, ribosomes)

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DIAGRAM 4: PATHOTYPES OF E. COLI (Exam Favorite - Table Diagram)

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DIAGRAM 5: COLONY MORPHOLOGY (Culture)

• Draw pink/red colonies (lactose fermenter) with surrounding bile precipitation
• Label: "Dry, pink, flat colonies with metallic sheen" (from EMB agar - more specific)

• Metallic green sheen colonies - pathognomonic of E. coli
• Draw colonies with a greenish iridescent shine

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KEY POINTS TO WRITE BESIDE DIAGRAM

Apurba Sastri exam tips:

1. "E. coli is the most common cause of UTI" - always state this
2. "ETEC - LT toxin works like cholera toxin (↑cAMP)" - mechanism diagram scores marks
3. "EHEC O157:H7 causes HUS" - Hemolytic Uremic Syndrome = microangiopathic hemolytic anemia + thrombocytopenia + acute renal failure
4. "Weil-Felix reaction is negative" - differentiates from Rickettsial disease
5. IMViC reactions: Indole (+), Methyl red (+), Voges-Proskauer (-), Citrate (-) = + + - - for E. coli

Draw the IMViC table in every E. coli answer - it is a guaranteed 1-2 marks question by itself!