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SYSTEMIC BACTERIOLOGY - COMPLETE EXAM ANSWERS

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2024 (22 BATCH)

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Q1. Case: 65-year-old diabetic woman, abscess on right upper arm post-IM injection. Gram stain: Gram +ve cocci in clusters. Blood agar: beta-hemolytic, golden yellow colonies.

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(A) Most likely organism

• Gram +ve cocci in clusters (grape-like arrangement)
• Beta-hemolysis on blood agar
• Golden yellow (aureus = gold) pigmented colonies
• Diabetic patient (immunocompromised) + post-injection abscess (skin/soft tissue infection)

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(B) Virulence Factors of S. aureus

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(C) Laboratory Diagnosis of S. aureus

• Gram +ve cocci in clusters ("bunch of grapes")
• Also seen as pairs and tetrads

• Blood Agar: Beta-hemolytic, golden-yellow, opaque colonies (2 mm diameter)
• MacConkey Agar: Does NOT grow (Gram +ve)
• Mannitol Salt Agar (MSA): Selective medium - ferments mannitol → yellow colonies (S. aureus is the only pathogenic staphylococcus that ferments mannitol)
• Nutrient Agar: Golden-yellow pigment best seen at room temperature

• Coagulase test is the gold standard to differentiate S. aureus from CoNS
  • Slide coagulase: detects bound coagulase (clumping factor)
  • Tube coagulase: detects free coagulase (confirms diagnosis)

• Kirby-Bauer disc diffusion
• Oxacillin/Cefoxitin disc for MRSA screening

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(D) Drug Resistance - MRSA (Methicillin-Resistant S. aureus)

• S. aureus acquires the mecA gene (carried on SCCmec - staphylococcal cassette chromosome mec)
• mecA gene encodes an altered penicillin-binding protein - PBP2a (PBP2')
• PBP2a has low affinity for all beta-lactam antibiotics
• Result: Resistant to ALL penicillins, cephalosporins, carbapenems

• HA-MRSA (Hospital-acquired): Multi-drug resistant (MDR), SCCmec types I, II, III
• CA-MRSA (Community-acquired): SCCmec type IV/V, often carries PVL toxin

• VISA (Vancomycin Intermediate S. aureus): Thickened cell wall traps vancomycin
• VRSA (Vancomycin Resistant S. aureus): Acquires vanA gene from VRE
• Beta-lactamase production: Destroys penicillin G and ampicillin

• Drug of choice: Vancomycin (IV)
• Alternatives: Linezolid, Daptomycin, Teicoplanin, Tigecycline

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Q2. Laboratory Diagnosis of Enteric Fever - Blood Culture Approach

• Bacteremia occurs in Week 1 (1st week of fever)
• Bacteria in blood BEFORE appearing in stool/urine

1. Collect 10 mL blood aseptically during pyrexia (before antibiotics)
2. Inoculate into bile broth (ox bile or bile broth - ratio 1:10 blood:broth) - bile stimulates growth
3. Incubate at 37°C for 7-10 days
4. Subculture every 48 hours onto:
   • MacConkey agar: Non-lactose fermenting (NLF), pale/colorless colonies
   • DCA (Deoxycholate Citrate Agar): Selective/differential - NLF colorless colonies
   • Wilson-Blair medium (Bismuth Sulphite Agar): Black colonies with metallic sheen (H2S production)
   • Blood agar: Non-hemolytic gray colonies
5. Biochemical tests on NLF colonies:
   • Oxidase: Negative
   • Catalase: Positive
   • H2S: Positive (in TSI agar - alkaline slant, acid butt, H2S)
   • Urease: Negative (differentiates from Proteus)
   • IMViC: - - + + (Indole -, MR -, VP -, Citrate +... actually IMViC: --+-)
   • Gas: S. Typhi = NO gas (S. Paratyphi = gas produced)
6. Serological confirmation (Kauffmann-White scheme):
   • O antigen: somatic (group D)
   • H antigen: flagellar (phase 1: d)
   • Vi antigen: virulence capsular antigen (in acute infection)
7. Widal Test (serology):
   • Agglutination test detecting antibodies against O and H antigens
   • Significant titre: O ≥1:80, H ≥1:160 (or 4-fold rise in paired sera)
   • Limitations: false positives (prior vaccination, cross-reactivity), false negatives (early disease, immunocompromised)

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Q3. Different Staining Methods in Diagnosis of M. tuberculosis

• Cell wall contains mycolic acids (long-chain fatty acids, C60-C90)
• They bind carbol fuchsin and resist decolorization with 3% acid-alcohol (20% H2SO4)
• This property is called acid-fastness

1. Heat-fix smear
2. Apply carbol fuchsin - heat gently for 5 min (hot method)
3. Wash with water
4. Decolorize with 3% acid-alcohol (20% H2SO4 for M. tuberculosis)
5. Counterstain with Methylene blue (or malachite green)
6. Examine under oil immersion

• 1-9 AFB per 100 fields = 1+ (scanty, report exact number)
• 10-99/100 fields = 1+
• 1-10/field = 2+

• 10/field = 3+

• NAAT/GeneXpert MTB/RIF: Detects M. tuberculosis DNA AND rifampicin resistance in 2 hours
• LAMP (Loop-mediated isothermal amplification): Point-of-care testing

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Q4. Infections Caused by Pseudomonas aeruginosa

• Produces pyocyanin (blue-green pigment) and pyoverdin (fluorescent yellow-green)
• Fruity/grape-like odor (aminoacetophenone)
• Grows on MacConkey (NLF), Cetrimide agar (selective) - green metallic sheen

• Pili, flagella (adhesion & motility)
• Exotoxin A (ADP-ribosylation of EF-2, like diphtheria toxin - inhibits protein synthesis)
• Exoenzyme S and T
• Alginate (slime/biofilm - esp. in cystic fibrosis)
• Proteases (elastase, alkaline protease) - tissue destruction
• Pyocyanin - generates reactive oxygen species

• Anti-pseudomonal penicillins: Piperacillin-tazobactam
• Cephalosporins: Cefepime, Ceftazidime
• Carbapenems: Imipenem, Meropenem (except Ertapenem)
• Fluoroquinolones: Ciprofloxacin
• Aminoglycosides: Amikacin, Tobramycin
• Usually requires combination therapy (to prevent resistance)

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2024 (21 BATCH)

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Q1. Case: 30-year-old male, 6 months - low-grade fever, weight loss, chronic cough with expectoration. Acid-fast bacilli on sputum smear.

(A) Provisional Diagnosis

• Chronic (>3 weeks) productive cough
• Constitutional symptoms: low-grade fever, night sweats (implied), weight loss, anorexia ("loss of weight and appetite")
• AFB seen on sputum smear
• Classic clinical triad of TB: Chronic cough + Hemoptysis + Constitutional symptoms

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(B) Laboratory Diagnosis and Treatment Regimens Based on Drug Susceptibility

• Early morning sputum (3 specimens on 3 consecutive days)
• For smear microscopy + culture + DST

• ZN stain or Auramine-rhodamine (fluorescence)
• AFB appear as red rods on blue background

• Lowenstein-Jensen (LJ) Medium: Solid egg-based medium
  • Colonies: Rough, buff/cream colored, cauliflower-like (described as bread crumb colonies)
  • Growth: Slow - 4-8 weeks
• MGIT (Mycobacteria Growth Indicator Tube): Liquid medium with fluorescent oxygen sensor
  • Faster: 2-3 weeks
  • Detects growth when oxygen is consumed

• Phenotypic: Proportion method on LJ medium (6-8 weeks)
• Molecular (rapid):
  • GeneXpert MTB/RIF: Detects TB + Rifampicin resistance in 2 hours
  • Line Probe Assay (LPA/Hain Genotype MTBDRplus): Detects resistance to rifampicin, isoniazid in 1-2 days

• R = Rifampicin (inhibits RNA polymerase)
• I = Isoniazid (inhibits mycolic acid synthesis - InhA)
• P = Pyrazinamide (disrupts membrane potential in acidic pH)
• E = Ethambutol (inhibits arabinosyl transferase - cell wall)

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(C) Pathogenesis of Pulmonary Tuberculosis

1. Inhalation of M. tuberculosis droplet nuclei (<5 microns) - reach alveoli
2. Phagocytosis by alveolar macrophages
3. Bacterial survival inside macrophages: inhibits phagosome-lysosome fusion, escapes oxidative killing
4. Intracellular multiplication → macrophage death → bacteria released
5. Ghon's focus (primary focus) formed in mid-zone of lung
6. Bacilli travel to hilar lymph nodes → Ghon's complex (primary focus + lymph node = primary complex)
7. Cell-mediated immunity (CMI) develops at 4-6 weeks (tuberculin sensitivity appears)
8. Granuloma formation (Tuberculous granuloma):
   • Central: Caseous necrosis (cheese-like)
   • Surrounded by: Epithelioid macrophages, Langhan's giant cells, lymphocytes, fibroblasts
9. Outcomes of primary infection:
   • Healing (90%) - calcification
   • Progressive primary TB (immunocompromised)
   • Dormant bacilli (latent TB)
10. Reactivation/Post-primary TB (secondary TB):
    • Upper lobe predilection (high O2 tension)
    • Cavitation → spread via bronchi → Cavity tuberculosis
    • Systemic spread → Miliary TB (hematogenous)

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(D) Drug Resistance in TB

• Irregular/incomplete treatment (most important)
• Inadequate drug dosage
• Poor drug quality
• Malabsorption of drugs
• Monotherapy

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Q2. MDRO (Multi-Drug Resistant Organism)

• Antibiotic stewardship programs
• Hand hygiene (most important infection control measure)
• Contact precautions
• Surveillance cultures
• Rational antibiotic use

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Q3. Specific Test for Syphilis

• Positive in: Primary (70%), Secondary (100%), Tertiary (70-75%) syphilis
• Prozone phenomenon: False negative due to antibody excess (dilute sample and retest)
• Quantitative VDRL: Monitor treatment - titres fall with successful treatment (4-fold decline = cure)
• False positives: SLE, malaria, leprosy, infections (Wassermann reactions)

• Treponemal tests remain positive for life even after treatment (cannot monitor cure)
• FTA-ABS is the most sensitive test for primary syphilis
• TPI (T. pallidum Immobilization) - oldest but replaced by FTA-ABS

• Examine exudate from chancre/moist lesion
• T. pallidum appears as corkscrew-shaped, motile spirochete with 6-14 coils
• Only useful in primary syphilis (open lesions)

1. Screen with VDRL/RPR
2. Confirm positive with TPHA/FTA-ABS
3. Use quantitative VDRL to monitor treatment

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Q4. NGU (Non-Gonococcal Urethritis)

• Scanty, mucoid/watery urethral discharge (vs. profuse, purulent in gonorrhea)
• Dysuria (mild)
• Often asymptomatic (esp. in women - cervicitis, PID)
• Incubation period: 1-5 weeks

1. Gram stain of urethral discharge:
   • Gonococcal: Gram -ve diplococci inside PMNs
   • NGU: PMNs present but NO Gram -ve diplococci
   • Criteria: ≥5 PMNs/high power field (x1000) without gonococci
2. NAAT (Nucleic Acid Amplification Test): Gold standard for C. trachomatis (urine/swab)
3. Chlamydial culture: McCoy cells (difficult, not routine)
4. Urethral smear: ≥2 PMN/oil immersion field = urethritis

• Azithromycin 1g single oral dose (first-line for C. trachomatis NGU)
• Doxycycline 100 mg twice daily x 7 days
• For M. genitalium (macrolide-resistant): Moxifloxacin
• Always treat sexual partners

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2023

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Q1. Describe Various Virulence Factors in Bacteria with Examples

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Q2. Principles of Presumptive Coliform Testing and Interpretation

• Inoculate water sample into MacConkey broth or Lactose broth (in Durham tubes)
• Incubate at 37°C for 24-48 hours
• Positive: Gas production in Durham tube (CO2 from lactose fermentation) + turbidity
• Positive = presumptive evidence of coliform

• From positive Durham tube → inoculate Brilliant Green Bile Lactose broth (BGBLB)
• 37°C for 48 hours
• Gas production = Total coliforms confirmed
• Then incubate a set at 44°C for fecal coliforms (E. coli specifically)
• Gas at 44°C + indole production = E. coli (confirmed fecal contamination)

• Subculture onto Eosin Methylene Blue (EMB) or MacConkey agar
• E. coli: Metallic green sheen on EMB, pink colonies on MacConkey
• Gram stain: Gram -ve non-sporing rods
• Confirm biochemically: IMViC = + + - - for E. coli

• Statistical estimate of coliform count from multiple tube dilutions
• WHO safe limit: 0 coliforms per 100 mL of drinking water

• Presence of E. coli = definite fecal contamination
• Presence of coliforms only = probable contamination

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Q3. Anaerobic Culture Method

• Clostridium tetani, C. botulinum, C. perfringens, Bacteroides fragilis, Fusobacterium, etc.

• Sealed metal jar
• Gas-pack (GasPak): Sachet produces H2 + CO2; catalytic palladium pellet combines H2 + O2 → H2O
• Indicator strip (methylene blue): colorless = anaerobic achieved

• Large glove box with N2/CO2/H2 atmosphere
• Best for strict anaerobes

• Burns candle to consume O2 and produce CO2
• Used for microaerophiles (Campylobacter, Helicobacter), NOT for strict anaerobes

• Liquid medium with reducing agents (thioglycollate, cysteine, sodium sulfite)
• Anaerobic conditions at bottom of tube

• Robertson's cooked meat medium: Anaerobic, enrichment broth for Clostridia
• Heated blood agar (chocolate agar) for anaerobes

• Neomycin blood agar: Selective for Gram +ve anaerobes
• Blood agar + aminoglycosides: Selective for Gram -ve anaerobes
• CCFA (Cycloserine-Cefoxitin Fructose Agar): Selective for C. difficile
• KVLB (Kanamycin-Vancomycin Lysed Blood agar): For Bacteroides

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Q4. Laboratory Diagnosis of Enteric Fever (Full)

• Week 1: Blood (90%) + Bone marrow (95%, even on antibiotics)
• Week 2-3: Urine (50%) + Stool (60-80%)
• Week 2 onwards: Serology (Widal test)

• Tube agglutination test (macroscopic)
• Patient serum (serial dilutions: 1:20, 1:40, 1:80...) vs. O and H antigens
• Significant titre: O agglutinins ≥1:80; H agglutinins ≥1:160
• Single high titre OR 4-fold rise in paired sera (2 weeks apart) = significant
• Limitations: Cross-reactions with other Salmonella spp., malaria, liver disease, previous immunization

• Typhidot: ELISA detecting IgM and IgG anti-outer membrane protein antibodies
  • Typhidot-M: Only IgM - detects acute infection even when IgG present
• Tubex test: Inhibition magnetic binding immunoassay - detects IgM anti-O9
• Blood culture remains gold standard (15-35 mL blood, 1:10 dilution in bile broth)

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Q5. Case: 50-year-old, 5-day fever, malaise, headache, yellow sclera, microalbuminuria.

• Gram negative spirochete (too thin to see on Gram stain)
• Hooked ends ("question mark" shaped under dark-field)
• Aerobic organism
• Zoonotic (rodents are main reservoir)

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2022

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Q1. Case: 7-year-old girl, high fever, headache, altered sensorium, seizures, neck rigidity. CSF: Gram +ve cocci in pairs, alpha-hemolytic colonies on blood agar.

(A) Probable Clinical Diagnosis and Etiological Agent

• Gram +ve cocci in pairs (diplococci) = Streptococcus or Enterococcus (pairs vs clusters = Staphylococcus)
• Alpha-hemolysis on blood agar (partial/green hemolysis) = S. pneumoniae or viridans streptococci
• Meningitis with gram +ve diplococci in CSF = strongly S. pneumoniae
• Capsule makes it virulent

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(B) Pathogenesis and Clinical Manifestations

1. Colonization of nasopharynx (carrier state in 5-40% healthy adults)
2. Invasion of bloodstream → bacteremia
3. Crossing blood-brain barrier (BBB) - via transcytosis or "Trojan horse" through infected macrophages
4. Bacterial multiplication in subarachnoid space
5. LTA (lipoteichoic acid) + peptidoglycan fragments trigger intense inflammatory response
6. Release of IL-1, TNF, IL-6, IL-8 → neutrophil recruitment
7. Cerebral edema + increased intracranial pressure → herniation
8. Vasculitis → reduced cerebral blood flow → ischemia

• Fever (high grade)
• Headache (severe, "worst of life")
• Neck rigidity (meningismus) - Kernig's sign, Brudzinski's sign positive
• Altered sensorium - confusion to coma
• Seizures (in children especially)
• Photophobia, phonophobia
• Focal neurological deficits in late stages

• Subdural empyema
• Hearing loss (most common sequela)
• Cerebral infarction
• Hydrocephalus
• Death (untreated mortality 100%)

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(C) Laboratory Diagnosis

• Gram +ve lancet-shaped diplococci (in pairs)
• Surrounded by clear halo (capsule)

• Blood agar (5% sheep blood): Alpha-hemolytic, mucoid (shiny/water-droplet-like) colonies - draughtsman/draughtsboard appearance (due to autolysis)
• Chocolate agar: Ideal medium (releases V and X factors)
• Incubation: 37°C, 5% CO2

• Optochin (Ethylhydrocupreine) sensitivity: Zone of inhibition ≥14mm = S. pneumoniae (viridans streptococci resistant)
• Bile solubility test: S. pneumoniae is bile-soluble (colonies lyse in 10% sodium desoxycholate) = positive

• Quellung reaction (Neufeld's reaction): Capsular swelling with specific antiserum (capsule appears enlarged/refractile)
• Latex agglutination: Detects capsular polysaccharide antigen in CSF
• Urinary antigen test: Detects pneumococcal antigen (rapid, bedside)

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Q2. Laboratory Diagnosis of TB

• Specimen: Sputum, BAL, pleural fluid, CSF, urine, tissue biopsy
• Smear: ZN stain / Auramine-rhodamine
• Culture: LJ medium (4-8 weeks), MGIT (2-3 weeks)
• Molecular: GeneXpert MTB/RIF (2 hours), LPA
• Histology: Caseating granulomas with Langhan's giant cells
• IGRA (Interferon Gamma Release Assay): Blood test for latent TB (QuantiFERON-TB Gold)

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Q3. Culture Media Used for Vibrio Cholerae

1. First inoculate into APW (alkaline peptone water) - incubate 6-8 hours at 37°C
2. Subculture to TCBS - overnight incubation
3. Pick yellow colonies → biochemical + serological confirmation

• V. cholerae O1: Two biotypes (Classical + El Tor) × Two serotypes (Ogawa + Inaba)
• V. cholerae O139 (Bengal): Non-O1, produces cholera (since 1992)
• Confirmed with slide agglutination using O1 antiserum

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Q4. MRSA (Methicillin-Resistant Staphylococcus aureus)

• mecA gene → PBP2a → resistance to ALL beta-lactams
• Detected by: Cefoxitin disc (30 μg) - zone ≤21mm = MRSA (better than oxacillin)
• Or by PCR for mecA gene
• MRSA screening media: MRSA chromogenic agar (colonies appear distinctive color)
• Treatment: Vancomycin (IV), Linezolid, Daptomycin

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2021

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Q1. Enumerate Causes of PUO. Etiopathogenesis and Lab Diagnosis of Enteric Fever

• Fever >38.3°C on multiple occasions
• Duration >3 weeks
• Diagnosis unknown after 1 week of investigation

1. Ingestion of S. Typhi in contaminated food/water (infective dose: 10^5 organisms)
2. Bacteria pass through gastric acid (acid-tolerant)
3. Reach small intestine (terminal ileum) → penetrate M cells of Peyer's patches
4. Phagocytosed by macrophages → intracellular survival (inhibit oxidative killing)
5. Multiply in mesenteric lymph nodes → primary bacteremia (incubation 1-3 days)
6. Seeding of liver, spleen, bone marrow (reticuloendothelial system)
7. Secondary bacteremia = onset of clinical fever (1st week)
8. Re-infection of intestine via bile → necrosis of Peyer's patches (2nd-3rd week) → risk of intestinal perforation/hemorrhage
9. Excretion in stool (2nd-4th week)

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Q2. Non-Suppurative Complications of Streptococcus pyogenes (Group A Streptococcus)

• Pure immune-mediated damage
• Latent period (weeks) after infection resolves

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Q3. Morphology and Cultural Characteristics of C. diphtheriae

• Gram +ve rods
• Pleomorphic (variable shapes - club-shaped, with bulbous ends)
• Arranged in palisades ("Chinese letter" or "picket fence" arrangement)
• Non-motile, non-capsulated, non-sporing
• Metachromatic granules (Babes-Ernst granules / volutin granules): Contain polymetaphosphate; stain dark blue/red with Albert's stain or purple with methylene blue while the rest of cell is blue → these are poles of the cell
• Albert's stain: Granules appear bluish-black, cell body greenish - characteristic appearance

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Q4. Significant Bacteriuria

• Presence of ≥10^5 CFU/mL (100,000 CFU/mL) of a single organism in a midstream clean-catch urine specimen
• Established by Kass (Kass's criterion)

• ≥10^3 CFU/mL in symptomatic women with dysuria/frequency
• ≥10^2 CFU/mL in catheterized specimens or suprapubic aspirates
• Any growth from suprapubic aspirate (gold standard for contamination-free specimen)

• E. coli (80% of community UTI)
• Klebsiella, Proteus, Enterococcus, S. saprophyticus (young women)
• Pseudomonas, Enterobacter (hospital-acquired)

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2020

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Q1. Human Pathogenic Mycobacteria - Morphology, Culture, Biochemical Characteristics and Lab Diagnosis of M. tuberculosis

• Gram +ve (but poorly Gram-staining), Acid-fast bacillus (AFB)
• Slender, slightly curved rods, 2-4 μm long
• Non-motile, non-capsulated, non-sporing
• Aerobic (strict)
• Slow growing (generation time 15-20 hours vs 20 min for E. coli)
• Cell wall: Abundant mycolic acids → acid-fastness, resistance to dehydration, chemicals, antibiotics
• Also contains: arabinogalactan, peptidoglycan, lipoarabinomannan (LAM)

• Composition: Glycerol egg medium + malachite green (inhibitor)
• Temperature: 37°C
• Growth time: 4-8 weeks
• Colonies: Rough, dry, buff/cream colored, wrinkled, cauliflower-like
• "Breadcrumb" or "coarse granular" texture
• Slopes up from base (eugonic growth with glycerol)

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Q2. Pathogenesis of Streptococcus pyogenes (Group A Streptococcus / GAS)

• Tonsillitis/pharyngitis, Otitis media, Mastoiditis
• Peritonsillar abscess, Sinusitis
• Erysipelas, Cellulitis, Impetigo
• Necrotizing fasciitis ("flesh-eating disease")
• Bacteremia, Meningitis, Osteomyelitis

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Q3. Etiopathogenesis and Lab Diagnosis of Cholera

1. Ingestion of V. cholerae (infective dose: 10^8 - 10^10 organisms, lower in achlorhydric patients)
2. Survives gastric acid → reaches small intestine (jejunum)
3. Adheres via TCP (Toxin-co-regulated pili) to enterocytes
4. Secretes Cholera Toxin (CT/CTX):
   • AB5 toxin: 1 A subunit + 5 B subunits
   • B subunit binds GM1 ganglioside on enterocyte
   • A1 fragment ADP-ribosylates Gs alpha (stimulatory G protein) → permanently activates adenylyl cyclase
   • cAMP rises → activates PKA → phosphorylates CFTR → massive Cl- secretion + Na+ (via paracellular) follows + water
5. Isotonic secretory diarrhea: up to 20 L/day

• Painless, profuse, watery diarrhea ("rice-water stool" - colorless with flecks of mucus)
• Vomiting (usually early)
• NO fever, NO abdominal pain (V. cholerae doesn't invade mucosa)
• Rapid severe dehydration → hypovolemic shock, metabolic acidosis, hypokalemia
• Washerwomen's hands, sunken eyes, skin turgor loss

• Wet mount (hanging drop): Comma-shaped bacilli with "shooting star" motility (darting motility)
• This motility is inhibited by V. cholerae O1 antiserum (vibriostatic test / immobilization test)

• Enrichment: Alkaline Peptone Water (APW) pH 8.6, 6-8 hours
• Selective: TCBS → yellow colonies (V. cholerae O1)
• Differential: TTGA (Monsur's) → black colonies with halo
• Biochemical: Oxidase +ve (key!), TSI: Alkaline/alkaline (no H2S), Indole +ve (biotype El Tor), Voges-Proskauer: El Tor = +ve, Classical = -ve

• Slide agglutination with O1 and O139 antisera
• Widal-like tube agglutination (not routine)

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Q4. Widal Test

• H antigen (flagellar) of S. Typhi (d phase)
• O antigen (somatic/LPS) of S. Typhi (group D: O9, O12)
• H antigen of S. Paratyphi A (a phase)
• H antigen of S. Paratyphi B (b phase)

• Patient serum serially diluted (1:20, 1:40, 1:80, 1:160, 1:320)
• Mixed with respective bacterial suspensions
• Incubate at 37°C for 18-24 hours (or 55°C for 4 hours for O)
• Read for agglutination

• Single titre ≥1:80 (O) or ≥1:160 (H) = significant in endemic areas
• 4-fold rise in paired sera (gold standard interpretation)
• O agglutinins appear first (early infection); H agglutinins are longer lasting
• O rising + H rising = active infection
• H high alone = previous infection or vaccination
• O high alone = early infection (H not yet risen)

• False positives: Malaria, liver disease, rheumatoid arthritis, other Salmonella sp., previous immunization, collagen diseases
• False negatives: Early disease, antibiotic treatment, immunocompromised host
• Prozone phenomenon (antibody excess) - rare with Widal

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2019

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Q1. Microbial Etiology of Diarrhea and Dysentery + Vibrio cholerae (Morphology, Culture, Lab Diagnosis)

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Q2. PUO and Lab Diagnosis of Tuberculosis

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Q3. Morphology and Culture Characteristics of M. tuberculosis

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2018

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Q1. Pathogenicity and Lab Diagnosis of Corynebacterium diphtheriae

• Encoded by tox gene carried on beta-phage (bacteriophage corynephage beta)
• Only lysogenized strains (carrying the phage) produce toxin
• Produced only in low-iron conditions (iron repressor DtxR normally blocks tox gene)

• Single polypeptide (62 kDa) cleaved into:
  • Fragment B: Binds to cell surface receptor (HB-EGF)
  • Fragment A: Enzymatically active - ADP-ribosylates EF-2 (Elongation Factor 2) → halts protein synthesis → cell death

1. Droplet infection → nasopharynx/tonsils
2. C. diphtheriae colonizes - does NOT invade tissues
3. Produces toxin locally → systemic absorption
4. Local: Pseudomembrane forms (fibrin, necrotic epithelium, bacteria, leucocytes) - gray-white membrane, bleeds on removal → "Leather-like" membrane
5. Systemic toxin effects:
   • Myocarditis (most common cause of death in diphtheria) - heart block, arrhythmia
   • Neuritis - palatal palsy (nasal voice, regurgitation), ocular palsy (accommodation paralysis), peripheral neuropathy
   • Renal tubular necrosis

• Intradermal injection of toxin (0.2 mL diluted DT)
• Positive (susceptible): Erythema + induration at 24-48 hours (no immunity)
• Negative (immune): No reaction

• Gram stain: Gram +ve pleomorphic rods in Chinese letter arrangement
• Albert's stain: Dark greenish blue granules (metachromatic) in blue-green cell bodies
• Ponder's stain: Granules appear red, cell body blue
• Methylene blue (Loeffler's): Granules appear dark blue/purple (metachromatic)

• Loeffler's serum slope: Primary isolation - grow well in 6-8 hours; excellent for showing granules
• Blood Tellurite Agar (Hoyle's/McLeod's): Black/gray colonies appear in 24-48 hours
• Colonial appearance depends on biotype (see 2021 Q3)

• Elek's gel precipitation test (in vitro):
  • Filter paper strip soaked in diphtheria antitoxin placed on Elek's agar
  • Test strain streaked perpendicular to the strip
  • Toxin diffuses → reacts with antitoxin → precipitation line at 45° = toxigenic strain
  • Result in 24-48 hours
• Guinea pig inoculation (in vivo):
  • Two guinea pigs: one given antitoxin (protected), one not
  • Both inoculated with organism
  • Unprotected animal dies → proves toxigenicity
• PCR for tox gene: Molecular - rapid and specific

• Diphtheria Antitoxin (DAT): Neutralizes free toxin (immediate, dose based on severity)
• Antibiotics: Penicillin G or Erythromycin (kills bacteria, stops toxin production)
• Prophylaxis: DTP vaccine (diphtheria toxoid - formalin-inactivated toxin)

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Exam Tips:

• Always draw a table when comparing organisms/tests
• For case-based questions: name the organism first, then justify with clues from the case
• Learn the "gold standard" for each diagnosis (blood culture for enteric fever, ZN stain for TB, TCBS for cholera, coagulase for S. aureus, Elek's for diphtheria toxin)
• Remember: culture media associations are high-yield (TCBS=cholera, LJ=TB, Tellurite=diphtheria, MacConkey=Gram -ve, MSA=S. aureus)
• Drug resistance mechanisms are frequently asked - especially MRSA (mecA/PBP2a) and MDR-TB (rpoB mutation)