Microbiology
High-yield MBBS Microbiology question bank — bacteriology, immunology, virology, mycology and parasitology, with laboratory diagnosis, at Ananthanarayan & Paniker / Jawetz depth.
12chapters144questions82High-Yield
Definition
Mycobacterium tuberculosis is a slender, acid-fast bacillus causing tuberculosis, a chronic granulomatous infection.
Morphology & Culture
- Slender, straight or slightly curved acid-fast bacilli
- High mycolic acid content in the wall → acid fastness
- Obligate aerobe, very slow growing (generation time ~18 h)
- Lowenstein-Jensen medium — rough, tough, buff colonies in 4–6 weeks
- Liquid culture (MGIT) faster (1–2 weeks)
Pathogenesis
- Droplet inhalation → alveolar macrophages
- Survives inside macrophages (inhibits phagolysosome fusion)
- Type IV hypersensitivity → caseating granuloma (tubercle)
- Primary complex (Ghon focus + lymph node); post-primary reactivation
Survival inside macrophages drives the granulomatous immune response. Feature Detail Stain Ziehl-Neelsen (red) Medium LJ medium Growth 4–6 weeks Applied
- Diagnosis: sputum AFB, CBNAAT/GeneXpert, culture
- Treatment: HRZE regimen under DOTS
🔑KEY POINTS TO REMEMBER- M. tuberculosis is an acid-fast, slow-growing obligate aerobe.
- Grows on LJ medium in 4–6 weeks; mycolic acid gives acid-fastness.
- Type IV hypersensitivity produces caseating granulomas.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Mycobacterium leprae is an acid-fast bacillus causing leprosy, a chronic infection of skin and peripheral nerves.
Organism
- Acid-fast bacilli in globi (cigar bundles)
- Cannot be cultured in artificial media
- Grown in armadillo and mouse footpad
- Longest generation time (~12–14 days); prefers cooler body sites
Spectrum of Disease
- Tuberculoid (TT) — good cell-mediated immunity, few lesions, bacilli scanty, lepromin positive
- Lepromatous (LL) — poor immunity, many lesions, bacilli abundant, lepromin negative
- Borderline forms in between
- Nerve involvement → anaesthesia, deformity
Host cell-mediated immunity determines which end of the spectrum develops. Feature Tuberculoid Lepromatous Bacilli Scanty Numerous Lepromin Positive Negative Lesions Few Many Applied
- Diagnosis: slit-skin smear, nerve thickening, anaesthetic patch
- Treatment: WHO multidrug therapy (rifampicin, dapsone, clofazimine)
🔑KEY POINTS TO REMEMBER- M. leprae is acid-fast and cannot be cultured in vitro.
- Tuberculoid = good immunity, few bacilli; lepromatous = poor immunity, many bacilli.
- Diagnosed by slit-skin smear; treated with WHO MDT.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Corynebacterium diphtheriae is a Gram-positive bacillus causing diphtheria, a toxin-mediated upper respiratory infection.
Morphology & Culture
- Club-shaped Gram-positive bacilli in Chinese letter arrangement
- Metachromatic (volutin) granules — Albert stain
- Media: Loeffler’s serum slope (rapid growth), potassium tellurite (black colonies)
- Biotypes: gravis, intermedius, mitis
Toxin & Disease
- Diphtheria toxin (A-B toxin) — phage-coded (lysogenic conversion)
- Inhibits protein synthesis by inactivating EF-2 (ADP-ribosylation)
- Grey adherent pseudomembrane in throat (bleeds if removed)
- Complications: myocarditis, palatal and other paralysis, airway obstruction
The organism stays local while its toxin causes systemic damage. Test/Medium Purpose Albert stain Volutin granules Loeffler’s slope Rapid growth Elek test Toxigenicity Applied
- Treatment: antitoxin immediately + penicillin/erythromycin
- Prevented by DPT immunisation (toxoid)
🔑KEY POINTS TO REMEMBER- C. diphtheriae: Chinese-letter bacilli with metachromatic granules (Albert stain).
- Phage-coded toxin inhibits EF-2 → pseudomembrane and myocarditis.
- Treat with antitoxin plus antibiotics; prevented by toxoid.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Clostridium tetani is an anaerobic, spore-forming Gram-positive bacillus causing tetanus through its neurotoxin.
Organism
- Gram-positive bacillus with terminal round spore (drumstick appearance)
- Strict anaerobe, motile; spores widespread in soil
- Non-invasive — remains at the wound site
Toxin & Clinical Features
- Tetanospasmin — travels along nerves to the CNS
- Blocks release of inhibitory neurotransmitters (glycine, GABA)
- → Unopposed motor activity: spasms and rigidity
- Trismus (lockjaw), risus sardonicus, opisthotonus
- Spasms triggered by light, sound, touch
Loss of inhibitory signals leaves muscles in continuous contraction. Feature Detail Spore Terminal (drumstick) Toxin Tetanospasmin Paralysis Spastic Applied
- Management: wound debridement, antitoxin (human immunoglobulin), metronidazole, diazepam, ventilation
- Prevention: tetanus toxoid (DPT, boosters)
🔑KEY POINTS TO REMEMBER- C. tetani: anaerobe with terminal drumstick spores.
- Tetanospasmin blocks glycine and GABA → spastic paralysis.
- Managed with antitoxin, metronidazole, sedation; prevented by toxoid.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Gas gangrene (clostridial myonecrosis) is a rapidly progressive, life-threatening infection of muscle caused mainly by Clostridium perfringens.
Organism
- Large Gram-positive bacilli with subterminal spores
- Non-motile anaerobe (unlike other clostridia)
- Nagler reaction positive — lecithinase (alpha toxin)
- Stormy clot reaction in litmus milk; double zone of haemolysis
Pathogenesis & Features
- Contaminated deep wound with devitalised muscle (anaerobic)
- Alpha toxin (lecithinase) — destroys cell membranes
- Gas production → crepitus
- Severe pain, oedema, foul discharge, toxaemia, shock
Dead tissue provides the anaerobic conditions the organism needs. Feature Detail Spore Subterminal Motility Non-motile Key toxin Alpha (lecithinase) Applied
- Treatment: urgent surgical debridement, high-dose penicillin, hyperbaric oxygen
- Also causes food poisoning (enterotoxin, reheated meat)
🔑KEY POINTS TO REMEMBER- C. perfringens: non-motile anaerobe with subterminal spores, Nagler-positive.
- Alpha toxin (lecithinase) causes myonecrosis with gas formation.
- Treatment is urgent debridement plus penicillin.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
The Mantoux test is an intradermal skin test detecting delayed (Type IV) hypersensitivity to tuberculoprotein.
Method
- 0.1 mL of 5 TU PPD injected intradermally on the forearm
- Raises a wheal of 6–10 mm
- Read at 48–72 hours
- Measure the induration (not erythema) transversely
Interpretation
- ≥ 10 mm — positive (usual criterion)
- ≥ 5 mm — positive in HIV or immunosuppressed
- Indicates infection or prior exposure, not active disease
- False negative — miliary TB, HIV, malnutrition, measles, steroids
- False positive — BCG vaccination, atypical mycobacteria
Sensitised T cells produce induration over two to three days. Reading Meaning ≥ 10 mm Positive ≥ 5 mm (HIV) Positive Negative May be anergy Applied
- Useful mainly in children and for latent TB screening
- IGRA tests are unaffected by BCG
🔑KEY POINTS TO REMEMBER- Mantoux: 0.1 mL of 5 TU PPD intradermally, read at 48–72 h.
- Measure induration; ≥10 mm positive (≥5 mm in HIV).
- Indicates exposure, not active disease; false negatives in anergy.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
BCG (Bacille Calmette-Guérin) is a live attenuated vaccine derived from Mycobacterium bovis, used against tuberculosis.
Features & Administration
- Live attenuated M. bovis, attenuated by repeated subculture
- Given intradermally, 0.1 mL (0.05 mL in newborns), left deltoid
- Given at birth in the national immunisation schedule
- Stored at 2–8°C, protected from light; reconstituted vaccine used within hours
Response & Protection
- Papule → ulcer → scar over 6–8 weeks
- Protects best against severe childhood TB (miliary TB, TB meningitis)
- Variable protection against adult pulmonary TB
- Converts the Mantoux test to positive
A controlled local infection primes cell-mediated immunity. Feature Detail Type Live attenuated Route Intradermal Best protection Childhood miliary TB Applied
- Contraindicated in symptomatic HIV and immunodeficiency
- Complications: abscess, regional lymphadenitis (BCG-itis)
🔑KEY POINTS TO REMEMBER- BCG is a live attenuated M. bovis vaccine given intradermally at birth.
- Best protection is against miliary TB and TB meningitis in children.
- Contraindicated in immunodeficiency; converts Mantoux to positive.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
The lepromin test assesses cell-mediated immunity in leprosy; it is a prognostic and classification test, not diagnostic.
Method & Readings
- 0.1 mL lepromin injected intradermally on the forearm
- Early (Fernandez) reaction — read at 48 h (erythema, induration)
- Late (Mitsuda) reaction — read at 21 days (nodule); more useful
- Positive = nodule >5 mm at 3 weeks
Interpretation
- Positive — tuberculoid leprosy (good cell-mediated immunity)
- Negative — lepromatous leprosy (poor immunity)
- Also positive in normal healthy individuals
- Used for classification and prognosis, not diagnosis
The delayed nodule reflects the strength of cell-mediated immunity. Result Type of leprosy Positive Tuberculoid Negative Lepromatous Applied
- Helps place a patient on the leprosy spectrum
- Not used to diagnose leprosy (slit-skin smear is)
🔑KEY POINTS TO REMEMBER- Lepromin test measures cell-mediated immunity, not infection.
- Mitsuda (late, 21-day) reading is the useful one.
- Positive in tuberculoid, negative in lepromatous leprosy.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Botulism is a severe flaccid paralysis caused by the neurotoxin of Clostridium botulinum.
Toxin & Mechanism
- Botulinum toxin — the most potent biological toxin known
- Blocks acetylcholine release at the neuromuscular junction
- → Flaccid (descending) paralysis
- Heat-labile toxin (destroyed by boiling), spores heat-resistant
Types & Features
- Food-borne — preformed toxin in canned/preserved food
- Infant botulism — honey, spore germination in gut (floppy baby)
- Wound botulism
- Features: diplopia, ptosis, dysphagia, descending weakness, respiratory failure; no fever, consciousness intact
Blocking acetylcholine release paralyses muscles from the head downwards. Feature Botulism Tetanus Paralysis Flaccid Spastic Toxin blocks Acetylcholine Glycine/GABA Applied
- Treatment: antitoxin + ventilatory support
- Therapeutic use: botulinum toxin for dystonia, spasticity, cosmesis
🔑KEY POINTS TO REMEMBER- Botulinum toxin blocks acetylcholine release → flaccid descending paralysis.
- Food-borne (preformed toxin), infant (honey) and wound types.
- Opposite of tetanus; treated with antitoxin and ventilation.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Pseudomembranous colitis is antibiotic-associated colitis caused by Clostridioides difficile toxins.
Pathogenesis
- Antibiotics disturb normal colonic flora
- C. difficile overgrows and produces toxins
- Toxin A (enterotoxin) and Toxin B (cytotoxin)
- Mucosal damage → yellowish pseudomembrane plaques
- Common culprits: clindamycin, cephalosporins, fluoroquinolones, ampicillin
Features & Management
- Watery diarrhoea, cramps, fever, leucocytosis
- Severe: toxic megacolon, perforation
- Diagnosis: toxin detection in stool (EIA, PCR); sigmoidoscopy shows plaques
- Treatment: stop the offending antibiotic, oral vancomycin or fidaxomicin; metronidazole in mild cases
- Recurrent cases: faecal microbiota transplant
Removing competing flora lets the organism flourish and release toxins. Toxin Action A Enterotoxin (fluid secretion) B Cytotoxin (cell damage) Applied
- Spores resist alcohol — use soap and water for hand hygiene
- Avoid antimotility drugs
🔑KEY POINTS TO REMEMBER- C. difficile overgrows after antibiotics and produces toxins A and B.
- Causes pseudomembranous colitis with watery diarrhoea.
- Stop the antibiotic; treat with oral vancomycin or fidaxomicin.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Anaerobic infections are caused by non-sporing anaerobes such as Bacteroides fragilis, typically arising from normal flora.
Organisms & Sites
- B. fragilis — commonest; Gram-negative anaerobic bacillus, gut flora
- Also Prevotella, Fusobacterium, Peptostreptococcus
- Infections: intra-abdominal abscess, appendicitis, peritonitis, pelvic infection, aspiration pneumonia, dental and brain abscess
Clues to Anaerobic Infection
- Foul-smelling pus or discharge
- Gas in tissues; necrotic tissue
- Infection near a mucosal surface
- Failure to grow on routine aerobic culture
- Usually polymicrobial
Loss of oxygen tension lets commensal anaerobes become pathogens. Feature Detail Chief organism B. fragilis Odour Foul Treatment Metronidazole Applied
- Treatment: metronidazole, clindamycin, β-lactam/β-lactamase inhibitor
- Drainage of abscess is essential
🔑KEY POINTS TO REMEMBER- B. fragilis is the commonest non-sporing anaerobe (gut flora).
- Suspect anaerobes with foul-smelling pus, gas and abscesses.
- Treat with metronidazole plus surgical drainage.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Actinomycosis is a chronic suppurative, granulomatous infection caused by Actinomyces israelii, an anaerobic filamentous bacterium.
Organism & Features
- Gram-positive, branching filamentous anaerobic bacterium (not a fungus)
- Normal flora of mouth, gut and female genital tract
- Discharging pus contains ‘sulphur granules’ (yellow colonies of organisms)
- Not acid-fast (unlike Nocardia)
Clinical Types
- Cervicofacial — commonest, after dental extraction (‘lumpy jaw’)
- Thoracic — after aspiration
- Abdominal — after appendicitis or surgery
- Pelvic — associated with intrauterine devices
- Forms multiple sinuses crossing tissue planes
The organism burrows across tissue planes, discharging sulphur granules. Feature Actinomyces Nocardia Oxygen Anaerobic Aerobic Acid-fast No Partially Granules Sulphur Rare Applied
- Treatment: prolonged high-dose penicillin (weeks to months) ± surgery
- Sulphur granules crushed and stained confirm diagnosis
🔑KEY POINTS TO REMEMBER- Actinomyces israelii: anaerobic branching Gram-positive bacterium, not acid-fast.
- Cervicofacial form commonest; sinuses discharge sulphur granules.
- Treated with prolonged high-dose penicillin.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).