Microbiology
High-yield MBBS Microbiology question bank — bacteriology, immunology, virology, mycology and parasitology, with laboratory diagnosis, at Ananthanarayan & Paniker / Jawetz depth.
12chapters144questions82High-Yield
Definition
Treponema pallidum is a thin, motile spirochaete causing syphilis, a chronic sexually transmitted disease with distinct stages.
Organism
- Slender spirochaete with regular coils, actively motile
- Cannot be cultured on artificial media
- Demonstrated by dark-ground microscopy
- Very delicate — killed rapidly by drying, heat, antiseptics
Stages of Disease
- Primary — painless indurated ulcer (hard chancre), 3 weeks after exposure
- Secondary — rash (palms and soles), condylomata lata, generalised lymphadenopathy
- Latent — asymptomatic, serology positive
- Tertiary — gumma, cardiovascular (aortitis), neurosyphilis
- Congenital — transplacental transmission
Untreated infection progresses through well-defined stages over years. Test type Example Non-treponemal VDRL, RPR Treponemal TPHA, FTA-ABS Applied
- Treatment: benzathine penicillin (drug of choice)
- Jarisch-Herxheimer reaction after starting treatment
🔑KEY POINTS TO REMEMBER- T. pallidum is a non-cultivable spirochaete seen by dark-ground microscopy.
- Stages: primary (hard chancre), secondary, latent, tertiary, congenital.
- Treated with benzathine penicillin.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Rickettsiae are small obligate intracellular Gram-negative bacteria transmitted by arthropod vectors, causing fever with rash and vasculitis.
General Features
- Obligate intracellular — multiply in endothelial cells
- Transmitted by lice, fleas, ticks, mites
- Cause vasculitis → rash, haemorrhage
- Cannot be grown on ordinary media (need cell culture, egg)
Important Diseases
- Epidemic typhus — R. prowazekii, louse-borne
- Endemic (murine) typhus — R. typhi, flea-borne
- Scrub typhus — Orientia tsutsugamushi, mite (chigger); eschar
- Rocky Mountain spotted fever — R. rickettsii, tick-borne
- Q fever — Coxiella burnetii (inhalation, no vector needed)
Endothelial infection causes the vasculitis behind fever and rash. Disease Vector Epidemic typhus Louse Scrub typhus Mite (chigger) RMSF Tick Applied
- Doxycycline is the drug of choice for all
- Weil-Felix test is a classic (though non-specific) screening test
🔑KEY POINTS TO REMEMBER- Rickettsiae are obligate intracellular, arthropod-borne bacteria.
- Cause vasculitis → fever with rash; scrub typhus shows an eschar.
- Doxycycline is the treatment of choice.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Chlamydiae are obligate intracellular bacteria with a unique biphasic life cycle of elementary and reticulate bodies.
Life Cycle
- Elementary body — infectious, extracellular, metabolically inert
- Reticulate body — replicating, intracellular form
- Inclusion bodies form in the cytoplasm
- Cannot make their own ATP (‘energy parasites’)
Species & Diseases
- C. trachomatis — trachoma (A–C), genital infection (D–K), lymphogranuloma venereum (L1–L3), neonatal conjunctivitis and pneumonia
- C. pneumoniae — atypical pneumonia
- C. psittaci — psittacosis (from birds)
The infectious form converts inside cells into the replicating form. Serotype Disease A–C Trachoma D–K Genital infection L1–L3 LGV Applied
- Treatment: azithromycin or doxycycline (β-lactams ineffective — no peptidoglycan)
- Leading infectious cause of preventable blindness (trachoma)
🔑KEY POINTS TO REMEMBER- Chlamydiae are obligate intracellular with elementary and reticulate bodies.
- C. trachomatis: A–C trachoma, D–K genital, L1–L3 LGV.
- Treated with azithromycin/doxycycline; β-lactams do not work.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Mycoplasmas are the smallest free-living organisms and are unique in lacking a cell wall.
Characteristics
- No cell wall → pleomorphic, Gram stain unhelpful
- Cell membrane contains sterols
- Smallest free-living organisms; can pass through bacterial filters
- Culture: enriched media → ‘fried-egg’ colonies (slow, 1–3 weeks)
Diseases
- M. pneumoniae — atypical (walking) pneumonia, commonest in young adults
- Extrapulmonary: haemolytic anaemia (cold agglutinins), rash, arthritis
- M. hominis, Ureaplasma urealyticum — genital infections
Attachment to respiratory epithelium causes a mild but persistent pneumonia. Feature Detail Cell wall Absent Colony Fried-egg Serology Cold agglutinins Applied
- β-lactams are ineffective (no cell wall)
- Treat with macrolides, doxycycline or fluoroquinolones
🔑KEY POINTS TO REMEMBER- Mycoplasmas lack a cell wall and have sterols in the membrane.
- M. pneumoniae causes atypical pneumonia with cold agglutinins.
- β-lactams useless; use macrolides or doxycycline.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Zoonotic bacterial infections are transmitted from animals to humans; important examples are brucellosis, anthrax and plague.
Brucellosis
- Brucella melitensis, abortus, suis; small Gram-negative coccobacilli
- Source: unpasteurised milk, contact with animals
- Undulant fever, sweating, arthralgia, hepatosplenomegaly
- Treatment: doxycycline + rifampicin/streptomycin (prolonged)
Anthrax
- Bacillus anthracis — large Gram-positive bacilli, bamboo-stick chains, capsulated (polypeptide)
- Cutaneous (malignant pustule / black eschar), pulmonary (woolsorter’s disease), intestinal
- McFadyean reaction — capsule staining
- Treatment: ciprofloxacin or doxycycline
Plague
- Yersinia pestis — bipolar (safety-pin) staining
- Rat flea bite; bubonic (bubo) and pneumonic forms
- Treatment: streptomycin, doxycycline
Each disease follows a different route from its animal reservoir. Disease Organism Key sign Brucellosis Brucella Undulant fever Anthrax B. anthracis Black eschar Plague Y. pestis Bubo Applied
- Anthrax and plague are potential bioterrorism agents
- Pasteurisation prevents brucellosis
🔑KEY POINTS TO REMEMBER- Brucellosis: undulant fever from unpasteurised milk.
- Anthrax: bamboo-stick bacilli, black eschar (malignant pustule).
- Plague: Y. pestis with safety-pin staining, causes buboes.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
The VDRL (Venereal Disease Research Laboratory) test is a non-treponemal flocculation test used to screen for syphilis.
Principle & Method
- Detects reagin antibody against cardiolipin-lecithin-cholesterol antigen
- Slide flocculation test; result reported as a titre
- Antigen is not treponemal — hence non-specific
- Becomes positive 1–4 weeks after the chancre appears
Uses & Limitations
- Screening and, importantly, monitoring response to treatment (titre falls)
- Biological false positives — pregnancy, malaria, leprosy, SLE, viral infections
- Prozone phenomenon → false negative in secondary syphilis (excess antibody)
- Positive result must be confirmed by a treponemal test (TPHA/FTA-ABS)
The test measures a non-specific antibody, so confirmation is essential. Test Type Use VDRL Non-treponemal Screening, follow-up TPHA Treponemal Confirmation Applied
- Falling titre indicates successful treatment
- Treponemal tests stay positive for life
🔑KEY POINTS TO REMEMBER- VDRL is a non-treponemal flocculation test detecting reagin antibody.
- Used for screening and monitoring treatment response (titre).
- Biological false positives common; confirm with TPHA/FTA-ABS.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Leptospirosis is a zoonotic infection caused by Leptospira interrogans, acquired through contact with water contaminated by rodent urine.
Organism & Transmission
- Thin, tightly coiled spirochaete with hooked ends
- Seen by dark-ground microscopy; cultured in EMJH medium
- Reservoir: rodents (urine); enters through skin abrasions or mucosa
- Associated with floods, sewage work, paddy fields
Clinical Features
- Anicteric — fever, severe myalgia (calf), conjunctival suffusion, headache
- Weil’s disease (severe) — jaundice, renal failure, haemorrhage
- Biphasic course: septicaemic then immune phase
- Diagnosis: MAT (gold standard), ELISA IgM, PCR
The organism spreads in blood, then immune-phase damage affects liver and kidney. Feature Detail Source Rodent urine Classic sign Conjunctival suffusion Severe form Weil’s disease Applied
- Treatment: doxycycline (mild) or penicillin (severe)
- Doxycycline prophylaxis during flood exposure
🔑KEY POINTS TO REMEMBER- Leptospira: hooked-end spirochaete from rodent urine, seen by dark-ground microscopy.
- Fever with severe myalgia and conjunctival suffusion.
- Weil’s disease = jaundice + renal failure; treat with doxycycline/penicillin.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Borrelia are large, loosely coiled spirochaetes causing relapsing fever and Lyme disease.
Relapsing Fever
- B. recurrentis (louse-borne), B. duttoni (tick-borne)
- Antigenic variation → repeated febrile relapses
- Organisms visible in peripheral blood smear during fever
- Treatment: doxycycline (watch for Jarisch-Herxheimer reaction)
Lyme Disease
- B. burgdorferi, transmitted by Ixodes tick
- Stage 1 — erythema chronicum migrans (bull’s-eye rash)
- Stage 2 — carditis, facial palsy, meningitis
- Stage 3 — chronic arthritis, neurological disease
- Treatment: doxycycline or ceftriaxone
One species evades immunity by changing antigens; the other spreads in stages. Disease Vector Hallmark Relapsing fever Louse/tick Recurrent fever Lyme disease Ixodes tick Bull’s-eye rash Applied
- Relapses explained by antigenic variation of surface proteins
- Early treatment of Lyme disease prevents later stages
🔑KEY POINTS TO REMEMBER- Borrelia are large loosely coiled spirochaetes.
- Relapsing fever: antigenic variation, seen on blood smear.
- Lyme disease: Ixodes tick, erythema chronicum migrans; treat with doxycycline.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Trachoma is a chronic keratoconjunctivitis caused by Chlamydia trachomatis serotypes A, B, Ba and C, and a leading preventable cause of blindness.
Transmission & Pathology
- Spread by eye-seeking flies, fingers, fomites (shared towels)
- Associated with poor hygiene and water scarcity
- Follicles on the upper tarsal conjunctiva → scarring
- Scarring → entropion and trichiasis → corneal opacity → blindness
Diagnosis & Control
- Conjunctival scraping → inclusion bodies (Halberstaedter-Prowazek) with Giemsa
- WHO SAFE strategy: Surgery, Antibiotics, Facial cleanliness, Environmental improvement
- Treatment: single-dose azithromycin or topical tetracycline
Repeated infections scar the lid, turning lashes inward onto the cornea. Element Detail Serotypes A, B, Ba, C Vector Flies Strategy SAFE Applied
- Blindness is preventable if treated early
- Mass azithromycin distribution in endemic areas
🔑KEY POINTS TO REMEMBER- Trachoma is caused by C. trachomatis serotypes A–C, spread by flies and fomites.
- Scarring → trichiasis → corneal opacity → blindness.
- Controlled by the WHO SAFE strategy and azithromycin.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
The Weil–Felix reaction is a heterophile agglutination test for rickettsial infections using Proteus OX antigens.
Principle
- Rickettsiae share antigens with certain Proteus strains
- Patient serum agglutinates Proteus OX-19, OX-2, OX-K antigens
- A heterophile (non-specific) reaction
- Titre ≥ 1:160 or a fourfold rise is significant
Interpretation & Limitations
- OX-19 — typhus group (epidemic, endemic)
- OX-K — scrub typhus
- OX-2 — spotted fever group
- Negative in Q fever
- Low sensitivity and specificity — largely replaced by ELISA/IFA
Shared antigens make Proteus a convenient surrogate test antigen. Antigen Disease OX-19 Typhus group OX-K Scrub typhus OX-2 Spotted fever Applied
- Still used where resources are limited
- Confirmation needs specific serology
🔑KEY POINTS TO REMEMBER- Weil-Felix uses Proteus OX antigens (heterophile agglutination).
- OX-19 typhus, OX-K scrub typhus, OX-2 spotted fever.
- Negative in Q fever; poor sensitivity and specificity.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Q fever is a zoonosis caused by Coxiella burnetii, unusual among rickettsial infections in being spread by inhalation.
Distinctive Features
- No arthropod vector needed — inhalation of contaminated aerosols
- Source: cattle, sheep, goats (placenta, milk, urine, faeces)
- Forms a spore-like resistant form → survives in the environment
- No rash (unlike other rickettsial infections)
- Weil-Felix negative
Clinical Features
- Acute: fever, severe headache, atypical pneumonia, hepatitis
- Chronic: culture-negative endocarditis
- Diagnosis: serology (phase I and II antibodies)
- Treatment: doxycycline; chronic cases need prolonged therapy
Airborne spread explains outbreaks among farm and abattoir workers. Feature Q fever Other rickettsiae Spread Inhalation Arthropod Rash Absent Present Weil-Felix Negative Positive Applied
- Occupational disease of farmers, abattoir and veterinary workers
- Consider in culture-negative endocarditis
🔑KEY POINTS TO REMEMBER- Q fever (C. burnetii) spreads by inhalation, not by vectors.
- No rash and Weil-Felix negative — unlike other rickettsial infections.
- Causes atypical pneumonia and culture-negative endocarditis; treat with doxycycline.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Genital chlamydial infection is caused by C. trachomatis serotypes D–K and is the commonest bacterial sexually transmitted infection.
Clinical Features
- Often asymptomatic (especially in women) → silent spread
- Men — non-gonococcal urethritis, epididymitis
- Women — cervicitis, pelvic inflammatory disease
- Complications: infertility, ectopic pregnancy, chronic pelvic pain
- Neonates — inclusion conjunctivitis and pneumonia (from birth canal)
Diagnosis & Treatment
- NAAT (PCR) on urine or swab — test of choice
- Giemsa-stained inclusion bodies; cell culture (McCoy cells)
- Treatment: azithromycin single dose or doxycycline 7 days
- Treat partners to prevent reinfection
Silent infection ascends the genital tract and damages the tubes. Group Presentation Men Urethritis Women Cervicitis, PID Neonate Conjunctivitis Applied
- Screening of sexually active young women is recommended
- Often coexists with gonorrhoea — treat both
🔑KEY POINTS TO REMEMBER- Serotypes D–K cause the commonest bacterial STI, often asymptomatic.
- Leads to PID, infertility and ectopic pregnancy; neonatal conjunctivitis.
- Diagnosed by NAAT; treated with azithromycin or doxycycline plus partner treatment.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).