Microbiology
High-yield MBBS Microbiology question bank — bacteriology, immunology, virology, mycology and parasitology, with laboratory diagnosis, at Ananthanarayan & Paniker / Jawetz depth.
12chapters144questions82High-Yield
Definition
Malaria is a protozoal infection caused by Plasmodium species, transmitted by the bite of the female Anopheles mosquito.
Species
- P. vivax — commonest in India; benign tertian; relapses (hypnozoites)
- P. falciparum — malignant tertian; most dangerous
- P. malariae — quartan; P. ovale — relapsing
- P. knowlesi — zoonotic
Life Cycle
- Man = intermediate host (asexual cycle); mosquito = definitive host (sexual cycle)
- Sporozoites injected → liver (exo-erythrocytic schizogony)
- Merozoites → red cells (erythrocytic schizogony) → fever paroxysms
- Hypnozoites in liver (vivax, ovale) → relapse
- Gametocytes taken up by mosquito
Synchronous rupture of infected red cells produces the periodic fever. Species Fever Relapse P. vivax Tertian (48 h) Yes P. falciparum Malignant tertian No P. malariae Quartan (72 h) No Applied
- Falciparum complications: cerebral malaria, blackwater fever, severe anaemia
- Treatment: ACT for falciparum; chloroquine + primaquine for vivax
🔑KEY POINTS TO REMEMBER- Malaria is spread by female Anopheles; man is the intermediate host.
- P. falciparum is the most dangerous; P. vivax relapses via hypnozoites.
- Fever paroxysms follow synchronous red-cell rupture.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Amoebiasis is infection with Entamoeba histolytica, causing dysentery and extraintestinal abscesses.
Parasite & Transmission
- Two stages: trophozoite (invasive) and cyst (infective)
- Quadrinucleate cyst is the infective form
- Faeco-oral spread through contaminated food and water
- Trophozoite contains ingested red cells (diagnostic)
Pathology & Clinical Features
- Invades colonic mucosa → flask-shaped ulcers (caecum, sigmoid)
- Amoebic dysentery — blood and mucus in stool, no/low fever
- Amoeboma (mass lesion)
- Extraintestinal: liver abscess (anchovy-sauce pus), lung, brain
Trophozoites invade the colon and may travel by portal vein to the liver. Stage Role Cyst Infective, transmission Trophozoite Invasive, causes disease Applied
- Diagnosis: stool microscopy, antigen detection, serology; ultrasound for abscess
- Treatment: metronidazole + luminal amoebicide (diloxanide furoate)
🔑KEY POINTS TO REMEMBER- E. histolytica: infective quadrinucleate cyst, invasive trophozoite.
- Causes flask-shaped colonic ulcers and amoebic liver abscess.
- Treated with metronidazole plus a luminal amoebicide.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Leishmaniasis is a protozoal disease caused by Leishmania species, transmitted by the bite of the sandfly.
Parasite & Transmission
- Two forms: amastigote (LD body) in man, promastigote in sandfly
- Vector: Phlebotomus argentipes (India)
- Amastigotes multiply within macrophages
- Reservoir: man (Indian kala-azar), dogs, rodents elsewhere
Clinical Forms
- Visceral (kala-azar) — L. donovani: prolonged fever, massive splenomegaly, pancytopenia, weight loss, darkening of skin
- Cutaneous — oriental sore
- Mucocutaneous — espundia (South America)
- Post-kala-azar dermal leishmaniasis (PKDL) — reservoir for transmission
The parasite survives inside the very cells meant to destroy it. Test Detail Splenic aspirate Most sensitive rK39 strip test Rapid serology Bone marrow LD bodies Applied
- Treatment: liposomal amphotericin B (first-line), miltefosine
- Vector control with insecticide spraying
🔑KEY POINTS TO REMEMBER- Leishmania: amastigote in man, promastigote in sandfly.
- Kala-azar causes fever with massive splenomegaly and pancytopenia.
- Diagnosed by rK39/splenic aspirate; treated with liposomal amphotericin B.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Giardiasis and trichomoniasis are flagellate protozoal infections of the small intestine and genital tract respectively.
Giardiasis (Giardia lamblia)
- Pear-shaped flagellate with two nuclei (‘face-like’ appearance)
- Faeco-oral spread; cyst is infective
- Attaches to duodenal mucosa by a sucking disc
- Malabsorption, steatorrhoea, foul-smelling bulky stools, no blood
- Diagnosis: stool cysts, duodenal aspirate, antigen ELISA
Trichomoniasis (Trichomonas vaginalis)
- Flagellate with no cyst stage — trophozoite only
- Sexually transmitted
- Women: frothy greenish offensive discharge, itching, strawberry cervix
- Men: usually asymptomatic carrier
- Diagnosis: wet mount showing jerky motile trophozoites
Both are flagellates, but their routes and sites differ entirely. Feature Giardia Trichomonas Cyst Present Absent Spread Faeco-oral Sexual Site Duodenum Vagina Applied
- Both treated with metronidazole
- Treat sexual partners in trichomoniasis
🔑KEY POINTS TO REMEMBER- Giardia: faeco-oral, duodenal attachment → malabsorption and steatorrhoea.
- Trichomonas: sexually transmitted, no cyst stage, strawberry cervix.
- Both respond to metronidazole.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Toxoplasmosis is infection with Toxoplasma gondii, an intracellular protozoan whose definitive host is the cat.
Transmission
- Ingestion of oocysts from cat faeces (soil, vegetables)
- Eating undercooked meat containing tissue cysts
- Transplacental transmission to the fetus
- Rarely by blood transfusion or transplantation
Clinical Forms
- Immunocompetent — usually asymptomatic; may cause lymphadenopathy
- Congenital — classic tetrad: hydrocephalus, intracranial calcification, chorioretinitis, convulsions
- AIDS — reactivation causing cerebral toxoplasmosis (ring-enhancing lesions)
- Diagnosis: serology (IgM/IgG), PCR, imaging
Latent tissue cysts reactivate when cell-mediated immunity falls. Host group Manifestation Healthy adult Asymptomatic Fetus Congenital tetrad AIDS Cerebral abscess Applied
- Treatment: pyrimethamine + sulfadiazine (with folinic acid)
- Pregnant women should avoid cat litter and undercooked meat
🔑KEY POINTS TO REMEMBER- T. gondii: cat is the definitive host; spread by oocysts, meat, placenta.
- Congenital tetrad: hydrocephalus, calcification, chorioretinitis, convulsions.
- Cerebral toxoplasmosis in AIDS; treat with pyrimethamine + sulfadiazine.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Laboratory diagnosis of malaria depends on demonstrating the parasite in blood by microscopy, antigen detection or molecular methods.
Microscopy (gold standard)
- Thick smear — detects parasite, higher sensitivity (screening)
- Thin smear — identifies species and parasite density
- Giemsa/Leishman stain
- Best collected during or just after a fever paroxysm; repeat if negative
Other Methods
- Rapid diagnostic tests (RDT) — detect HRP-2 (falciparum) and pLDH; useful in the field
- PCR — most sensitive, species identification, research and mixed infections
- QBC (fluorescence); serology only for epidemiology
- Supporting findings: anaemia, thrombocytopenia
Thick films find the parasite; thin films identify which species it is. Method Advantage Thick smear Sensitive detection Thin smear Species identification RDT Rapid, field use Applied
- HRP-2 may stay positive for weeks after cure
- A single negative smear does not exclude malaria — repeat
🔑KEY POINTS TO REMEMBER- Peripheral smear is the gold standard: thick for detection, thin for species.
- Stained with Giemsa; sample during fever paroxysm.
- RDTs detect HRP-2/pLDH; PCR is the most sensitive.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Amoebic liver abscess is the commonest extraintestinal complication of Entamoeba histolytica infection.
Pathogenesis & Features
- Trophozoites reach the liver via the portal vein
- Usually single abscess in the right lobe (postero-superior)
- Contents: ‘anchovy-sauce’ chocolate-brown pus (necrotic liver, sterile)
- Fever, tender hepatomegaly, right upper quadrant pain, referred shoulder pain
- Often no history of dysentery
Diagnosis & Treatment
- Ultrasound — hypoechoic lesion; raised right dome of diaphragm on X-ray
- Serology (antibody) positive in most cases; leucocytosis, deranged liver enzymes
- Treatment: metronidazole (excellent response) + luminal amoebicide
- Aspiration if large, left lobe, or not responding — risk of rupture
Trophozoites carried by the portal vein digest liver tissue. Feature Detail Site Right lobe Pus Anchovy sauce, sterile Treatment Metronidazole Applied
- Rupture into pleura, pericardium or peritoneum is the danger
- Pus is sterile on bacterial culture
🔑KEY POINTS TO REMEMBER- Amoebae reach the liver via the portal vein, usually the right lobe.
- Anchovy-sauce sterile pus; often no preceding dysentery.
- Metronidazole is curative; aspirate if large or unresponsive.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Trypanosomiasis is caused by flagellate Trypanosoma species, producing African sleeping sickness and American Chagas disease.
African Trypanosomiasis (sleeping sickness)
- T. brucei gambiense (chronic) and T. b. rhodesiense (acute)
- Vector: tsetse fly (Glossina)
- Chancre at bite site → fever, lymphadenopathy (Winterbottom’s sign)
- CNS stage: daytime somnolence, personality change, coma
- Antigenic variation allows immune evasion
American Trypanosomiasis (Chagas disease)
- T. cruzi; vector: reduviid (kissing) bug — infected faeces rubbed in
- Acute: Romana’s sign (periorbital oedema), fever
- Chronic: cardiomyopathy, megaoesophagus, megacolon
- Diagnosis: blood smear (acute), serology (chronic)
Different vectors and continents produce quite different diseases. Disease Vector Hallmark Sleeping sickness Tsetse fly Somnolence Chagas Reduviid bug Cardiomyopathy Applied
- African: suramin/pentamidine (early), melarsoprol (CNS stage)
- Chagas: benznidazole, nifurtimox
🔑KEY POINTS TO REMEMBER- African trypanosomiasis: tsetse fly, Winterbottom’s sign, sleeping sickness.
- Chagas disease: reduviid bug, Romana’s sign, cardiomyopathy and megacolon.
- Antigenic variation helps African trypanosomes evade immunity.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Cryptosporidiosis is diarrhoeal disease caused by Cryptosporidium parvum/hominis, an important opportunistic infection in AIDS.
Parasite & Transmission
- Coccidian protozoan; oocyst is infective and immediately infectious when passed
- Faeco-oral spread; water-borne outbreaks common
- Chlorine-resistant oocysts — survive water treatment
- Very low infective dose
Clinical Features & Diagnosis
- Immunocompetent — self-limiting watery diarrhoea (1–2 weeks)
- AIDS (CD4 <100/μL) — chronic profuse watery diarrhoea, wasting; biliary disease
- Diagnosis: modified Ziehl-Neelsen (acid-fast) stain of stool → pink oocysts
- Also antigen ELISA, PCR
Immune status decides whether illness is brief or unrelenting. Feature Detail Stain Modified ZN (acid-fast) Water Chlorine-resistant AIDS Chronic diarrhoea Applied
- Main treatment in AIDS is antiretroviral therapy (restore CD4)
- Nitazoxanide in immunocompetent patients; boil drinking water
🔑KEY POINTS TO REMEMBER- Cryptosporidium oocysts are chlorine-resistant and water-borne.
- Self-limiting in healthy people; chronic severe diarrhoea in AIDS.
- Diagnosed by modified Ziehl-Neelsen stain of stool.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Antimalarial drugs act on different stages of the parasite for treatment of the acute attack, radical cure or prophylaxis.
By Stage of Action
- Blood schizonticides — artemisinins, chloroquine, quinine (clinical cure)
- Tissue schizonticide — primaquine (kills hypnozoites → radical cure)
- Gametocidal — primaquine (blocks transmission)
- Prophylaxis — doxycycline, mefloquine, chloroquine
Current Practice
- Falciparum — ACT (artemisinin-based combination therapy) + single-dose primaquine
- Vivax — chloroquine + primaquine for 14 days
- Severe malaria — IV artesunate
- Combination therapy prevents resistance
Clearing blood stages cures the attack; primaquine prevents relapse. Drug Stage Purpose Artemisinin Blood Clinical cure Primaquine Liver Radical cure Artesunate IV Blood Severe malaria Applied
- Check G6PD before primaquine (haemolysis risk)
- Chloroquine resistance is widespread in falciparum
🔑KEY POINTS TO REMEMBER- Blood schizonticides (ACT, chloroquine) treat the acute attack.
- Primaquine gives radical cure by killing hypnozoites; check G6PD.
- Severe malaria is treated with IV artesunate.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Balantidiasis is colonic infection with Balantidium coli, the only ciliate pathogenic to humans and the largest protozoan parasite.
Parasite & Transmission
- Largest protozoan parasite of man; covered with cilia
- Two stages: trophozoite (invasive) and cyst (infective)
- Pig is the natural reservoir
- Faeco-oral spread; occupational risk for pig handlers
Clinical Features & Treatment
- Often asymptomatic carriage
- Invades colonic mucosa → ulcers resembling amoebic ulcers
- Balantidial dysentery — diarrhoea with blood and mucus, abdominal pain
- Rarely perforation and extraintestinal spread
- Diagnosis: stool microscopy — large ciliated motile trophozoite
- Treatment: tetracycline (drug of choice) or metronidazole
Like amoebiasis, invasion of colonic mucosa produces bloody dysentery. Feature Detail Group Ciliate (only one) Reservoir Pig Treatment Tetracycline Applied
- Consider in dysentery among pig farm workers
- Prevented by hygiene and safe disposal of pig faeces
🔑KEY POINTS TO REMEMBER- B. coli is the only ciliate pathogen and the largest protozoan parasite.
- Pig is the reservoir; spread is faeco-oral.
- Causes dysentery with colonic ulcers; treated with tetracycline.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).Definition
Cutaneous leishmaniasis (oriental sore) is a localised skin infection caused by Leishmania species, transmitted by the sandfly.
Organism & Features
- L. tropica, L. major (Old World); L. mexicana, L. braziliensis (New World)
- Vector: sandfly (Phlebotomus)
- Papule at bite site → nodule → painless ulcer with raised indurated edge
- Exposed parts (face, limbs); usually single or few lesions
- Heals over months leaving a depressed scar
Diagnosis & Treatment
- Slit-skin smear from the ulcer edge → amastigotes (LD bodies) in macrophages
- Culture in NNN medium; PCR for species
- Montenegro (leishmanin) skin test positive
- Treatment: intralesional or systemic antimonials; ketoconazole, cryotherapy, paromomycin
- L. braziliensis may progress to mucocutaneous (espundia) disease
The parasite stays confined to the skin, unlike visceral disease. Feature Detail Lesion Painless ulcer, raised edge Sample Slit-skin smear (edge) Medium NNN Applied
- Self-healing but leaves disfiguring scars
- Vector control and bed nets prevent transmission
🔑KEY POINTS TO REMEMBER- Cutaneous leishmaniasis (oriental sore) is spread by sandflies.
- Painless ulcer with a raised indurated edge on exposed skin.
- Diagnosed by slit-skin smear/NNN culture; treated with antimonials.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Medical Parasitology (Paniker).