Free NEET PG & INI-CET Microbiology MCQs

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Gram-positive bacteria retain the crystal violet-iodine complex in their thick peptidoglycan wall and appear purple. Gram-negatives lose it during decolourisation and take up the safranin counterstain, appearing pink.

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Standard autoclaving uses saturated steam at 121 degrees C and 15 psi for 15 minutes, killing bacteria, viruses, fungi and spores. Moist heat denatures proteins more effectively than dry heat.

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The Ziehl-Neelsen stain identifies acid-fast bacilli such as Mycobacterium tuberculosis, whose mycolic-acid-rich walls retain carbol fuchsin despite acid-alcohol decolourisation, appearing red on a blue background.

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Cell-wall-active antibiotics such as beta-lactams work best in the log phase, when bacteria are actively dividing and synthesising peptidoglycan. Dormant cells in stationary phase are less susceptible.

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Lowenstein-Jensen, an egg-based medium, is used to grow Mycobacterium tuberculosis. Thayer-Martin is for gonococci, Loeffler for diphtheria and Bordet-Gengou for pertussis.

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Ethylene oxide gas sterilises heat- and moisture-sensitive items such as endoscopes and plastics. It is effective but toxic, requiring adequate aeration before the item is used.

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Both toxins ADP-ribosylate elongation factor-2, halting protein synthesis and killing the cell. Diphtheria toxin also produces a pseudomembrane in the throat and can cause myocarditis.

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Tetanospasmin blocks release of the inhibitory neurotransmitters GABA and glycine from spinal interneurons, causing unopposed motor activity with rigidity and spasms such as lockjaw and opisthotonos. Botulinum toxin by contrast blocks acetylcholine release, causing flaccid paralysis.

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Cholera toxin ADP-ribosylates the Gs protein, permanently activating adenylate cyclase and raising cAMP. This drives massive chloride and water secretion into the gut lumen, producing rice-water stools.

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Staphylococcus aureus is coagulase-positive, distinguishing it from coagulase-negative staphylococci such as S. epidermidis. All staphylococci are catalase-positive, which separates them from streptococci.

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Group A streptococcus, S. pyogenes, is beta-haemolytic, producing complete clearing on blood agar, and is bacitracin-sensitive. S. pneumoniae and viridans streptococci are alpha-haemolytic, giving partial greenish haemolysis.

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In neonates, Group B Streptococcus, Escherichia coli and Listeria monocytogenes are the leading causes of meningitis. In older children and adults, S. pneumoniae and N. meningitidis predominate.

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The high mycolic-acid content of the mycobacterial cell wall makes it acid-fast, resisting decolourisation by acid-alcohol. This waxy wall also confers resistance to many disinfectants and to drying.

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H. pylori produces urease, which splits urea into ammonia and neutralises gastric acid locally. The urea breath test detects this activity by measuring labelled carbon dioxide produced from ingested urea.

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Pseudomonas aeruginosa produces the blue-green pigment pyocyanin and a characteristic fruity, grape-like odour. It is an important cause of burn-wound, hospital-acquired and ventilator-associated infections.

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Clostridioides difficile overgrows after antibiotic use, producing toxins A and B that cause pseudomembranous colitis. Diagnosis is by stool toxin testing or PCR, and treatment is oral vancomycin or fidaxomicin.

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The Weil-Felix test detects cross-reacting antibodies against Proteus OX antigens in rickettsial infections. It is a non-specific test, now largely replaced by more specific serology.

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Blood culture is most sensitive in the first week of enteric fever. Stool and urine cultures become positive later, in the second and third weeks. The Widal test has limited specificity and is supportive only.

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Treponema pallidum, a spirochaete, causes syphilis and is visualised by dark-ground microscopy as it is too thin for routine light microscopy. Serology uses non-treponemal tests such as VDRL and RPR alongside treponemal tests such as TPHA.

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Endotoxin is the lipid A portion of lipopolysaccharide in the gram-negative outer membrane. Released on cell lysis, it triggers cytokine release, fever and septic shock. Unlike exotoxins it is not secreted and cannot be converted into a toxoid.

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In the window period HBsAg has cleared and anti-HBs has not yet appeared, so anti-HBc IgM is the only marker indicating recent infection. It is essential for diagnosis at this stage.

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HBeAg positivity reflects active viral replication and high infectivity. Anti-HBs indicates immunity from vaccination or recovery, while isolated HBsAg indicates infection but not necessarily high replication.

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The CD4 T-cell count reflects immune status and susceptibility to opportunistic infections, and a count below 200 cells per microlitre defines AIDS. Viral load, by contrast, monitors treatment response and infectivity.

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A reactive HIV screening ELISA is confirmed with a Western blot or an antigen-antibody differentiation immunoassay. Modern algorithms use a fourth-generation antigen-antibody test followed by a differentiation assay and, if needed, nucleic acid testing.

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Negri bodies are eosinophilic intracytoplasmic inclusions in neurons, especially in the hippocampus and cerebellum, and are diagnostic of rabies. Rabies is almost invariably fatal once symptomatic, which makes post-exposure prophylaxis crucial.

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Antigenic shift is a major, abrupt change from reassortment of influenza A genome segments, producing novel strains capable of causing pandemics. Antigenic drift is gradual point mutation causing seasonal epidemics.

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High-risk human papillomavirus types 16 and 18 cause most cervical cancers via the E6 and E7 oncoproteins, which inactivate p53 and Rb. HPV vaccination targets these types.

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EBV is linked to infectious mononucleosis, Burkitt lymphoma, nasopharyngeal carcinoma and Hodgkin lymphoma. Cervical carcinoma is caused by human papillomavirus, not EBV.

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Hepatitis A and E spread by the faecal-oral route and cause acute, self-limiting hepatitis, though E is dangerous in pregnancy. Hepatitis B, C and D spread parenterally and can cause chronic disease.

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Dengue is transmitted by the day-biting Aedes aegypti mosquito, which also spreads chikungunya and Zika. Anopheles transmits malaria, Culex transmits Japanese encephalitis and filariasis, and sandflies transmit leishmaniasis.

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OPV is a live attenuated vaccine that induces both mucosal IgA and systemic immunity. The injectable IPV is a killed vaccine. Live vaccines are generally avoided in pregnancy and severe immunodeficiency.

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Reverse transcriptase transcribes the retroviral RNA genome into DNA, which integrase then inserts into the host genome. Reverse transcriptase inhibitors are a cornerstone of antiretroviral therapy.

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Plasmodium falciparum causes the most severe malaria, including cerebral malaria and high parasitaemia, because it infects red cells of all ages and causes cytoadherence. Its gametocytes are banana-shaped.

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Plasmodium vivax and P. ovale form dormant liver hypnozoites that reactivate to cause relapses. Primaquine is added to clear hypnozoites, after checking G6PD status to avoid haemolysis.

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Amoebic liver abscess yields reddish-brown 'anchovy sauce' fluid, made of necrotic hepatocytes and blood, and typically affects the right lobe. Treatment is metronidazole followed by a luminal agent to clear cysts.

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Wuchereria bancrofti, transmitted by Culex mosquitoes, causes most lymphatic filariasis and elephantiasis. Nocturnal blood smears show microfilariae, and treatment is diethylcarbamazine.

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Echinococcus granulosus causes hydatid disease; humans are accidental intermediate hosts and dogs the definitive host. Cyst rupture risks anaphylaxis, so care is needed during surgery, with albendazole cover.

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Kala-azar, caused by Leishmania donovani, is transmitted by the sandfly and causes fever, hepatosplenomegaly, pancytopenia and hyperpigmentation. Diagnosis is by splenic or marrow aspirate showing amastigotes, the LD bodies.

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Cryptococcus neoformans causes meningitis in immunocompromised patients, and India ink reveals its thick capsule as a clear halo. Cryptococcal antigen testing is more sensitive for diagnosis.

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Dimorphic fungi such as Histoplasma, Blastomyces, Coccidioides and Sporothrix grow as mould in the environment at 25 degrees C and as yeast in tissue at body temperature. Mould in the cold, yeast in the heat.

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Candida albicans causes oral thrush, presenting as removable white plaques. It is common with immunosuppression, diabetes, antibiotics or inhaled steroids. Treatment includes topical nystatin or oral fluconazole.

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Secretory IgA is the predominant antibody in mucosal secretions, providing local defence at mucosal surfaces. IgG is the main serum antibody and the only one crossing the placenta.

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IgG is the only immunoglobulin that crosses the placenta, giving the newborn passive immunity in early life. IgM is the first antibody produced in a primary response and indicates recent infection.

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IgM is the first antibody produced in a primary response, and its presence indicates recent or acute infection. It is a pentamer and a potent activator of complement.

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IgE bound to mast cells and basophils mediates type I hypersensitivity, including anaphylaxis and allergy. Allergen cross-linking of IgE triggers degranulation with histamine release.

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CD8 cytotoxic T cells recognise antigen on MHC class I molecules, present on all nucleated cells. CD4 helper T cells recognise antigen on MHC class II, found on professional antigen-presenting cells.

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BCG, against tuberculosis, is a live attenuated vaccine, as are MMR, oral polio and varicella. Hepatitis B is a subunit vaccine, tetanus is a toxoid and IPV is inactivated.

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Giving preformed antibodies confers immediate but short-lived passive immunity, useful after exposure to tetanus, rabies or hepatitis B. Active immunity from vaccination is slower to develop but longer-lasting.

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Terminal complement deficiency, affecting C5 to C9 and therefore the membrane attack complex, predisposes to recurrent Neisseria infections, both meningococcal and gonococcal, because the complex is needed to lyse these organisms.

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The enzyme-linked immunosorbent assay detects and quantifies antigens or antibodies using an enzyme-linked colour change. It is widely used for HIV screening, hormone assays and serology.