PGIMER Entrance Microbiology Flashcards

24 sample cards from the Microbiology deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. What are the distinguishing features of Vibrio cholerae and its toxin?

   Curved (comma-shaped), Gram-negative, oxidase-positive, motile (single polar flagellum) bacillus showing darting motility; grows on TCBS agar as yellow colonies. Cholera toxin ADP-ribosylates Gs protein, raising cAMP, causing massive secretory ('rice-water stool') diarrhea. Serogroups O1 and O139 cause epidemics.

2. Describe the key features of Pseudomonas aeruginosa.

   Gram-negative, oxidase-positive, non-lactose-fermenting, obligate aerobe producing blue-green pigment (pyocyanin) and fluorescein, with a grape/fruity odor. Causes burn-wound infections, hospital-acquired pneumonia, UTIs, otitis externa, and infections in cystic fibrosis/neutropenic patients. Often multidrug-resistant.

3. What are the main respiratory Gram-negative pathogens and their key features?

   Haemophilus influenzae: requires factors X (hemin) and V (NAD), satellitism around S. aureus; type b causes meningitis/epiglottitis. Bordetella pertussis: whooping cough, Bordet-Gengou medium, pertussis toxin. Legionella pneumophila: atypical pneumonia from water sources, BCYE medium, silver stain. Klebsiella pneumoniae: lobar pneumonia (red-currant jelly sputum).

4. Describe Neisseria meningitidis and N. gonorrhoeae and how they are distinguished.

   Both are Gram-negative, oxidase-positive, kidney-shaped diplococci. N. meningitidis: capsulated, causes meningitis/meningococcemia, ferments maltose AND glucose, has a vaccine. N. gonorrhoeae: causes gonorrhea/PID/ophthalmia neonatorum, ferments glucose ONLY, no useful capsule vaccine, often intracellular in neutrophils.

5. Why is Mycobacterium tuberculosis described as acid-fast, and how is it stained?

   Its cell wall is rich in mycolic acids (long-chain lipids) that resist decolorization by acid-alcohol after staining. Demonstrated by Ziehl-Neelsen (carbol fuchsin, acid-alcohol decolorizer, methylene blue counterstain) showing red bacilli, or by auramine-rhodamine fluorescent stain. It is a slow-growing obligate aerobe.

6. Which media and key tests are used for Mycobacterium tuberculosis diagnosis?

   Culture on Lowenstein-Jensen (LJ) medium (egg-based, takes 3-6 weeks; rough, tough, buff colonies) or liquid MGIT. Niacin test positive and nitrate reduction positive (differentiates from M. bovis). Rapid molecular tests: CBNAAT/GeneXpert (detects DNA + rifampicin resistance). Tuberculin (Mantoux) is a type IV hypersensitivity test.

7. Differentiate tuberculoid and lepromatous leprosy (Mycobacterium leprae).

   Tuberculoid (paucibacillary): strong cell-mediated immunity, few bacilli, few well-defined anesthetic lesions, lepromin test positive. Lepromatous (multibacillary): weak CMI, abundant bacilli (globi/foam cells), numerous symmetrical lesions, leonine facies, lepromin test negative. M. leprae cannot be cultured in vitro (grown in armadillo/mouse footpad).

8. Name the major spirochetes and the diseases they cause.

   Treponema pallidum: syphilis. Borrelia burgdorferi: Lyme disease; Borrelia recurrentis: relapsing fever. Leptospira interrogans: leptospirosis (Weil's disease). Spirochetes are thin, helical, motile (axial filaments/endoflagella) bacteria, mostly not seen on Gram stain and visualized by dark-field microscopy or silver staining.

9. List the serological tests for syphilis and classify them.

   Non-treponemal (cardiolipin, screening, become negative after treatment): VDRL and RPR. Treponemal (specific, confirmatory, remain positive for life): TPHA, FTA-ABS, TP-PA. Dark-field microscopy demonstrates motile T. pallidum from primary chancre. Biological false-positive VDRL occurs in SLE, pregnancy, leprosy.

10. Name the major Gram-positive and Gram-negative anaerobes of medical importance.

    Gram-positive anaerobes: Clostridium (tetani, botulinum, perfringens, difficile), Actinomyces, Peptostreptococcus. Gram-negative anaerobes: Bacteroides fragilis (most common in intra-abdominal abscesses), Fusobacterium, Prevotella. Anaerobic infections are often polymicrobial, foul-smelling, with gas/abscess formation.

11. Describe the mechanism of action of tetanus and botulinum toxins.

    Both are neurotoxins. Tetanospasmin (C. tetani) blocks release of inhibitory neurotransmitters (GABA/glycine) at spinal inhibitory neurons, causing spastic paralysis (lockjaw, risus sardonicus, opisthotonus). Botulinum toxin (C. botulinum) blocks acetylcholine release at the neuromuscular junction, causing flaccid (descending) paralysis.

12. Name important zoonotic bacteria and their animal reservoirs/transmission.

    Brucella (brucellosis): cattle/goats, unpasteurized milk. Yersinia pestis (plague): rodents via flea bite. Bacillus anthracis (anthrax): herbivores, animal products. Francisella tularensis (tularemia): rabbits/ticks. Leptospira: rodent urine in water. Coxiella burnetii (Q fever): cattle, inhalation. Rickettsia: ticks/lice/mites.

13. Classify medically important DNA viruses by their families.

    Herpesviridae (HSV-1/2, VZV, CMV, EBV, HHV-8): dsDNA, enveloped, latency. Hepadnaviridae (HBV): partial dsDNA. Adenoviridae: dsDNA, naked. Papovaviridae/Papillomaviridae (HPV): dsDNA, naked. Poxviridae (smallpox): largest, dsDNA. Parvoviridae (B19): ssDNA, naked (smallest DNA virus).

14. Classify medically important RNA viruses with key examples.

    Picornaviridae (polio, hepatitis A, rhino, coxsackie): ssRNA naked. Orthomyxoviridae (influenza): segmented ssRNA, antigenic shift/drift. Paramyxoviridae (measles, mumps, RSV, parainfluenza). Togaviridae (rubella). Flaviviridae (dengue, yellow fever, JE, hepatitis C). Rhabdoviridae (rabies, bullet-shaped). Retroviridae (HIV, reverse transcriptase).

15. What is the difference between antigenic drift and antigenic shift in influenza?

    Antigenic drift: minor, gradual point mutations in hemagglutinin (HA)/neuraminidase (NA) genes causing seasonal epidemics. Antigenic shift: major, abrupt change from reassortment of genome segments (e.g., from animal strains) producing a new subtype, which can cause pandemics because of lack of population immunity.

16. Distinguish the serological markers of hepatitis B infection (HBsAg, anti-HBs, HBeAg, anti-HBc, HBV DNA).

    HBsAg: active infection (acute or chronic). Anti-HBs: immunity (recovery or vaccination). HBeAg: high infectivity/active replication. Anti-HBe: low infectivity. IgM anti-HBc: recent/acute infection (also the marker in the 'window period'). Anti-HBc IgG: past or chronic infection. HBV DNA: direct measure of viral load.

17. Compare the five hepatitis viruses by genome and transmission route.

    HAV: ssRNA (Picornavirus), fecal-oral, no chronicity. HBV: DNA (Hepadnavirus), blood/sexual/perinatal, can become chronic, has vaccine. HCV: ssRNA (Flavivirus), blood-borne, high chronicity, no vaccine. HDV: defective RNA, needs HBV coat, blood-borne. HEV: ssRNA, fecal-oral, severe in pregnancy, no chronicity.

18. Describe the structure and replication of HIV.

    HIV is an enveloped retrovirus with two copies of ssRNA, reverse transcriptase, integrase, and protease enzymes; envelope glycoproteins gp120 (binds CD4 and CCR5/CXCR4 co-receptor) and gp41 (fusion). It infects CD4+ T cells and macrophages, reverse-transcribes RNA into DNA that integrates into the host genome (provirus), progressively depleting CD4 cells and causing AIDS.

19. What are the laboratory tests used to diagnose and monitor HIV infection?

    Screening: ELISA for anti-HIV antibodies (and 4th-generation p24 antigen/antibody combo). Confirmation: Western blot or a second different antibody assay. Early/window-period diagnosis and infant diagnosis: HIV RNA PCR (viral load). Monitoring: CD4 count (immune status; AIDS-defining when below 200 cells/uL) and viral load (treatment response).

20. Name the major systemic (dimorphic) fungi and the diseases they cause.

    Dimorphic fungi (mould in environment at 25C, yeast in tissue at 37C): Histoplasma capsulatum (histoplasmosis, intracellular in macrophages), Blastomyces dermatitidis (blastomycosis), Coccidioides immitis (spherules, coccidioidomycosis), Paracoccidioides, and Sporothrix schenckii (rose-gardener's, subcutaneous).

21. List the major opportunistic fungal infections and the typical host risk factors.

    Candida albicans (germ tube positive; thrush, candidemia) in diabetics/immunosuppressed/catheters. Aspergillus fumigatus (invasive aspergillosis, aspergilloma; acute-angle septate hyphae) in neutropenia. Cryptococcus neoformans (capsulated yeast, India ink, meningitis) in HIV/AIDS. Mucor/Rhizopus (mucormycosis, broad aseptate hyphae) in diabetic ketoacidosis. Pneumocystis jirovecii pneumonia in AIDS.

22. What is the difference between intestinal and tissue (extraintestinal) amoebiasis caused by Entamoeba histolytica?

    Entamoeba histolytica is transmitted by the fecal-oral route (cyst is infective form, trophozoite is invasive). Intestinal amoebiasis causes amoebic dysentery with flask-shaped colonic ulcers. Extraintestinal/tissue spread (via portal vein) causes amoebic liver abscess ('anchovy sauce' pus). Diagnosed by stool microscopy, serology, and antigen detection.

23. Differentiate the four Plasmodium species causing human malaria.

    P. falciparum: most severe (cerebral malaria, blackwater fever), malignant tertian, multiple ring forms/banana-shaped gametocytes, no relapse. P. vivax and P. ovale: benign tertian (48-hr cycle), form hypnozoites causing relapses. P. malariae: quartan (72-hr cycle), can cause nephrotic syndrome. Diagnosis by thick and thin blood smears (Giemsa) or rapid antigen tests.

24. Name important intestinal helminths and a key diagnostic/clinical feature of each.

    Ascaris lumbricoides: largest roundworm, Loeffler's syndrome from larval migration. Ancylostoma/Necator (hookworm): iron-deficiency anemia, skin penetration. Enterobius vermicularis (pinworm): perianal itching, diagnosed by cellophane (NIH) swab. Trichuris trichiura (whipworm): rectal prolapse. Strongyloides stercoralis: autoinfection, hyperinfection in immunosuppressed. Taenia (tapeworm): from undercooked pork/beef; T. solium causes cysticercosis.

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