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Microbiology Flashcards

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  1. A patient with recurrent Neisseria infections is found to have a complement deficiency. Which complement components are most critical for defense against encapsulated Neisseria species?

    Answer: C5-C9 (membrane attack complex)

    Deficiency of terminal complement components C5-C9 (the membrane attack complex) predisposes patients to recurrent infections with Neisseria meningitidis and N. gonorrhoeae.

  2. A clinical isolate of Staphylococcus aureus is resistant to all beta-lactam antibiotics, including extended-spectrum penicillins and carbapenems. Which mechanism is responsible?

    Answer: Acquisition of mecA gene encoding an altered penicillin-binding protein (PBP2a)

    MRSA acquires the mecA gene (carried on SCCmec) encoding PBP2a, which has very low affinity for all beta-lactam antibiotics, conferring pan-beta-lactam resistance.

  3. A Gram-negative organism isolated from a catheter-associated UTI produces an enzyme that inactivates aminoglycosides by covalent modification. Which type of enzyme is responsible?

    Answer: Aminoglycoside acetyltransferase/phosphotransferase

    Aminoglycoside resistance most commonly results from transferase enzymes (acetyltransferases, phosphotransferases, nucleotidyltransferases) that chemically modify the drug and prevent it from binding the 30S ribosome.

  4. An elderly nursing home resident develops watery diarrhea after a prolonged course of clindamycin. Stool toxin assay is positive. The pathogen produces two toxins: Toxin A (enterotoxin) and Toxin B (cytotoxin). Which treatment is first-line for a non-severe initial episode?

    Answer: Oral vancomycin or fidaxomicin

    Current IDSA guidelines recommend oral vancomycin or fidaxomicin as first-line therapy for initial non-severe Clostridioides difficile infection; fidaxomicin has lower recurrence rates.

  5. A 19-year-old develops sudden-onset fever, petechial rash, and nuchal rigidity. CSF shows low glucose, elevated protein, and Gram-negative diplococci. Prophylaxis is recommended for close contacts. Which drug is used for prophylaxis?

    Answer: Rifampin or ciprofloxacin

    Rifampin (or a single dose of ciprofloxacin or ceftriaxone) is given as chemoprophylaxis to household and close contacts of Neisseria meningitidis cases to eliminate nasopharyngeal carriage.

  6. A 7-year-old develops a malar rash, arthritis, and carditis 3 weeks after an untreated streptococcal pharyngitis. Which immunological mechanism is thought to cause the cardiac damage in acute rheumatic fever?

    Answer: Molecular mimicry between streptococcal M protein and cardiac myosin

    Antibodies against Group A Streptococcus M protein cross-react with cardiac myosin (molecular mimicry), leading to autoimmune damage of the mitral and aortic valves in rheumatic fever.

  7. A patient on immunosuppressive therapy develops fever and ground-glass opacities on chest CT. BAL staining with Grocott's methenamine silver shows cup-shaped cysts. Which drug is first-line treatment?

    Answer: Trimethoprim-sulfamethoxazole

    Pneumocystis jirovecii pneumonia (PCP) is treated with high-dose trimethoprim-sulfamethoxazole (TMP-SMX), which inhibits dihydropteroate synthase in the organism's folate synthesis pathway.