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Microbiology: Antimicrobial Susceptibility Testing Flashcards

7 cards from real AP practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.

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  1. Which test is used to detect inducible clindamycin resistance caused by the erm gene in staphylococci?

    Answer: D-zone test (double-disk diffusion)

    The D-zone test places erythromycin and clindamycin disks adjacent to each other; flattening of the clindamycin zone toward the erythromycin disk indicates inducible resistance.

  2. What does the term 'bactericidal' mean in the context of antimicrobial testing?

    Answer: The drug kills at least 99.9% of the original bacterial inoculum

    A bactericidal drug kills at least 99.9% (3 log reduction) of the original inoculum, distinguishing it from bacteriostatic agents that only inhibit growth.

  3. An organism that tests susceptible to a beta-lactam in vitro but carries a mecA gene should be reported as:

    Answer: Resistant, because mecA confers oxacillin/methicillin resistance regardless of phenotypic results

    mecA encodes an altered penicillin-binding protein (PBP2a) conferring MRSA resistance; organisms carrying mecA must be reported resistant to all beta-lactams regardless of in vitro appearance.

  4. What is the significance of the minimum bactericidal concentration (MBC) relative to the MIC?

    Answer: MBC is the lowest concentration that kills 99.9% of bacteria, typically equal to or greater than the MIC

    The MBC is the lowest drug concentration that kills 99.9% of the original inoculum and is equal to or higher than the MIC because killing requires higher concentrations than inhibition.

  5. Which method of AST is best suited for testing slow-growing or fastidious organisms like Haemophilus influenzae?

    Answer: Disk diffusion on Haemophilus Test Medium (HTM) with appropriate supplements

    Haemophilus Test Medium (HTM) provides the growth factors (NAD and hemin) needed for H. influenzae and gives reliable, reproducible susceptibility results.

  6. What does a zone of inhibition of exactly zero (no zone) around an antibiotic disk indicate?

    Answer: The organism is fully resistant to that antibiotic

    No zone of inhibition means the organism grew up to the disk, indicating complete resistance to the antibiotic.

  7. In E-test (epsilometer test) strips, how is the MIC determined?

    Answer: At the point where the elliptical inhibition zone intersects the MIC scale on the strip

    The MIC is read at the point where the elliptical zone of inhibition intersects the printed concentration gradient scale on the E-test strip.