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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. What is the inoculum effect in antimicrobial susceptibility testing?

    Answer: A false increase in MIC when a higher bacterial inoculum is used, especially with beta-lactamase producers

    The inoculum effect occurs when a higher-than-standard bacterial concentration produces falsely elevated MICs, particularly significant with beta-lactam antibiotics against beta-lactamase-producing organisms.

  2. Which phenotypic test is used to confirm extended-spectrum beta-lactamase (ESBL) production in Enterobacteriaceae?

    Answer: Combination disk test using cefotaxime or ceftazidime with and without clavulanic acid

    ESBL confirmation uses cephalosporin disks alone and in combination with clavulanic acid; a ≥5 mm increase in zone diameter with the inhibitor confirms ESBL production.

  3. Which carbapenem resistance detection method relies on the release of CO₂ when carbapenemases hydrolyze carbapenems, detected by a colorimetric indicator?

    Answer: CARBA NP test

    The CARBA NP test detects carbapenemase activity by a pH change (color shift) caused by CO₂ released when carbapenemases hydrolyze imipenem.

  4. A patient's blood culture isolate tests susceptible to vancomycin with an MIC of 1 µg/mL, but therapy is failing. Which phenomenon might explain this outcome?

    Answer: Heteroresistance, where a small subpopulation of cells is resistant and selected during therapy

    Heteroresistance means most cells appear susceptible but a small resistant subpopulation expands under treatment pressure, explaining clinical failure despite a low MIC.

  5. What is the standard bacterial inoculum concentration used to prepare suspensions for disk diffusion testing according to CLSI guidelines?

    Answer: 1–2 × 10⁸ CFU/mL (0.5 McFarland turbidity)

    CLSI recommends a 0.5 McFarland turbidity standard, which corresponds to approximately 1–2 × 10⁸ CFU/mL, for preparing the bacterial inoculum for disk diffusion.

  6. Which antibiotic is used as a surrogate to detect methicillin resistance in Staphylococcus aureus using disk diffusion?

    Answer: Cefoxitin

    A 30-µg cefoxitin disk is the recommended surrogate marker for oxacillin/methicillin resistance because it reliably induces mecA expression and gives clearer zones.

  7. What key advantage does broth microdilution have over disk diffusion in clinical microbiology?

    Answer: It provides an actual numerical MIC value used for dose optimization

    Broth microdilution yields a specific MIC value that clinicians and pharmacists can use to optimize antibiotic dosing, which disk diffusion's categorical result cannot provide.