FREE AP-MICRO Antimicrobial Susceptibility Testing Questions and Answers

Question 1

A Staphylococcus aureus isolate from a wound culture shows resistance to erythromycin but appears susceptible to clindamycin on an automated susceptibility testing panel. Which of the following tests should be performed to detect potential inducible resistance?

Accepted Answer

D-test

Answer

The D-test is specifically designed to detect inducible clindamycin resistance in staphylococci and streptococci that are resistant to erythromycin but appear susceptible to clindamycin. A flattening of the clindamycin zone of inhibition adjacent to the erythromycin disk (forming a 'D' shape) indicates the presence of an erm gene, which can methylate the ribosomal target and confer resistance to clindamycin upon exposure, leading to clinical failure if clindamycin is used.

Question 2

When performing a Kirby-Bauer disk diffusion test, a technologist uses a Mueller-Hinton agar plate with a depth of 6 mm, which is thicker than the CLSI recommended standard of 4 mm. What effect will this have on the zone sizes?

Accepted Answer

Zone sizes will be smaller, potentially leading to false-resistant results.

Answer

The depth of the agar is a critical variable in disk diffusion testing. An agar depth greater than the standard 4 mm will slow the vertical and lateral diffusion of the antimicrobial agent from the disk. This reduced diffusion rate results in smaller zones of inhibition, which can lead to an interpretation of false resistance or a lower level of susceptibility.

Question 3

A laboratory is performing the CLSI-recommended phenotypic confirmatory test for Extended-Spectrum Beta-Lactamase (ESBL) production in a Klebsiella pneumoniae isolate. A positive test is indicated by which of the following results?

Accepted Answer

A ≥ 5 mm increase in the zone diameter for ceftazidime tested with clavulanic acid versus ceftazidime tested alone.

Answer

The standard CLSI phenotypic confirmatory test for ESBLs relies on the principle that ESBLs are inhibited by beta-lactamase inhibitors like clavulanic acid. The test compares the zone of inhibition of a third-generation cephalosporin (like cefotaxime or ceftazidime) alone with the zone of the same cephalosporin combined with clavulanic acid. A ≥ 5 mm increase in the zone diameter with the combination disk indicates the presence of an ESBL.

Question 4

Which of the following sets of organisms are the standard, non-fastidious quality control strains recommended by CLSI for routine disk diffusion and MIC testing?

Accepted Answer

Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 27853

Answer

The primary, most frequently used quality control (QC) strains for routine antimicrobial susceptibility testing of non-fastidious bacteria are Escherichia coli ATCC 25922 (representing Gram-negative bacilli), Staphylococcus aureus ATCC 25923 (representing Gram-positive cocci), and Pseudomonas aeruginosa ATCC 27853 (representing non-fermenting Gram-negative bacilli). These strains have well-established, expected ranges for various antimicrobials.

Question 5

A clinical laboratory reports a vancomycin MIC of 2 µg/mL for a Staphylococcus aureus isolate from a blood culture. The CLSI interpretive breakpoint for susceptible is ≤2 µg/mL. How should this result be interpreted?

Accepted Answer

Susceptible

Answer

According to CLSI guidelines, an organism is considered susceptible if the Minimum Inhibitory Concentration (MIC) is less than or equal to (≤) the susceptible breakpoint. Since the MIC of 2 µg/mL is equal to the susceptible breakpoint of ≤2 µg/mL for vancomycin against S. aureus, the isolate is correctly interpreted as Susceptible.

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FREE AP-MICRO Antimicrobial Susceptibility Testing Questions and Answers