Free AP-MICRO Antimicrobial Susceptibility Testing Questions and Answers 1 — 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?
- Cefoxitin disk screen
- D-test (Correct answer)
- Modified Hodge test
- Oxacillin salt agar screen
Correct answer: D-test
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?
- Zone sizes will be larger, potentially leading to false-susceptible results.
- Zone sizes will be unaffected by the agar depth.
- Zone sizes will be smaller, potentially leading to false-resistant results. (Correct answer)
- Zone sizes will be inconsistent and the test will be invalid regardless of the results.
Correct answer: Zone sizes will be smaller, potentially leading to false-resistant results.
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?
- A zone of inhibition around a meropenem disk that is ≤ 21 mm.
- Resistance to both cefotaxime and ceftazidime.
- A ≥ 5 mm increase in the zone diameter for ceftazidime tested with clavulanic acid versus ceftazidime tested alone. (Correct answer)
- Growth on Mueller-Hinton agar containing a 4 µg/mL concentration of cefoxitin.
Correct answer: A ≥ 5 mm increase in the zone diameter for ceftazidime tested with clavulanic acid versus ceftazidime tested alone.
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?
- Enterococcus faecalis ATCC 51299, Klebsiella pneumoniae ATCC 700603, Escherichia coli ATCC 35218
- Staphylococcus aureus ATCC 43300, Haemophilus influenzae ATCC 49247, Streptococcus pneumoniae ATCC 49619
- Candida albicans ATCC 90028, Aspergillus fumigatus ATCC 204305, Cryptococcus neoformans ATCC 90112
- Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 27853 (Correct answer)
Correct answer: Escherichia coli ATCC 25922, Staphylococcus aureus ATCC 25923, Pseudomonas aeruginosa ATCC 27853
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?
- Resistant
- Susceptible (Correct answer)
- Intermediate
- Invalid, test must be repeated
Correct answer: Susceptible
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.
Question 6: Antimicrobial susceptibility testing for Streptococcus pneumoniae requires specific modifications to the standard Kirby-Bauer procedure. Which of the following is a required modification?
- Incubation in an anaerobic environment for 24-48 hours.
- Use of Haemophilus Test Medium (HTM) agar.
- Inoculation of a 1.0 McFarland standard suspension.
- Incubation in an atmosphere of 5-7% CO2 on Mueller-Hinton agar with 5% sheep blood. (Correct answer)
Correct answer: Incubation in an atmosphere of 5-7% CO2 on Mueller-Hinton agar with 5% sheep blood.
Streptococcus pneumoniae is a fastidious organism that requires an enriched medium and specific atmospheric conditions for optimal growth. For susceptibility testing, CLSI guidelines mandate the use of Mueller-Hinton agar supplemented with 5% sheep blood and incubation in a CO2-enriched atmosphere (5-7% CO2) to support its growth.
Free AP-MICRO Antimicrobial Susceptibility Testing Questions and Answers 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?