AP-Micro Microbiology: Antimicrobial Susceptibility Testing 2 — Questions and Answers
Question 1: Which CLSI breakpoint category indicates that an organism is inhibited by drug concentrations achievable with standard dosing?
- Resistant
- Intermediate
- Susceptible (Correct answer)
- Non-susceptible
Correct answer: Susceptible
The 'Susceptible' category means the organism is inhibited at drug levels achievable with standard dosing regimens.
Question 2: In the Kirby-Bauer disk diffusion method, what does a larger zone of inhibition indicate?
- Greater resistance to the antibiotic
- Greater susceptibility to the antibiotic (Correct answer)
- A bactericidal rather than bacteriostatic drug
- Poor quality agar medium
Correct answer: Greater susceptibility to the antibiotic
A larger zone of inhibition indicates that the organism is more susceptible because fewer bacteria survive at greater distances from the drug-laden disk.
Question 3: What is the purpose of using Mueller-Hinton agar specifically for disk diffusion susceptibility testing?
- It supports growth of fastidious organisms only
- It has low thymidine content and supports reproducible results for most pathogens (Correct answer)
- It contains antibiotics that enhance zone formation
- It is the only agar that allows blood to be added for hemolysis detection
Correct answer: It has low thymidine content and supports reproducible results for most pathogens
Mueller-Hinton agar is recommended because its low thymidine content avoids interference with sulfonamide and trimethoprim testing and gives reproducible results.
Question 4: Which mechanism of resistance involves a bacterium producing an enzyme that destroys an antibiotic before it can act?
- Target site modification
- Efflux pump overexpression
- Enzymatic inactivation (Correct answer)
- Reduced outer membrane permeability
Correct answer: Enzymatic inactivation
Enzymatic inactivation, such as beta-lactamase production, destroys the antibiotic molecule before it reaches its target.
Question 5: A laboratory reports an MIC of 0.25 µg/mL for an antibiotic with a susceptible breakpoint of ≤0.5 µg/mL. How should this result be interpreted?
- Resistant
- Intermediate
- Susceptible (Correct answer)
- Indeterminate
Correct answer: Susceptible
Because the MIC (0.25 µg/mL) is at or below the susceptible breakpoint (≤0.5 µg/mL), the organism is classified as susceptible.
Question 6: What is the role of quality control strains such as Staphylococcus aureus ATCC 25923 in disk diffusion testing?
- They serve as the test organism for each patient sample
- They verify that the testing conditions and reagents produce expected results (Correct answer)
- They provide antibiotic disks for each test run
- They are used to calculate patient MIC values
Correct answer: They verify that the testing conditions and reagents produce expected results
QC strains with known susceptibility profiles are tested alongside patient isolates to ensure that methods, media, and disks are performing correctly.
Question 7: Which antibiotic class is specifically tested using a cation-adjusted Mueller-Hinton broth to avoid falsely elevated MICs?
- Penicillins
- Fluoroquinolones
- Aminoglycosides (Correct answer)
- Macrolides
Correct answer: Aminoglycosides
Aminoglycoside MIC testing requires cation-adjusted broth because Mg²⁺ and Ca²⁺ concentrations directly affect drug activity against certain organisms.
AP-Micro Microbiology: Antimicrobial Susceptibility Testing 2
Which CLSI breakpoint category indicates that an organism is inhibited by drug concentrations achievable with standard dosing?