Microbiology: Antimicrobial Susceptibility Testing Flashcards
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Read the first 7 Microbiology: Antimicrobial Susceptibility Testing flashcards as text
Which CLSI breakpoint category indicates that an organism is inhibited by drug concentrations achievable with standard dosing?
Answer: Susceptible
The 'Susceptible' category means the organism is inhibited at drug levels achievable with standard dosing regimens.
In the Kirby-Bauer disk diffusion method, what does a larger zone of inhibition indicate?
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.
What is the purpose of using Mueller-Hinton agar specifically for disk diffusion susceptibility testing?
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.
Which mechanism of resistance involves a bacterium producing an enzyme that destroys an antibiotic before it can act?
Answer: Enzymatic inactivation
Enzymatic inactivation, such as beta-lactamase production, destroys the antibiotic molecule before it reaches its target.
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?
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.
What is the role of quality control strains such as Staphylococcus aureus ATCC 25923 in disk diffusion testing?
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.
Which antibiotic class is specifically tested using a cation-adjusted Mueller-Hinton broth to avoid falsely elevated MICs?
Answer: Aminoglycosides
Aminoglycoside MIC testing requires cation-adjusted broth because Mg²⁺ and Ca²⁺ concentrations directly affect drug activity against certain organisms.