Quality control results for disk diffusion AST using appropriate ATCC strains of bacteria showed zone size diameters decreasing over time for a few antimicrobials. Which of the follow-ing statements is a possible explanation?
a. Antimicrobial disks were stored in a frost-free freezer.
b. Concentration of the bacterial inoculum was 5 105 CFU/mL.
c. Depth of Mueller-Hinton agar plates measured 3 mm.
d. Plates were incubated less than 16 hours in ambient air.
A
Decreasing zone sizes would indicate that the bacteria are becoming resistant to the antimicro-bials tested. However, it can be that antimicrobial disks could lose their potency, which is likely if they are stored in a frost-free freezer that goes through cycles of freezing and thawing. An-swers b-d would all be reasons why zone sizes would be increased in size. Accurate and repro-ducible AST results depend on standardized conditions for inoculum size, depth of agar, and du-ration of incubation.
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