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 following 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.

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Decreasing zone sizes would indicate that the bacteria are becoming resistant to the antimicrobials 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.Answers b-d would all be reasons why zone sizes would be increased in size.Accurate and reproducible AST results depend on standardized conditions for inoculum size,depth of agar,and duration of incubation.

Quality Control Results for Disk Diffusion AST Using Appropriate ATCC