Kirby-Bauer disk diffusion testing yields what measurement to indicate antibiotic susceptibility?

• A. The amount of bacterial growth in broth
• B. Zones of inhibition around antibiotic discs
• C. Color change of colonies
• D. Time to visible growth

Answer: (B)

The measurement you get from Kirby-Bauer disk diffusion is the zone of inhibition around the antibiotic discs. In this test, paper discs containing antibiotics are placed on an agar plate that has been inoculated with the bacteria in question. The antibiotic diffuses into the surrounding agar; if the bacteria are susceptible, they cannot grow near the disc, creating a clear, circular area—the zone of inhibition. The size of this zone, measured in millimeters, is then compared to standardized breakpoints to determine if the organism is susceptible, intermediate, or resistant to that antibiotic.

This method isn’t about the total amount of growth in broth, which would relate to different tests like broth dilution/MIC. It’s not based on color changes of colonies or on how quickly visible growth occurs, which are used in other assays or indicators.