Briefly describe the Kirby-Bauer disk diffusion assay and result interpretation.

• A. Disks impregnated with antibiotics placed on a bacterial lawn; zones of inhibition measured and compared to CLSI standards to classify as susceptible, intermediate, or resistant.
• B. Bacteria are grown in antibiotic-free media to observe natural resistance.
• C. DNA sequencing is used to detect resistance genes.
• D. Antibiotics are added directly to broth and growth is observed.

Answer: (A)

The main idea here is to determine if a bacterial strain is inhibited by certain antibiotics by letting those antibiotics diffuse from paper disks into a uniform bacterial lawn. In practice, a standardized amount of bacteria is spread on Mueller-Hinton agar to form a confluent lawn, antibiotic-impregnated disks are placed on the surface, and after incubation the clear halos where growth is prevented appear around the disks. The size of each inhibition zone is measured and compared to standardized breakpoints (such as CLSI guidelines) for that organism and antibiotic. If the zone is large enough, the organism is labeled susceptible; if it’s too small, it’s resistant; there’s an intermediate range in between. This method gives rapid, practical guidance about which antibiotics are likely to be effective, using qualitative categories rather than exact concentrations.