Kirby-Bauer disk diffusion testing is used for what purpose?

• A. To identify the genus of bacteria via biochemical tests
• B. To quantify bacterial load in a sample
• C. To assess antibiotic susceptibility by measuring zones of inhibition around antibiotic discs
• D. To determine the fastest-growing culture

Answer: (C)

The main idea is using disk diffusion to determine antibiotic susceptibility. In this test, a uniform lawn of the bacterial isolate is spread on an agar plate, and paper discs containing specific antibiotics are placed on the surface. After incubation, the antibiotic diffuses into the surrounding agar. If the bacteria are susceptible to a drug, they won’t grow near the disc, creating a clear zone of inhibition around it. The size of that zone is measured and compared to standardized guidelines to decide whether the organism is susceptible, intermediate, or resistant to that antibiotic.

This method isn’t used to identify the bacteria or to measure how many organisms are present in a sample. It’s a practical, cost-effective way to guide treatment by showing which antibiotics are likely effective. Results rely on standardized conditions—such as the type of agar, the inoculum density, and incubation time and temperature—to ensure accuracy and comparability across labs.