The agar used in Kirby-Bauer testing should have a pH in which range?

• A. 7.0-7.2
• B. 7.2-7.4
• C. 7.4-7.6
• D. 7.6-7.8

Answer: (B)

Crucial to this test is how evenly antibiotics diffuse through the agar and how the bacteria grow. The pH of the agar medium directly affects both diffusion of drugs and microbial growth, so keeping it tightly controlled yields reliable, interpretable zone sizes. Mueller-Hinton agar used for Kirby-Bauer is buffered to maintain a pH in the range of 7.2 to 7.4. Within this window, diffusion behaves predictably and zone diameters correlate with true susceptibility, which is why this range is specified. If the pH strays from this range, diffusion can change and growth can shift, leading to inconsistent or misleading results. So, the best range to ensure accuracy and reproducibility is 7.2–7.4.