What is the optimal pH range for the Kirby-Bauer test agar?

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

Answer: (C)

Maintaining a near-neutral pH in Kirby-Bauer diffusion plates is crucial because how antibiotics diffuse through agar and how bacteria grow are both influenced by pH. The Mueller-Hinton agar used for this test is prepared to a pH around 7.2 to 7.4, which provides consistent diffusion patterns and reliable zone diameters across many drug‑bacteria combinations. If the pH drifts toward more alkaline values, such as 7.6–7.8, diffusion rates for some drugs can change and organisms may grow differently, leading to readings that don’t match reference standards. Staying within the 7.2–7.4 range helps ensure the results are accurate and comparable to interpretive criteria. In practice, labs routinely check and adjust the agar pH to stay within this window before performing the test.