The pH of the agar used for the Kirby-Bauer test should be within which range?

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

Answer: (B)

Standardization of the growth medium’s pH is crucial for the Kirby-Bauer disk diffusion test because antibiotic diffusion through the agar and bacterial growth are both pH-dependent. Mueller-Hinton agar is prepared to have a pH of about 7.2 to 7.4 at the testing temperature to ensure consistent and reproducible zone sizes across different runs and laboratories. If the pH strays from this range, diffusion rates can change and some antibiotics may alter in activity, leading to unreliable results. Maintaining 7.2–7.4 provides stable conditions so that the measured zones accurately reflect bacterial susceptibility.