The medium recommended for routine susceptibility testing of nonfastidious bacteria is:

• A. MacConkey Agar
• B. Middlebrook 7H10 Agar
• C. Mueller-Hinton Agar
• D. Trypticase Soy Agar

Answer: (C)

For reliable antibiotic susceptibility testing across a wide range of nonfastidious bacteria, you need a medium that supports growth without altering how antibiotics diffuse through the agar. Mueller-Hinton Agar is precisely that: it’s nonselective and noninhibitory, has a defined chemical composition, and is standardized in terms of pH, depth, and ion content. This standardization produces reproducible zone sizes in disk diffusion and consistent results in broth microdilution, which is why it’s the recommended medium by guidelines for routine susceptibility testing.

Other media either are designed for specific groups or have characteristics that can affect diffusion. MacConkey agar is selective for Gram-negative enterics and lactose fermenters, which can skew results and isn’t the universal standard for AST. Middlebrook 7H10 is specialized for mycobacteria, not for routine nonfastidious bacteria. Trypticase Soy Agar is general-purpose but does not have the standardized properties needed for validated, comparable susceptibility testing across many organisms.