Which medium is recommended by CLSI for routine susceptibility testing of nonfastidious bacteria?

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

Answer: (C)

For routine susceptibility testing, you need a medium that supports consistent diffusion of antibiotics and reliable growth of a wide range of nonfastidious bacteria so that inhibition zones can be interpreted against standard breakpoints. Mueller-Hinton Agar provides just that: a simple, well-defined formulation with controlled calcium and magnesium levels, near-neutral pH, and a standardized thickness. This combination minimizes variability in how antibiotics diffuse through the gel, producing reproducible zone sizes across laboratories and products, which CLSI uses to establish breakpoints.

Other media don’t fit this standard system. MacConkey agar is selective and differential for Gram-negative enterics and lactose fermenters, which can alter diffusion and growth patterns in ways that aren’t aligned with routine diffusion-based testing. Middlebrook 7H10 agar is specialized for Mycobacteria and requires different growth conditions, not for general nonfastidious organisms. Trypticase Soy Agar is a general-purpose medium, but it lacks the standardized diffusion properties that make Mueller-Hinton agar the preferred choice for routine susceptibility testing.