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

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

• A. Mueller-Hinton agar
• B. Trypticase soy agar
• C. MacConkey agar
• D. MiddlebrookVHIOagar

Answer: (C)

For routine susceptibility testing, you need a medium that yields uniform antibiotic diffusion and supports growth of a wide range of nonfastidious bacteria. Mueller-Hinton agar provides a defined, stable composition with the right depth and pH, allowing consistent diffusion of antimicrobials and reliable, reproducible results in disk diffusion and MIC testing. That standardization is why CLSI specifies Mueller-Hinton agar as the go-to medium for nonfastidious organisms. MacConkey agar is excellent for isolating and differentiating Gram-negative enterics, but its selective components can affect diffusion and growth in ways that aren’t ideal for routine AST. The other media mentioned serve different purposes—trypticase soy agar is a general-purpose medium without the needed standardization for diffusion, and Middlebrook VHIO agar is designed for mycobacteria, not nonfastidious bacteria.

For routine susceptibility testing, you need a medium that yields uniform antibiotic diffusion and supports growth of a wide range of nonfastidious bacteria. Mueller-Hinton agar provides a defined, stable composition with the right depth and pH, allowing consistent diffusion of antimicrobials and reliable, reproducible results in disk diffusion and MIC testing. That standardization is why CLSI specifies Mueller-Hinton agar as the go-to medium for nonfastidious organisms. MacConkey agar is excellent for isolating and differentiating Gram-negative enterics, but its selective components can affect diffusion and growth in ways that aren’t ideal for routine AST. The other media mentioned serve different purposes—trypticase soy agar is a general-purpose medium without the needed standardization for diffusion, and Middlebrook VHIO agar is designed for mycobacteria, not nonfastidious bacteria.