Which medium should be used for in vitro susceptibility testing of Streptococcus pneumoniae?

• A. Chocolate agar
• B. Charcoal yeast extract agar
• C. Mueller-Hinton base supplemented with 5% lysed horse blood
• D. Mueller-Hinton base supplemented with 1% hemoglobin and 1% IsoVitaleX

Answer: (C)

When evaluating antibiotic susceptibility by disk diffusion, the medium must both support growth of the organism and provide consistent, interpretable inhibition zones. Streptococcus pneumoniae is a fastidious organism, so it needs a blood-containing medium to grow well and to produce reliable zone sizes that match established breakpoints. Using Mueller-Hinton agar supplemented with 5% lysed horse blood supplies the necessary nutrients while maintaining standardization, making the results reproducible across laboratories.

Chocolate agar is great for isolating certain organisms because it supplies extra nutrients, but it isn’t the standardized medium used for disk diffusion susceptibility testing, so its diffusion characteristics can lead to non-comparable results. Charcoal yeast extract agar is a specialized medium for other purposes and isn’t the CLSI-recommended choice for pneumococcal susceptibility testing. The option with 1% hemoglobin and IsoVitaleX targets very fastidious organisms that require X and V factors (like Haemophilus species); it isn’t the standard for Streptococcus pneumoniae susceptibility testing and would complicate interpretation against pneumococcal breakpoints.