Which of the following media is both selective and differential?

• A. Sheep blood agar
• B. Chocolate agar
• C. Mannitol salt agar
• D. Mueller-Hinton agar

Answer: (C)

A medium that is selective and differential both inhibits growth of some organisms and reveals differences among those that do grow. Mannitol salt agar does this by using a high salt concentration to select for staphylococci, which tolerate high osmolarity. It also contains mannitol and a pH indicator; if an organism can ferment mannitol, acid is produced and the surrounding medium turns yellow, signaling a positive fermentation. Non-fermenters do not acidify the medium, so the color stays the same. This combination of selecting for the intended genus and differentiating based on mannitol fermentation makes it the best choice. The other media are not both selective and differential: sheep blood agar and chocolate agar are enriched (not selective), while Mueller-Hinton agar is non-selective and used primarily for antibiotic susceptibility testing.