Which media differentiates Staphylococcus species and differentiates S. aureus by mannitol fermentation?

• A. MacConkey agar differentiates lactose fermenters.
• B. Mannitol Salt Agar selects for Streptococcus species.
• C. Blood agar differentiates hemolysis patterns.
• D. Mannitol Salt Agar selects for Staphylococcus and differentiates S. aureus by mannitol fermentation turning yellow.

Answer: (D)

Mannitol Salt Agar is used because its high salt concentration selects for Staphylococcus species, which tolerate salty conditions that many other bacteria cannot endure. It also contains mannitol and a pH indicator; if a Staphylococcus ferments mannitol, acid is produced and the medium around the colony turns yellow. Staphylococcus aureus is typically mannitol-positive, so it produces yellow colonies, while other Staphylococcus species that don’t ferment mannitol remain red or colorless. Other media mentioned don’t provide both the selective pressure for Staphylococcus and the differential signal for mannitol fermentation, so they don’t identify S. aureus in the same way.