Mannitol Salt Agar is a medium for Staphylococcus aureus which is selective because the high sodium chloride level inhibits most other species with the exception of halophilic Vibrios.
The majority of S. aureus ferment mannitol producing yellow colonies, occasional strains of coagulase-negative staphylococci may also ferment mannitol. It is necessary to confirm the identity of presumptive S. aureus colonies by other means e.g. coagulase, protein A, DN’ase, thermonuclease or latex agglutination.