Mannitol Salt Agar

Mannitol Salt Agar (MSA) is a selective and differential medium. The high concentration of salt (7.5%) selects for members of the genus Staphylococcus, since they can tolerate high saline levels. Organisms from other genera may grow, but they typically grow very weakly.

MSA also contains the sugar mannitol and the pH indicator phenol red.  If an organism can ferment mannitol, an acidic byproduct is formed that will cause the phenol red in the agar to turn yellow. Most pathogenic staphylococci, such as Staphylococcus aureus, will ferment mannitol.  Most non-pathogenic staphylococci will not ferment mannitol.

The Staphylococcus aureus ferments mannitol and turns the medium yellow. 
The Serratia marcescens does not grow because of the high salt content.