Mannitol salt agar (MSA) only allows the growth of halophiles. Among the halophiles, mannitol fermenters release acid that turns the pH indicator yellow; mannitol nonfermenters leave the medium red. Onto MSA you inoculate a halophilic mannitol fermenter, a nonhalophilic mannitol fermenter, and a halophilic mannitol nonfermenter. Thus, the medium is acting as a medium.

Mannitol salt agar can act as both a selective and differentiating medium. A selective medium is one in which inhibits the growth of a particular organism, and allows others to thrive in said conditions. In this case, MSA favors halophilic organisms. MSA is also a differentiating medium because of the mannitol content. Mannitol is a sugar and mannitol fermenters, like s. saprophyticus, use mannitol and release acid as a byproduct. The pH indicator within the agar changes from red to yellow.

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