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.
