Mannitol Salt Agar
Mannitol Salt Agar (MSA) is both selective and differential. It contains 7.5% sodium chloride which selects for organisms that are halotolerant. It also contains the carbohydrate mannitol, and a pH indicator, phenol red, which allow distinction between organisms which can ferment mannitol and those that can not. Organisms that are able to ferment the mannitol will produce acid fermentation products which lower the pH, causing the phenol red indicator to turn yellow.
Both organisms growing on the plate above are halotolerant. The organism (Staphylococcus aureus) indicated by the orange arrow is capable of mannitol fermentation, signified by the color change in the surrounding media. The organism (Staphylococcus epidermidis) indicated by the blue arrow is not capable of mannitol fermentation.