Mannitol Salt Agar is a selective and differential growth medium used primarily to isolate and identify Staphylococcus species. It contains a high concentration of salt, which inhibits the growth of most bacteria, allowing only salt-tolerant organisms to thrive. The medium also includes mannitol, a sugar alcohol, and a pH indicator that changes color when mannitol is fermented. When Staphylococcus aureus ferments mannitol, it produces acid, leading to a color change in the agar from red to yellow. This characteristic helps differentiate it from other Staphylococcus species that do not ferment mannitol, which will leave the medium red.