Mannitol salt agar is a selective as well as a differential medium used in the isolation of the staphylococci bacteria. It is selective because it contains 7.5 percent sodium chloride, which promotes the growth of some microbes while preventing the growth of others.
Salt in the surrounding medium draws water from cells as a result of diffusion and osmosis. The bacteria that can deal with the stress placed on the cell in a high-salt concentration grow in this condition. The bacteria that are unable to deal with the high level of salt tend to dehydrate. Thus, the inclusion of high salt content in a solution produces a selective medium.
Staphylococcus withstands the osmotic pressure created by a 7.5 percent sodium chloride concentration, while this concentration inhibits the growth of most of the other gram-negative and gram-positive bacteria. A person can determine the growth of organisms in 7.5 percent sodium chloride by inspecting the inoculated mannitol salt dish after 24 hours of incubation to check for the presence of colonies.
Mannitol salt agar sorts the bacteria based on the capability to ferment the sugar mannitol that is the only carbohydrate in the medium. Mannitol salt agar uses phenol red as a pH indicator. The organisms that can ferment mannitol produce acid byproducts, thus causing a color change.