Mannitol Salt Agar is used for the isolation of staphylococci in a laboratory setting. Mannitol Salt Agar is not intended for use in the diagnosis of disease or other conditions in humans. Conforms to Harmonized USP/EP/JP Requirements.
Chapman formulated Mannitol Salt Agar to isolate staphylococci by inhibiting growth of most other bacteria with a high salt concentration. Chapman added 7.5% Sodium Chloride to Phenol Red Mannitol Agar and noted pathogenic strains of staphylococci (coagulase-positive staphylococci) grew luxuriantly and produced yellow colonies with yellow zones. Non-pathogenic staphylococci produced small red colonies with no color change to the surrounding medium.
Mannitol Salt Agar is highly selective and samples from heavily contaminated sources may be streaked onto this medium without danger of overgrowth. Mannitol Salt Agar is recommended for isolating pathogenic staphylococci from samples, cosmetics, and microbial limit tests.
Bacteria that grow in the presence of a high salt concentration and ferment mannitol produce acid products, turning the phenol red pH indicator from red to yellow. Typical pathogenic staphylococci ferment mannitol and form yellow colonies with yellow zones. Typical non-pathogenic staphylococci do not ferment mannitol and form red colonies.
Enzymatic digest of casein, enzymatic digest of animal tissue, and beef extract provide the nitrogen, vitamins, and carbon in Mannitol Salt Agar. D-Mannitol is the carbohydrate source. In high concentrations, sodium chloride inhibits most bacteria other than staphylococci. Phenol red is the pH indicator. Agar is the solidifying agent.
