MANNITOL SALT AGAR (MSA) is a selective medium prepared according to the recommendations of Chapman for the isolation of presumptive pathogenic staphylococci.The coagulase-positive species of Staphylococcus i.e. Staphylococcus aureus is well-documented as a human opportunistic pathogen. S. aureus is also isolated from recreational water like swimming pools, and are thus indicators of health risk. S. aureus is relatively resistant to the effect of disinfectant like chlorine and sodium chloride. Staphylococci have the unique ability of growing on a high salt containing media. Isolation of coagulase-positive staphylococci on Phenol Red Mannitol Agar supplemented with 7.5% NaCl was studied by Chapman. The resulting Mannitol Salt Agar Base is recommended for the isolation of coagulase positive staphylococci from cosmetics, milk, food and other specimens.
Peptone special and beef extract serve as sources of essential nutrients required for bacterial growth. Sodium chloride in higher concentration makes the medium selective for Staphylococcus by inhibiting accompanying flora. D-Mannitol is the fermentable carbohydrate, fermentation of which leads to acid production, detected by the pH indicator dye, namely phenol red. S. aureus ferment mannitol and produce yellow coloured colonies surrounded by yellow zones. Coagulase-negative strains of S. aureus are usually mannitol non-fermenters and therefore produce pink to red colonies surrounded by red-purple zones. The additional property of lipase activity of Staphylococcus aureus can be detected by the addition of the Egg Yolk Emulsion (TS 002). The lipase activity can be visualized as yellow opaque zones around the colonies.