MANNITOL SALT AGAR (USP/EP/BP/JP/IP)

Mannitol Salt Agar is a selective and differential medium for the isolation of pathogenic staphylococci from pharmaceutical products as described in the USP/EP/BP/JP/IP microbial limit test methods. Its high salt concentration promotes the growth of staphylococci and inhibits the growth of most other bacteria. It is also suitable for testing cosmetics, milk, food and other specimens.
Beef extract, pancreatic digest of casein and peptic digest of animal tissue provide carbon, nitrogenous compounds, amino acids, vitamins and other necessary ingredients for growth of the microorganism. Mannitol is the fermentable sugar and phenol red is the pH indicator. Organisms that ferment mannitol produce acid, causing the indicator to turn from red to yellow.
Staphylococcus aureus forms yellow colonies with yellow zones around. When Egg Yolk Emulsion is added, its lipase activity can also lead to formation of yellow opaque zones around colonies. Mannitol non-fermenting staphylococci generally produce pink to red colonies with red-purple zones. Presumptive coagulase-positive S. aureus colonies should undergo coagulase testing for confirmation. Agar serves as the solidifying agent and the medium for the development and differentiation of colonies.

Selective conditions favour the recovery of pathogenic staphylococci, while mannitol fermentation and phenol red help differentiate presumptive Staphylococcus aureus from other staphylococci.