MANNITOL SALT AGAR

Mannitol Salt Agar is a selective differential medium for the isolation of pathogenic staphylococci from clinical and nonclinical specimens. It is especially suitable for the detection of presumptive Staphylococcus aureus in cosmetics, milk, food and other specimens. The medium is prepared as recommended by Chapman and is also recommended by USP for the isolation of S. aureus during microbiological examination of non-sterile products.
The high concentration of sodium chloride has a selective action in that it inhibits the growth of many bacteria other than staphylococci. Peptones and beef extract are nutrients required for the growth of microbes, and mannitol is the fermentable carbohydrate. S. aureus ferments mannitol and produces acid resulting in a reduction in pH and change in colour of phenol red indicator.
Presumptive S. aureus colonies usually are yellow and may be surrounded by yellow zones. Coagulase-negative staphylococci usually produce pink to red colonies but not the typical yellow reaction. Presumptive positive colonies should be confirmed by suitable biochemical tests such as coagulase tests. Egg yolk emulsion is also incorporated for the detection of lipase activity.

A selective and differential medium for isolating presumptive pathogenic staphylococci and differentiating mannitol-fermenting strains from other staphylococci.