Mannitol Salt Agar is used for the selective isolation and enumeration of staphylococci from clinical and nonclinical materials.
SUMMARY AND EXPLANATION
Koch, in 1942, reported that only staphylococci grow on agar media containing 7.5% sodium chloride. Chapman further studied this phenomenon in greater detail and concluded that the addition of 7.5% sodium chloride to phenol red mannitol agar results in an improved medium for the isolation of plasma-coagulating staphylococci. Mannitol Salt Agar is listed as one of several media recommended for the enumeration of gram-positive bacteria in cosmetics, clinical specimens, and pharmaceutical products.
Mannitol Salt Agar is a nutritive medium due to its content of peptones and beef extract, which supply essential growth factors, such as nitrogen, carbon, sulfur and trace nutrients. The 7.5% concentration of sodium chloride results in the partial or complete inhibition of bacterial organisms other than staphylococci. Mannitol fermentation, as indicated by a change in the phenol red indicator, aids in the differentiation of staphylococcal species. Agar is a solidifying agent.