In 1942 it was discovered by Koch that a 7.5% NaCl concentration is a selective agent for the isolation of staphylococci. Then in 1945, Chapman supplemented a 7.5% NaCl concentration into Phenol Red Mannitol Agar, and noted that the addition of a 75 g / Liter concentration of Sodium Chloride results in the partial or complete inhibition of bacterial organisms other than staphylococci1ü. In Mannitol Salt Agar (MSA), mannitol fermentation helps differentiate between staphylococcal species with phenol red as an indicator. Coagulase positive staphylococci produce yellow colonies and agar color change from red to yellow, and coagulase negative staphylococci produce white colonies with no color change of the phenol red indicator. Peptones and beef extract act as a nutrient and vitamin source.
Mannitol Salt Agar conforms to the Harmonized United States Pharmacopeia (USP), European Pharmacopeia (EP) and Japanese Pharmacopeia (JP) Standards.