Chapman's Agar or Mannitol Salt Agar (MSA) is a semi-synthetic selective culture medium used for the selection of halophilic bacteria, particularly those that ferment mannitol. It is particularly used for the isolation of Staphilicoccus. This medium is important in medical laboratories, as it allows to distinguish pathogenic microbes in a short period of time. It contains a high concentration (approximately 7.5 to 10%) of salt (NaCl), which makes it selective for most Gram-negative bacteria and some Gram-positive bacteria (Staphylococcus, Enterococcus and Micrococcaceae) since this level of salt is inhibitory for most other bacteria. It is also a differential medium for mannitol fermenting staphylococci, containing carbohydrate mannitol and phenol red indicator, a pH indicator to detect the acid produced by mannitol fermenting staphylococci. Staphylococcus aureus produces yellow colonies with yellow areas, while other coagulase-negative staphylococci produce small pink or red colonies without changing the color of the medium. If an organism can ferment mannitol, an acidic by-product is formed that causes the phenol red in the agar to turn yellow. It is used for the selective isolation of suspected pathogenic Staphylococcus species (pp).