The presence of a low level of peptone lessens the physiological shock normally experienced by bacterial cells when they are introduced to a diluent such as Ringer’s Solution. The level of peptone is such that multiplication of the organisms is not possible in the time in which the sample will be present in the diluent (1-2 hours). This formula is recommended by ISO 6887: BS5763.
The majority of S. aureus ferment mannitol producing yellow colonies, occasional strains of coagulase-negative staphylococci may also ferment mannitol. It is necessary to confirm the identity of presumptive S. aureus colonies by other means e.g. coagulase, protein A, DN’ase, thermonuclease or latex agglutination.