<P> Mannitol salt agar or MSA is a commonly used selective and differential growth medium in microbiology . It encourages the growth of a group of certain bacteria while inhibiting the growth of others . This medium is important in medical laboratories by distinguishing pathogenic microbes in a short period of time . It contains a high concentration (about 7.5% - 10%) of salt (NaCl), making it selective for Gram - positive bacteria (Staphylococcus and Micrococcaceae) since this level of salt is inhibitory to most other bacteria . It is also a differential medium for mannitol - fermenting staphylococci, containing carbohydrate mannitol and the indicator phenol red, a pH indicator for detecting acid produced by mannitol - fermenting staphylococci . Staphylococcus aureus produces yellow colonies with yellow zones, whereas other coagulase - negative staphylococci produce small pink or red colonies with no colour change to the medium . If an organism can ferment mannitol, an acidic byproduct is formed that causes the phenol red in the agar to turn yellow . It is used for the selective isolation of presumptive pathogenic (pp) Staphylococcus species . </P> <Ul> <Li> Gram + Staphylococcus: fermenting mannitol: medium turns yellow (e.g. S. aureus) </Li> <Li> Gram + Staphylococcus: not fermenting mannitol, medium does not change color (e.g. S. epidermidis) </Li> <Li> Gram + Streptococcus: inhibited growth </Li> <Li> Gram -: inhibited growth </Li> </Ul>

What is the role of phenol red in the mannitol salt medium