Staphylococcus aureus and Staphylococcus epidermidis on mannitol salt agar
Mannitol salt agar is a commonly used growth medium in microbiology. It encourages the growth of a group of certain bacteria while inhibiting the growth of others. It contains a high concentration (~7.5%-10%) of salt (NaCl), making it selective for gram positive bacteria Staphylococci (and Micrococcaceae) since this level of NaCl is inhibitory to most other bacteria.
It is also a differential medium for mannitol fermentors, containing mannitol and the indicator phenol red. Staphylococcus aureus produce yellow colonies with yellow zones, whereas other 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 will cause the phenol red in the agar to turn yellow.