This Petri dish culture plate contained mannitol-salt agar (MSA) growth medium, which had been inoculated with a Staphylococcus sp. bacterium. Of importance is the coloration of both the colonies, and surrounding medium, which had turned yellow. If the Staphylococcus sp. bacterium is able to cause fermentation of the contained mannitol constituent, as in the case of S. aureus, an acidic environment will be created, imparting a yellowish coloration to the medium, when the phenol red indicator in the MSA turns yellow, which it does in response to acids. If the bacterium introduced to the medium is unable to ferment mannitol, as in the case of S. epidermidis, the phenol red will remain unaffected, and the color of the medium will in turn, remain red to pink. The high concentration of salt in the MSA medium inhibits the growth of bacterium other than Staphylococci.