Here's why:
* MSA is a selective and differential medium.
* Selective: It contains a high salt concentration (7.5% NaCl) that inhibits the growth of most bacteria except staphylococci.
* Differential: It contains the sugar mannitol and a pH indicator (phenol red).
* Staphylococcus epidermidis is a common skin commensal and does not ferment mannitol. This means it doesn't produce acid, so the pH indicator remains pink/red.
* Staphylococcus aureus, on the other hand, ferments mannitol, producing acid and turning the medium yellow.
Therefore, a white colony on MSA indicates a staphylococcal species that does not ferment mannitol, like S. epidermidis.
