Mannitol Salt Agar (MSA) is a selective and differential medium used to isolate and identify Staphylococcus aureus.

* Selective: MSA contains a high concentration of salt (7.5% NaCl) which inhibits the growth of most bacteria except staphylococci.

* Differential: MSA contains mannitol, a sugar, and a pH indicator (phenol red). *Staphylococcus aureus* ferments mannitol, producing acid that turns the pH indicator yellow. Other staphylococci that do not ferment mannitol will grow on the medium but the color remains red.

Bacillus subtilis is a gram-positive bacterium that is not a staphylococcus. It does not ferment mannitol and will not cause a color change on MSA.

Therefore, Bacillus subtilis will grow on MSA but will not ferment mannitol and the agar will remain red.