Final answer:
When S. aureus grows on mannitol salts, it turns yellow.
Step-by-step explanation:
When S. aureus grows on mannitol salts, it turns yellow.
The mannitol salt agar (MSA) contains a pH indicator called phenol red, which turns yellow when the medium is acidified by the products of fermentation. S. aureus is capable of fermenting mannitol, producing acids that cause the color change.