Purpose: Isolation and differentiation of Staphylococcus aureus.
Phenol red - pH indicator. Yellow below 6.8, red at 7.4 to 8.4, and pink at 8.4 and above.
Mannitol - substrate for fermentation (makes medium differential).
Sodium chloride - High concentration of salt (makes medium selective). Kills most bacteria, but Staphylococci can thrive here.