Why s.aures colony yellow on msa?

Staphylococcus aureus colonies appear yellow on Mannitol Salt Agar (MSA) because they ferment mannitol, producing acid as a byproduct. The acid lowers the pH of the medium, causing the phenol red indicator in the agar to change from red to yellow. This color change is a key characteristic used to differentiate S. aureus from other staphylococci that do not ferment mannitol, which typically result in red colonies.