Staphylococcus aureus yellow colonies on sheep blood agar
Typical colonies of Staphylococcus aureus on blood agar (CAP agar) after 24 hours, aerobic atmosphere, 37°C.
Yellow colonies are surroundend by a zone of beta-hemolysis.
Some strains of S. aureus are capable of producing staphyloxanthin - a golden coloured carotenoid pigment. This pigment acts as a virulence factor, primarily by being a bacterial antioxidant which helps the microbe evade the reactive oxygen species which the host immune system uses to kill pathogens.