S. aureus is a facultatively anaerobic, Gram-positive coccus, which appears as grape-like clusters when viewed through a microscope, and has round, usually golden-yellow colonies,
often with hemolysis, when grown on blood agar plates. The golden appearance is the etymological root of the bacterium's name; aureus means "golden" in Latin.
Some strains of Staphylococcus 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.
Mutant strains of S. aureus modified to lack staphyloxanthin are less likely to survive incubation with an oxidizing chemical, such as hydrogen peroxide than pigmented strains.
Mutant colonies are quickly killed when exposed to human neutrophils, while many of the pigmented colonies survive.
