Staphylococcus aureus is a facultative anaerobic, Gram-positive coccus and is the most common cause of staph infections. It is frequently part of the skin flora found in the nose and on skin. About 20% of the human population are long-term carriers of S. aureus. The carotenoid pigment staphyloxanthin is responsible for S. aureus' characteristic golden colour, which may be seen in colonies of the organism. This pigment acts as a virulence factor with an antioxidant action that helps the microbe evade death by reactive oxygen species used by the host immune system. Staph organisms which lack the pigment are more easily killed by host defenses.