Staphylococcus aureus is a bacterium that is a member of the Firmicutes, and is frequently found in the human respiratory tract and on the skin. Although S. aureus is not always pathogenic, it is a common cause of skin infections, respiratory disease , and food poisoning. Disease-associated strains often promote infections by producing potent protein toxins, and expressing cell-surface proteins that bind and inactivate antibodies. The emergence of antibiotic-resistant forms of pathogenic S. aureus is a worldwide problem in clinical medicine.
Gram stain of S. aureus cells which typically occur in clusters. The cell wall readily absorbs the crystal violet stain.
Yellow colonies of S. aureus on a blood agar plate, note regions of clearing around colonies caused by lysis of red cells in the agar.
S. aureus is a facultative anaerobic Gram-positive coccal bacterium, also known as "golden staph" and Oro staphira. In medical literature the bacteria is often referred to as S. aureus or Staph aureus. Staphylococcus should not be confused with the similarly named and medically relevant genus Streptococcus. S. aureus appears as grape-like clusters when viewed through a microscope, and has large, round, golden-yellow colonies, often with hemolysis, when grown on blood agar plates.