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 (e.g. boils), respiratory disease (e.g. sinusitis), and food poisoning. Disease-associated strains often promote infections by producing potent protein toxins, and expressing cell-surface proteins that bind and inactivate antibodies. 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.