Staphylococcus aureus is a Gram-positive, round-shaped bacterium a member of the Firmicutes, and is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive for catalase and nitrate reduction and is a facultative anaerobe that can grow without the need for oxygen. Although "S. aureus" usually acts as a commensal of the human microbiota it can also become an opportunistic pathogen, being a common cause of skin infections including abscesses, respiratory infections such as sinusitis, and food poisoning. Pathogenic strains often promote infections by producing virulence factors such as potent protein toxins, and the expression of a cell-surface protein that binds and inactivates antibodies. The emergence of antibiotic-resistant strains of "S. aureus" such as methicillin-resistant "S. aureus" (MRSA) is a worldwide problem in clinical medicine. Despite much research and development, no vaccine for "S. aureus" has been approved.