A genus of spherical nonmotile Gram-positive bacteria that occur widely as saprotrophs or parasites. The cells occur in grapelike clusters. Many species inhabit the skin and mucous membranes, and some cause disease in humans and animals. S. aureus infection can lead to boils and abscesses in humans; this species also produces toxins that irritate the gastrointestinal tract and result in staphylococcal food poisoning. Certain strains are resistant to antibiotics, and infection with these is very difficult to treat. For example, some strains of methicillin-resistant S. aureus (MRSA) are now resistant to nearly all antibiotics and pose a grave threat to patients, causing lethal infections of the skin, lungs, and blood.