Staphylococcus aureus organisms are pyogenic, nonmotile, Gram-positive cocci that form grapelike clusters. These bacteria cause a myriad of skin lesions (boils, carbuncles, impetigo, and scalded skin) and also cause osteomyelitis, pneumonia, endocarditis, food poisoning, and toxic shock syndrome.
organ infections are described in other chapters. Staphylococcus epidermidis, a species that is related to Staphylococcus aureus, causes opportunistic infections in catheterized patients, patients with prosthetic cardiac valves, and drug addicts. Staphylococcus saprophyticus is a common cause of urinary tract infections in young women.
Pathogenesis
Staphylococcus aureus and other virulent staphylococci possess a multitude of virulence factors, which include surface proteins involved in adherence, secreted enzymes that degrade proteins, and secreted toxins that damage host cells.
Staphylococci are distinguished by their large number of plasmids, which encode proteins involved in antibiotic resistance and other virulence factors.
Staphylococcus aureus expresses surface receptors for fibrinogen (called clumping factor), fibronectin, and vitronectin, and uses these molecules as a bridge to bind to host endothelial cells.
Staphylococci infecting prosthetic valves and catheters have a polysaccharide capsule that allows them to attach to the artificial materials and to resist host cell phagocytosis.
The lipase of Staphylococcus aureus degrades lipids on the skin surface, and its expression is correlated with the ability of the bacteria to produce skin abscesses. Staphylococci also have protein A on their surface, which binds the Fc portion of immunoglobulins.