Staphylococci

This chapter looks at staphylococci, which are Gram-positive cocci that form clusters, but can occur singly, in pairs, chains, or tetrads. They are classically distinguished from other Gram-positive cocci by presence of catalase, an enzyme that degrades hydrogen peroxide (H2O2). S. aureus is distinguished from other coagulase-negative staphylococci, which are generally less virulent, by the presence of coagulase, an enzyme that coagulates plasma. Many toxins and regulatory elements enhance virulence in staphylococci. Staphylococci are skin commensals. About 20% of adults are persistently colonized by S. aureus, 60% are intermittently colonized, and 20% are never colonized. High-risk groups for S. aureus colonization include infants, insulin-dependent diabetics, intravenous drug users, HIV-infected patients, and renal dialysis patients. Methicillin-resistant Staphylococcus aureus (MRSA) was once predominantly seen in hospitals. However, over the past twenty years the epidemiology of MRSA has significantly changed, with MRSA infections now being observed in community settings.