Staphylococcus epidermidis — the 'accidental' pathogen

The commensal bacteriumStaphylococcus epidermidis is a colonizer of the human skin. Despite lacking recognized virulence factors, S. epidermidiscan cause infection, often on the surface of indwelling medical devices. In this Review, Michael Otto highlights how normally benign bacterial factors take on more virulent roles during host infection with this 'accidental' pathogen. Although nosocomial infections by Staphylococcus epidermidis have gained much attention, this skin-colonizing bacterium has apparently evolved not to cause disease, but to maintain the commonly benign relationship with its host. Accordingly, S. epidermidis does not produce aggressive virulence determinants. Rather, factors that normally sustain the commensal lifestyle of S. epidermidis seem to give rise to additional benefits during infection. Furthermore, we are beginning to comprehend the roles of S. epidermidis in balancing the epithelial microflora and serving as a reservoir of resistance genes. In this Review, I discuss the molecular basis of the commensal and infectious lifestyles of S. epidermidis.