SummaryThe ability of Staphylococcus aureus to invade mammalian cells may explain its capacity to colonize mucosa and to persist in tissues after bacteraemia. To date, the underlying molecular mechanisms of cellular invasion by S. aureus are unknown, despite its high prevalence and dif®culties in treatment. Here, we show cellular invasion as a novel function for an S. aureus adhesin, previously implicated solely in attachment. S. aureus, but not S. epidermidis, invaded epithelial 293 cells in a temperature-and F-actindependent manner. Formaldehyde-®xed and live bacteria were equally invasive, suggesting that no active bacterial process was involved. All clinical S. aureus isolates analysed, but only a subset of laboratory strains, were invasive. Fibronectin-binding proteins (FnBPs) acted as S. aureus invasins, because: (i) FnBP deletion mutants of invasive laboratory strains lost invasiveness; (ii) expression of FnBPs in noninvasive strains conferred invasiveness; and (iii) the soluble isolated ®bronectin-binding domain of FnBP (D1±D4) completely blocked invasion. Integrin a 5 b 1 served as host cell receptor, which interacted with staphylococcal FnBPs through cellular or soluble ®bronectin. FnBP-de®cient mutants lost invasiveness for epithelial cells, endothelial cells and ®broblasts. Thus, ®bronectin-dependent bridging between S. aureus FnBPs and host cell integrin a 5 b 1 is a conserved mechanism for S. aureus invasion of human cells. This may prove useful in developing new therapeutic and vaccine strategies for S. aureus infections.