Fibronectin binding protein and host cell tyrosine kinase are required forinternalization of Staphylococcus aureus by epithelial cells

Staphylococcus aureus expresses several surface proteins that promote adher ence to host cell extracellular matrix proteins, including fibronectin (Fn) . Since this organism has recently been shown to be internalized by nonprof essional phagocytes, a process that typically requires high-affinity bindin g to host cell receptors, we investigated the role of its Fn binding protei ns (FnBPs) and other surface proteins in internalization by the bovine mamm ary gland epithelial cell line (MAC-T). Efficient internalization of S. aur eus 8325-4 required expression of FnBPs; an isogenic mutant (DU5883), not e xpressing FnBPs, aas reduced by more than 95% in its ability to invade MAC- T cells. Moreover, D3, a synthetic peptide derived from the ligand binding domain of FnBP, inhibited the internalization of the 8325-4 strain in a dos e-dependent fashion and the efficiency of staphylococcal internalization wa s partially correlated with Fn binding ability. Interestingly, Fn also inhi bited the internalization and adherence of S. aureus 8325-4 in a dose-depen dent manner. In contrast to internalization, adherence of DU5883 to MAC-T w as reduced by only approximately 40%, suggesting that surface binding prote ins, other than FnBPs, can mediate bacterial adherence to cells. Adherence via these proteins, however, does not necessarily result in internalization of the staphylococci. An inhibitor of protein tyrosine kinase, genistein, reduced MAC-T internalization of S. aureus by 95%, indicating a requirement for a host signal transduction system in this process. Taken together, the se results indicate that S, aureus invades nonprofessional phagocytes by a mechanism requiring interaction between FnBP and the host cell, leading to signal transduction and subsequent rearrangement of the host cell cytoskele ton.