Cellular invasion by Staphylococcus aureus involves a fibronectin bridge between the bacterial fibronectin-binding MSCRAMMs and host cell beta 1 integrins

Although Staphylococcus aureus is primarily considered an extracellular pat hogen, recent evidence suggests that this bacterium can invade a variety of nonprofessional phagocytic cells. Here we investigate the early stages of cellular invasion by S. aureus and determine the bacterial and host compone nts that are required for this process. S. aureus expresses two cell surfac e-associated fibronectin (FN)-binding proteins (FnbpA and FnbpB) that media te the interaction of the bacteria with both soluble and solid-phase FN in vitro. Using a mutant of S. aureus that larks the expression of both Fnbps, we show that the expression of either protein is necessary for efficient u ptake by the mouse fibroblast line GD25 beta 1A. Invasion could be inhibite d by soluble recombinant proteins encompassing either the FN-binding D repe at region or the A region (and B repeats) of FnbpA, suggesting that the act ivities of both regions are important in this process. We demonstrate that FN is also required for invasion of this cell line. In the presence of FN-d epleted fetal bovine serum, the invasion level was reduced by similar to 40 % compared to in the presence of whole fetal bovine serum. Invasion could b e further reduced by the addition of anti-mouse FN antibodies to the assay. Finally, we utilize a mutant mouse fibroblast line, which lacks beta1 inte grin expression, to demonstrate that host cell beta1 integrins are necessar y for efficient cellular invasion. The level of invasion of the mutant cell line GD25 was reduced by similar to 97% compared to the beta1-expressing c omplemented cell line GD25 beta 1A, In addition, invasion of the GD25 beta 1A cell line could be inhibited by an RGD-containing peptide, further impli cating a role for integrins in this process. Based on these observations, w e put forward a model of S. aureus invasion in which host FN forms a bridge between the bacterial Fnbps and host cell beta1 integrins, leading to bact erial uptake.