Staphylococcus Epidermidis Induced Platelet Aggregation Is Mediated by the Fibrinogen Binding Surface Protein SdrG.

S. epidermidis is normally present on the surface of the skin, however in a hospital setting, it is responsible for many infections involving implanted medical devices resulting in potentially fatal complications such as infective endocarditis and septicemia. S. epidermidis has previously been shown to induce platelet aggregation (Ohshima et al. Microbiol Immunol 1991) through an unknown mechanism. The surface protein SdrG (serine-aspartate repeat G protein) is important in the initial colonization of the device, as it binds fibrinogen, which is one of the first proteins to coat foreign devices. We thus postulated that this SdrG fibrinogen complex may mediate the S. epidermidis interaction with platelets. In order to investigate the specific effects of SdrG on platelets, we expressed SdrG in Lactococcus lactis which does not activate platelets. L. lactis SdrG was able to support platelet adhesion in the absence of fibrinogen, and to a greater extent in the presence of fibrinogen suggesting both a direct interaction with the platelet and an indirect interaction via a fibrinogen bridge. Fibrinogen dependent adhesion was inhibited by both abciximab and tirofiban confirming an interaction with GPIIb/IIIa. Furthermore, SdrG when expressed in L. lactis can induce platelet aggregation with a lag time of 1.5 ± 0.4 min. This aggregation was inhibited by abciximab suggesting that it is true aggregation rather than agglutination. We have also shown this to be dependent on cyclooxygenase signalling, as it is inhibited by aspirin. SdrG binds to the C-terminus of the beta chain of fibrinogen (Ponnuraj K et al. , Cell 2003). We synthesized a 15-mer peptide mimicking the binding region of the fibrinogen beta chain in order to determine whether fibrinogen binding was important for this aggregation. The native fibrinopeptide NEEGFFSARGHRPLD increased the lag time for aggregation to 9.2 ± 1.4 min, however, the control peptide NEEGFFAARGHRPLD did not affect aggregation (1.4 ± 0.3 min). The lag time did not increase further with saturating concentrations of peptide concentrations suggesting instability of the peptide, or a second mechanism involved in the aggregation which is consistent with the fibrinogen independent interaction seen in the adhesion assay. This work shows that SdrG alone is sufficient to support platelet adhesion and aggregation. SdrG mediates platelet activation through a novel mechanism via GPIIb/IIIa, and an as yet unknown direct interaction. Thus SdrG appears to be the dominant virulence factor for S. epidermidis induced thrombosis, therefore, inhibition of these interactions have potential for treatment of thrombotic complications associated with S. epidermidis infections.