Elucidating the role of Staphylococcus epidermidis serine-aspartate repeat protein G in platelet activation

Background: Staphylococcus epidermidis is a commensal of the human skin that has been implicated in infective endocarditis and infections involving implanted medical devices. S. epidermidis induces platelet aggregation by an unknown mechanism. The fibrinogen-binding protein serine-aspartate repeat protein G (SdrG) is present in 67-91% of clinical strains. Objectives: To determine whether SdrG plays a role in platelet activation, and if so to investigate the role of fibrinogen in this mechanism. Methods: SdrG was expressed in a surrogate host, Lactococcus lactis, in order to investigate its role in the absence of other staphylococcal components. Platelet adhesion and platelet aggregation assays were employed. Results: L. lactis expressing SdrG stimulated platelet aggregation (lag time: 2.9 +/- 0.5 min), whereas the L. lactis control did not. L. lactis SdrG-induced aggregation was inhibited by alpha(IIb)beta(3) antagonists and aspirin. Aggregation was dependent on both fibrinogen and IgG, and the platelet IgG receptor Fc gamma RIIa. Preincubation of the bacteria with B beta-chain fibrinopeptide inhibited aggregation (delaying the lag time six-fold), suggesting that fibrinogen acts as a bridging molecule. Platelets adhered to L. lactis SdrG in the absence of fibrinogen. Adhesion was inhibited by alpha(IIb)beta(3) antagonists, suggesting that this direct interaction involves alpha(IIb)beta(3). Investigation using purified fragments of SdrG revealed a direct interaction with the B-domains. Adhesion to the A-domain involved both a fibrinogen and an IgG bridge. Conclusion: SdrG alone is sufficient to support platelet adhesion and aggregation through both direct and indirect mechanisms.