The low-affinity IgG receptor, FcgammaRIIA, has been implicated in Streptococcus sanguis-induced platelet aggregation. Therefore, it is likely that signal transduction is at least partly mediated by FcgammaRIIA activation and a tyrosine kinase-dependent pathway. In this study the signal transduction mechanisms associated with platelet activation in response to the oral bacterium, S. sanguis were characterised. In the presence of IgG, S. sanguis strain 2017-78 caused the tyrosine phosphorylation of FcgammaRIIA 30s following stimulation, which led to the phosphorylation of Syk, LAT, and PLCgamma2. These early events were dependent on Src family kinases but independent of either TxA(2) or the engagement of the alpha(IIb)beta(3) integrin. During the lag phase prior to platelet aggregation, FcgammaRIIA, Syk, LAT, and PLCgamma2 were each dephosphorylated, but were re-phosphorylated as aggregation occurred. Platelet stimulation by 2017-78 also induced the tyrosine phosphorylation of PECAM-1, an ITIM-containing receptor that recruits protein tyrosine phosphatases. PECAM-1 co-precipitated with the protein tyrosine phosphatase SHP-1 in the lag phase. SHP-1 was also maximally tyrosine phosphorylated during this phase, suggesting a possible role for SHP-1 in the observed dephosphorylation events. As aggregation occurred, SHP-1 was dephosphorylated, while FcgammaRIIA, Syk, LAT, and PLCgamma2 were rephosphorylated in an RGDS-sensitive, and therefore alpha(IIb)beta(3)-dependent, manner. Additionally, TxA(2) release, 5-hydroxytryptamine secretion and phosphatidic acid formation were all blocked by RGDS. Aspirin also abolished these events, but only partially inhibited alpha(IIb)beta(3) -mediated re-phosphorylation. Therefore, S. sanguis -bound IgG cross links FcgammaRIIA and initiates a signaling pathway that is down-regulated by PECAM-1-bound SHP-1. Subsequent engagement of alpha(IIb)beta(3) leads to SHP-1 dephosphorylation permiting a second wave of signaling leading to TxA(2) release and consequent platelet aggregation.