Site directed mutagenesis and in vivo studies in a rat model of infective endocarditis verified that this carbohydrate binding site within GspBBR mediates binding of S. gordonii strain M99 to sialyl-T antigen, the host carbohydrates of the platelet membrane receptor GPIbα, and intact platelets, and that this interaction is important for virulence. The gene discussed is GP1BA; the disease is infective endocarditis.