Clumping factor A mediates binding of Staphylococcus aureus to human platelets

The direct binding of bacteria to platelets may be an important virulence m echanism in the pathogenesis of infective endocarditis. We have preciously described Staphylococcus aureus strain PS12, a Tn551-derived mutant of stra in ISP479, with reduced ability to bind human platelets in vitro, When test ed in an animal model of endocarditis, tile PS12 strain was less virulent t han its parental strain, as measured by bacterial densities in endocardial vegetations and incidence of systemic embolization. We hare now characteriz ed the gene disrupted in PS12 and its function in platelet binding. DNA seq uencing, Southern blotting, and PCR analysis indicate that PS12 contained t wo Tn551 insertions within the clumping factor A (ClfA) locus (clfA). The f irst copy was upstream from the clfA start codon and appeared to have no ef fect on ClfA production. The second insertion was within the region encodin g the serine aspartate repeat of ClfA and resulted in the production of a t runcated ClfA protein that was secreted from the cell. A purified, recombin ant form of the ClfA A region, encompassing amino acids 40 through 559, sig nificantly. reduced the binding of ISP479C to human platelets by 41% (P = 0 .0001), Immunoprecipitation of recombinant ClfA that had been incubated wit h solubilized platelet membranes coprecipitated a 118-kDa platelet membrane protein. This protein does not appear to be glycoprotein IIb. These result s indicate that platelet binding by S. aureus is mediated in part by the di rect binding of ClfA to a novel 118-kDa platelet membrane receptor.