Streptococcus sanguis interaction with salivary MG2 and human platelets

Members of the serine-rich repeat family of streptococcal surface proteins (srp) have been shown to bind to the low molecular weight salivary mucin, MG2. Recently the srp of Streptococcus sanguis strain 133-79 has also been shown to mediate adhesion to human platelets via GPIb, which is thought to be important in the pathogenesis of infective endocarditis. Objective: The aim of this study was to determine whether the ability of a range of strains of S.sanguis to interact with MG2 is related to their ability to bind to platelet GPIb. Methods: Eleven strains of S.sanguis were assessed for their agglutination in submandibular and parotid saliva by spectrophotometry and for binding to purified GPIb using a whole cell fluorescence assay. Also, ability of GPIb to competitively inhibit binding to MG2 on blots was assessed with some strains. Results: 9/11 S.sanguis strains agglutinated in parotid saliva, including an srp insertional mutant of S.sanguis. 7/11 S.sanguis strains agglutinated in submandibular saliva but not the srp mutant. The same 7 strains, but not the non-agglutinating strains, also bound to purified GPIb. After 10 min agitation in submandibular saliva, the degree of agglutination (reduction in OD 600nm of cell suspensions) correlated with the degree of binding of FITC-labelled cells to GPIb (0.11+0.05 v. 7000+853 respectively; Pearson r2 = 0.65). Also, incubation of biotinylated S.sanguis SK36 cells with purified GPIb, inhibited their binding to submandibular MG2 on blots. Conclusion: Binding of S.sanguis to both GPIb and MG2 is mediated by the srp. The pathogenicity of S.sanguis in infective endocarditis may be an accident of its evolution to interact with MG2.