Streptococcus sanguis expresses a 150-kilodalton two-domain adhesin: characterization of several independent adhesin epitopes.

Streptococcus sanguis binds to saliva-coated hydroxylapatite (sHA), an in vitro model of the enamel pellicle. To learn if more than one adhesin functions during adhesion, 12 reactive monoclonal antibodies (MAbs) were isolated by screening against both adhesive and nonadhesive strains. Two of these MAbs, 1.1 and 1.2, inhibited adhesion in a dose-dependent fashion, although maximum inhibition with either was only 37%. When these two MAbs plus a polyclonal antibody to P1-like adhesin were combined, the inhibition was additive to about 82%. These data indicated that there were at least three distinct, functional adhesion epitopes on the surface of S. sanguis. Western blot analyses of S. sanguis surface macromolecules showed antigens at 36 and 56 (with MAb 1.2), 87 and 150 (with both MAb 1.1 and MAb 1.2), and 100, 130, and 170 kDa (with anti-P1 antibody). The antigens were eluted from gels. Isolated antigens and corresponding antibodies inhibited adhesion similarly. Additivity experiments suggested the distinct epitopes were in three groups: (i) 36/56 kDa, (ii) 87/150 kDa, and (iii) 100/130/170 kDa. The 150-kDa antigen reacting with both MAbs was isolated from gels and digested with trypsin. The digestion revealed a series of tryptic bands. A band at 38 kDa reacted with MAb 1.1 whereas a band at 54 kDa reacted with MAb 1.2 in Western blot analysis, indicating two distinct adhesive epitopes on the 150-kDa antigen. These data strongly suggest that S. sanguis adhesion to sHA is maximized when several adhesin epitopes are coexpressed on surface antigens of different sizes.