Cloning of a Streptococcus sanguis adhesin which mediates binding to saliva-coated hydroxyapatite

Chromosomal DNA from a salivary aggregating strain of Streptococcus sanguis 12 was partially digested with PstI and ligated into the plasmid vector pUC18 and transformed into Escherichia coli JM83. A total of 1,700 recombinant clones of E. coli were examined by a colony immunoassay with antisera raised against either S. sanguis 12 whole cells or S. sanguis 12 surface fibrils. Five clones which reacted with one or the other antiserum were shown to be unique by Western blotting (immunoblotting) and restriction endonuclease digestion. One recombinant plasmid pSA2 expressed two proteins with Mrs of 20,000 and 36,000. The 36,000-Mr protein has been designated SsaB. Both proteins were purified to homogeneity by Sephadex G-75 and ion-exchange chromatography. The proteins were present in mutanolysin digests of whole-cell lysates of S. sanguis 12 and in the non-saliva-aggregating variant 12na and the hydrophilic variant 12L. Polyclonal antiserum raised against the SsaB protein reacted strongly with the cell surfaces of S. sanguis 12 and 12na but not with that of 12L. SsaB inhibited the adhesion of S. sanguis 12na to saliva-coated hydroxyapatite, indicating that the adhesin mediates the binding to the pH-sensitive receptor.