Discrete protein determinant directs the species-specific adherence of Porphyromonas gingivalis to oral streptococci

For pathogens to survive in the human oral cavity, they must identify a sui table niche in the complex multispecies biofilm that exists on oral tissues . The periodontal pathogen Porphyromonas gingivalis adheres to Streptococcu s gordonii by interacting with a specific region of the streptococcal SspB polypeptide, designated BAR. However, it does not adhere to Streptococcus m utans, which expresses SpaP, a highly conserved homolog of SspB. Comparison of the predicted secondary structure of BAR with the corresponding region of SpaP suggested that the substitution of Asn for Gly(1182) and Val for Pr o(1185) in SspB may confer a unique local structure that is not conserved i n SpaP. A synthetic peptide of 26 amino acids that encompassed residues 116 7 to 1193 of SspB promoted avid adherence of P. gingivalis, whereas a pepti de derived from the region corresponding to BAR in SpaP was inactive. Subst itution of Gly(1182) and Pro(1185) for Asn(1182) and Val(1185) in SspB by s ite-specific mutation generated proteins that were predicted to assume an S paP-like secondary structure, and the purified proteins did not promote P. gingivalis adherence. Furthermore, Enterococcus faecalis strains expressing the site-specific mutants did not support adherence of A gingivalis cells. In contrast, A gingivalis adhered efficiently to E. faecalis strains expre ssing intact SspB or SspB-SpaP chimeric proteins containing BAR. These resu lts suggest that a region of SspB consisting of 26 amino acids is sufficien t to mediate the adherence of A gingivalis to S. gordonii and that the spec ies specificity of adherence arises from its interaction with a discrete st ructural determinant of SspB that is not conserved in SpaP.