Tandem genes encode cell-surface polypeptides SspA and SspB which mediate adhesion of the oral bacterium Streptococcus gordonii to human and bacterial receptors.

The highly conserved antigen I/II family of polypeptides produced by oral streptococci are believed to be colonization determinants and may mediate adhesion of bacterial cells to salivary glycoproteins absorbed to cells and tissues in the human oral cavity. Streptococcus gordonii is shown to express, on the cell surface, two antigen I/II polypeptides designated SspA and SspB (formerly Ssp-5) that are the products of tandemly arranged chromosomal genes. The structure and arrangement of these genes is similar in two independently isolated strains, DL1 and M5, of S. gordonii. The mature polypeptide sequences of M5 SspA (1539 amino acid (aa) residues) and SspB (1462 aa residues) are almost wholly conserved (98% identical) in the C-terminal regions (from residues 796 in SspA and 719 in SspB, to the respective C-termini), well-conserved (84%) at the N-terminal regions (residues 1-429), and divergent (only 27% identical residues) within the intervening central regions. Insertional inactivation of the sspA gene in S. gordonii DL1 resulted in reduced binding of cells to salivary agglutinin glycoprotein (SAG), human erythrocytes, and to the oral bacterium Actinomyces naeslundii. Further reductions in streptococcal cell adhesion to SAG and to two strains of A. naeslundii were observed when both sspA and sspB genes were inactivated. The results suggest that both SspA and SspB polypeptides are involved in adhesion of S. gordonii cells to human and bacterial receptors.