Figure 4. Streptococcal invasion of dentinal tubules (upper diagram) and co-invasion with P. gingivalis (lower diagram). Streptococcal cells (•) adhere to unmineralized collagen type I ( ) via antigen I/II polypeptide adhesin (). Growth of streptococci in the presence of collagen peptides leads to up-regulation of antigen I/II production (), long-chaining of cells, and colonization along the length of the tubule. In the lower diagram, P. gingivalis cells (•) and S. gordonii cells both adhere to collagen (1), but P. gingivalis is unable to penetrate the tubules further in monoculture. The presence of S. gordonii (2) provides an additional binding substrate for P. gingivalis and promotes intratubular colonization by P. gingivalis. Up-regulation of streptococcal antigen I/II adhesin production (3) provides additional binding sites for P. gingivalis. These bacteria remain in association with the streptococci (4), and the dentinal tubules become invaded by a mixed bacterial population.