Oral bacteria coadhere to specific partners during biofilm formation that in turn may affect the species and spatial distribution in dental plaque. Bacterial coaggregation occurs when the adhesins on the surface of one cell type are able to bind their specific cognate receptors on the surface of a different cell type. Adhesins of the antigen I/II family, SspA and SspB, are surface proteins expressed by Streptococcus gordonii DL1 and are known to be largely responsible for the coaggregation of strain DL1 with Actinomyces naeslundii. We hypothesize that antigen I/II family members are integral for mediating coaggregation and coadherence of early colonizers of the tooth surface. Objective: To test for the presence of these adhesins on other early colonizing streptococci, we examined Streptococcus oralis H1, Streptococcus oralis So34, Streptococcus oralis C104, Streptococcus mitis J22, Streptococcus gordonii PK488, and Streptococcus SM PK509, representative members of the six known streptococcal coaggregation groups. Methods: Cell surface preparations of strains H1, So34, C104, J22, PK488, and PK509 were examined through immunoblot analysis using anti-SspB antiserum. A region of the genes encoding antigen I/II family members from strains So34 and J22 was PCR-amplified and sequenced. These sequences were compared to the sequence of sspB from DL1. Results: Strains H1, So34, C104, J22, and PK488 expressed surface proteins that react with anti-SspB antiserum while strain PK509 did not. The cloned DNA showed high sequence identity to sspB in regions known to be conserved among family members and low similarity in regions known to diverge among family members. Conclusion: Strains H1, So34, C104, J22, and PK488 each express antigen I/II family members likely to be novel, previously uncharacterized adhesins that may contribute to early bacterial colonization events.

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