Identification of independent Streptococcus gordonii SspA and SspB functions in coaggregation with Actinomyces naeslundii

The initial stages of dental plaque formation involve the adherence of earl y colonizing organisms such as Streptococcus gordonii and Actinomyces naesl undii to the saliva-coated tooth surface and to each other. The S. gordonii surface proteins SspA and SspB are known to play a role in adherence to sa livary proteins and mediate coaggregation with other bacteria. Coaggregatio n is the adhesin receptor-mediated interaction between genetically distinct cell types and appears to be ubiquitous among oral isolates. To define the function of SspA and SspB separately on the surface of their natural host, we constructed and analyzed the coaggregation properties of an isogenic ss pB mutant of S. gordonii DL1, an sspAB double mutant, and a previously desc ribed sspA mutant. A. naeslundii strains have been previously classified in to six coaggregation groups based on the nature of their coaggregations wit h S. gordonii DL1 and other oral streptococci. Coaggregation assays with th e ssp and sspB mutants showed that SspA and SspB are the streptococcal prot eins primarily responsible for defining these coaggregation groups and, thu s, are highly significant in the establishment of early dental plaque. SspA exhibited two coaggregation-specific functions. It participated in lactose -inhibitable and -noninhibitable interactions, while SspB mediated only lac tose-noninhibitable coaggregations. Accordingly, the sspAB double mutant la cked these functions and allowed us to detect a third coaggregation interac tion with one of these organisms. These proteins may play an important role in development of S. gordonii-A. naeslundii communities in early dental pl aque. Understanding these adhesin proteins will aid investigations of compl ex microbial communities that characterize periodontal diseases.