One of the major uncertainties raised by the wide-spread ability of cells of oral bacteria to interact physically in vitro is whether these coaggregations function in vivo. Using direct immunofluorescence, we have demonstrated the coincident juxtaposition in dental plaque of streptococci and actinomyces, suggesting that these known coaggregating partners may use coaggregation as a means to colonize enamel. Our overall goal is to determine spatial organization of different viridans streptococci in biofilms that form during initial colonization of the tooth surface. These bacteria include two groups with distinctive properties. One group contains strains of S. sanguis and S. gordonii with GalNAc-sensitive adhesions and the other contains strains of S. oralis and other species with complementary receptors for intrageneric coaggregation. The models for biofilm formation will include an in vitro flow-cell model in which streptococci with known adhesive properties attach and grow on a saliva-coated surface and an in vivo model in which enamel chips are colonized within the oral cavity for periods of time up to 72 hours. The results of these studies should provide significant insights into the involvement of interbacterial adhesion in biofilm development. Our goal from these studies is to investigate the spatial organization of streptococci and their coaggregation partner actinomyces and veillonellae in biofilms formed in vivo compared to in vitro. We hope to make significant advances in understanding the distribution of these species in initial dental plaque formation. Since the streptococci are the predominant organism in early dental plaque, we propose that understanding their contribution to microbial communities will lead to important advances in designing effective oral health regimens.