Spatial Organization of Viridans Streptococci in Oral Biofilms

Dental caries and periodontal disease are closely associated with dental plaque, the biofilm that results from microbial colonization of the tooth surface. Various experimental models have been described for the initial phase of colonization, including one that utilizes enamel chips positioned within the oral cavity for periods of time up to 24 hours. Scanning electron microscopy of chips removed after 4 hrs in vivo revealed individual bacteria attached to the pellicle-coated surface. Growth of these pioneer organisms resulted in microcolonies that merged to form a spreading monolayer of cocci and rods. Over 80% of the bacteria present in early biofilms were identified as members of four closely related viridans streptococcal species, Streptococcus sanguis, S. oralis, S. mitis and S. gordonii. Veillonella spp, and Actinomyces naeslundii each composed about 5%. Interestingly, veillonellae, actinomyces and streptococci all increased during the first 24 hrs of colonization thereby suggesting important roles for these bacteria in normal daily plaque accumulation [early biofilm development]. 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.