Competition between pioneer colonizing bacteria may determine polymicrobial succession during dental plaque development, but the ecological constraints are poorly understood. For example, more Streptococcus sanguinis than Streptococcus gordonii organisms are consistently isolated from the same intraoral sites, yet S. gordonii fails to be excluded and survives as a species over time. To explain this observation, we hypothesized that S. gordonii could compete with S. sanguinis to adhere to saliva-coated hydroxyapatite (sHA), an in vitro model of the tooth surface. Both species bound similarly to sHA, yet 10-to 50-fold excess S. gordonii DL1 reduced binding of S. sanguinis SK36 by 85 to >95%. S. sanguinis, by contrast, did not significantly compete with S. gordonii to adhere. S. gordonii competed with S. sanguinis more effectively than other species of oral streptococci and depended upon the salivary film on HA. Next, putative S. gordonii adhesins were analyzed for contributions to interspecies competitive binding. Like wild-type S. gordonii, isogenic mutants with mutations in antigen I/II polypeptides (sspAB), amylase-binding proteins (abpAB), and Csh adhesins (cshAB) competed effectively against S. sanguinis. By contrast, an hsa-deficient mutant of S. gordonii showed significantly reduced binding and competitive capabilities, while these properties were restored in an hsa-complemented strain. Thus, Hsa confers a selective advantage to S. gordonii over S. sanguinis in competitive binding to sHA. Hsa expression may, therefore, serve as an environmental constraint against S. sanguinis, enabling S. gordonii to persist within the oral cavity, despite the greater natural prevalence of S. sanguinis in plaque and saliva.Within the oral cavity there are hundreds of species with the potential to colonize the tooth surface. Thus, microbes that have a competitive binding advantage are likely to be successful. Competition occurs both between and within species and is strongest for those microbes that target a similar pool of receptors. Furthermore, the outcome of competitive events for one set of bacteria can be expected to influence that of another. This is particularly pertinent for pioneer colonizers, since initial adhesion to the tooth surface by oral microbes is essential for the development and composition of dental plaque. Understanding the factors that influence initial colonization could, therefore, contribute to our understanding of the ecological constraints that govern this polymicrobial community and may suggest novel preventive and control measures for dental plaque.The pioneer colonizers Streptococcus sanguinis and Streptococcus gordonii are readily able to attach to the tooth surface (37). The promiscuous adhesive capabilities of these bacteria are facilitated, in part, by specific cell surface adhesins (16,43,59). S. sanguinis and S. gordonii have a high level of 16S rRNA sequence homology and, until recently, were classified as the same species (19). In addition, these two streptococcal species are isolated ...