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Streptococcus gordonii Hsa Environmentally Constrains Competitive Binding by Streptococcus sanguinis to Saliva-Coated Hydroxyapatite

Angela H. Nobbs,^1, Yongshu Zhang,¹ Ali Khammanivong,¹ and Mark C. Herzberg^1,2*

Department of Diagnostic and Biological Sciences, School of Dentistry, University of Minnesota, Minneapolis, Minnesota 55455,¹ Mucosal and Vaccine Research Center, Minneapolis VA Medical Center, Minneapolis, Minnesota 55417²

Received 2 October 2006/ Accepted 24 January 2007

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.

Published ahead of print on 2 February 2007.

Present address: Novartis Vaccines, Via Fiorentina 1, 53100 Siena, Italy.

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