ABSTRACT The role of S. mutans in the etiology of dental caries has been well investigated. S. mutans resides in the dental plaque with up to 70 other bacterial species. Recent advances in research suggest that the cariogenic potential of dental plaque may be detemiined by the combined interactions ofthe entire oral bacteria. These interactions include modulation of gene expression, co-aggregation and growth inhibition. We hypothesize that oral streptococci compete In part by influencing virulence gene expression in the antagonistic species'by direct physical Interactions and/or metabolic signaling. The proposed Aims are designed to characterize the virulence genes of S. mutans and S. sanguinis and to identify the mechanisms how oral bacteria influence the expression of virulence gene(s) of other species. We chose the interaction of S. mutans and S. sanguinis as ideal model system because of their long known mutual exclusion. To test our hypothesis that interspecies interactions influence the outcome of the competition between S. mutans and S. sanguinis, we will: [unreadable] Develop a dual-species biofilm model based on saliva coated hydroxyapatite for reproducible investigation of S. mutans and S. sanguinis biofilm formation [unreadable] Search for new genes involved in the interspecies interaction between S. mutans and S. sanguinis [unreadable] Determine the expression profile of S. mutans and S. sanguinis virulence genes in response to each other [unreadable] Further characterize the direct physical interactions between S. mutans and S. sanguinis by atomic force microscopy