The cell wall receptor polysaccharides (RPS) of viridans streptococci have receptors for lectins found on other oral bacterial species that contribute to coaggregation phenomena important in microbial ecology of the teeth. We have previously reported complete chemical structures of polysaccharides from numerous strains as determined by high resolution NMR spectroscopy. Gene clusters for the synthesis of these RPS have been identified and polysaccharide production modified by replacing specific genes for glycosyl transferases and polymerases leading to production of mutant strains having novel structures. Our methods for facile determination of these structures have given insight into the specificity of numerous glycosyl transferases and their range of acceptor specificities in an in vivo context. This strategy has also led to insights into the specificity of glycosyl-phosphotransferases, O-acetylases and polysaccharide polymerases (wzy).