Salivary amylase binding and gene expression in Streptococcus gordonii
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Streptococcus gordonii and other commensal oral streptococci initiate the formation of dental plaque biofilm by attaching to the dental pellicle on the clean tooth surface. A prominent characteristic of S. gordonii and many other oral streptococcal species is their ability to interact with human salivary amylase. Salivary amylase, an abundant enzyme in saliva and the dental pellicle, is primarily responsible for starch hydrolysis. Amylase-binding streptococci are prevalent only in animals with evident amylase activity in their saliva, which suggest that bacteria-amylase interaction is an important factor in oral colonization. S. gordonii is able to bind host salivary amylase to its surface by the action of surface expressed amylase binding protein A (AbpA). The first aim of this study was to investigate the role of amylase binding in bacterial gene expression. Microarray analysis of S. gordonii gene expression in response to the binding of salivary amylase revealed differential expression of a number of genes, particularly, the upregulation of genes involved in fatty acid synthesis (FAS). In addition, changes in bacterial phenotype were evident including increased proliferation, and resistance to acidic conditions and the antimicrobial agent (triclosan). An abpA -deficient strain failed to bind salivary amylase to the bacterial surface and failed to produce a similar effect on gene expression and phenotype in response to exposure to salivary amylase. These results suggest that salivary amylase elicits differential gene expression and phenotype adjustment of S. gordonii and that the AbpA protein could play a key role this observed effect. The second aim of this study was to determine the role of starch and amylase in the regulation of AbpA expression. Previously, it was found that glucose regulates the expression of AbpA through catabolite repression. Our study presents the evidence that abpA gene and the AbpA protein were both highly upregulated in S. gordonii in the presence of starch and amylase, as well as maltose/maltodextrin, products of starch degradation by the action of salivary amylase. Starch alone or amylase alone did not affect AbpA expression. These results suggest that maltose/maltodextrin plays regulatory role in AbpA expression as a substrate induction mechanism in addition to catabolite repression by glucose.