Uptake and metabolism of N-acetylglucosamine and glucosamine by Streptococcus mutans

Glucosamine and N-acetylglucosamine are among the most abundant sugars on the planet, and their introduction into the oral cavity via the diet and host secretions, and through bacterial biosynthesis, provides oral biofilm bacteria with a source of carbon, nitrogen, and energy. In this study, we demonstrated that the dental caries pathogen Streptococcus mutans possesses an inducible system for the metabolism of N-acetylglucosamine and glucosamine. These amino sugars are transported by the phosphoenolpyruvate:sugar phosphotransferase system (PTS), with the glucose/mannose enzyme II permease encoded by manLMN playing a dominant role. Additionally, a previously uncharacterized gene product encoded downstream of the manLMN operon, ManO, was shown to influence the efficiency of uptake and growth on N-acetylglucosamine and, to a lesser extent, glucosamine. A transcriptional regulator, designated NagR, was able to bind the promoter regions in vitro, and repress the expression in vivo, of the nagA and nagB genes, encoding N-acetylglucosamine-6-phosphate deacetylase and glucosamine-6-phosphate deaminase, respectively. The binding activity of NagR could be inhibited by glucosamine-6-phosphate in vitro. Importantly, in contrast to the case with certain other Firmicutes, the gene for de novo synthesis of glucosamine-6-phosphate in S. mutans, glmS, was also shown to be regulated by NagR, and NagR could bind the glmS promoter region in vitro. Finally, metabolism of these amino sugars by S. mutans resulted in the production of significant quantities of ammonia, which can neutralize cytoplasmic pH and increase acid tolerance, thus contributing to enhanced persistence and pathogenic potential.