Glucose-PTS Involvement in Maltose Metabolism by Streptococcus mutans

Streptococcus mutans grows with starch-derived maltose in the presence of saliva. Maltose transported into the cells is mediated by the MalQ protein (4-alpha-glucanotransferase) to produce glucose and maltooligosaccharides. Glucose can be phosphorylated to glucose 6-phosphate, which can enter the glycolysis pathway. The MalQ enzyme is essential in the catabolism of maltose when it is the sole carbon source, suggesting the presence of a downstream glucokinase of the MalQ enzyme reaction. However, a glucokinase gene-inactivated mutant (glk mutant) grew with maltose as the sole carbon source, with no residual glucokinase activity. This left a phosphoenolpyruvate-dependent phosphotransferase system (PTS) as the only candidate pathway for the phosphorylation of glucose in its transport as a substrate. Our hypothesis was that intracellular glucose derived from maltose mediated by the MalQ protein was released into the extracellular environment, and that such glucose was transported back into the cells by a PTS. The mannose PTS encoded by the manL, manM, and manN genes transports glucose into cells as a high affinity system with concomitant phosphorylation. The purpose of this study was to investigate extracellular glucose by using an enzyme-linked photometrical method, monitoring absorbance changes at 340 nm in supernatant of S. mutans cells. A significant amount of glucose was detected in the extracellular fluid of a glk, manLM double mutant. These results suggest that the glk and manLMN genes participate in maltose catabolism in this organism. The significance of multiple metabolic pathways for important energy sources, including maltose, in the oral environment is discussed.