SORBITOL INHIBITION OF GLUCOSE-METABOLISM BY STREPTOCOCCUS SANGUIS-160

Clinical studies in Sweden have shown that the proportion of sorbitol-utilizing strains of Streptococcus sanguis increases in dental plaque from individuals using sorbitol-containing products for prolonged periods. We have undertaken to study the metabolism of glucose and sorbitol by S. sanguis 160, isolated from a subject consuming sorbitol-containing chewing-gum 4 times a day for 4 years. Growth on glucose was inhibited by the presence of sorbitol in the growth medium and sorbitol was utilized in the presence of glucose, albeit, at a slower rate than glucose. In addition, pulses of glucose added to cultures growing on sorbitol resulted in the expulsion of sorbitol from the cell. In order to examine further the relationship of sorbitol and glucose, uptake assays were carried out with S. sanguis 160 grown in continuous culture (pH 7.0, dilution rate = 0.1 h-1) with glucose, sorbitol or nitrogen (sorbitol excess) limitations. The uptake of [C-14]-glucose by sorbitol-limited cells, but not by glucose-limited cells, was inhibited by sorbitol, as was glycolysis. Kinetic experiments with glucose-limited cells showed 2 transport systems for glucose with K(s) values of 5.2 and 40-mu-M, and glucose phosphorylation activity by decryptified cells indicated transport by the P-enolpyruvate (PEP) phosphotransferase system (PTS) with lesser activity for an ATP-dependent transport process. Transition from glucose-limited growth to sorbitol-limited growth revealed repression of total [C-14]-glucose uptake by intact cells and activity for Enzyme II for glucose (Ell(glc)) of the PTS measured in membrane preparations in the presence of an excess of the soluble PTS proteins in crude cell-free supernatant fractions. In a 'crossover' experiment, Ell(glc) activity in membranes of glucose-limited cells was repressed when the glucose-limited soluble fraction was replaced by the soluble supernatant derived from sorbitol-limited and sorbitol-excess cells. This suggests that the transition from a glucose limitation to a sorbitol limitation resulted in the repression of a soluble PTS component, possibly Enzyme III for glucose (III(glc)), necessary for glucose transport. In addition, the 8-fold lower Ell(glc) activity in sorbitol-limited membranes when combined with the glucose-limited soluble components revealed repression of Ell(glc) by sorbitol. Higher repression (103-fold) of Ell(glc) activity was observed with sorbitol-excess membranes. This evidence indicates that sorbitol is an effective catabolite repressor of the glucose-PTS, exhibiting negative regulation over synthesis of both Ell(glc) and a soluble component, presumably III(glc).