Sorbitol transport and metabolism by oral streptococci

Sorbitol transport by oral streptococci was mediated by a phosphoenolpyruvate phosphotransferase system (PTS). The transition of S. sanguis 160 from continuous growth on limiting glucose to limiting sorbitol resulted in induction of EIIsor of the sorbitol-PTS, as well as sorbitol-6-P dehydrogenase which converts sorbitol-6-P to fructose-6-P. Sorbitol transport activity required the presence of a soluble sorbitol-specific component of the PTS, tentatively identified as an enzyme III for sorbitol (IIIsor). In addition, the results indicated that sorbitol transport can be mediated by the ELLglc, particularly in the presence of the sorbitol-specific component. S. sanguis 160 utilized sorbitol in a manner different from that reported for S mutans. Growth on glucose by S. sanguis 160 was inhibited by the presence of sorbitol in the growth medium and sorbitol was utilized in the presence of glucose. In addition, pulses of glucose added to cultures growing on sorbitol resulted in expulsion of sorbitol from the cell. Sorbitol was shown to interfere with glucose metabolism by S. sanguis 160 by inhibiting glucose transport by sorbitol-limited cells, but not by glucose-limited cells, and by inhibiting glycolytic activity with glucose as the substrate. Furthermore, sorbitol was an effective repressor of the glucose-PTS, exhibiting negative regulation over synthesis of both EIIglc and a soluble component of the glucose-PTS, presumably IIIglc. Oral streptococci metabolize sorbitol to large amounts of formate and ethanol in addition to smaller amounts of lactate and acetate. The metabolism of sorbitol by S. sanguis and S. gordonii was less sensitive to oxygen than that by S. mutans and S. mitis.