Sorbitol transport by Streptococcus sanguis 160

Sorbitol metabolism was examined with a sorbitol‐fermenting strain (160) of Streptococcus sanguis isolated from the dental plaque of a subject using sorbitol‐containing chewing‐gum for 4 years. S. sanguis 160 was grown in continuous culture (pH, 7.0; dilution rate, 0.1 h −1 ) with glucose, sorbitol and nitrogen (sorbitol‐excess) limitations. Cells grown with a glucose limitation exhibited low, but detectable, uptake of [ 14 C]‐sorbitol and transition to medium limiting in sorbitol resulted in a 5‐fold increase in sorbitol uptake. Kinetic data revealed that both glucose and sorbitol‐limited cells possessed 2 transport systems for sorbitol (K s = 3.3–6.7 and 36–64 μM), but continued growth of the organism on limiting sorbitol resulted in the loss of the high‐affinity system. Decryptified, sorbitol‐limited cells phosphorylated sorbitol in the presence of phosphoenolpyruvate (PEP), but not with ATP, indicating sorbitol transport solely via the PEP phosphotransferase (PTS) system. PEP‐dependent activity in glucose‐limited and sorbitol‐excess cells was 6‐ and 4‐fold lower than that of the sorbitol‐limited cells. Uptake of [ 14 C]‐sorbitol and activity for Ell for sorbitol [E11 sor ] of the PTS in cells in transition from a glucose to sorbitol limitation confirmed the induction of the sorbitol‐PTS and the repression of the glucose‐PTS in the presence of sorbitol. Cells grown with an excess of sorbitol exhibited very low EII sor activity. A crossover experiment with membranes and soluble fractions from glucose‐, sorbitol‐ and nitrogen‐limited cells of S. sanguis 160 demonstrated the induction of a soluble PTS component in sorbitol‐limited cells essential for sorbitol transport via the PTS. In addition, this factor, tentatively identified as lll sor , was shown to stimulate 7‐fold sorbitol phosphorylation by glucose‐limited membranes, indicating the possibility that sorbitol can be transported via Ell glc in the presence of the sorbitol‐specific, soluble component. This observation was confirmed by demonstrating that glucose and sorbitol compete for 1 of the 2 sorbitol transport systems in the organism. Our results indicate that transition of S. sanguis 160 from glucose to sorbitol‐limited growth results in sorbitol transport initially via Ell glc and a sorbitol‐specific soluble factor (lll sor ) and this is followed by the relatively slow induction of Ell sor and the concomitant repression of E11 glc .