Evidence that glucose and sucrose uptake in oral streptococcal bacteria involves independent phosphotransferase and proton-motive force-mediated mechanisms

Keevil, C. W., Williamson, M. I., Marsh, P. D. and Ellwood, D. C. (1984) Evidence that glucose and sucrose uptake in oral streptococcal bacteria involves independent phosphotransferase and proton-motive force-mediated mechanisms. Archives of Oral Biology, 29 (11), 871-878. (doi:10.1016/0003-9969(84)90085-2).

Abstract

Sugar transport and glycolysis in Streptococcus sanguis NCTC 7865, Streptococcus mitis ATCC 903, Streptococcus salivarius NCTC 8606 and several strains of Streptococcus mutans were investigated by following the rate of acid production by washed bacteria at a constant pH of 7.0. The phosphoenolpyruvate-phosphotransferase system (PTS) was inhibited by low concentrations of chlorhexidine. When this PTS-inhibitory concentration of chlorhexidine was added to cells washed and re-suspended in KCl, glucose uptake and glycolysis continued at a greatly-reduced rate. Chlorhexidine abolished glucose and sucrose uptake and metabolism in bacteria washed and incubated in saline. The Na⁺-inhibition was reproduced in KCl-washed bacteria using the cyclic peptide ionophores, valinomycin and gramicidin, to dissipate K⁺ and H⁺ gradients across the cell membrane. Glucose metabolism by Strep, mutans B13 was more resistant to chlorhexidine than that of Strep, mutans NCTC 10449 or Strep, sanguis but was more sensitive to the ionophores. Valinomycin had a greater inhibitory effect on strain B13 than the other two. That ion gradients are important in the chlorhexidine-resistant glucose-uptake mechanism was confirmed using the classical uncoupling agents, carbonylcyanide-m-chlorophenylhydrazone, 2,4-dinitrophenol and KSCN. Glucose metabolism was inhibited in the presence of both the uncouplers and the PTS-inhibitory concentration of chlorhexidine and significant inhibition was also observed in the absence of the PTS inhibitor. Lactate or the ATPase inhibitor, dicyclohexyl carbodiimide (DCCD), had similar inhibitory effects on the non-PTS uptake system. The results are consistent with the existence of an alternative (non-PTS) sugar-uptake system driven by proton-motive force (pmf) which is relatively insensitive to chlorhexidine but is inhibited by Na⁺, lactate, ionophores and uncouplers. The partial inhibition by DCCD in the absence of chlorhexidine indicates a role for ATP in the pmf-driven system but not in the PTS, and is probably required by the intracellular glucokinase for glucose phosphorylation. Comparison of published molar growth yields with theoretical yields predicted from the metabolic pathways in oral streptococci suggested that energy derived from substrate-level phosphorylation could not account for the high yields obtained but that pmf-driven processes could satisfy the discrepancies. Hexose uptake by oral streptococci is apparently driven by pmf when PTS activities are low. Such conditions are likely to occur in dental plaque.

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Contributors

Author: C. W. Keevil

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