Effect of endogenous phosphoenolpyruvate potential on fluoride inhibition of glucose uptake by Streptococcus mutans

Divalent cations enhance fluoride binding to Streptococcus mutans and Streptococcus sanguinis cells and subsequently inhibit bacterial acid production

One preventive effect of topical fluoride application is derived from the fact that fluoride can inhibit bacterial acid production. Furthermore, divalent cations such as Ca(2+) and Mg(2+) increase the binding of fluoride to bacterial cells. These findings suggest that exposure of oral bacteria to fluoride in the presence of divalent cations increases fluoride binding to bacterial cells and subsequently enhances fluoride-induced inhibition of bacterial acid production. This study investigated the effects of fluoride exposure (0-20,000 ppm F) in the presence of Ca(2+) or Mg(2+) prior to glucose challenge on pH fall ability by bacterial sugar fermentation, as well as fluoride binding to bacterial cells by exposure to fluoride, and fluoride release from bacterial cells during bacterial sugar fermentation, using caries-related bacteria, Streptococcus mutans and Streptococcus sanguinis. The pH fall by both streptococci was inhibited by exposure to over 250 ppm F in the presence of Ca(2+) (p < 0.01), whereas in the presence of Mg(2+), the pH fall by S. mutans and S. sanguinis was inhibited after exposure to over 250 and 950 ppm F, respectively (p < 0.05). The amounts of fluoride binding to and released from streptococcal cells increased with the concentration of fluoride the cells were exposed to in the presence of Mg(2+), but were high enough even after 250 ppm F exposure in the presence of Ca(2+). The enhanced inhibition of acid production in the presence of divalent cations is probably due to the improved efficiency of fluoride binding to bacterial cells being improved via these divalent cations.

Possible link between glycolysis and apoptosis induced by sodium fluoride

Fluoride has been used to prevent caries in the dentition, but the possible underlying mechanisms of cytotoxicity induction by this compound are still unclear. Since fluoride is known as an inhibitor of glycolytic enzymes, we investigated the possible connection between NaF-induced apoptosis and glycolysis in human promyelocytic leukemia HL-60 cells. NaF-induced apoptotic cell death is characterized by caspase activation, internucleosomal DNA fragmentation, loss of mitochondrial membrane potential, and production of apoptotic bodies. Higher activation of caspases-3 and -9, as compared with that of caspase-8, suggested the involvement of an extrinsic pathway. Utilization of glucose was nearly halted by NaF, whereas that of glutamine was rather enhanced. NaF enhanced the expression of Bad protein, but not that of Bcl-2 and Bax proteins, and reduced HIF-1alpha mRNA expression. Analysis of these data suggests a possible link between glycolysis and apoptosis.

Biochemical mechanisms of enhanced inhibition of fluoride on the anaerobic sugar metabolism by Streptococcus sanguis

The effect of fluoride on acid production by Streptococcus sanguis ATCC 10556 was compared under anaerobic and aerobic conditions. The rate of acid production under constant-pH and pH-free-fall conditions was determined during glucose metabolism by resting cells. Anaerobic glycolysis was inhibited more strongly by fluoride than was aerobic glycolysis. Intracellular levels of 3-phosphoglyceric, 2-phosphoglyceric, and phosphoenolpyruvic acids were lower under anaerobic conditions than under aerobic conditions. Thus, S. sanguis had a low phosphoenolpyruvate (PEP) potential under anaerobic conditions. This low PEP potential was suggested to account for the more effective fluoride inhibition of enolase and, consequently, the reduced transport of sugar by the PEP-dependent phosphotransferase system of this micro-organism.

Biochemical effects of fluoride on oral bacteria

Fluoride inhibition of carbohydrate metabolism by the acidogenic plaque microflora is well-established, although it has not always been appreciated that oral bacteria vary considerably in their susceptibility to fluoride. Early studies demonstrated that the F-induced reduction in acid production was due, in part, to the inhibition of the glycolytic enzyme, enolase, which converts 2-P-glycerate to P-enolpyruvate. The decreased output of PEP in the presence of F, in turn, results in the inhibition of sugar transport via the PEP phosphotransferase system (PTS). Bacterial accumulation of fluoride involves the transport of HF, a process requiring a transmembrane pH difference or pH gradient, which is generated only by metabolically active cells. The uptake of HF into the more alkaline cytoplasm results in the dissociation of HF to H+ and F- and, if allowed to continue, the accumulation of protons acidifies the cytoplasm, causing a reduction in both the proton gradient and enzyme activity. Current information indicates that in addition to enolase, F- also inhibits the membrane-bound, proton-pumping H+/ATPase, which is involved in the generation of proton gradients through the efflux of protons from the cell at the expense of ATP. Thus, fluoride has the dual action of dissipating proton gradients and preventing their generation through its action on H+/ATPase. The collapse of transmembrane proton gradient, in turn, reduces the ability of cells to transport solutes via mechanisms involving proton motive force. In spite of these known effects on the bacterial cell, there is no general agreement that the anti-microbial effects of F contribute to the anti-caries effect of fluoride.(ABSTRACT TRUNCATED AT 250 WORDS)

Lactic acid bacteria: model systems for in vivo studies of sugar transport and metabolism in gram-positive organisms

Lactic acid bacteria provide a model system for the in vivo study of mechanisms pertaining to the regulation of sugar transport and metabolism by microorganisms. Recent studies with resting and growing cells of the homofermentative Streptococci and Lactobacilli have yielded evidence for hitherto unsuspected regulatory mechanisms in this group of industrial and medically important bacteria. These regulatory mechanisms mediate the exclusion and expulsion of sugars, the preferential transport of sugar from sugar mixtures, resistance to non-metabolizable sugar analogs and participate in the establishment of energy-dissipating futile cycles. Transport experiments conducted with novel sugar analogs, data from enzymatic analyses and 31P-NMR spectroscopy studies with wild type and mutant strains of Streptococci, have provided new insight into the fine- and coarse-controls responsible for the modulation of activity of the sugar transport: glycolysis cycle. The purpose of this review is to summarize our current knowledge of these regulatory mechanisms and to suggest avenues for future investigation. Although specifically addressed to the lactic acid bacteria, it seems likely that some of the mechanisms described will be found in other Gram-positive species.