Our previous continuous culture studies with strains of Streptococcus mutans have indicated that the organism has the capacity of adapt to growth in acidic environments. This study was undertaken to examine this question in more detail. S. mutans Ingbritt and the phosphotransferase system (PTS)-defective mutant, S. mutans DR0001/6, were grown in continuous culture at pH 7.5 and 5.5 or 5.1, and the pH optimum for glucose uptake and glycolysis and the capacity of the cells to generate pH gradients were determined over the pH range 4.5 to 8.0 with steady state, washed cells. In addition, the proton permeability of the cells was measured over the pH range by an acid pulse technique. The results indicate that the pH optimum for glucose uptake by S. mutans Ingbritt grown at pH 7.5 was 7.5 and this optimum shifted to 7.0 and 6.0 for cells grown at pH 5.5 and 5.1, whereas with the S. mutans DR0001/6, the optimum shifted from 7.5 to 7.0 for the pH 5.5 cells. A similar shift in the pH optimum for glycolysis was observed for the 2 strains, and this was particularly pronounced for cells incubated with glucose in the presence of gramicidin to dissipate proton gradients. The capacity of the cells to generate pH gradients was related to their metabolic activity, and although larger gradients were not formed by the pH 5.5 cells, these cells were nevertheless capable of maintaining gradients at a lower pH; S. mutans DR0001/6 generated 2-fold larger pH gradients at pH 5.5 than S. mutans Ingbritt.(ABSTRACT TRUNCATED AT 250 WORDS)

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As a participant in the mucosal immune response, the intestinal epithelial cell must respond to a variety of stimuli, including lactic acid bacteria (LAB) consumed in the diet. The objective of this study was to compare the abilities of several strains of LAB to modulate cytokine secretion by human intestinal epithelial cell (IEC) line HT-29. Certain strains of Lactobacillus rhamnosus, Lactobacillus delbrueckii, and Lactobacillus acidophilus suppressed the production of the chemokine RANTES by stimulated HT-29 IEC, although the magnitude of this suppression varied depending on the nature of the bacterial growth medium. Similarly, specific strains showed growth condition-dependent suppression of HT-29 interleukin-8 (IL-8) production. Strain-dependent effects were also seen for the suppression of tumor necrosis factor alpha (TNF-alpha) and transforming growth factor beta (TGF-beta) production. The binding of several of these bacterial strains to the HT-29 cell line was also examined. Different strains were found to have differing abilities to interact with IEC, with L. rhamnosus R0011 being the strain that generally had the most extensive effects on HT-29 cytokine production and also bound to HT-29 IEC most effectively. Modulation of IEC cytokine production has the potential to profoundly affect the mucosal microenvironment, influencing the immune response to pathogens and other ingested antigens.

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