Intracellular polysaccharide production by human oral strains of Lactobacillus casei

Effect of reduced nutritional supply on the metabolic activity and survival of cariogenic bacteria in vitro

Sealed cariogenic bacteria are deprived from dietary carbohydrate, but could be provided with nutrients by pulpal fluids, with adaptive strain-specific activities being possible. We investigated survival and metabolic activity of the cariogenic bacteria Streptococcus sobrinus, Actinomyces naeslundii and Lactobacillus rhamnosus in different carbohydrate-limited media without carbon source (CLM), or containing glucose (CLM-G), albumin (CLM-A), or α1-acid glycoprotein (CLM-AGP) in vitro. Bacterial metabolite concentrations (lactate, pyruvate, oxaloacetate, citrate, acetate, formate, ethanol, acetoin) after 20 and 4 hours incubation, and bacterial numbers (CFU) after 24 hours incubation were analyzed using multivariate-analysis-of-variance (MANOVA). The medium (p = 0.02/MANOVA), strain and incubation-time (both p < 0.001) had significant impact on metabolite concentrations. Bacteria secreted mainly lactate (80.3 µg/10⁶ bacteria S. sobrinus) and acetate (54.5 µg/10⁶ bacteria A. naeslundii). Nearly all metabolites were produced in higher concentrations in S. sobrinus than in A. naeslundii or L. rhamnosus (p < 0.05/HSD). Metabolite concentration was significantly higher in CLM-G than in other media for most metabolites (p < 0.05). L. rhamnosus showed significantly lower survival than S. sobrinus and A. naeslundii (p < 0.05/HSD) regardless of the media, while S. sobrinus and A. naeslundii showed medium-specific survival. Survival of carbon starvation was strain- and medium-specific. Sustained organic acid production was found for all strains and media.

Glucose metabolism by Prevotella intermedia and Prevotella nigrescens

Glucose metabolism by Prevotella intermedia and Prevotella nigrescens were investigated. Glucose increased the anaerobic growth of these bacteria and promoted the accumulation of intracellular polysaccharide. The polysaccharide was confirmed to be glycogen-like glucan by the absorption spectrum of iodinepolysaccharide complex and the sugar composition. The washed cells consumed glucose anaerobically and converted a part of glucose into the metabolic end-products acetate, formate and succinate. The rest of glucose was confirmed to be accumulated as intracellular polysaccharide. The cells grown in the presence of glucose produced acetate, formate and succinate without exogenous glucose along with the consumption of intracellular polysaccharide. The metabolism of glucose and intracellular polysaccharide required bicarbonate. Prevotella cells had hexokinase and a set of the usual enzymes of the Embden-Meyerhof-Parnas pathway except that phosphofructokinase was pyrophosphate-dependent. A series of enzymes, including phosphoenolpyruvate carboxylase, phosphoenolpyruvate carboxykinase, malate dehydrogenase, fumarase and fumarate reductase, was found for succinate formation. Another series of enzymes, pyruvate oxidoreductase, pyruvate formate-lyase, phosphotransacetylase and acetate kinase was found for acetate and formate formation. Glucose 1,6-bisphosphate-dependent phosphoglucomutase and fructose 1,6-bisphosphate-activated UDP-glucose pyrophosphorylase were detected for glycogen synthesis, while glycogen phosphorylase was for glycogen degradation. The capacity of intracellular polysaccharide formation in addition to glucose fermentation could be advantageous for survival in the supragingival area as well as in the subgingival area.

Metabolism of intracellular polysaccharide in the cells of Streptococcus mutans under strictly anaerobic conditions

Streptococcus mutans, which had accumulated glycogen-like iodophilic intracellular polysaccharide (IPS), produced large amounts of formate, acetate and ethanol from the IPS by pyruvate formate-lyase (PFL) under strictly anaerobic conditions without exogenous sugar. Under aerobic conditions, the same S. mutans produced exclusively lactate and pyruvate from the IPS because of the inactivation of PFL by oxygen. The total amount of acid produced under anaerobic conditions was larger than that under aerobic conditions. The analysis of intracellular glycolytic intermediates revealed that levels of fructose 1,6-bisphosphate (lactate dehydrogenase (LDH) activator) and glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (PFL inhibitors) were low when IPS was used as a glycolytic substrate, implying that PFL functions more efficiently than LDH in IPS metabolism. These findings suggest that the PFL pathway contributes to the acid production from IPS, and may explain partially why the acids in starved dental plaque are mainly acetate and formate.