Oxygen sensitivity of sugar metabolism and interconversion of pyruvate formate-lyase in intact cells of Streptococcus mutans and Streptococcus sanguis

Thiamine-Starved Lactococcus lactis for Producing Food-Grade Pyruvate

Lactococcus lactis is a safe lactic acid bacterium widely used in dairy fermentations. Normally, its main fermentation product is lactic acid; however, L. lactis can be persuaded into producing other compounds, e.g., through genetic engineering. Here, we have explored the possibility of rewiring the metabolism of L. lactis into producing pyruvate without using genetic tools. Depriving the thiamine-auxotrophic and lactate dehydrogenase-deficient L. lactis strain RD1M5 of thiamine efficiently shut down two enzymes at the pyruvate branch, the thiamine pyrophosphate (TPP) dependent pyruvate dehydrogenase (PDHc) and α-acetolactate synthase (ALS). After eliminating the remaining enzyme acting on pyruvate, the highly oxygen-sensitive pyruvate formate lyase (PFL), by simple aeration, the outcome was pyruvate production. Pyruvate could be generated by nongrowing cells and cells growing in a substrate low in thiamine, e.g., Florisil-treated milk. Pyruvate is a precursor for the butter aroma compound diacetyl. Using an α-acetolactate decarboxylase deficient L. lactis strain, pyruvate could be converted to α-acetolactate and diacetyl. Summing up, by starving L. lactis for thiamine, secretion of pyruvate could be attained. The food-grade pyruvate produced has many applications, e.g., as an antioxidant or be used to make butter aroma.

Glucose Phosphotransferase System Modulates Pyruvate Metabolism, Bacterial Fitness, and Microbial Ecology in Oral Streptococci

Spontaneous mutants with defects in the primary glucose phosphotransferase permease (manLMNO) of Streptococcus sanguinis SK36 showed enhanced fitness at low pH. Transcriptomics and metabolomics with a manL deletion mutant (SK36/manL) revealed redirection of pyruvate to production of acetate and formate, rather than lactate. These observations were consistent with measurements of decreased lactic acid accumulation and increased excretion of acetate, formate, pyruvate, and H2O2. Genes showing increased expression in SK36/manL included those encoding carbohydrate transporters, extracellular glycosidases, intracellular polysaccharide metabolism, and arginine deiminase and pathways for metabolism of acetoin, ethanolamine, ascorbate, and formate, along with genes required for membrane biosynthesis and adhesion. Streptococcus mutans UA159 persisted much better in biofilm cocultures with SK36/manL than with SK36, an effect that was further enhanced by culturing the biofilms anaerobically but dampened by adding arginine to the medium. We posited that the enhanced persistence of S. mutans with SK36/manL was in part due to excess excretion of pyruvate by the latter, as addition of pyruvate to S. mutans-S. sanguinis cocultures increased the proportions of UA159 in the biofilms. Reducing the buffer capacity or increasing the concentration of glucose benefited UA159 when cocultured with SK36, but not with SK36/manL, likely due to the altered metabolism and enhanced acid tolerance of the mutant. When manL was deleted in S. mutans or Streptococcus gordonii, the mutants presented altered fitness characteristics. Our study demonstrated that phosphotransferase system (PTS)-dependent modulation of central metabolism can profoundly affect streptococcal fitness and metabolic interactions, revealing another dimension in commensal-pathogen relationships influencing dental caries development. IMPORTANCE Dental caries is underpinned by a dysbiotic microbiome and increased acid production. As beneficial bacteria that can antagonize oral pathobionts, oral streptococci such as S. sanguinis and S. gordonii can ferment many carbohydrates, despite their relative sensitivity to low pH. We characterized the molecular basis for why mutants of glucose transporter ManLMNO of S. sanguinis showed enhanced production of hydrogen peroxide and ammonia and improved persistence under acidic conditions. A metabolic shift involving more than 300 genes required for carbohydrate transport, energy production, and envelope biogenesis was observed. Significantly, manL mutants engineered in three different oral streptococci displayed altered capacities for acid production and interspecies antagonism, highlighting the potential for targeting the glucose-PTS to modulate the pathogenicity of oral biofilms.

Green Tea-Derived Epigallocatechin Gallate Inhibits Acid Production and Promotes the Aggregation of Streptococcus mutans and Non-Mutans Streptococci

It has been suggested that green tea-derived epigallocatechin gallate (EGCG), which has antimicrobial properties, might help prevent dental caries. However, the detailed properties of EGCG remain unclear. In this study, the antimicrobial properties of EGCG were evaluated by examining its bactericidal activity, its inhibitory effects against bacterial growth, acid production, acidic end-product formation, and sugar uptake (phosphoenolpyruvate-dependent phosphotransferase system, PEP-PTS activity), and its effects on bacterial aggregation, using monocultured planktonic cells of Streptococcus mutans and non-mutans streptococci. Coincubating S. mutans with EGCG (1 mg/mL) for 4 h had no bactericidal effects, while it decreased the growth and acid production of S. mutans by inhibiting the activity of the PEP-PTS. EGCG (2 mg/mL) caused rapid bacterial cell aggregation and had reduced the optical density of S. mutans cell suspension by 86.7% at pH 7.0 and 90.7% at pH 5.5 after 2 h. EGCG also reduced the acid production of non-mutans streptococci, including S. sanguinis, S. gordonii, and S. salivarius, and promoted the aggregation of these non-mutans streptococci. Furthermore, these antimicrobial effects of short-term EGCG treatment persisted in the presence of saliva. These results suggest that EGCG might have short-term antibacterial effects on caries-associated streptococci in the oral cavity.

Metatranscriptomic analysis of modified atmosphere packaged poultry meat enables prediction of Brochothrix thermosphacta and Carnobacterium divergens in situ metabolism

In this study, in situ-expressed metabolic routes of Brochothrix (B.) thermosphacta and Carnobacterium (C.) divergens were evaluated based on a metatranscriptomic dataset from bacteria growing on MAP chicken meat (O₂/CO₂; N₂/CO₂). Both species exhibited no (C. divergens) or minor transcription regulation (B. thermosphacta) within their main metabolic routes in response to different atmospheres. Both employ pathways related to glucose and ribose. Gluconeogenesis from lipid-borne glycerol is active in the progressing lack of carbohydrates. Pyruvate fates in both species comprise lactate, ethanol, acetate, CO₂, formate, C4-compounds and H₂O₂ (only B. thermosphacta). Both species express genes for a minimal aerobic respiratory chain, but do not possess the genetic setting for a functional citric acid cycle. While products of carbohydrate and glycerol metabolism display mild to medium sensorial off-characteristics, predicted end products of their amino acid metabolism comprise, e.g., isobutyrate and isovalerate (B. thermosphacta) or cadaverine and tyramine (C. divergens) as potent spoilage compounds.

Sugar Metabolism of Scardovia wiggsiae, a Novel Caries-Associated Bacterium

Scardovia wiggsiae has been detected from caries in children and adolescents and has been suggested to be a caries-associated microorganism. To investigate the cariogenic potential of S. wiggsiae, we examined carbohydrate metabolism and acid productivity, the fluoride sensitivity of carbohydrate metabolism and the mechanism by which fluoride inhibits carbohydrate metabolism, and the acid sensitivity of carbohydrate metabolism in this bacterium. S. wiggsiae metabolized glucose and reduced the environmental pH to 3.5. It mainly produced acetic acid from glucose, together with small amounts of lactic and formic acid. The 50% inhibitory concentration of fluoride for acid production was 8.0 mM at pH 7.0 and 1.5 mM at pH 5.5, which were much higher than those of representative caries-associated bacteria, such as Streptococcus mutans. Metabolomic profiles showed the accumulation of 3-phosphoglycerate and a marked reduction in the pyruvate concentration in the presence of fluoride, suggesting that fluoride inhibits the latter half of glycolysis, including enolase activity. Enolase activity was inhibited by fluoride in S. wiggsiae, but it was more fluoride-tolerant than the enolase activity of S. mutans. Unlike in S. mutans, lactic acid did not inhibit acid production by S. wiggsiae at acidic pH. These results indicate that S. wiggsiae exhibits high acid production and tolerance to fluoride and lactic acid. S. wiggsiae possesses a unique metabolic pathway, the F6PPK shunt, which might allow it to avoid the lactate-formate pathway, including fluoride-sensitive enolase activity, and enable metabolic flow to the fluoride-tolerant acetate pathway. The fluoride tolerance of S. wiggsiae's enolase activity also increases the fluoride tolerance of its carbohydrate metabolism. The lactic acid tolerance of S. wiggsiae's acid production might result in S. wiggsiae having high acidogenic and aciduric potential and make it ecologically competitive in acidic environments, such as caries lesions, where lactic acid predominates.

Characterization of LrgAB as a stationary phase-specific pyruvate uptake system in Streptococcus mutans

BACKGROUND

Our recent '-omics' comparisons of Streptococcus mutans wild-type and lrgAB-mutant revealed that this organism undergoes dynamic cellular changes in the face of multiple exogenous stresses, consequently affecting its comprehensive virulence traits. In this current study, we further demonstrate that LrgAB functions as a S. mutans pyruvate uptake system.

RESULTS

S. mutans excretes pyruvate during growth as an overflow metabolite, and appears to uptake this excreted pyruvate via LrgAB once the primary carbon source is exhausted. This utilization of excreted pyruvate was tightly regulated by glucose levels and stationary growth phase lrgAB induction. The degree of lrgAB induction was reduced by high extracellular levels of pyruvate, suggesting that lrgAB induction is subject to negative feedback regulation, likely through the LytST TCS, which is required for expression of lrgAB. Stationary phase lrgAB induction was efficiently inhibited by low concentrations of 3FP, a toxic pyruvate analogue, without affecting cell growth, suggesting that accumulated pyruvate is sensed either directly or indirectly by LytS, subsequently triggering lrgAB expression. S. mutans growth was inhibited by high concentrations of 3FP, implying that pyruvate uptake is necessary for S. mutans exponential phase growth and occurs in a Lrg-independent manner. Finally, we found that stationary phase lrgAB induction is modulated by hydrogen peroxide (H₂O₂) and by co-cultivation with H₂O₂-producing S. gordonii.

CONCLUSIONS

Pyruvate may provide S. mutans with an alternative carbon source under limited growth conditions, as well as serving as a buffer against exogenous oxidative stress_. Given the hypothesized role of LrgAB in cell death and lysis, these data also provide an important basis for how these processes are functionally and mechanically connected to key metabolic pathways such as pyruvate metabolism.

Metabolic property of acetaldehyde production from ethanol and glucose by oral Streptococcus and Neisseria

Acetaldehyde is known to be carcinogenic and produced by oral bacteria. Thus, bacterial acetaldehyde production might contribute to oral cancer. Therefore, we examined bacterial acetaldehyde production from ethanol and glucose under various conditions mimicking the oral cavity and clarified the metabolic pathways responsible for bacterial acetaldehyde production. Streptococcus mitis, S. salivarius, S. mutans, Neisseria mucosa and N. sicca were used. The bacterial metabolism was conducted at pH 5.0–8.0 under aerobic and anaerobic conditions. The production of acetaldehyde and organic acids was measured with gas chromatography and HPLC, respectively. Bacterial enzymes were also assessed. All of the bacteria except for S. mutans exhibited their greatest acetaldehyde production from ethanol at neutral to alkaline pH under aerobic conditions. S. mutans demonstrated the greatest acetaldehyde from glucose under anaerobic conditions, although the level was much lower than that from ethanol. Alcohol dehydrogenase and NADH oxidase were detected in all of the bacteria. This study revealed that oral indigenous bacteria, Streptococcus and Neisseria can produce acetaldehyde, and that such acetaldehyde production is affected by environmental conditions. It was suggested that alcohol dehydrogenase and NADH oxidase are involved in ethanol-derived acetaldehyde production and that the branched-pathway from pyruvate is involved in glucose-derived acetaldehyde production.

Acidogenic Potential of Oral Bifidobacterium and Its High Fluoride Tolerance

Bifidobacterium is frequently detected in early childhood caries and white spot lesions, indicating that it is a novel caries-associated bacterium. Bifidobacterium is known to possess a unique metabolic pathway, the “bifid shunt,” which might give it cariogenic potential by increasing its acid production. Thus, we evaluated the acid-producing activity of Bifidobacterium and its sensitivity to fluoride, a caries preventive reagent. Bifidobacterium longum, Bifidobacterium dentium, and Streptococcus mutans were used. Acid-producing activity was measured using a pH-stat in the absence and presence of fluoride under anaerobic conditions. Furthermore, metabolomic analysis was performed to elucidate the mechanism underlying the inhibitory effects of fluoride. The acid production of Bifidobacterium at pH 5.5 was as high as that seen at pH 7.0, indicating that Bifidobacterium has high cariogenic potential, although it produced less acid than S. mutans. In addition, Bifidobacterium produced acid in the absence of extracellular carbohydrates, suggesting that it can store intracellular polysaccharides. Bifidobacterium produced more acid from lactose than from glucose. Bifidobacterium mainly produced acetate, whereas S. mutans mainly produced lactate. The 50% inhibitory concentration (IC₅₀) of fluoride for acid production was 6.0–14.2 times higher in Bifidobacterium than in S. mutans. Fluoride inhibited enolase in the glycolysis, resulting in the intracellular accumulation of 3-phosphoenolpyruvate, glucose 6-phosphate, and erythrose 4-phosphate. However, the bifid shunt provides a bypass pathway that can be used to produce acetate, suggesting that Bifidobacterium is able to metabolize carbohydrates in the presence of fluoride. It is suggested that its exclusive acetate production contributes to the pathogenesis of dental caries.

Plasticity of the Pyruvate Node Modulates Hydrogen Peroxide Production and Acid Tolerance in Multiple Oral Streptococci

Commensal Streptococcus sanguinis and Streptococcus gordonii are pioneer oral biofilm colonizers. Characteristic for both is the SpxB-dependent production of H₂O₂, which is crucial for inhibiting competing biofilm members, especially the cariogenic species Streptococcus mutans H₂O₂ production is strongly affected by environmental conditions, but few mechanisms are known. Dental plaque pH is one of the key parameters dictating dental plaque ecology and ultimately oral health status. Therefore, the objective of the current study was to characterize the effects of environmental pH on H₂O₂ production by S. sanguinis and S. gordoniiS. sanguinis H₂O₂ production was not found to be affected by moderate changes in environmental pH, whereas S. gordonii H₂O₂ production declined markedly in response to lower pH. Further investigation into the pyruvate node, the central metabolic switch modulating H₂O₂ or lactic acid production, revealed increased lactic acid levels for S. gordonii at pH 6. The bias for lactic acid production at pH 6 resulted in concomitant improvement in the survival of S. gordonii at low pH and seems to constitute part of the acid tolerance response of S. gordonii Differential responses to pH similarly affect other oral streptococcal species, suggesting that the observed results are part of a larger phenomenon linking environmental pH, central metabolism, and the capacity to produce antagonistic amounts of H₂O₂IMPORTANCE Oral biofilms are subject to frequent and dramatic changes in pH. S. sanguinis and S. gordonii can compete with caries- and periodontitis-associated pathogens by generating H₂O₂ Therefore, it is crucial to understand how S. sanguinis and S. gordonii adapt to low pH and maintain their competitiveness under acid stress. The present study provides evidence that certain oral bacteria respond to environmental pH changes by tuning their metabolic output in favor of lactic acid production, to increase their acid survival, while others maintain their H₂O₂ production at a constant level. The differential control of H₂O₂ production provides important insights into the role of environmental conditions for growth competition of the oral flora.

Homeostasis of metabolites in Escherichia coli on transition from anaerobic to aerobic conditions and the transient secretion of pyruvate

We have developed a method for rapid quenching of samples taken from chemostat cultures of Escherichia coli that gives reproducible and reliable measurements of extracellular and intracellular metabolites by ¹H NMR and have applied it to study the major central metabolites during the transition from anaerobic to aerobic growth. Almost all metabolites showed a gradual change after perturbation with air, consistent with immediate inhibition of pyruvate formate-lyase, dilution of overflow metabolites and induction of aerobic enzymes. Surprisingly, although pyruvate showed almost no change in intracellular concentration, the extracellular concentration transiently increased. The absence of intracellular accumulation of pyruvate suggested that one or more glycolytic enzymes might relocate to the cell membrane. To test this hypothesis, chromosomal pyruvate kinase (pykF) was modified to express either PykF-green fluorescent protein or PykF-FLAG fusion proteins. Measurements showed that PykF-FLAG relocates to the cell membrane within 5 min of aeration and then slowly returns to the cytoplasm, suggesting that on aeration, PykF associates with the membrane to facilitate secretion of pyruvate to maintain constant intracellular levels.

Oxygen Affects Gut Bacterial Colonization and Metabolic Activities in a Gnotobiotic Cockroach Model

The gut microbiota of termites and cockroaches represents complex metabolic networks of many diverse microbial populations. The distinct microenvironmental conditions within the gut and possible interactions among the microorganisms make it essential to investigate how far the metabolic properties of pure cultures reflect their activities in their natural environment. We established the cockroach Shelfordella lateralis as a gnotobiotic model and inoculated germfree nymphs with two bacterial strains isolated from the guts of conventional cockroaches. Fluorescence microscopy revealed that both strains specifically colonized the germfree hindgut. In diassociated cockroaches, the facultatively anaerobic strain EbSL (a new species of Enterobacteriaceae) always outnumbered the obligately anaerobic strain FuSL (a close relative of Fusobacterium varium), irrespective of the sequence of inoculation, which showed that precolonization by facultatively anaerobic bacteria does not necessarily favor colonization by obligate anaerobes. Comparison of the fermentation products of the cultures formed in vitro with those accumulated in situ indicated that the gut environment strongly affected the metabolic activities of both strains. The pure cultures formed the typical products of mixed-acid or butyrate fermentation, whereas the guts of gnotobiotic cockroaches accumulated mostly lactate and acetate. Similar shifts toward more-oxidized products were observed when the pure cultures were exposed to oxygen, which corroborated the strong effects of oxygen on the metabolic fluxes previously observed in termite guts. Oxygen microsensor profiles of the guts of germfree, gnotobiotic, and conventional cockroaches indicated that both gut tissue and microbiota contribute to oxygen consumption and suggest that the oxygen status influences the colonization success.

Oral Microbiome Metabolism: From "Who Are They?" to "What Are They Doing?"

Recent advances in molecular biology have facilitated analyses of the oral microbiome ("Who are they?"); however, its functions (e.g., metabolic activities) are poorly understood ("What are they doing?"). This review aims to summarize our current understanding of the metabolism of the oral microbiome. Saccharolytic bacteria-including Streptococcus, Actinomyces, and Lactobacillus species-degrade carbohydrates into organic acids via the Embden-Meyerhof-Parnas pathway and several of its branch pathways, resulting in dental caries, while alkalization and acid neutralization via the arginine deiminase system, urease, and so on, counteract acidification. Proteolytic/amino acid-degrading bacteria, including Prevotella and Porphyromonas species, break down proteins and peptides into amino acids and degrade them further via specific pathways to produce short-chain fatty acids, ammonia, sulfur compounds, and indole/skatole, which act as virulent and modifying factors in periodontitis and oral malodor. Furthermore, it is suggested that ethanol-derived acetaldehyde can cause oral cancer, while nitrate-derived nitrite can aid caries prevention and systemic health. Microbial metabolic activity is influenced by the oral environment; however, it can also modify the oral environment, enhance the pathogenicity of bacteria, and induce microbial selection to create more pathogenic microbiome. Taking a metabolomic approach to analyzing the oral microbiome is crucial to improving our understanding of the functions of the oral microbiome.

Facultative Anaerobe Caldibacillus debilis GB1: Characterization and Use in a Designed Aerotolerant, Cellulose-Degrading Coculture with Clostridium thermocellum

Development of a designed coculture that can achieve aerotolerant ethanogenic biofuel production from cellulose can reduce the costs of maintaining anaerobic conditions during industrial consolidated bioprocessing (CBP). To this end, a strain of Caldibacillus debilis isolated from an air-tolerant cellulolytic consortium which included a Clostridium thermocellum strain was characterized and compared with the C. debilis type strain. Characterization of isolate C. debilis GB1 and comparisons with the type strain of C. debilis revealed significant physiological differences, including (i) the absence of anaerobic metabolism in the type strain and (ii) different end product synthesis profiles under the experimental conditions used. The designed cocultures displayed unique responses to oxidative conditions, including an increase in lactate production. We show here that when the two species were cultured together, the noncellulolytic facultative anaerobe C. debilis GB1 provided respiratory protection for C. thermocellum, allowing the synergistic utilization of cellulose even under an aerobic atmosphere.

The ADP-glucose pyrophosphorylase from Streptococcus mutans provides evidence for the regulation of polysaccharide biosynthesis in Firmicutes

Streptococcus mutans is the leading cause of dental caries worldwide. The bacterium accumulates a glycogen-like internal polysaccharide, which mainly contributes to its carionegic capacity. S.mutans has two genes (glgC and glgD) respectively encoding putative ADP-glucose pyrophosphorylases (ADP-Glc PPase), a key enzyme for glycogen synthesis in most bacteria. Herein, we report the molecular cloning and recombinant expression of both genes (separately or together) followed by the characterization of the respective enzymes. When expressed individually GlgC had ADP-Glc PPase activity, whereas GlgD was inactive. Interestingly, the coexpressed GlgC/GlgD protein was one order of magnitude more active than GlgC alone. Kinetic characterization of GlgC and GlgC/GlgD pointed out remarkable differences between them. Fructose-1,6-bis-phosphate activated GlgC by twofold, but had no effect on GlgC/GlgD. Conversely, phospho-enol-pyruvate and inorganic salts inhibited GlgC/GlgD without affecting GlgC. However, in the presence of fructose-1,6-bis-phosphate GlgC acquired a GlgC/GlgD-like behaviour, becoming sensitive to the stated inhibitors. Results indicate that S. mutans ADP-Glc PPase is an allosteric regulatory enzyme exhibiting sensitivity to modulation by key intermediates of carbohydrates metabolism in the cell. The particular regulatory properties of the S.mutans enzyme agree with phylogenetic analysis, where GlgC and GlgD proteins found in other Firmicutes arrange in distinctive clusters.

Inhibition kinetics of catabolic dehydrogenases by elevated moieties of ATP and ADP--implication for a new regulation mechanism in Lactococcus lactis

ATP and ADP inhibit, in varying degrees, several dehydrogenases of the central carbon metabolism of Lactococcus lactis ATCC 19435 in vitro, i.e. glyceraldehyde-3-phosphate dehydrogenase (GAPDH), lactate dehydrogenase (LDH) and alcohol dehydrogenase (ADH). Here we demonstrate mixed inhibition for GAPDH and competitive inhibition for LDH and ADH by adenine nucleotides in single inhibition studies. The nonlinear negative co-operativity was best modelled with Hill-type kinetics, showing greater flexibility than the usual parabolic inhibition equation. Because these natural inhibitors are present simultaneously in the cytoplasm, multiple inhibition kinetics was determined for each dehydrogenase. For ADH and LDH, the inhibitor combinations ATP plus NAD and ADP plus NAD are indifferent to each other. Model discrimination suggested that the weak allosteric inhibition of GAPDH had no relevance when multiple inhibitors are present. Interestingly, with ADH and GAPDH the combination of ATP and ADP exhibits lower dissociation constants than with either inhibitor alone. Moreover, the concerted inhibition of ADH and GAPDH, but not of LDH, shows synergy between the two nucleotides. Similar kinetics, but without synergies, were found for horse liver and yeast ADHs, indicating that dehydrogenases can be modulated by these nucleotides in a nonlinear manner in many organisms. The action of an elevated pool of ATP and ADP may effectively inactivate lactococcal ADH, but not GAPDH and LDH, providing leverage for the observed metabolic shift to homolactic acid formation in lactococcal resting cells on maltose. Therefore, we interpret these results as a regulation mechanism contributing to readjusting the flux of ATP production in L. lactis.

Proteins involved in difference of sorbitol fermentation rates of the toxigenic and nontoxigenic Vibrio cholerae El Tor strains revealed by comparative proteome analysis

BACKGROUND

The nontoxigenic V. cholerae El Tor strains ferment sorbitol faster than the toxigenic strains, hence fast-fermenting and slow-fermenting strains are defined by sorbitol fermentation test. This test has been used for more than 40 years in cholera surveillance and strain analysis in China. Understanding of the mechanisms of sorbitol metabolism of the toxigenic and nontoxigenic strains may help to explore the genome and metabolism divergence in these strains. Here we used comparative proteomic analysis to find the proteins which may be involved in such metabolic difference.

RESULTS

We found the production of formate and lactic acid in the sorbitol fermentation medium of the nontoxigenic strain was earlier than of the toxigenic strain. We compared the protein expression profiles of the toxigenic strain N16961 and nontoxigenic strain JS32 cultured in sorbitol fermentation medium, by using fructose fermentation medium as the control. Seventy-three differential protein spots were found and further identified by MALDI-MS. The difference of product of fructose-specific IIA/FPR component gene and mannitol-1-P dehydrogenase, may be involved in the difference of sorbitol transportation and dehydrogenation in the sorbitol fast- and slow-fermenting strains. The difference of the relative transcription levels of pyruvate formate-lyase to pyruvate dehydrogenase between the toxigenic and nontoxigenic strains may be also responsible for the time and ability difference of formate production between these strains.

CONCLUSION

Multiple factors involved in different metabolism steps may affect the sorbitol fermentation in the toxigenic and nontoxigenic strains of V. cholerae El Tor.

The involvement of the pyruvate dehydrogenase E1alpha subunit, in Streptococcus mutans acid tolerance

Streptococcus mutans, an etiological agent of dental caries, is a normal inhabitant of dental plaque. Two main virulence factors of S. mutans are acidogenicity and aciduricity - the ability to produce acid and survive at low pH, respectively. Metabolic processes, including the catabolism of pyruvate, are finely regulated following acid exposure in S. mutans. Proteome analysis of the S. mutans acid response has shown pyruvate dehydrogenase A (PdhA) is upregulated. PdhA is the E1alphasubunit of the four-enzyme pyruvate dehydrogenase complex, responsible for the heterofermentative catalysis of pyruvate into acetyl-CoA. Acetyl-CoA is subsequently catalyzed into ethanol and acetate yielding additional ATP. This investigation examined the relationships between PdhA, aciduricity, and metabolism in S. mutans. An S. mutans pdhA knockout (PDHAKO) revealed an acid sensitive phenotype, displayed by increased doubling times, and decreased competitiveness in a biofermentor. Quantitative real-time PCR showed pdhA expression increased dramatically during acidic growth and under acid adaptation. Additionally, pdhA expression responded to conditions favoring heterofermentative growth; decreased in the presence of excess glucose, and increased during stationary phase compared with mid-log phase growth. This study demonstrated that in S. mutans, pdhA expression responds to conditions conducive to heterofermentation and deletion of pdhA resulted in decreased aciduricity.

Effects of oxygen on biofilm formation and the AtlA autolysin of Streptococcus mutans

The Streptococcus mutans atlA gene encodes an autolysin required for biofilm maturation and biogenesis of a normal cell surface. We found that the capacity to form biofilms by S. mutans, one of the principal causative agents of dental caries, was dramatically impaired by growth of the organism in an aerated environment and that cells exposed to oxygen displayed marked changes in surface protein profiles. Inactivation of the atlA gene alleviated repression of biofilm formation in the presence of oxygen. Also, the formation of long chains, a characteristic of AtlA-deficient strains, was less evident in cells grown with aeration. The SMu0629 gene is immediately upstream of atlA and encodes a product that contains a C-X-X-C motif, a characteristic of thiol-disulfide oxidoreductases. Inactivation of SMu0629 significantly reduced the levels of AtlA protein and led to resistance to autolysis. The SMu0629 mutant also displayed an enhanced capacity to form biofilms in the presence of oxygen compared to that of the parental strain. The expression of SMu0629 was shown to be under the control of the VicRK two-component system, which influences oxidative stress tolerance in S. mutans. Disruption of vicK also led to inhibition of processing of AtlA, and the mutant was hyperresistant to autolysis. When grown under aerobic conditions, the vicK mutant also showed significantly increased biofilm formation compared to strain UA159. This study illustrates the central role of AtlA and VicK in orchestrating growth on surfaces and envelope biogenesis in response to redox conditions.

Difference in the xylitol sensitivity of acid production among Streptococcus mutans strains and the biochemical mechanism

Xylitol inhibits the glycolysis and growth of Streptococcus mutans, but to different degrees among strains. Thus, we studied the biochemical mechanism through which the inhibition varies, using S. mutans strains ATCC 31989, NCTN 10449, and NCIB 11723, which are highly sensitive, moderately sensitive, and resistant to xylitol, respectively, under strictly anaerobic conditions such as those found in deep layers of dental plaque. Xylitol (30 mM) decreased the rate of acid production from glucose (10 mM) in ATCC 31989, NCTC 10449, and NCIB 11723 by 86, 26, and 0%, respectively. The activities of the xylitol : phosphoenolpyruvate phosphotransferase system (PEP-PTS) relative to those of glucose : PEP-PTS were 120, 16, and 3%, respectively. In ATCC 31989 and NCTC 10449, intracellular accumulation of xylitol 5-phosphate and decreases of fructose 1,6-bisphosphate and glucose 6-phosphate were observed. Furthermore, in the presence of xylitol (30 mM), glucose : PEP-PTS activities decreased by 34, 17, and 0%, respectively. These findings indicated that the higher the xylitol : PEP-PTS activity was and the more effectively xylitol decreased glucose : PEP-PTS activity, the more sensitive the strain was to xylitol. These results suggest that the following inhibitory mechanisms are active in the xylitol-sensitive mutans streptococci: direct inhibition of glycolytic enzymes by xylitol 5-phosphate derived from xylitol : PEP-PTS and, possibly, indirect inhibition through competition for the phosphoryl donor, HPr-P, between glucose and xylitol : PEP-PTSs.

Synergistic inhibition by combination of fluoride and xylitol on glycolysis by mutans streptococci and its biochemical mechanism

The purpose of this study was to evaluate the combined inhibitory effect of fluoride and xylitol on acid production by mutans streptococci, Streptococcus mutans NCTC10449 and Streptococcus sobrinus 6715, from glucose under strictly anaerobic conditions at fixed pH 5.5 and 7.0. The bacteria were grown in a tryptone-yeast extract broth under strictly anaerobic conditions (N2: 80%; H2: 10%; CO2: 10%). Reaction mixtures for acid production from glucose contained bacterial cells with fluoride (0-6.4 mM) and/or xylitol (60 mM). Acidic end products of glucose fermentation and intracellular glycolytic intermediates were assayed. The combination of fluoride and xylitol inhibited acid production more effectively than fluoride or xylitol alone. In the presence of fluoride and xylitol, the proportion of lactic acid in the total amount of acidic end products decreased, while the proportion of formic and acetic acids increased. Analyses of intracellular glycolytic intermediates revealed that xylitol inhibited the upper part of the glycolytic pathway, while fluoride inhibited the lower part. This study indicates that fluoride and xylitol together have synergistic inhibitory effects on the acid production of mutans streptococci and suggests that xylitol has the potential to enhance inhibitory effects of low concentrations of fluoride.

Physiological characterization of a heme-deficient mutant of Staphylococcus aureus by a proteomic approach

The high-resolution two-dimensional (2D) protein gel electrophoresis technique combined with matrix-assisted laser desorption ionization-time of flight mass spectrometry was used for identification of proteins whose levels were changed by a mutation in hemB. Cytoplasmic protein extracts obtained from the mutant and the wild type (strain COL) at different stages of growth in tryptone soya broth (exponential, transitional, and stationary growth phases) were separated on 2D protein gels. Comparison of the 2D patterns of the protein extracts of the two strains revealed major differences. Because the electron transport chain of the mutant is interrupted due to the deficiency of heme, this organism should be unable to use oxygen or nitrate as a terminal electron acceptor. Consistent with this hypothesis, proteins involved in the glycolytic pathway and related pathways (glyceraldehyde-3-phosphate dehydrogenase, enolase, and phosphoglycerate kinase) and in fermentation pathways (lactate dehydrogenase, alcohol dehydrogenase, and pyruvate formate lyase) were induced in exponentially growing cells of the mutant. These results strongly indicate that the hemB mutant generates ATP from glucose or fructose only by substrate phosphorylation. Analyses of the fermentation reactions showed that the main product was lactate. Although pyruvate formate lyase (Pfl) and pyruvate dehydrogenase were present, neither ethanol nor acetate was detected in significant amounts. Presumably, Pfl was not activated in the presence of oxygen, and pyruvate dehydrogenase might have very low activity. Transcriptional analysis of citB, encoding the aconitase, revealed that the activity of the citrate cycle enzymes was down-regulated in the hemB mutant. The arginine deiminase pathway was also induced, and it could provide ATP as well. Furthermore, the amounts of most of the extracellular virulence factors were significantly reduced by a mutation in hemB, which is consistent with previous reports.

Biochemical and functional properties of a pyruvate formate-lyase (PFL)-activating system in Streptococcus mutans

Streptococcus mutans has an oxygen-sensitive enzyme, pyruvate formate-lyase (PFL), which is a key enzyme in anaerobic sugar fermentation. We have shown that S. mutans has an activating system, including a PFL-activating enzyme (PFL-activase) and an electron transport system; the latter transfers an electron from NADPH to PFL-activase, as occurs in Escherichia coli. NADPH was a physiological electron donor for the electron transport system and as little as 0.02 mM NADPH activated over 80% of PFL of S. mutans. The optimum pH of the PFL-activating system was around 6.8, whereas the optimum of the E. coli system is at alkaline pH. In addition, small dialyzable molecules in cell-free extracts participated in keeping PFL active in S. mutans. These results suggest that, in dental plaque under anaerobic conditions where sugar supply is often limited or pH frequently falls below neutrality, S. mutans always keeps PFL active through the activating system.

Xylitol inhibition of anaerobic acid production by Streptococcus mutans at various pH levels

Xylitol inhibits the glycolysis and growth of Streptococcus mutans. We studied the inhibitory effect of xylitol on the acid production of S. mutans at several pH levels under the strictly anaerobic conditions found in the deep layer of dental plaque. Xylitol inhibited the rate of acid production from glucose and changed the profile of acidic end products to formate-acetate dominance, with a decrease in the intracellular level of fructose 1,6-bisphosphate and an intracellular accumulation of xylitol 5-phosphate (X5P). These results were notable at pH 5.5-7.0, but were not evident at pH 5.0. Since the activity of phosphoenolpyruvate phosphotransferase for xylitol was greater at higher pH, it is suggested that xylitol could be incorporated more efficiently at higher pH and that the resultant accumulation of X5P could inhibit the glycolysis of S. mutans more effectively.

Acid-neutralizing activity during amino acid fermentation by Porphyromonas gingivalis, Prevotella intermedia and Fusobacterium nucleatum

Acid-neutralizing activity during amino acid fermentation by washed cells of Porphyromonas gingivalis, Prevotella intermedia and Fusobacterium nucleatum was studied. When the washed cells of these strains were anaerobically incubated in the presence of aspartylaspartic acid or glutamylglutamic acid for P. gingivalis, aspartic acid for P. intermedia and glutamic acid for F. nucleatum at an initial pH of 5.0 or 5.5, the pH of the incubation mixtures rose toward neutral. F. nucleatum had the highest acid-neutralizing activity, followed by P. intermedia and P. gingivalis. The P. intermedia and F. nucleatum cells were used to measure the amounts of base produced at a fixed pH of 5.0. These cells generated significant amounts of base at pH 5.0 along with the production of organic acids and ammonia from aspartic or glutamic acid. Acid-base balance theoretically calculated from the amounts of consumed substrate and end products implies that the acid-neutralizing activity was derived from the decrease in acidity during the fermentation of amino acid into organic acids and ammonia.

Novel two-component regulatory system involved in biofilm formation and acid resistance in Streptococcus mutans

The abilities of Streptococcus mutans to form biofilms and to survive acidic pH are regarded as two important virulence determinants in the pathogenesis of dental caries. Environmental stimuli are thought to regulate the expression of several genes associated with virulence factors through the activity of two-component signal transduction systems. Yet, little is known of the involvement of these systems in the physiology and pathogenicity of S. mutans. In this study, we describe a two-component regulatory system and its involvement in biofilm formation and acid resistance in S. mutans. By searching the S. mutans genome database with tblastn with the HK03 and RR03 protein sequences from S. pneumoniae as queries, we identified two genes, designated hk11 and rr11, that encode a putative histidine kinase and its cognate response regulator. To gain insight into their function, a PCR-mediated allelic-exchange mutagenesis strategy was used to create the hk11 (Em(r)) and rr11 (Em(r)) deletion mutants from S. mutans wild-type NG8 named SMHK11 and SMRR11, respectively. The mutants were examined for their growth rates, genetic competence, ability to form biofilms, and resistance to low-pH challenge. The results showed that deletion of hk11 or rr11 resulted in defects in biofilm formation and resistance to acidic pH. Both mutants formed biofilms with reduced biomass (50 to 70% of the density of the parent strain). Scanning electron microscopy revealed that the biofilms formed by the mutants had sponge-like architecture with what appeared to be large gaps that resembled water channel-like structures. The mutant biofilms were composed of longer chains of cells than those of the parent biofilm. Deletion of hk11 also resulted in greatly diminished resistance to low pH, although we did not observe the same effect when rr11 was deleted. Genetic competence was not affected in either mutant. The results suggested that the gene product of hk11 in S. mutans might act as a pH sensor that could cross talk with one or more response regulators. We conclude that the two-component signal transduction system encoded by hk11 and rr11 represents a new regulatory system involved in biofilm formation and acid resistance in S. mutans.

Dipeptide utilization by the periodontal pathogens Porphyromonas gingivalis, Prevotella intermedia, Prevotella nigrescens and Fusobacterium nucleatum

Porphyromonas gingivalis, Prevotella intermedia, Prevotella nigrescens and Fusobacterium nucleatum, which can frequently be isolated from periodontal pockets, preferentially utilize proteins and peptides as growth substrates. In this study, we determined the size of peptide that is preferentially utilized as a source of energy and material for cell growth by P. gingivalis, P. intermedia, P. nigrescens and F. nucleatum using various sizes of poly amino acids consisting of two to approximately 100 molecules of aspartate or glutamate. Resting cells of P. gingivalis, P. intermedia and P. nigrescens utilized aspartylaspartate, while cells of P. gingivalis and F. nucleatum utilized glutamylglutamate. The addition of aspartylaspartate to the culture medium increased the growth of P. gingivalis, P. intermedia and P. nigrescens, while the addition of glutamylglutamate promoted the growth of P. gingivalis and F. nucleatum. These results clearly indicate that dipeptides such as aspartylaspartate and glutamylglutamate can be utilized as growth substrates for P. gingivalis, P. intermedia, P. nigrescens and F. nucleatum.

Preferential utilization of dipeptides by Porphyromonas gingivalis

Although Porphyromonas gingivalis is known to utilize peptides preferentially, instead of free amino acids, as the source of energy and cell material, there is only limited information on what sizes and kinds of peptide this bacterium preferentially utilizes. In this study, therefore, we tested aspartate or glutamate monopolymers consisting of from 2 to 100 amino acids as metabolic substrates for P. gingivalis. The washed cells of P. gingivalis consumed aspartylaspartate and glutamylglutamate, and produced large amounts of ammonia and organic acids such as propionate and butyrate, while the cells formed only small amounts of end-products from aspartate, glutamate, and other peptides longer than a dipeptide. P. gingivalis also metabolized valylvaline and leucylleucine and produced isobutyrate and isovalerate, respectively, only in the presence of aspartylaspartate or glutamylglutamate. This suggests a metabolic linkage between these dipeptides. These results clearly indicate that P. gingivalis utilizes dipeptides preferentially as its metabolic substrates.

Inhibitory effect of sorbitol on sugar metabolism of Streptococcus mutans in vitro and on acid production in dental plaque in vivo

This study was conducted to find out whether sorbitol inhibits the sugar metabolism of Streptococcus mutans in vitro and the acid production in dental plaque in vivo. S. mutans NCIB 11723 was anaerobically grown in sorbitol-containing medium. The rate of acid production from sugars was estimated with a pH stat. The rate of acid production from glucose or sucrose was not changed at various concentrations of oxygen. By the addition of sorbitol to sugar, however, the acid production was decreased with increasing levels of oxygen. Intracellular NADH/NAD+ ratio and (dihydroxyacetone-phosphate+glyceraldehyde-phosphate)/3-phosphoglycerate ratio were high whenever the acid production was inhibited by sorbitol. Sorbitol also inhibited the acid production in dental plaque in vivo. These results suggest that the increased NADH/NAD+ ratio during sorbitol metabolism through the inactivation of pyruvate formate-lyase by oxygen inhibited glyceraldehyde-phosphate dehydrogenase and then the acid production of S. mutans and the one in dental plaque.

Metabolic pathways for cytotoxic end product formation from glutamate- and aspartate-containing peptides by Porphyromonas gingivalis

Metabolic pathways involved in the formation of cytotoxic end products by Porphyromonas gingivalis were studied. The washed cells of P. gingivalis ATCC 33277 utilized peptides but not single amino acids. Since glutamate and aspartate moieties in the peptides were consumed most intensively, a dipeptide of glutamate or aspartate was then tested as a metabolic substrate of P. gingivalis. P. gingivalis cells metabolized glutamylglutamate to butyrate, propionate, acetate, and ammonia, and they metabolized aspartylaspartate to butyrate, succinate, acetate, and ammonia. Based on the detection of metabolic enzymes in the cell extracts and stoichiometric calculations (carbon recovery and oxidation/reduction ratio) during dipeptide degradation, the following metabolic pathways were proposed. Incorporated glutamylglutamate and aspartylaspartate are hydrolyzed to glutamate and aspartate, respectively, by dipeptidase. Glutamate is deaminated and oxidized to succinyl-coenzyme A (CoA) by glutamate dehydrogenase and 2-oxoglutarate oxidoreductase. Aspartate is deaminated into fumarate by aspartate ammonia-lyase and then reduced to succinyl-CoA by fumarate reductase and acyl-CoA:acetate CoA-transferase or oxidized to acetyl-CoA by a sequential reaction of fumarase, malate dehydrogenase, oxaloacetate decarboxylase, and pyruvate oxidoreductase. The succinyl-CoA is reduced to butyryl-CoA by a series of enzymes, including succinate-semialdehyde dehydrogenase, 4-hydroxybutyrate dehydrogenase, and butyryl-CoA oxidoreductase. A part of succinyl-CoA could be converted to propionyl-CoA through the reactions initiated by methylmalonyl-CoA mutase. The butyryl- and propionyl-CoAs thus formed could then be converted into acetyl-CoA by acyl-CoA:acetate CoA-transferase with the formation of corresponding cytotoxic end products, butyrate and propionate. The formed acetyl-CoA could then be metabolized further to acetate.

Synthesis and posttranslational regulation of pyruvate formate-lyase in Lactococcus lactis

The enzyme pyruvate formate-lyase (PFL) from Lactococcus lactis was produced in Escherichia coli and purified to obtain anti-PFL antibodies that were shown to be specific for L. lactis PFL. It was demonstrated that activated L. lactis PFL was sensitive to oxygen, as in E. coli, resulting in the cleavage of the PFL polypeptide. The PFL protein level and its in vivo activity and regulation were shown by Western blotting, enzyme-linked immunosorbent assay, and metabolite measurement to be dependent on the growth conditions. The PFL level during anaerobic growth on the slowly fermentable sugar galactose was higher than that on glucose. This shows that variation in the PFL protein level may play an important role in the regulation of metabolic shift from homolactic to mixed-acid product formation, observed during growth on glucose and galactose, respectively. During anaerobic growth in defined medium, complete activation of PFL was observed. Strikingly, although no formate was produced during aerobic growth of L. lactis, PFL protein was indeed detected under these conditions, in which the enzyme is dispensable due to the irreversible inactivation of PFL by oxygen. In contrast, no oxygenolytic cleavage was detected during aerobic growth in complex medium. This observation may be the result of either an effective PFL deactivase activity or the lack of PFL activation. In E. coli, the PFL deactivase activity resides in the multifunctional alcohol dehydrogenase ADHE. It was shown that in L. lactis, ADHE does not participate in the protection of PFL against oxygen under the conditions analyzed. Our results provide evidence for major differences in the mechanisms of posttranslational regulation of PFL activity in E. coli and L. lactis.

Characterization of the Streptococcus mutans pyruvate formate-lyase (PFL)-activating enzyme gene by complementary reconstitution of the In vitro PFL-reactivating system

The act gene was identified and an act mutant as well as the pfl mutant was constructed in Streptococcus mutans. Pyruvate formate-lyase (PFL) activity was regenerated with the mixture of the respective cell extracts from these mutants by complementary reconstitution of the in vitro reactivating system. The S. mutans act gene encoded the sole enzyme able to activate the PFL protein in this organism.

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.

Pathways for amino acid metabolism by Prevotella intermedia and Prevotella nigrescens

Pathways for amino acid metabolism by Prevotella intermedia and Prevotella nigrescens were investigated. Prevotella strains grew anaerobically in tryptone-based medium and their growth increased upon the addition of aspartate to the medium. Washed cells of tryptone-grown strains metabolized aspartate to succinate, acetate, fumarate, malate, formate and ammonia, while from tryptone they produced isobutyrate and isovalerate in addition to the end products from aspartate. Cell extracts obtained from the tryptone-grown cells had aspartate ammonia-lyase for the conversion of aspartate to fumarate. Methylviologen-dependent fumarate reductase was found to reduce fumarate to succinate. A series of enzymatic activities, including fumarase, NAD-dependent malate dehydrogenase, oxaloacetate decarboxylase, methylviologen-dependent pyruvate oxidoreductase, phosphotransacetylase and acetate kinase, was detected for the oxidative conversion of fumarate to acetate. Pyruvate formate-lyase and NAD-dependent formate dehydrogenase were also found for the production and consumption of formate, respectively. Methylviologen: NAD(P) oxidoreductase was found to be responsible for linkage between these reductive and oxidative pathways. Furthermore, the cell extracts had branched-chain amino acid aminotransferase and methylviologen-dependent branched-chain 2-oxoacid oxidoreductase, concomitantly with NAD-dependent glutamate dehydrogenase. Valine and leucine could be converted to isobutyryl CoA and isovaleryl CoA, respectively, through the sequential catalyses of these enzymes, and consequently to isobutyrate and isovalerate, respectively.

Functions of two types of NADH oxidases in energy metabolism and oxidative stress of Streptococcus mutans

We have previously identified two distinct NADH oxidases corresponding to H(2)O(2)-forming oxidase (Nox-1) and H(2)O-forming oxidase (Nox-2) induced in Streptococcus mutans. Sequence analyses indicated a strong similarity between Nox-1 and AhpF, the flavoprotein component of Salmonella typhimurium alkyl hydroperoxide reductase; an open reading frame upstream of nox-1 also showed homology to AhpC, the direct peroxide-reducing component of S. typhimurium alkyl hydroperoxide reductase. To determine their physiological functions in S. mutans, we constructed knockout mutants of Nox-1, Nox-2, and/or the AhpC homologue; we verified that Nox-2 plays an important role in energy metabolism through the regeneration of NAD(+) but Nox-1 contributes negligibly. The Nox-2 mutant exhibited greatly reduced aerobic growth on mannitol, whereas there was no significant effect of aerobiosis on the growth on mannitol of the other strains or growth on glucose of any of the strains. Although the Nox-2 mutants grew well on glucose aerobically, the end products of glucose fermentation by the Nox-2 mutant were substantially shifted to higher ratios of lactic acid to acetic acid compared with wild-type cells. The resistance to cumene hydroperoxide of Escherichia coli TA4315 (ahpCF-defective mutant) transformed with pAN119 containing both nox-1 and ahpC genes was not only restored but enhanced relative to that of E. coli K-12 (parent strain), indicating a clear function for Nox-1 as part of an alkyl hydroperoxide reductase system in vivo in combination with AhpC. Surprisingly, the Nox-1 and/or AhpC deficiency had no effect on the sensitivity of S. mutans to cumene hydroperoxide and H(2)O(2), implying that the existence of some other antioxidant system(s) independent of Nox-1 in S. mutans compensates for the deficiency.

Effects of pH on the glucose and lactate metabolisms by the washed cells of Actinomyces naeslundii under anaerobic and aerobic conditions

Effects of pH on the glucose and lactate metabolism by the washed cells of Actinomyces naeslundii genospecies 1 and 2 under anaerobic and aerobic conditions were studied. The rate of acid production from glucose was the highest at pH 7.0 and decreased as the pH lowered to 4.5, irrespective of atmospheric conditions. The anaerobic end-product in the absence of bicarbonate was mainly lactate, while in the presence of bicarbonate the rate of acid production increased 1.8-2.5 times with the production of formate, acetate and succinate in addition to lactate. Under aerobic conditions, the cells produced acids from glucose along with oxygen consumption and the end-product was mainly acetate. In contrast to the glucose metabilism, the cells produced base from lactate along with oxygen consumption. The rates of base production and oxygen consumption were the highest at pH 5.5. The end-products from lactate were acetate and pyruvate. These results indicate that oral actinomyces has a various activity of glucose and lactate metabolism at a wide range of environmental pH and suggest its flexibility in surviving in dental plaque, where the environmental factors fluctuate.

Catabolic pathway for aerobic degradation of lactate by Actinomyces naeslundii

The aerobic metabolism of lactate by oral Actinomyces was studied. Six of 7 strains of Actinomyces naeslundii increased their growth in the presence of lactate under aerobic conditions. Washed cells grown on lactate aerobically degraded lactate and pyruvate to acetate with a concomitant consumption of oxygen. In the presence of catalase, the molar ratios of oxygen consumed to acetate produced were 1 for lactate degradation and 0.5 for pyruvate degradation. The enzymatic activities found in cell extracts revealed that lactate could be converted to pyruvate by NAD-independent lactate dehydrogenase (iLDH) and further to acetyl CoA by pyruvate dehydrogenase (PDH). The acetyl CoA formed could be metabolized into acetate by phosphotransacetylase (PTA) and acetate kinase (AK) with the formation of ATP. These results indicate that A. naeslundii metabolizes lactate into acetate by the sequential enzymatic reactions iLDH, PDH, PTA and AK and that hydrogens produced by iLDH and PDH are transferred to oxygen. The activity of lactate degradation and oxygen consumption may modify the environmental conditions of dental plaque.

Cloning and sequence analysis of the pfl gene encoding pyruvate formate-lyase from Streptococcus mutans

We have isolated a sorbitol-negative mutant of Streptococcus mutans GS-5 following random mutagenesis with plasmid pVA891 clone banks. This mutant did not metabolize sorbitol anaerobically but did so aerobically. A 10-kb chromosomal DNA fragment flanking the pVA891 insertion was deleted in this mutant. The corresponding region from the parental strain GS-5 was then recovered by a marker rescue method with Escherichia coli. The pyruvate formate-lyase gene, pfl, was identified within a 3-kb PstI-XbaI fragment located in the middle of the deleted region of the chromosome, and its inactivation in S. mutans produced the same sorbitol-negative phenotype. Nucleotide sequence analysis of the pfl gene revealed a 2.3-kb open reading frame (ORF) preceded by potential ribosome-binding and promoter-like sequences. The ORF specified a putative protein of 775 amino acid residues with a calculated molecular weight of 87,533. The amino acid sequence deduced from the ORF exhibited significant similarity to that of the E. coli pfl gene.

Effect of acetate on sorbitol fermentation by oral lactobacilli

The rate of acid production and end-products from sorbitol were measured under anaerobic conditions in washed-cell suspensions of oral strains of Lactobacillus casei subsp. casei and Lactobacillus casei subsp. rhamnosus. The enzymatic activities were assayed in cell extracts of these strains. The cells fermented sorbitol to lactate, formate, ethanol and acetate under anaerobic conditions. Exposure of the cells to air (oxygen) led to inactivation of pyruvate formate-lyase and inhibition of anaerobic sorbitol fermentation. In the presence of acetate, air-exposed cells fermented sorbitol with a concomitant consumption of acetate and production of ethanol and lactate. Acetate also enhanced acid production from sorbitol in cells kept under anaerobic conditions and resulted in formation of lactate and ethanol. Cell extracts of all the strains had NADH-coupled acetate-reducing activity, which consisted of sequential reactions of acetate kinase, phosphotransacetylase, acylating aldehyde dehydrogenase and alcohol dehydrogenase. These findings indicate that oral lactobacilli can utilize acetate as an electron acceptor for maintaining their intracellular redox balance during anaerobic sorbitol fermentation in the absence of pyruvate formate-lyase activity.

The effect of fluorhydroxyapatite-derived fluoride on acid production by streptococci

The effect of fluoride derived from fluorhydroxyapatite (FHAp) minerals on bacterial glycolysis under aerobic and strictly anaerobic conditions was studied to validate the claims that this mineral could be used as a reservoir of fluoride in plaque. To isolate the direct effect of fluoride on bacterial glycolysis from that of an indirect pH-buffering effect of hydroxyl or phosphate ions which are also dissolved from the mineral, we equalized the pH-fall time course of reactions by manually adding KOH or HCl. This ensured that pH effects on glycolysis were minimized. Under controlled pH-fall and strictly anaerobic conditions, fluoride derived from the dissolution of FHAp containing more than 30,100 ppm fluoride (i.e., when the substitution of OH by F in the mineral was greater than 80%) had a direct inhibitory effect on lactic acid production in Streptococcus mutans. Under free pH-fall and strictly anaerobic conditions, increasing amounts of fluoride in FHAp (starting as low as 2000 ppm fluoride), appeared to have a pronounced indirect inhibitory effect on lactic acid production. This was probably mediated through a reducing pH buffer effect of the mineral. Even in the presence of high-fluoride FHAp, only 0.01 to 0.025 mmol/L fluoride was found in the reaction mixtures, a probable result of non-stoichiometric dissolution of FHAp. In spite of such low levels of fluoride, marked inhibitory effects on bacterial glycolysis were demonstrated. The results of this study suggest that high-fluoride FHAp may serve as a reservoir of fluoride for the inhibition of anaerobic acid production by S. mutans.

The role of the succinate pathway in sorbitol fermentation by oral Actinomyces viscosus and Actinomyces naeslundii

The sorbitol fermentation by Actinomyces viscosus and Actinomyces naeslundii was studied with washed sorbitol-grown cells. The fermentation was followed by titration of acids produced at pH 7.0 under anaerobic conditions. Metabolic end-products and intracellular levels of NAD, NADH and glycolytic intermediates during the fermentation were also analyzed. Cell extracts were examined for certain enzyme activities. Bicarbonate was required for acid production from sorbitol and from a mixture of glucose and sorbitol. Malate and fumarate could also support the acid production of A. viscosus. The main end-products were succinate and lactate but not ethanol. Cell extracts showed no activities of alcohol and aldehyde dehydrogenases, but they had activities of malate dehydrogenase and fumarate reductase. In the absence of bicarbonate, malate or fumarate, the intracellular NADH/NAD ratio increased and the levels of 3- and 2-phosphoglycerate and phosphoenolpyruvate decreased. The results indicate that oral sorbitol-fermenting actinomyces lack the ethanol pathway that can contribute to NADH oxidation. To maintain intracellular redox balance during anaerobic sorbitol fermentation, these bacteria can oxidize surplus NADH through a succinate pathway.

Acid production by streptococci growing at low pH in a chemostat under anaerobic conditions

Streptococcus mutans and other oral streptococci were grown in continuous culture under strictly anaerobic conditions. When the cultural pH was kept at 7.0, the main acid products were formate and acetate, as reported previously. However, more lactate was produced at pH values of 5.5 or 6.0, with a concomitant decrease in formate and acetate production. This change in fermentation products could partly be ascribed to a change in intracellular pH and difference in the pH optima between pyruvate formate-lyase (PFL) and lactate dehydrogenase (LDH). At extracellular pH values of 7.0 and 5.5, the intracellular pH values of S. mutans NCIB 11723 were 7.5 and 6.6, respectively. The pH optima of PFL and LDH were 7.8 and 5.5-6.3, respectively. The cells had also a larger amount of LDH during growth at pH 5.5 than at pH 7.0.

Stimulatory effect of bicarbonate on the glycolysis of Actinomyces viscosus and its biochemical mechanism

The effects of bicarbonate on acid production by 4 human strains of Actinomyces viscosus were estimated under anaerobic conditions. The rate of acid production was accelerated by bicarbonate 3-4 times as much as that without bicarbonate. The analyses of intracellular glycolytic intermediates, NAD and NADH revealed a decrease in NADH:NAD ratio and an increase in the level of 3-phosphoglycerate in the cells when bicarbonate was present. Furthermore, when bicarbonate was available, malate dehydrogenase and fumarate reductase in the succinate pathway were expected to function as NADH-oxidizing enzymes in addition to lactate dehydrogenase. These observations indicate the efficient regeneration of NAD in the presence of bicarbonate. Thus, the stimulation of A. viscosus glycolysis by bicarbonate was thought to stem from the activation of glyceraldehyde 3-phosphate dehydrogenase (G3PDH) by the decrease in the level of NADH, because NADH was a strong inhibitor of G3PDH in this microorganism.

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.

Acid profile in carious dentin

Organic acids in carious dentin from 69 permanent teeth were analyzed by gas chromatography. Lactate, acetate, propionate, and butyrate were detected in most samples, and limited amounts of isobutyrate, valerate, isovalerate, caproate, and isocaproate were occasionally detected. Lactate, acetate, and propionate were major acids and altogether accounted for about 90% of total acid in most samples of carious dentin. However, the proportion of these three acids varied among the samples. Some samples contained over 85% lactate, while others contained mainly acetate and propionate. A high percentage of acetate was usually accompanied by an appreciable amount of propionate. All seven samples in carious dentin under fillings or restorations had little lactate, but a high percentage of acetate plus propionate. The differences in acid profiles of carious dentin may reflect differences in the microbial ecology of carious dentin, and a stage of progress of dentin caries or a type of dentin caries.