Phosphoenolpyruvate carboxylase and ammonium metabolism in oral streptococci

Anti-bacterial and anti-biofilm activities of arachidonic acid against the cariogenic bacterium Streptococcus mutans

Streptococcus mutans is a Gram-positive, facultative anaerobic bacterium, which causes dental caries after forming biofilms on the tooth surface while producing organic acids that demineralize enamel and dentin. We observed that the polyunsaturated arachidonic acid (AA) (ω-6; 20:4) had an anti-bacterial activity against S. mutans, which prompted us to investigate its mechanism of action. The minimum inhibitory concentration (MIC) of AA on S. mutans was 25 μg/ml in the presence of 5% CO2, while it was reduced to 6.25-12.5 μg/ml in the absence of CO2 supplementation. The anti-bacterial action was due to a combination of bactericidal and bacteriostatic effects. The minimum biofilm inhibitory concentration (MBIC) was the same as the MIC, suggesting that part of the anti-biofilm effect was due to the anti-bacterial activity. Gene expression studies showed decreased expression of biofilm-related genes, suggesting that AA also has a specific anti-biofilm effect. Flow cytometric analyses using potentiometric DiOC2(3) dye, fluorescent efflux pump substrates, and live/dead SYTO 9/propidium iodide staining showed that AA leads to immediate membrane hyperpolarization, altered membrane transport and efflux pump activities, and increased membrane permeability with subsequent membrane perforation. High-resolution scanning electron microscopy (HR-SEM) showed remnants of burst bacteria. Furthermore, flow cytometric analysis using the redox probe 2',7'-dichlorofluorescein diacetate (DCFHDA) showed that AA acts as an antioxidant in a dose-dependent manner. α-Tocopherol, an antioxidant that terminates the radical chain, counteracted the anti-bacterial activity of AA, suggesting that oxidation of AA in bacteria leads to the production of cytotoxic radicals that contribute to bacterial growth arrest and death. Importantly, AA was not toxic to normal Vero epithelial cells even at 100 μg/ml, and it did not cause hemolysis of erythrocytes. In conclusion, our study shows that AA is a potentially safe drug that can be used to reduce the bacterial burden of cariogenic S. mutans.

Nitrogenous compounds stimulate glucose-derived acid production by oral Streptococcus and Actinomyces

Both Streptococcus and Actinomyces can produce acids from dietary sugars and are frequently found in caries lesions. In the oral cavity, nitrogenous compounds, such as peptides and amino acids, are provided continuously by saliva and crevicular gingival fluid. Given that these bacteria can also utilize nitrogen compounds for their growth, it was hypothesized that nitrogenous compounds may influence their acid production; however, no previous studies have examined this topic. Therefore, the present study aimed to assess the effects of nitrogenous compounds (tryptone and glutamate) on glucose-derived acid production by Streptococcus and Actinomyces. Acid production was evaluated using a pH-stat method under anaerobic conditions, whereas the amounts of metabolic end-products were quantified using high performance liquid chromatography. Tryptone enhanced glucose-derived acid production by up to 2.68-fold, whereas glutamate enhanced Streptococcus species only. However, neither tryptone nor glutamate altered the end-product profiles, indicating that the nitrogenous compounds stimulate the whole metabolic pathways involving in acid production from glucose, but are not actively metabolized, nor do they alter metabolic pathways. These results suggest that nitrogenous compounds in the oral cavity promote acid production by Streptococcus and Actinomyces in vivo.

Purification of recombinant non-phosphorylating NADP-dependent glyceraldehyde-3-phosphate dehydrogenase from Streptococcus pyogenes expressed in E. coli

Streprococcus pyogenes gapN was cloned and expressed by functional complementation of the Escherichia gap mutant W3CG. The IPTG-induced NADP non-phosphorylating GAPDH (GAPN) has been purified about 75.4 fold from E. coli cells, using a procedure involving conventional ammonium sulfate fractionation, anion-exchange chromatography, hydrophobic chromatography and hydroxyapatite chromatography. The purified protein was characterised: it's an homotetrameric structure with a native molecular mass of 224 kDa, have an acid pI of 4.9 and optimum pH of 8.5. Studies on the effect of assay temperature on enzyme activity revealed an optimal value of about 60 degrees C with activation energy of 51 KJ mole(-1). The apparent Km values for NADP and D-G3P or DL-G3P were estimated to be 0.385 +/- 0.05 and 0.666 +/- 0.1 mM, respectively and the Vmax of the purified protein was estimated to be 162.5 U mg(-1). The S. pyogenes GAPN was markedly inhibited by sulfydryl-modifying reagent iodoacetamide, these results suggest the participation of essential sulfydryl groups in the catalytic activity.

Effects of acidification on growth and glycolysis of Streptococcus sanguis and Streptococcus mutans

After carbohydrate intake, pH in dental plaque decreases rapidly and reaches about 4 within a few minutes. The acidification not only promotes demineralization of tooth surface but can also cause damage to bacteria in dental plaque. We, therefore, investigated the effect of acidification on the dental plaque bacteria Streptococcus sanguis and Streptococcus mutans. At pH 4.0 and 4.2, both growth and glycolytic activities in these streptococci were repressed. Prolonged acidification (for 60 min at pH 4.0) not only repressed both growth and glycolytic activities but also impaired them in S. sanguis cells with concomitant inactivation of the glycolytic enzymes, hexokinase, phosphofructokinase, glyceraldehydephosphate dehydrogenase and enolase. The impaired abilities of glycolysis and growth recovered following incubation at pH 7.0 for 80-90 min, and this was accompanied by reactivation of the glycolytic enzymes. On the other hand, these impairments were not observed in S. mutans cells exposed to prolonged acidification. These results indicate that the low pH frequently occurring in dental plaque may transiently impair streptococcal glycolysis and growth and that S. mutans is more durable to the acidification than S. sanguis.

Role of the citrate pathway in glutamate biosynthesis by Streptococcus mutans

In work previously reported (J. A. Gutierrez, P. J. Crowley, D. P. Brown, J. D. Hillman, P. Youngman, and A. S. Bleiweis, J. Bacteriol. 178:4166-4175, 1996), a Tn917 transposon-generated mutant of Streptococcus mutans JH1005 unable to synthesize glutamate anaerobically was isolated and the insertion point of the transposon was determined to be in the icd gene encoding isocitrate dehydrogenase (ICDH). The intact icd gene of S. mutans has now been isolated from an S. mutans genomic plasmid library by complementation of an icd mutation in Escherichia coli host strain EB106. Genetic analysis of the complementing plasmid pJG400 revealed an open reading frame (ORF) of 1,182 nucleotides which encoded an enzyme of 393 amino acids with a predicted molecular mass of 43 kDa. The nucleotide sequence contained regions of high (60 to 72%) homology with icd genes from three other bacterial species. Immediately 5' of the icd gene, we discovered an ORF of 1,119 nucleotides in length, designated citZ, encoding a homolog of known citrate synthase genes from other bacteria. This ORF encoded a predicted protein of 372 amino acids with a molecular mass of 43 kDa. Furthermore, plasmid pJG400 was also able to complement a citrate synthase (gltA) mutation of E. coli W620. The enzyme activities of both ICDH, found to be NAD+ dependent, and citrate synthase were measured in cell extracts of wild-type S. mutans and E. coli mutants harboring plasmid pJG400. The region 5' from the citZ gene also revealed a partial ORF encoding 264 carboxy-terminal amino acids of a putative aconitase gene. The genetic and biochemical evidence indicates that S. mutans possesses the enzymes required to convert acetyl coenzyme A and oxalacetate to alpha-ketoglutarate, which is necessary for the synthesis of glutamic acid. Indeed, S. mutans JH1005 was shown to assimilate ammonia as a sole source of nitrogen in minimal medium devoid of organic nitrogen sources.

Sequence, expression, and function of the gene for the nonphosphorylating, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase of Streptococcus mutans

We report the sequencing of a 2,019-bp region of the Streptococcus mutans NG5 genome which contains a 1,428-bp open reading frame (ORF) whose putative translation product had 50% identity to the amino acid sequences of the nonphosphorylating, NADP-dependent glyceraldehyde-3-phosphate dehydrogenases (GAPN) from maize and pea. This ORF is located approximately 200 bp downstream of the ptsI gene coding for enzyme I of the phosphoenolpyruvate:sugar phosphotransferase transport system. Mutant BCH150, in which the putative gapN gene had been inactivated, lacked GAPN activity that was present in the wild-type strain, thus positively identifying the ORF as the S. mutans gapN gene. Another strain of S. mutans, DC10, which contains an insertionally inactivated ptsI gene, still possessed GAPN activity, as did S. salivarius ATCC 25975, which contains an insertion element between the ptsI and gapN genes. Since the wild-type S. mutans NG5 lacks both glucose-6-phosphate dehydrogenase and NADH:NADP oxidoreductase activities, the NADP-dependent glyceraldehyde-3-phosphate dehydrogenase is important as a means of generating NADPH for biosynthetic reactions.

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.

Role of NADH oxidase in the oxidative inactivation of Streptococcus salivarius fructosyltransferase

A cell-associated fructosyltransferase produced by Streptococcus salivarius was irreversibly inactivated in a time-dependent manner when resting or permeabilized cell suspensions were incubated with low concentrations (less than 1.0 microM) of copper. In addition to copper, the inactivation was dependent on oxygen and on a fermentable carbon source (endogenous intracellular polysaccharide or glucose). In starved, permeabilized cell suspensions, the fermentable carbon source could be replaced by NADH but not by NADPH or ATP. Of several other S. salivarius enzymes tested, only fructosyltransferase was inactivated under these conditions. The available evidence indicated that NADH oxidase is the enzyme responsible for fructosyltransferase inactivation. Results from oxygen radical scavenger studies implicated one or more species of oxygen radicals and hydrogen peroxide in the inactivation reaction.

Regulation of glycolytic rate in Streptococcus sanguis grown under glucose-limited and glucose-excess conditions in a chemostat

The biochemical mechanisms of the acidogenic potential of Streptococcus sanguis ATCC 10556 grown in glucose-excess and glucose-limited continuous culture were studied. The rate of acid production during the glucose metabolism by the cells grown under glucose limitation (glucose-limited cells) was 2.1 to 2.6 times that by the cells grown in an excess of glucose (glucose-excess cells). When the glucose-limited cells were metabolizing glucose, intracellular concentrations of glucose 6-phosphate, fructose 6-phosphate, 3-phosphoglycerate, and pyruvate were higher, and that of glyceraldehyde 3-phosphate was lower, than those when the glucose-excess cells were metabolizing glucose. The levels of fructose 1,6-bisphosphate and dihydroxyacetone phosphate were not significantly different between these cells. The activities of glucose-phosphoenolpyruvate phosphotransferase system in decriptified cells and glyceraldehyde-3-phosphate dehydrogenase in cell-free extracts of the glucose-limited cells were higher than those in the glucose-excess cells. The activities of glucokinase, phosphoglycerate kinase, and pyruvate kinase in cell-free extracts of these cells were not different significantly. We conclude that the high glycolytic activity of the glucose-limited cells results from the increase in the synthesis of glucose-phosphoenolpyruvate phosphotransferase and glyceraldehyde-3-phosphate dehydrogenase.

Hydrogen peroxide excretion by oral streptococci and effect of lactoperoxidase-thiocyanate-hydrogen peroxide

Approved type strains of Streptococcus sanguis, S. mitis, S. mutans, and S. salivarius were grown under aerobic and anaerobic conditions. The rate of hydrogen peroxide excretion, oxygen uptake, and acid production from glucose by washed-cell suspensions of these strains were studied, and the levels of enzymes in cell-free extracts which reduced oxygen, hydrogen peroxide, or hypothiocyanite (OSCN-) in the presence of NADH or NADPH were assayed. The effects of lactoperoxidase-thiocyanate-hydrogen peroxide on the rate of acid production and oxygen uptake by intact cells, the activity of glycolytic enzymes in cell-free extracts, and the levels of intracellular glycolytic intermediates were also studied. All strains consumed oxygen in the presence of glucose. S. sanguis, S. mitis, and anaerobically grown S. mutans excreted hydrogen peroxide. There was higher NADH oxidase and NADH peroxidase activity in aerobically grown cells than in anaerobically grown cells. NADPH oxidase activity was low in all species. Acid production, oxygen uptake, and, consequently, hydrogen peroxide excretion were inhibited in all the strains by lactoperoxidase-thiocyanate-hydrogen peroxide. S. sanguis and S. mitis had a higher capacity than S. mutans and S. salivarius to recover from this inhibition. Higher activity in the former strains of an NADH-OSCN oxidoreductase, which converted OSCN- into thiocyanate, explained this difference. The change in levels of intracellular glycolytic intermediates after inhibition of glycolysis by OSCN- and the actual activity of glycolytic enzymes in cell-free extracts in the presence of OSCN- indicated that the primary target of OSCN- in the glycolytic pathway was glyceraldehyde 3-phosphate dehydrogenase.

Purification and properties of pyruvate kinase from Streptococcus sanguis and activator specificity of pyruvate kinase from oral streptococci

It was found that pyruvate kinases with two different regulatory characteristics were distributed among oral streptococci. The pyruvate kinases of Streptococcus mutans, Streptococcus salivarius, and Streptococcus bovis were activated by glucose 6-phosphate, whereas the enzymes of both Streptococcus sanguis and Streptococcus mitis were activated by fructose 1,6-bisphosphate. Pyruvate kinase (EC 2.7.1.40) from S. sanguis NCTC 10904 was purified, giving a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The enzyme had a molecular weight of 250,000 to 260,000 and consisted of four identical subunits. Whereas the pyruvate kinase from S. mutans was completely dependent on glucose 6-phosphate (K. Abbe and T. Yamada, J. Bacteriol. 149:299-305, 1982), the enzyme from S. sanguis was activated by fructose 1,6-bisphosphate. In the presence of 0.5 mM fructose 1,6-bisphosphate, the saturation curves for the substrates, phosphoenolpyruvate and ADP, were hyperbolic, and the Km values were 0.13 and 0.30 mM, respectively. Without fructose 1,6-bisphosphate, however, saturation curves for both substrates were sigmoidal. GDP, IDP, and UDP could replace ADP. Like the enzyme from S. mutans, the enzyme from S. sanguis required a divalent cation, Mg2+ or Mn2+, and a monovalent cation, K+ or NH4+, for activity, and it was strongly inhibited by Pi. When the concentration of Pi was increased, the half-saturating concentration and Hill coefficient for fructose 1,6-bisphosphate increased. The remarkable fluctuation of intracellular levels of fructose 1,6-bisphosphate and phosphoenolpyruvate observed in the cells growing under glucose limitation and nitrogen limitation implies that the intracellular concentration of fructose 1,6-bisphosphate, in cooperation with that of Pi, may regulate pyruvate kinase activity in S. sanguis in vivo.

Identification of PK 1 bacteriophage DNA in Streptococcus mutans

A lysogenic bacteriophage PK 1 and plasmid DNA's in Streptococcus mutans PK 1 have been characterized by electron microscopy PK 1 phage DNA molecules were observed in both linear and circular forms, which gave the molecular weights of (28.9 +/- 0.4) x 10(6) and (27.4 +/- 0.2) X 10(6) daltons, respectively. Plasmid DNA has a molecular weight of 4.0 X 10(6) daltons. No difference of density in CsCl density gradient between linear and circular forms of phage DNA and plasmid DNA was observed.

Purification and properties of pyruvate kinase from Streptococcus mutans

Pyruvate kinase (EC 2.7.1.40) from Streptococcus mutans strain JC2 was purified, giving a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The molecular weight of the native enzyme was 180,000 to 190,000, and the enzyme was considered to consist of four identical subunits. This enzyme was completely dependent on glucose 6-phosphate for activity, and the saturation curve for activation by glucose 6-phosphate was sigmoidal. In the presence of 0.5 mM glucose 6-phosphate, the saturation curves for the substrates phosphoenolpyruvate and ADP were hyperbolic, and the Km values were 0.22 and 0.39 mM, respectively. GDP, IDP, and UDP could replace ADP, and the Km for GDP (0.026 mM) was 0.067 of that for ADP. The enzyme required not only divalent cations, Mg2+ or Mn2+, but also monovalent cations, K+ or NH4+, for activity, and it was strongly inhibited by Pi. When the concentration of Pi was increased, the half-saturating concentration and Hill coefficient for glucose 6-phosphate increased. However, the enzyme was immediately inactivated in a solution without Pi. The intracellular concentration of glucose 6-phosphate, in cooperation with that of Pi, may regulate pyruvate kinase activity in S. mutans.

Regulation and function of ammonia-assimilating enzymes in Streptococcus mutans

The ability of Streptococcus mutans to synthesize amino acids was examined. A total of 8 of 12 laboratory strains grew anaerobically on solid-defined medium that contained no amino acids. Several isolates, therefore, assimilated ammonia for the biosynthesis of amino acids. These strains included representatives of five serotypes. One strain, DR0001, was also grown in liquid-defined medium. The enzymes of two pathways by which ammonia can be fixed were detected in this strain DR0001 could use either a reduced nicotinamide adenine dinucleotide phosphate-coupled glutamate dehydrogenase or the combined action of adenosine 5'-triphosphate-driven glutamine synthetase with a reduced nicotinamide adenine dinucleotide-coupled glutamate synthase to assimilate ammonia for the biosynthesis of amino acids. Evidence that both pathways were functional was provided by an analysis of the influence of the nitrogen source on enzyme levels and by the isolation and characterization of glutamate dehydrogenase-negative mutants.

Glucose-6-phosphate-dependent pyruvate kinase in Streptococcus mutans

Pyruvate kinase of Streptococcus mutans JC 2 had an absolute and specific requirement for glucose-6-phosphate. Inorganic phosphate was a strong inhibitor. The enzyme required K+ or NH4+ and Mg2+ or Mn2+. S. mutans FIL and E 49, Streptococcus bovis ATCC 9809, and Streptococcus salivarius ATCC 13419 had also glucose-6-phosphate-dependent pyruvate kinases, whereas Streptococcus sanguis NCTC 10904 had an enzyme activated by fructose-1,6-diphosphate.

Regulation of lactate dehydrogenase and change of fermentation products in streptococci

Streptococcus mutans JC 2 produced mainly lactate as a fermentation product when grown in nitrogen-limited continuous culture in the presence of an excess of glucose and produced formate, acetate, and ethanol, but no lactate, under glucose-limited conditions. The levels of lactate dehydrogenase (LDH) in these cultures were of the same order of magnitude, and the activity of LDH was completely dependent on fructose-1,6-diphosphate (FDP). The intracellular level of FDP was high and the level of phosphoenolpyruvate (PEP) was low under the glucose-excess conditions. In the glucose-limited cultures, all glycolytic intermediates studied, except PEP, were low. S. mutans FIL, which had an FDP-independent LDH and similar levels of glycolytic intermediates as S. mutans JC2, produced mainly lactate under glucose-excess or under glucose-limited conditions. LDH of Streptococcus bovis ATCC 9809 was dependent on FDP for activity at a low concentration of pyruvate but had a significant activity without FDP at a high concentration of pyruvate. This strain also produced mainly lactate both under glucose-excess and glucose-limited conditions. The levels and characteristics of these LDHs were not changed by the culture conditions. These results indicate that changes in the intracellular level of FDP regulate LDH activity, which in turn influences the type of fermentation products produced by streptococci. PEP, adenosine 5'-monophosphate, adenosine 5'-diphosphate, and inorganic phosphate significantly inhibited LDH activity from S. mutans JC 2 and may also participate in the regulation of LDH activity in other streptococci.

Amino acid requirements of Streptococcus mutans and other oral streptococci

The amino acid requirements of Streptococcus mutans strains AHT, OMZ-61, FA-1, BHT, GS-5, JC-2, Ingbritt, At6T, OMZ-176, 6715, Streptococcus salivarius HHT, Streptococcus sanguis OMZ-9, and strain 72x46 were determined in a chemically defined medium. When grown anaerobically in the presence of sodium carbonate (or bicarbonate for a few strains), few amino acids were required. All strains tested required cystine (or cystine) as a nutrient. Three strains (S. mutans OMZ-176, FA-1, and BHT) required glutamate (and/or glutamine). A third amino acid (lysine for S. mutans FA-1 and histidine for S. mutans OMZ-176) was required by two of the three strains which required glutamate. The amino acids mentioned above were required for all conditions of incubation (and inoculum) tested. The requirements for several other amino acids were conditional, that is, dependent on the incubation conditions and inoculum used. For example, when carbonate was not added, glutamate was required by S. mutans GS-5. Aerobic incubations, with carbonate or bicarbonate added, resluted in requirements for glutamate and leucine by several strains. With these incubation conditions, one strain required isoleucine (S. mutans FA-1), another valine (S. mutans AHT), and a third tyrosine (72x46). Aerobic incubations in the absence of carbonate or bicarbonate further increased the number of amino acids required by several strains. Furthermore, when stationary-phase cultures replaced exponentially growing cultures as an inoculum, several strains required additional amino acids, presumably for the initiation of growth.

Transmission of Lactobacillus jensenii and Lactobacillus acidophilus from mother to child at time of delivery

The presence of Lactobacillus jensenii and Lactobacillus acidophilus has been studied in specimens from the rectum and vagina of the mother, from the mouth of the infant at the time of delivery, and from the mouth and rectum of infants six days of age. L. jensenii could be differentiated from other species of lactobacilli by the following combination of characteristics: production of only D-lactate, hydrolysis of arginine, and fermentation of cellobiose, galactose, and ribose, but not of lactose. L. jensenii and L. acidophilus were common inhabitants of the vagina. In spite of a contamination of the infant's mouth by L. jensenii and L. acidophilus during delivery, neither of these organisms became established in the mouth of the newborn infants.