Differential toxic effects of lactate and acetate on the metabolism of Streptococcus mutans and Streptococcus sanguis

Metabolism mechanism of glycosaminoglycans by the gut microbiota: Bacteroides and lactic acid bacteria: A review

Glycosaminoglycans (GAGs), as a class of biopolymers, play pivotal roles in various biological metabolisms such as cell signaling, tissue development, cell apoptosis, immune modulation, and growth factor activity. They are mainly present in the colon in free forms, which are essential for maintaining the host's health by regulating the colonization and proliferation of gut microbiota. Therefore, it is important to explain the specific members of the gut microbiota for GAGs' degradation and their enzymatic machinery in vivo. This review provides an outline of GAGs-utilizing entities in the Bacteroides, highlighting their polysaccharide utilization loci (PULs) and the enzymatic machinery involved in chondroitin sulfate (CS) and heparin (Hep)/heparan sulfate (HS). While there are some variations in GAGs' degradation among different genera, we analyze the reputed GAGs' utilization clusters in lactic acid bacteria (LAB), based on recent studies on GAGs' degradation. The enzymatic machinery involved in Hep/HS and CS metabolism within LAB is also discussed. Thus, to elucidate the precise mechanisms utilizing GAGs by diverse gut microbiota will augment our understanding of their effects on human health and contribute to potential therapeutic strategies for diseases.

Propionate Attenuates Growth of Oral Streptococci through Enhancing Methionine Biosynthesis

Oral streptococci are considered as an opportunistic pathogen associated with initiation and progression of various oral diseases. However, since the currently-available treatments often accompany adverse effects, alternative strategy is demanded to control streptococci. In the current study, we investigated whether short-chain fatty acids (SCFAs), including sodium acetate (NaA), sodium propionate (NaP), and sodium butyrate (NaB), can inhibit the growth of oral streptococci. Among the tested SCFAs, NaP most potently inhibited the growth of laboratory and clinically isolated strains of Streptococcus gordonii under anaerobic culture conditions. However, the growth inhibitory effect of NaP on six different species of other oral streptococci was different depending on their culture conditions. Metabolic changes such as alteration of methionine biosynthesis can affect bacterial growth. Indeed, NaP enhanced intracellular methionine levels of oral streptococci as well as the mRNA expression level of methionine biosynthesis-related genes. Collectively, these results suggest that NaP has an inhibitory effect on the growth of oral streptococci, which might be due to alteration of methionine biosynthesis. Thus, NaP can be used an effective bacteriostatic agent for the prevention of oral infectious diseases caused by oral streptococci.

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.

Responses of Staphylococcus aureus exposed to HCl and organic acid stress

Staphylococcus aureus is an important food poisoning bacterium. In food preservation, acidification is a well-known method. Permeant weak organic acids, like lactic and acetic acids, are known to be more effective against bacteria than inorganic strong acids (e.g., HCl). Growth experiments and metabolic and transcriptional analyses were used to determine the responses of a food pathogenic S. aureus strain exposed to lactic acid, acetic acid, and HCl at pH 4.5. Lactic and acetic acid stress induced a slower transcriptional response and large variations in growth patterns compared with the responses induced by HCl. In cultures acidified with lactic acid, the pH of the medium gradually increased to 7.5 during growth, while no such increase was observed for bacteria exposed to acetic acid or HCl. Staphylococcus aureus increased the pH in the medium mainly through accumulation of ammonium and the removal of acid groups, resulting in increased production of diacetyl (2,3-butanedione) and pyrazines. The results showed flexible and versatile responses of S. aureus to different types of acid stress. As measured by growth inhibition, permeant organic acid stress introduced severe stress compared with the stress caused by HCl. Cells exposed to lactic acid showed specific mechanisms of action in addition to sharing many of the mechanisms induced by HCl stress.

Characterization of a glutamate transporter operon, glnQHMP, in Streptococcus mutans and its role in acid tolerance

Glutamate contributes to the acid tolerance response (ATR) of many Gram-negative and Gram-positive bacteria, but its role in the ATR of the oral bacterium Streptococcus mutans is unknown. This study describes the discovery and characterization of a glutamate transporter operon designated glnQHMP (Smu.1519 to Smu.1522) and investigates its potential role in acid tolerance. Deletion of glnQHMP resulted in a 95% reduction in transport of radiolabeled glutamate compared to the wild-type UA159 strain. The addition of glutamate to metabolizing UA159 cells resulted in an increased production of acidic end products, whereas the glnQHMP mutant produced less lactic acid than UA159, suggesting a link between glutamate metabolism and acid production and possible acid tolerance. To investigate this possibility, we conducted a microarray analysis with glutamate and under pH 5.5 and pH 7.5 conditions which showed that expression of the glnQHMP operon was downregulated by both glutamate and mild acid. We also measured the growth kinetics of UA159 and its glnQHMP-negative derivative at pH 5.5 and found that the mutant doubled at a much slower rate than the parent strain but survived at pH 3.5 significantly better than the wild type. Taken together, these findings support the involvement of the glutamate transporter operon glnQHMP in the acid tolerance response in S. mutans.

Proteome analysis of Streptococcus mutans metabolic phenotype during acid tolerance

Two-dimensional gel electrophoretic analysis of the proteome ofStreptococcus mutansgrown at a steady state in a glucose-limited anaerobic continuous culture revealed a number of proteins that were differentially expressed when the growth pH was lowered from pH 7·0 to pH 5·0. Changes in the expression of metabolic proteins were generally limited to three biochemical pathways: glycolysis, alternative acid production and branched-chain amino acid biosynthesis. The relative level of expression of protein spots representing all of the enzymes associated with the Embden–Meyerhof–Parnas pathway, and all but one of the enzymes involved in the major alternative acid fermentation pathways ofS. mutans, was identified and measured. Proteome data, in conjunction with end-product and cell-yield analyses, were consistent with a phenotypic change that allowedS. mutansto proliferate at low pH by expending energy to extrude excess H+from the cell, while minimizing the detrimental effects that result from the uncoupling of carbon flux from catabolism and the consequent imbalance in NADH and pyruvate production. The changes in enzyme levels were consistent with a reduction in the formation of the strongest acid, formic acid, which was a consequence of the diversion of pyruvate to both lactate and branched-chain amino acid production whenS. mutanswas cultivated in an acidic environment.

Fluoride and organic weak acids as modulators of microbial physiology

Fluoride is widely used as an anticaries agent in drinking water and a variety of other vehicles. This use has resulted in major health benefits. However, there are still open questions regarding the mechanisms of anticaries action and the importance of antimicrobial effects in caries reduction. Fluoride acts in multiple ways to affect the metabolism of cariogenic and other bacteria in the mouth. F(-)/HF can bind directly to many enzymes, for example, heme-containing enzymes or other metalloenzymes, to modulate metabolism. Fluoride is able also to form complexes with metals such as aluminum or beryllium, and the complexes, notably AlF(4)(-) and BeF(3)(-).H(2)O, can mimic phosphate with either positive or negative effects on a variety of enzymes and regulatory phosphatases. The fluoride action that appears to be most important for glycolytic inhibition at low pH in dental plaque bacteria derives from its weak-acid properties (pK(a)=3.15) and the capacity of HF to act as a transmembrane proton conductor. Since many of the actions of fluoride are related to its weak-acid character, it is reasonable to compare fluoride action to those of organic weak acids, including metabolic acids, food preservatives, non-steroidal anti-inflammatory agents and fatty acids, all of which act to de-energize the cell membrane by discharging DeltapH. Moreover, with the realization that the biofilm state is the common lifestyle for most microorganisms in nature, there is need to consider interactions of fluoride and organic weak acids with biofilm communities. Hopefully, this review will stimulate interest in the antimicrobial effects of fluoride or other weak acids and lead to more effective use of the agents for disease control and other applications.

Influence of starvation and biofilm formation on acid resistance of Streptococcus mutans

The aim of this study was to investigate acid resistance induced by starvation or biofilm formation in Streptococcus mutans ATCC 25175. The artificial biofilms were made on cover glasses, starved for 24 h and immersed in 0.1 M lactate buffer at pH 3.8 for 10 min. The biofilms were also exposed to 5% sucrose solution for 20 min to simulate acid shock produced by sucrose metabolism. Confocal laser scanning microscopy with fluorescein isothiocyanate staining measured the resultant minimum pH in biofilms. Live and dead organisms in biofilms were differentiated by confocal laser scanning microscopy with proidium iodide and SYTO9 staining. The same processes were used to treat planktonic organisms. The results showed that starved biofilms or planktonic cells showed significantly more viable bacteria after acid shock induced either by lactic acid or during sucrose consumption than non-starved biofilms or planktonic cells. In addition, biofilms showed greater resistance to acid shock induced by lactic acid than planktonic cells, whereas similar results were obtained where sucrose was used as a carbon source to reduce pH in biofilms and planktonic cells. Thus, it is suggested that starvation protects both biofilm and planktonic S. mutans from acid shock induced either by lactic acid or during sucrose consumption, while biofilm formation seemed to protect bacteria only from acid shock induced by pH 3.8 lactate buffer but not the acid shock of a slightly higher pH produced during sucrose consumption.

Defects in D-alanyl-lipoteichoic acid synthesis in Streptococcus mutans results in acid sensitivity

In the cariogenic organism, Streptococcus mutans, low pH induces an acid tolerance response (ATR). To identify acid-regulated proteins comprising the ATR, transposon mutagenesis with the thermosensitive plasmid pGh9:ISS1 was used to produce clones that were able to grow at neutral pH, but not in medium at pH 5.0. Sequence analysis of one mutant (IS1A) indicated that transposition had created a 6.3-kb deletion, one end of which was in dltB of the dlt operon encoding four proteins (DltA-DltD) involved in the synthesis of D-alanyl-lipoteichoic acid. Inactivation of the dltC gene, encoding the D-alanyl carrier protein (Dcp), resulted in the generation of the acid-sensitive mutant, BH97LC. Compared to the wild-type strain, LT11, the mutant exhibited a threefold-longer doubling time and a 33% lower growth yield. In addition, it was unable to initiate growth below pH 6.5 and unadapted cells were unable to survive a 3-h exposure in medium buffered at pH 3.5, while a pH of 3.0 was required to kill the wild type in the same time period. Also, induction of the ATR in BH97LC, as measured by the number of survivors at a pH killing unadapted cells, was 3 to 4 orders of magnitude lower than that exhibited by the wild type. While the LTA of both strains contained a similar average number of glycerolphosphate residues, permeabilized cells of BH97LC did not incorporate D-[(14)C]alanine into this amphiphile. This defect was correlated with the deficiency of Dcp. Chemical analysis of the LTA purified from the mutant confirmed the absence of D-alanine-esters. Electron micrographs showed that BH97LC is characterized by unequal polar caps and is devoid of a fibrous extracellular matrix present on the surface of the wild-type cells. Proton permeability assays revealed that the mutant was more permeable to protons than the wild type. This observation suggests a mechanism for the loss of the characteristic acid tolerance response in S. mutans.

Effects of organic acid anions on growth, glycolysis, and intracellular pH of oral streptococci

Oral streptococci produce large quantities of organic acids as the end-products of carbohydrate fermentation. In an approach to determine if oral streptococci exhibit differential sensitivities to organic acid anions, we determined the effects of formate, lactate, and acetate on intracellular pH maintenance, glycolysis, and growth of Streptococcus mutans and Streptococcus sanguis. Growth was determined as maximum culture optical density in the presence of the organic acid anions at pH 7.1, 6.7, 6.3, and 6.1, and the effects of the anions on glycolytic activity and intracellular pH were determined at pH 7.0 and 5.0. At pH 7.1, the organic acid anions had little effect on growth of either species. At the lower pH values, all of the anions reduced the maximum culture optical density of both species in a pH- and concentration-dependent manner, with S. sanguis being more sensitive to growth inhibition than S. mutans. The organic acid anions had little or no effect on glycolytic activity of either species at pH 7.0. However, all of the organic acid anions tested reduced glycolytic activity at pH 5.0 in a concentration-dependent manner, with S. sanguis being more sensitive than S. mutans. The inhibition of glycolysis could be related to the pKa of the organic acid, with formate and lactate being more inhibitory than acetate. The organic acid anions decreased the intracellular pH of S. mutans and S. sanguis, glycolyzing at an extracellular pH of 5.0, such that the reduction in glycolytic activity caused by the organic acid anions could be directly attributed to the fall in intracellular pH. In conclusion, the production of lactic acid in plaque would not only lower pH, thereby having a disadvantageous effect on less aciduric oral streptococci, such as S. sanguis, but would also increase their sensitivity to the effects of low pH, helping S. mutans to become more dominant.

Multiple stress responses in Streptococcus mutans and the induction of general and stress-specific proteins

The authors have previously demonstrated that Streptococcus mutans shows an exponential-phase acid-tolerance response following an acid shock from pH 7.5 to 5.5 that enhances survival at pH 3.0. In this study the response of S. mutans H7 to acid shock was compared with the responses generated by salt, heat, oxidation and starvation. Prior induction of the acid-tolerance response did not cross-protect the cells from a subsequent challenge by the other stresses; however, prior adaptation to the other stresses, except heat (42 degrees C), protected the cells during a subsequent acid challenge at pH 3.5. Starvation by fivefold dilution of the basal medium (BM) plus fivefold reduction of its glucose content increased the numbers of survivors 12-fold, whereas elimination of glucose from fivefold-diluted BM led to a sevenfold enhancement compared to the control cells; this indicated a relationship between the acid and starvation responses. The stress responses were further characterized by comparing the 2D electrophoretic protein profiles of exponential-phase cells subjected to the various stress conditions. Cells were grown to exponential phase at pH 7.5 (37 degrees C) and then incubated for 30 min under the various stress conditions in the presence of 14C-labelled amino acids followed by cell extraction, protein separation by 2D gel electrophoresis and image analysis of the resulting autoradiograms. Using consistent twofold or greater changes in IOD % as a measure, oxidative stress resulted in the upregulation of 69 proteins, 15 of which were oxidation-specific, and in the downregulation of 24 proteins, when compared to the control cells. An acid shock from pH 7.5 to 5.5 enhanced synthesis of 64 proteins, 25 of them acid-specific, while 49 proteins exhibited diminished synthesis. The dilution of BM resulted in the increased formation of 58 proteins, with 11 starvation-specific proteins and 20 showing decreased synthesis. Some 52 and 40 proteins were enhanced by salt and heat stress, with 10 and 6 of these proteins, respectively, specific to the stress. The synthesis of a significant number of proteins was increased by more than one, but not all stress conditions; six proteins were enhanced by all five stress conditions and could be classified as general stress proteins. Clearly, the response of S. mutans to adverse environmental conditions results in complex and diverse alterations in protein synthesis to further cell survival.

Acid-regulated proteins induced by Streptococcus mutans and other oral bacteria during acid shock

Our previous research has demonstrated that with the more aciduric oral bacteria, an acid shock to sub-lethal pH values results in the induction of an acid tolerance response that protects the cells at extremely low pH (pH 3.0-4.0) that kills unadapted control cells maintained at pH 7.5 (Oral Microbiol Immunol 1997: 12: 266-273). In this study, we were interested in comparing the protein profiles of acid-shocked and control cells of nine organisms from three acid-ogenic genera that could be categorized as strong, weak and non-acid responders in an attempt to identify proteins that could be classified as acid-regulated proteins and which may be important in the process of survival at very low pH. For this, log-phase cultures were rapidly acidified from pH 7.5 to 5.5 in the presence of [14C]-amino acids for varying periods up to 2 h, the period previously shown to be required for maximum induction of the acid response. The cells were extracted for total protein and subjected to one-dimensional sodium dodecyl sulfate-polyacrylamide chromatography with comparable control and acid-shocked protein profiles compared by scanning and computer analysis. Of particular interest were the proteins in the acid-shocked cells that showed enhanced labeling (i.e., synthesis) over the control cells, since these were considered acid-regulated proteins of importance in pH homeostasis. Streptococcus mutans LT11 generated the most rapid and complex pattern: a total of 36 acid-regulated proteins showing enhanced synthesis, with 25 appearing within the first 30 min of acid shock. The enhanced synthesis was transient with all proteins, with the exception of two with molecular weights of 50/49 and 33/32 kDa. Within the acid-regulated proteins were proteins having molecular weights comparable to the heat shock proteins and the various subunits of the membrane H+/ATPase. By comparison, the strong responder, Lactobacillus casei 151, showed the enhanced formation of only nine proteins within the first 30 min of the acid shock, with a total of 11 acid-regulated proteins formed during the 2-h adaptation period with enhanced synthesis transient for seven of these proteins. Streptococcus salivarius AT2 and Streptococcus gordonii TH12 had the formation of 6 and 8 proteins enhanced, while the weakly responding organisms, Streptococcus sanguis ATCC 10,556 and Streptococcus oralis ATCC 10,557, exhibited 8 and 6 such proteins, respectively. Even non-responding strains unable to survive at very low pH, such as Streptococcus sobrinus CH125/43, Streptococcus mitis ATCC 12,261 and Actinomyces naeslundii 301-13 showed the initial formation of 3-9 acid-regulated proteins, but protein synthesis was not sustained over the entire adaptation period. Clearly, the survival of oral bacteria at very low pH is related, not to the total number of the acid-regulated proteins induced per se but to the formation of key proteins that function to augment normal pH homeostasis.

Nucleotide sequence and molecular characterization of a gene encoding GTP-binding protein from Streptococcus gordonii

A 1286-bp fragment of chromosomal DNA from Streptococcus gordonii strain Challis was cloned and sequenced. The gene sgg consisted of 897-bp nucleotides encoding a 299-amino acid polypeptide (33,200 Da). The deduced amino acid sequence exhibited significant similarity to Era, G protein of Escherichia coli. The nucleotide binding assay demonstrated that recombinant Sgg bound [32P]GTP but not [32P]ATP, [32P]CTP, or [32P]UTP. These findings indicate that Sgg is a member of the G protein superfamily in the genus Streptococcus.

Acid tolerance response and survival by oral bacteria

Using 21 species of oral bacteria, representing six acidogenic genera, we undertook to determine whether the pH-limiting exponential growth is related to the ability of the organisms to generate an acid-tolerance response that results in enhanced survival at low pH. The lower pH limit of exponential growth varied by more than two units with that of Neisseria A182 at pH 6.34; growth of Lactobacillus casei RB1014 stopped at pH 3.81, with species of Actinomyces, Enterococcus, Prevotella and Streptococcus falling between these limits. The working hypothesis was that the organisms with the higher pH limits for growth are unable to respond to acidic environments in order to survive, whereas the more aciduric organisms would possess or acquire acid tolerance. Adaptation to acid tolerance was tested by determining whether the prior exposure of exponential-phase cells to a low, sub-lethal pH would trigger the induction of a mechanism that would enhance survival at a pH killing pH 7.5 control cells. The killing pH varied from pH 4.5 for Prevotella intermedia ATCC 25611 to pH 2.3 for the three Lactobacillus casei strains in the study, with the three Streptococcus mutans strains killed at pH 3.0 for 3 h. The adaptation experiments revealed three groups of organisms: non-acid-responders, generally representing strains with the highest terminal pH values; weak acid-responders in the middle of the pH list, generating low numbers of survivors at one or two pH values, and the aciduric, strong responders generating a high number of survivors at pH values in the range 6.0 to 3.5, but not at pH 7.5. Predominant among the latter group were the S. mutans and Lactobacilli casei strains, with the most significant adaptive response exhibited by S. mutans LT11 and S. mutans Ingbritt, involving a process that required protein synthesis. Time course experiments with the latter organisms indicated that 90-120 min was required after exposure to the triggering pH before the acid response was fully functional. These results indicate that the sudden exposure of strains of oral streptococci and lactobacilli, as well as Enterococcus faecalis, to pH values between 6.0 and 3.5 results in the induction of an acid tolerance response that enhances the survival of these strains at or below pH 3.5.