Effect of growth rate and pH on intracellular levels and activities of the components of the phosphoenolpyruvate: sugar phosphotransferase system in Streptococcus mutans Ingbritt

Metabolic and transcriptional analysis of acid stress in Lactococcus lactis, with a focus on the kinetics of lactic acid pools

The effect of pH on the glucose metabolism of non-growing cells of L. lactis MG1363 was studied by in vivo NMR in the range 4.8 to 6.5. Immediate pH effects on glucose transporters and/or enzyme activities were distinguished from transcriptional/translational effects by using cells grown at the optimal pH of 6.5 or pre-adjusted to low pH by growth at 5.1. In cells grown at pH 5.1, glucose metabolism proceeds at a rate 35% higher than in non-adjusted cells at the same pH. Besides the upregulation of stress-related genes (such as dnaK and groEL), cells adjusted to low pH overexpressed H(+)-ATPase subunits as well as glycolytic genes. At sub-optimal pHs, the total intracellular pool of lactic acid reached approximately 500 mM in cells grown at optimal pH and about 700 mM in cells grown at pH 5.1. These high levels, together with good pH homeostasis (internal pH always above 6), imply intracellular accumulation of the ionized form of lactic acid (lactate anion), and the concomitant export of the equivalent protons. The average number, n, of protons exported with each lactate anion was determined directly from the kinetics of accumulation of intra- and extracellular lactic acid as monitored online by (13)C-NMR. In cells non-adjusted to low pH, n varies between 2 and 1 during glucose consumption, suggesting an inhibitory effect of intracellular lactate on proton export. We confirmed that extracellular lactate did not affect the lactate: proton stoichiometry. In adjusted cells, n was lower and varied less, indicating a different mix of lactic acid exporters less affected by the high level of intracellular lactate. A qualitative model for pH effects and acid stress adaptation is proposed on the basis of these results.

Glucokinase contributes to glucose phosphorylation in d-lactic acid production by Sporolactobacillus inulinus Y2-8

Sporolactobacillus inulinus, a homofermentative lactic acid bacterium, is a species capable of efficient industrial d-lactic acid production from glucose. Glucose phosphorylation is the key step of glucose metabolism, and fine-tuned expression of which can improve d-lactic acid production. During growth on high-concentration glucose, a fast induction of high glucokinase (GLK) activity was observed, and paralleled the patterns of glucose consumption and d-lactic acid accumulation, while phosphoenolpyruvate phosphotransferase system (PTS) activity was completely repressed. The transmembrane proton gradient of 1.3–1.5 units was expected to generate a large proton motive force to the uptake of glucose. This suggests that the GLK pathway is the major route for glucose utilization, with the uptake of glucose through PTS-independent transport systems and phosphorylation of glucose by GLK in S. inulinus  d-lactic acid production. The gene encoding GLK was cloned from S. inulinus and expressed in Escherichia coli. The amino acid sequence revealed significant similarity to GLK sequences from Bacillaceae. The recombinant GLK was purified and shown to be a homodimer with a subunit molecular mass of 34.5 kDa. Strikingly, it demonstrated an unusual broad substrate specificity, catalyzing phosphorylation of 2-deoxyglucose, mannitol, maltose, galactose and glucosamine, in addition to glucose. This report documented the key step concerning glucose phosphorylation of S. inulinus, which will help to understand the regulation of glucose metabolism and d-lactic acid production.

A combined physiological and proteomic approach to reveal lactic-acid-induced alterations in Lactobacillus casei Zhang and its mutant with enhanced lactic acid tolerance

Lactobacillus casei has traditionally been recognized as a probiotic and frequently used as an adjunct culture in fermented dairy products, where acid stress is an environmental condition commonly encountered. In the present study, we carried out a comparative physiological and proteomic study to investigate lactic-acid-induced alterations in Lactobacillus casei Zhang (WT) and its acid-resistant mutant. Analysis of the physiological data showed that the mutant exhibited 33.8% higher glucose phosphoenolpyruvate:sugar phosphotransferase system activity and lower glycolytic pH compared with the WT under acidic conditions. In addition, significant differences were detected in both cells during acid stress between intracellular physiological state, including intracellular pH, H(+)-ATPase activity, and intracellular ATP pool. Comparison of the proteomic data based on 2D-DIGE and i-TRAQ indicated that acid stress invoked a global change in both strains. The mutant protected the cells against acid damage by regulating the expression of key proteins involved in cellular metabolism, DNA replication, RNA synthesis, translation, and some chaperones. Proteome results were validated by Lactobacillus casei displaying higher intracellular aspartate and arginine levels, and the survival at pH 3.3 was improved 1.36- and 2.10-fold by the addition of 50-mM aspartate and arginine, respectively. To our knowledge, this is the first demonstration that aspartate may be involved in acid tolerance in Lactobacillus casei. Results presented here may help us understand acid resistance mechanisms and help formulate new strategies to enhance the industrial applications of this species.

Characterization of the individual glucose uptake systems of Lactococcus lactis: mannose-PTS, cellobiose-PTS and the novel GlcU permease

According to previous reports, Lactococcus lactis imports glucose via two distinct phosphoenolpyruvate:phosphotransferase systems (mannose-PTS and cellobiose-PTS) and one or more unknown non-PTS permease(s). GlcU was identified as the sole non-PTS permease involved in the transport of glucose. Additionally, the biochemical properties of PTS(Man), PTS(Cel) and GlcU were characterized in double knockout mutants with glucose uptake restricted to a single system. Transport susceptibility to protonophores indicated that glucose uptake via GlcU is proton-motive force dependent. Competition assays revealed a high specificity of GlcU for glucose. Furthermore, the permease has low affinity for glucose and displays strong preference for the beta-anomer as shown by the profiles of consumption of the two glucose anomers studied by (13)C-NMR. Similar kinetic properties were found for PTS(Cel), while PTS(Man) is a high-affinity system recognizing equally well the two anomeric forms of glucose. Transcripts of the genes encoding the three transporters are present simultaneously in the parent strain NZ9000 as shown by reverse transcription-PCR. Investigation of the distribution of GlcU homologues among bacteria showed that these proteins are restricted to the low-GC Gram-positive Firmicutes. This work completes the identification of the glucose transport systems in L. lactis MG1363.

Isolation and solubilization of cellular membrane proteins from bacteria

Membrane proteins are rarely identified in two-dimensional electrophoretic (2-DE) proteomics maps. This is because of low abundance, poor solubility, and inherent hydrophobicity. In this study, membrane preparations from the Gram-positive bacterium Streptococcus mutans were isolated from protoplasts and by mechanical grinding. Membrane proteins were extracted using a mixture of trifluroethanol and chloroform, solubilized using highly chaotropic buffer containing ASB-14 and Triton X-100 and subjected to two-dimensional gel electrophoresis.

Assessment of genes associated with Streptococcus mutans biofilm morphology

Streptococcus mutans, the major pathogen responsible for dental caries in humans, is a biofilm-forming bacterium. In the present study, 17 different pulsed-field gel electrophoresis patterns of genomic DNA were identified in S. mutans organisms isolated clinically from whole saliva. The S. mutans isolates showed different abilities to form biofilms on polystyrene surfaces in semidefined minimal medium cultures. Following cultivation in a flow cell system in tryptic soy broth with 0.25% sucrose and staining using a BacLight LIVE/DEAD system, two strains, designated FSC-3 and FSC-4, showed the greatest and least, respectively, levels of biofilm formation when examined with confocal laser scanning microscopy. Further, image analyses of spatial distribution and architecture were performed to quantify the merged green (live cells) and red (dead cells) light. The light intensity of the FSC-3 biofilm was greater than that of the FSC-4 biofilm in the bottom area but not in the top area. S. mutans whole-genome array results showed that approximately 3.8% of the genes were differentially expressed in the two strains, of which approximately 2.2%, including bacitracin transport ATP-binding protein gene glrA and a BLpL-like putative immunity protein gene, were activated in FSC-3. In addition, about 1.6% of the genes, including those associated with phosphotransferase system genes, were repressed. Analyses of the glrA-deficient strains and reverse transcription-PCR confirmed the role of the gene in biofilm formation. Differential assessment of biofilm-associated genes in clinical strains may provide useful information for understanding the morphological development of streptococcal biofilm, as well as for colonization of S. mutans.

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.

Up-regulation of competence- but not stress-responsive proteins accompanies an altered metabolic phenotype in Streptococcus mutans biofilms

Mature biofilm and planktonic cells of Streptococcus mutans cultured in a neutral pH environment were subjected to comparative proteome analysis. Of the 242 protein spots identified, 48 were significantly altered in their level of expression (P<0.050) or were unique to planktonic or biofilm-grown cells. Among these were four hypothetical proteins as well as proteins known to be associated with the maintenance of competence or found to possess a cin-box-like element upstream of their coding gene. Most notable among the non-responsive genes were those encoding the molecular chaperones DnaK, GroEL and GroES, which are considered to be up-regulated by sessile growth. Analysis of the rest of the proteome indicated that a number of cellular functions associated with carbon uptake and cell division were down-regulated. The data obtained were consistent with the hypothesis that a reduction in the general growth rate of mature biofilms of S. mutans in a neutral pH environment is associated with the maintenance of transformation without the concomitant stress response observed during the transient state of competence in bacterial batch cultures.

Solubilization of cellular membrane proteins fromStreptococcus mutans for two-dimensional gel electrophoresis

Membrane proteins are rarely identified in two-dimensional electrophoretic (2-DE) proteomics maps. This is due to low abundancy, poor solubility, and inherent hydrophobicity leading to self-aggregation during the first dimension. In this study, membrane proteins from the Gram-positive bacterium Streptococcus mutans were solubilized using three different methods and evaluated by 2-DE. In the first method, the extraction was performed using sodium dodecyl sulfate (SDS) followed by solubilization with a chaotropic buffer and precipitation with methanol/chloroform. The second method was based on temperature-dependent phase partitioning using Triton X-114 followed by purification using the ReadyPrep 2-D clean-up kit from Bio-Rad. The third method involved extraction using the organic solvents trifluoroethanol (TFE) and chloroform, which produced three separate phases. The upper aqueous phase, enriched with TFE, gave the highest overall protein yield and best 2-DE resolution. Protein spot identification by nanoelectrospray quadrupole time of flight (QTOF)-tandem mass spectrometry (MS/MS) revealed known membrane and surface-associated proteins. This is the first report describing the successful solubilization and 2-D electrophoresis of membrane proteins from a Gram-positive bacterium.

Adaptive acid tolerance response of Streptococcus sobrinus

Streptococcus mutans and Streptococcus sobrinus are the bacteria most commonly associated with human dental caries. A major virulence attribute of these and other cariogenic bacteria is acid tolerance. The acid tolerance mechanisms of S. mutans have begun to be investigated in detail, including the adaptive acid tolerance response (ATR), but this is not the case for S. sobrinus. An analysis of the ATR of two S. sobrinus strains was conducted with cells grown to steady state in continuous chemostat cultures. Compared with cells grown at neutral pH, S. sobrinus cells grown at pH 5.0 showed an increased resistance to acid killing and were able to drive down the pH through glycolysis to lower values. Unlike what is found for S. mutans, the enhanced acid tolerance and glycolytic capacities of acid-adapted S. sobrinus were not due to increased F-ATPase activities. Interestingly though, S. sobrinus cells grown at pH 5.0 had twofold more glucose phosphoenolpyruvate:sugar phosphotransferase system (PTS) activity than cells grown at pH 7.0. In contrast, glucose PTS activity was actually higher in S. mutans grown at pH 7.0 than in cells grown at pH 5.0. Silver staining of two-dimensional gels of whole-cell lysates of S. sobrinus 6715 revealed that at least 9 proteins were up-regulated and 22 proteins were down-regulated in pH 5.0-grown cells compared with cells grown at pH 7.0. Our results demonstrate that S. sobrinus is capable of mounting an ATR but that there are critical differences between the mechanisms of acid adaptation used by S. sobrinus and 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.

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.

Analysis of Streptococcus mutans proteins modulated by culture under acidic conditions

Streptococcus mutans, a major etiological agent of dental caries, causes demineralization of the tooth tissue due to the formation of acids from dietary carbohydrates. Dominant among the virulence determinants of this organism are aciduricity and acidogenicity, the abilities to grow at low pH and to produce acid, respectively. The mechanisms underlying the ability of S. mutans to survive and proliferate at low pH are currently under investigation. In this study we cultured S. mutans at pH 5.2 or 7.0 and extracted soluble cellular proteins. These were analyzed using high-resolution two-dimensional gel electrophoresis, and replicate maps of proteins expressed under each of the two conditions were generated. Proteins with modulated expression at low pH, as judged by a change in the relative integrated optical density, were excised and digested with trypsin by using an in-gel protocol. Tryptic digests were analyzed using matrix-assisted laser desorption ionization mass spectrometry to generate peptide mass fingerprints, and these were used to assign putative functions according to their homology with the translated sequences in the S. mutans genomic database. Thirty individual proteins exhibited altered expression as a result of culture of S. mutans at low pH. Up-regulated proteins (n = 18) included neutral endopeptidase, phosphoglucomutase, 60-kDa chaperonin, cell division proteins, enolase, lactate dehydrogenase, fructose bisphosphate aldolase, acetoin reductase, superoxide dismutase, and lactoylglutathione lyase. Proteins down-regulated at pH 5.2 (n = 12) included protein translation elongation factors G, Tu, and Ts, DnaK, small-subunit ribosomal protein S1P, large-subunit ribosomal protein L12P, and components of both phosphoenolpyruvate:protein phosphotransferase and multiple sugar binding transport systems. The identification of proteins differentially expressed following growth at low pH provides new information regarding the mechanisms of survival and has identified new target genes for mutagenesis studies to further assess their physiological significance.

Genetics of acid adaptation in oral streptococci

A growing body of information has provided insights into the mechanisms by which the oral streptococci maintain their niches in the human mouth. In at least one case, Streptococcus mutans, the organism apparently uses a panel of proteins to survive in acidic conditions while it promotes the formation of dental caries. Oral streptococci, which are not as inherently resistant to acidification, use protective schemes to ameliorate acidic plaque pH values. Existing information clearly shows that while the streptococci are highly related, very different strategies have evolved for them to take advantage of their particular location in the oral cavity. The picture that emerges is that the acid-adaptive regulatory mechanisms of the oral streptococci differ markedly from those used by Gram-negative bacteria. What future research must determine is the extent and complexity of the acid-adaptive systems in these organisms and how they permit the organisms to maintain themselves in the face of a low-pH environment and the microbial competition present in their respective niches.

Adaptation of oral streptococci to low pH

The strategies employed by oral streptococci to resist the inimical influences of acidification reflect the diverse and dynamic niches of the human mouth. All of the oral streptococci are capable of rapid degradation of sugar to acidic end-products. As a result, the pH value of their immediate environment can plummet to levels where glycolysis and growth cease. At this point, the approaches for survival in acid separate the organisms. Streptococcus mutans, for example, relies on its F-ATPase, to protect itself from acidification by pumping protons out of the cells. S. salivarius responds by degrading urea to ammonia and S. sanguis produces ammonia by arginolysis. The mechanisms by which these organisms regulate their particular escape route are now being explored experimentally. The picture that emerges is that the acid-adaptive regulatory mechanisms of the oral streptococci differ markedly from those employed by Gram-negative bacteria. What remains to be elucidated are the breadth of the acid-response systems in these organisms and how they permit the microbes to sustain themselves in the face of low pH and the bacterial competition present in their respective niches. In this article, we summarize reports concerning the means by which oral streptococci either utilize acidification to subdue their competitors or protect themselves until pH values return to a more favorable level.

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.

Survival of oral bacteria

The global distribution of individual species of oral bacteria demonstrates their ability to survive among their human hosts. Such an ubiquitous existence is the result of efficient transmission of strains and their persistence in the oral environment. Genetic analysis has identified specific clones of pathogenic bacteria causing infection. Presumably, these express virulence-associated characteristics enhancing colonization and survival in their hosts. A similar situation may occur with the oral resident flora, where genetic variants may express specific phenotypic characteristics related to survival. Survival in the mouth is enhanced by dental plaque formation, where persistence is associated with the bacteria's capacity not only to adhere and grow, but also to withstand oxygen, wide fluctuations in pH and carbohydrate concentration, and a diverse array of microbial interactions. Streptococcus mutans has been discussed as a 'model' organism possessing the biochemical flexibility that permits it to persist and dominate the indigenous microflora under conditions of stress.

Analysis of Streptococcus salivarius urease expression using continuous chemostat culture

Alkali production from urea by bacterial ureases in the oral cavity is thought to have a major impact on oral health and on the physiology and ecology of oral bacteria. Using continuous chemostat culture, urease activity in Streptococcus salivarius 57.I was examined as a function of growth pH, carbohydrate availability and growth rate. A portion of the S. salivarius ureC gene was amplified by polymerase chain reactions (PCRs) using degenerate primers encoding highly conserved sequences from known ureases. The nucleotide sequence of the PCR product was determined, and was used to compare the level of urease gene expression under different growth conditions. The data indicated that urease was highly expressed at low pH, and expression was also modulated by glucose availability and growth rate. Differential expression was controlled, at least in part, at the transcriptional level.

Glucose transport by a mutant of Streptococcus mutans unable to accumulate sugars via the phosphoenolpyruvate phosphotransferase system

Streptococcus mutans transports glucose via the phosphoenolpyruvate (PEP)-dependent sugar phosphotransferase system (PTS). Earlier studies indicated that an alternate glucose transport system functions in this organism under conditions of high growth rates, low pH, or excess glucose. To identify this system, S. mutans BM71 was transformed with integration vector pDC-5 to generate a mutant, DC10, defective in the general PTS protein enzyme I (EI). This mutant expressed a defective EI that had been truncated by approximately 150 amino acids at the carboxyl terminus as revealed by Western blot (immunoblot) analysis with anti-EI antibody and Southern hybridizations with a fragment of the wild-type EI gene as a probe. Phosphotransfer assays utilizing 32P-PEP indicated that DC10 was incapable of phosphorylating HPr and EIIAMan, indicating a nonfunctional PTS. This was confirmed by the fact that DC10 was able to ferment glucose but not a variety of other PTS substrates and phosphorylated glucose with ATP and not PEP. Kinetic assays indicated that the non-PTS system exhibited an apparent Ks of 125 microM for glucose and a Vmax of 0.87 nmol mg (dry weight) of cells-1 min-1. Sugar competition experiments with DC10 indicated that the non-PTS transport system had high specificity for glucose since glucose transport was not significantly by a 100-fold molar excess of several competing sugar substrates, including 2-deoxyglucose and alpha-methylglucoside. These results demonstrate that S. mutans possesses a glucose transport system that can function independently of the PEP PTS.

Kinetics of growth and product formation in cultures from streptococci of groups A and C

During growth of streptococci of Lancefield groups A and C in a culture medium containing glucose, yeast extract and peptone, two main growth phases occur: growth phase I and growth phase II (diauxic growth). They are separated by a short stationary phase (1st stationary phase). The diauxic growth is caused by transient limitations as well as the availability of new sources of the amino acids L-serine and L-arginine. Growth phase I consists of an exponential and a nearly linear part. These growth kinetics are reflected by the kinetics of gas metabolism as well as by product formation. Hyaluronic acid is formed during the nearly linear phase whereas the enzyme alkaline phosphatase, is exclusively excreted in the 1st stationary phase. Also carbon dioxide and L-lactate are mainly produced in a growth phase-dependent mode. In the late stationary phase (2nd stationary phase) more oxygen is consumed whereas the demand for oxygen in the 1st stationary phase is nearly zero.

Sequence and expression of the genes for HPr (ptsH) and enzyme I (ptsI) of the phosphoenolpyruvate-dependent phosphotransferase transport system from Streptococcus mutans

We report the sequencing of a 2,242-bp region of the Streptococcus mutants NG5 genome containing the genes for ptsH and ptsI, which encode HPr and enzyme I (EI), respectively, of the phosphoenolpyruvate-dependent phosphotransferase transport system. The sequence was obtained from two cloned overlapping genomic fragments; one expresses HPr and a truncated EI, while the other expresses a full-length EI in Escherichia coli, as determined by Western immunoblotting. The ptsI gene appeared to be expressed from a region located in the ptsH gene. The S. mutans NG5 pts operon does not appear to be linked to other phosphotransferase transport system proteins as has been found in other bacteria. A positive fermentation pattern on MacConkey-glucose plates by an E. coli ptsI mutant harboring the S. mutans NG5 ptsI gene on a plasmid indicated that the S. mutans NG5 EI can complement a defect in the E. coli gene. This was confirmed by protein phosphorylation experiments with 32P-labeled phosphoenolpyruvate indicating phosphotransfer from the S. mutans NG5 EI to the E. coli HPr. Two forms of the cloned EI, both truncated to varying degrees in the C-terminal region, were inefficiently phosphorylated and unable to complement fully the ptsI defect in the E. coli mutant. The deduced amino acid sequence of HPr shows a high degree of homology, particularly around the active site, to the same protein from other gram-positive bacteria, notably, S. salivarius, and to a lesser extent with those of gram-negative bacteria. The deduced amino acid sequence of S. mutans NG5 EI also shares several regions of homology with other sequenced EIs, notably, with the region around the active site, a region that contains the only conserved cystidyl residue among the various proteins and which may be involved in substrate binding.

Amino-terminal methionine processing of the protein HPr in Streptococcus salivarius grown in continuous culture

HPr is a protein of the phosphoenolpyruvate:sugar phosphotransferase system (PTS). Streptococci possess two forms of HPr which differ by the presence or the absence of the N-terminal methionine (Met). These forms are called HPr-1 (without Met) and HPr-2 (with Met). In order to determine whether the ratio of these two forms varies with growth conditions, we measured the amount of HPr-1 and HPr-2 present in Streptococcus salivarius grown in continuous culture at pH 7.5. The results indicated that the HPr-1/HPr-2 ratio: 1) was not related to the cellular amount of total HPr; 2) was highest (10.2 +/- 3.5) under glucose (a PTS sugar) limitation (10 mM) and low dilution rate (D = 0.1 h-1; g = 6.9 h); 3) was decreased 2.4- to 5.7-fold when the amount of glucose and/or D was increased; 4) was not influenced by D when cells were cultured on galactose (a non-PTS sugar) but was two-fold higher under conditions of galactose excess (200 mM). We suggest that the cleavage of the N-terminal HPr Met is not a stochastic phenomenon but is dictated by growth conditions.

Evidence for catabolite inhibition in regulation of pentose utilization and transport in the ruminal bacterium Selenomonas ruminantium

Pentose sugars can be an important energy source for ruminal bacteria, but there has been relatively little study regarding the regulation of pentose utilization and transport by these organisms. Selenomonas ruminantium, a prevalent ruminal bacterium, actively metabolizes xylose and arabinose. When strain D was incubated with a combination of glucose and xylose or arabinose, the hexose was preferentially utilized over pentoses, and similar preferences were observed for sucrose and maltose. However, there was simultaneous utilization of cellobiose and pentoses. Continuous-culture studies indicated that at a low dilution rate (0.10 h-1) the organism was able to co-utilize glucose and xylose. This co-utilization was associated with growth rate-dependent decreases in glucose phosphotransferase activity, and it appeared that inhibition of pentose utilization was due to catabolite inhibition by the glucose phosphotransferase transport system. Xylose transport activity in strain D required induction, while arabinose permease synthesis did not require inducer but was subject to repression by glucose. Since an electrical potential or a chemical gradient of protons drove xylose and arabinose uptake, pentose-proton symport systems apparently contributed to transport.

Cloning, sequencing and expression in Escherichia coli of the ptsI gene encoding enzyme I of the phosphoenolpyruvate:sugar phosphotransferase transport system from Streptococcus salivarius

We present the cloning and sequencing of the ptsI gene, encoding enzyme I (EI) of the phosphoenolpyruvate (PEP): sugar phosphotransferase (PTS) transport system from Streptococcus salivarius. The ptsI gene corresponds to an open reading frame of 1731 nucleotides, which translates into a putative 577-amino acid (aa) protein with a M(r) of 62,948 and a pI of 4.49. The EI was produced in Escherichia coli under the control of its own promoter located immediately upstream of ptsI, a situation never previously reported for any other gene coding for an EI. The deduced aa sequence of the S. salivarius EI shows a high degree of similarity with the E. coli EI and the EI moiety of the multiphosphoryl transfer protein from Rhodobacter capsulatus. The S. salivarius EI also shares a highly conserved aa cluster with a non-PTS protein, the maize pyruvate:orthophosphate dikinase. The conserved cluster is located in a domain which is hypothesized to be the PEP-binding site.