Genetic regulation of fructosyltransferase in Streptococcus mutans

Propolis nanoparticle enhances the potency of antimicrobial photodynamic therapy against Streptococcus mutans in a synergistic manner

Less invasive removal approaches have been recommended for deep caries lesions. Antimicrobial photodynamic therapy (aPDT) and propolis nanoparticle (PNP) are highlighted for the caries management plan. Evidence is lacking for an additive effect of combination PNP with photosensitizer (PS) in aPDT. This study aimed to investigate the individual and synergistic effects of chlorophyllin-phycocyanin mixture (PhotoActive⁺) and toluidine blue O (TBO) as PSs in combination with PNP in the aPDT process (aPDT^plus) against major important virulence factors of Streptococcus mutans. Following characterization, biocompatibility of the PSs alone, or in combination with PNP were investigated on human gingival fibroblast cell. The in vitro synergy of PhotoActive⁺ or TBO and PNP was evaluated by the checkerboard method. The bacteria's virulence properties were surveyed in the presence of the PSs, individually as well as in combination. When the PSs were examined in combination (synergistic effect, FIC Index < 0.5), a stronger growth inhibitory activity was exhibited than the individual PSs. The biofilm formation, as well as genes involved in biofilm formation, showed greater suppression when the PSs were employed in combination. Overall, the results of this study suggest that the combination of PhotoActive⁺ or TBO with PNP with the least cytotoxicity effects and the highest antimicrobial activites would improve aPDT outcomes, leading to synergistic effects and impairing the virulence of S. mutans.

A new small molecule specifically inhibits the cariogenic bacterium Streptococcus mutans in multispecies biofilms

Streptococcus mutans is a major cariogenic bacterium. It has adapted to the biofilm lifestyle, which is essential for pathogenesis of dental caries. We aimed to identify small molecules that can inhibit cariogenic S. mutans and to discover lead structures that could give rise to therapeutics for dental caries. In this study, we screened a focused small-molecule library of 506 compounds. Eight small molecules which inhibited S. mutans at a concentration of 4 μM or less but did not affect cell growth or biofilm formation of commensal bacteria, represented by Streptococcus sanguinis and Streptococcus gordonii, in monospecies biofilms were identified. The active compounds share similar structural properties, which are characterized by a 2-aminoimidazole (2-AI) or 2-aminobenzimidazole (2-ABI) subunit. In multispecies biofilm models, the most active compound also inhibited cell survival and biofilm formation of S. mutans but did not affect commensal streptococci. This inhibitor downregulated the expression of six biofilm-associated genes, ftf, pac, relA, comDE, gbpB, and gtfB, in planktonic S. mutans cells, while it downregulated the expression of only ftf, pac, and relA in the biofilm cells of S. mutans. The most potent compound also inhibited production of two key adhesins of S. mutans, antigen I/II and glucosyltransferase (GTF). However, the compound did not alter the expression of the corresponding genes in both S. sanguinis and S. gordonii, indicating that it possesses a selective inhibitory activity against S. mutans.

Transcriptional regulation and signal-peptide-dependent secretion of exolevanase (LsdB) in the endophyte Gluconacetobacter diazotrophicus

Gluconacetobacter diazotrophicus utilizes plant sucrose with a constitutively expressed levansucrase (LsdA), producing extracellular levan, which may be degraded under energetically unfavored conditions. Reverse transcriptase-PCR analysis revealed that lsdA and the downstream exolevanase gene (lsdB) form an operon. lsdB transcription was induced during growth with low fructose concentrations (0.44 to 33 mM) and repressed by glucose. Transport of LsdB to the periplasm involved N-terminal signal peptide cleavage. Type II secretion mutants failed to transfer LsdB across the outer membrane, impeding levan hydrolysis.

Expression of biofilm-associated genes of Streptococcus mutans in response to glucose and sucrose

Streptococcus mutans is known as a primary pathogen of dental caries, one of the most common human infectious diseases. Exopolysaccharide synthesis, adherence to tooth surface and biofilm formation are important physiological and virulence factors of S. mutans. In vitro comparative gene expression analysis was carried out to differentiate 10 selected genes known to be mostly involved in S. mutans biofilm formation by comparing the expression under biofilm and planktonic environments. Real-time RT-PCR analyses indicated that all of the genes tested were upregulated in the biofilm compared to cells grown in planktonic conditions. The influence of simple dietary carbohydrates on gene expression in S. mutans biofilm was tested also. Among the tested genes, in the biofilm phase, the greatest induction was observed for gtf and ftf, which are genes encoding the extracellular polysaccharide-producing enzymes. Biofilm formation was accompanied by a 22-fold induction in the abundance of mRNA encoding glucosyltransferase B (GTFB) and a 14.8 -fold increase in mRNA encoding GTFC. Levels of mRNA encoding fructosyltransferase were induced approximately 11.8-fold in biofilm-derived cells. Another notable finding of this study suggests that glucose affects the expression of S. mutans GS5 biofilm genes. In spite of a significant upregulation in biofilm-associated gene expression in the presence of sucrose, the presence of glucose with sucrose reduced expression of most tested genes. Differential analysis of the transcripts from S. mutans, grown in media with various nutrient contents, revealed significant shifts in the expression of the genes involved in biofilm formation. The results presented here provide new insights at the molecular level regarding gene expression in this bacterium when grown under biofilm conditions, allowing a better understanding of the mechanism of biofilm formation by S. mutans.

Sucrose utilization and impact of sucrose on glycosyltransferase expression in Lactobacillus reuteri

Glycosyltransferases of lactic acid bacteria are associated with biofilm formation, bacterial stress response and sucrose metabolism. The aim of this study was to determine the contribution of glycosyltransferases to sucrose metabolism in Lactobacillus reuteri TMW1.106 expressing the glucosyltransferase GtfA and the inulosucrase Inu, and L. reuteri LTH 5448 expressing the fructosyltransferase FtfA. Transcriptional analysis using quantitative real time PCR revealed that expression of ftfA of L. reuteri LTH5448 was induced by sucrose, while sucrose had no effect on gtfA and inu expression of strain TMW 1.106. Inactivation of ftfA had no influence on growth of L. reuteri LTH5448 and only a minor impact on sucrose turnover. L. reuteri TMW1.106 and its gtfA and inu mutants reached similar cell counts when maltose was offered as substrate. Mutation of gtfA or inu impaired growth in media containing sucrose as sole carbon source despite the expression of sucrose phosphorylase as an alternative sucrose-hydrolysing enzyme. Moreover, the gtfA and inu mutants formed less lactate and ethanol and tolerated lower lactate levels compared to L. reuteri TMW1.106. The inu mutant constitutively overexpressed GtfA. We show here that the impact of different glycosyltransferases on sucrose metabolism of L. reuteri is strain dependent. In strain L. reuteri TMW 1.106, GtfA accounts for sucrose utilization, metabolism, and growth of the organism. In contrast, FtfA of L. reuteri LTH5448 contributes to sucrose turnover but alternative routes for sucrose metabolism are functional in this strain. Our data thus indicate that these glycosyltransferases affect the competitiveness of some L. reuteri strains in ecosystems where sucrose is present.

Differential expression profiles of Streptococcus mutans ftf, gtf and vicR genes in the presence of dietary carbohydrates at early and late exponential growth phases

Dental caries is one of the most common infectious diseases that affects humans. Streptococcus mutans, the main pathogenic bacterium associated with dental caries, produces a number of extracellular sucrose-metabolizing enzymes, such as glucosyltransferases (GTFB, GTFC and GTFD) and fructosyltransferase (FTF). The cooperative action of these enzymes is essential for sucrose-dependent cellular adhesion and biofilm formation. A global response regulator (vicR) plays important roles in S. mutans ftf and gtf expression in response to a variety of stimuli. A real-time reverse-transcription polymerase chain-reaction was used to quantify the relative levels of ftf, gtfB, gtfC, gtfD and vicR transcription of S. mutans in the presence of various dietary carbohydrates: sucrose, D-glucose, D-fructose, D-glucitol (D-sorbitol), D-mannitol and xylitol. Ftf was highly expressed at late exponential phase in the presence of sorbitol and mannitol. GtfB was highly expressed in the presence of all the above carbohydrates except for xylitol at early exponential growth phase and glucose and fructose at late exponential growth phase. Similar to gtfB, the expression of gtfC was also induced with the presence of all the tested carbohydrates except for xylitol at early growth and glucose and fructose at late exponential phase. In addition, no effect of mannitol on gtfC expression at early exponential phase was observed. GtfD was less influenced compared to the gtfB and gtfC, demonstrating enhanced expression especially in the presence of sorbitol, glucose, mannitol and xylitol at early exponential phase and mannitol at late exponential phase. VicR expression was induced only at the presence of xylitol at late exponential phase, and a decrease in expression was recorded at early exponential phase. Our findings show that dietary carbohydrates have a major influence on the transcription of ftf, gtfB, gtfC and gtfD, but less on vicR. Sorbitol and mannitol, which are considered as noncariogenic sugar substitutes, may indirectly affect caries by promoting biofilm formation via enhanced expression of gtfs and ftf. These results suggest regulatory circuits for exopolysaccharide gene expression in S. mutans.

A pleiotropic regulator, Frp, affects exopolysaccharide synthesis, biofilm formation, and competence development in Streptococcus mutans

Exopolysaccharide synthesis, biofilm formation, and competence are important physiologic functions and virulence factors for Streptococcus mutans. In this study, we report the role of Frp, a transcriptional regulator, on the regulation of these traits crucial to pathogenesis. An Frp-deficient mutant showed decreased transcription of several genes important in virulence, including those encoding fructosyltransferase (Ftf), glucosyltransferase B (GtfB), and GtfC, by reverse transcription and quantitative real-time PCR. Expression of Ftf was decreased in the frp mutant, as assessed by Western blotting as well as by the activity assays. Frp deficiency also inhibited the production of GtfB in the presence of glucose and sucrose as well as the production of GtfC in the presence of glucose. As a consequence of the effects on GtfB and -C, sucrose-induced biofilm formation was decreased in the frp mutant. The expression of competence mediated by the competence-signaling peptide (CSP) system, as assessed by comC gene transcription, was attenuated in the frp mutant. As a result, the transformation efficiency was decreased in the frp mutant but was partially restored by adding synthetic CSP. Transcription of the frp gene was significantly increased in the frp mutant under all conditions tested, indicating that frp transcription is autoregulated. Furthermore, complementation of the frp gene in the frp mutant restored transcription of the affected genes to levels similar to those in the wild-type strain. These results suggest that Frp is a novel pleiotropic effector of multiple cellular functions and is involved in the modulation of exopolysaccharide synthesis, sucrose-dependent biofilm formation, and competence development.

Surface plasmon resonance for real-time evaluation of immobilized fructosyltransferase activity

The extracellular enzyme fructosyltransferase (FTF) is considered to be a significant virulence factor in the dental biofilm. We have developed a method using surface plasmon resonance to detect the activity of immobilized FTF in situ. This real time technique provides a sensitive direct assay for characterizing functional properties of immobilized enzymes such as FTF.

Effect of carbohydrates on fructosyltransferase expression and distribution in Streptococcus mutans GS-5 biofilms

Streptococcus mutans produces a fructosyltransferase (FTF) enzyme, which synthesizes fructan polymers from sucrose. Fructans contribute to the virulence of the biofilm by acting as binding sites for S. mutans adhesion and as extracellular nutrition reservoir for the oral bacteria. Antibodies raised against a recombinant S. mutans FTF were used to test the effect of glucose, fructose, and sucrose on FTF expression in S. mutans GS-5 biofilms. Biofilms formed in the presence of fructose and glucose showed a higher ratio of FTF compared to biofilms formed in the presence of sucrose. Confocal laser scanning microscopy images of S. mutans biofilms indicated a carbohydrate-dependent FTF distribution. The layer adjacent to the surface and those at the liquid interface displayed high amounts cell-free FTF with limited amount of bacteria while the in-between layers demonstrated both cell-free FTF and cells expressing cell-surface FTF. Biofilm of S. mutans grown on hydroxyapatite surfaces expressed several FTF bands with molecular masses of 160, 125, 120, 100, and 50 kDa, as detected by using FTF specific antibodies. The results show that FTF expression and distribution in S. mutans GS-5 biofilms is carbohydrate regulated.

RegM is required for optimal fructosyltransferase and glucosyltransferase gene expression inStreptococcus mutans

Glucosyltransferases (Gtfs) and fructosyltransferase (Ftf), and the exopolysaccharides they produce, facilitate bacterial adherence and biofilm formation, and enhance the virulence of Streptococcus mutans. In this study, we used continuous chemostat cultures and reporter gene fusions to study the expression of ftf and gtfBC in response to carbohydrate availability and pH, and to asses the role of a protein similar to catabolite control protein A (CcpA), RegM, in regulation of these genes. Expression of ftf was efficient at pH 7.0 and 6.0, but was repressed at pH 5.0 under glucose-excess conditions. At pH 7.0, ftf expression was 5-fold lower under glucose-limiting conditions than in cells growing with an excess of glucose. Expression of gtfBC was also sensitive, albeit to a lesser extent, to pH and glucose availability. Inactivation of regM resulted in decreases of as much as 10-fold in both ftf and gtfBC expression, depending on growth conditions. These findings reinforce the importance of pH and carbohydrate availability for expression of two primary virulence attributes of S. mutans and reveal a critical role for RegM in regulation of expression of both gtfBC and ftf.

A two-component covRS regulatory system regulates expression of fructosyltransferase and a novel extracellular carbohydrate in Streptococcus mutans

The expression of fructosyltransferase (FTF), the enzyme that synthesizes fructan from sucrose, is regulated in the cariogenic bacterium Streptococcus mutans. However, the exact mechanism of FTF regulation is unknown. In this study, the role of a two-component regulatory system (covRS) in FTF expression was investigated. A CovR-defective mutant of S. mutans NG8 was constructed by homologous recombination. By use of immunoblotting, the mutant was shown to overexpress FTF in the absence of sucrose, while the wild type and a covRS-complemented mutant showed sucrose-inducible FTF expression. Reverse transcription-PCR showed that the ftf transcript levels were increased in the covR mutant, suggesting regulation at the transcriptional level. The covR mutant was also found to overproduce extracellular carbohydrate, and this phenotype was reversed by covRS complementation. Paper chromatographic studies and chemical tests showed that the extracellular carbohydrate contained glucose and glucuronic acid but not fructose. These results suggest that the extracellular carbohydrate was not fructan. The production of a glucose- and glucuronic acid-containing extracellular carbohydrate has not been reported for S. mutans and may be considered novel. In conclusion, the results indicate that the expression of FTF and a glucose- and glucuronic acid-containing carbohydrate was negatively regulated by the covRS two-component regulatory system in S. mutans.

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.

Regulation of the gtfBC and ftf genes of Streptococcus mutans in biofilms in response to pH and carbohydrate

Streptococcus mutans produces a number of extracellular sucrose-metabolizing enzymes that contribute to the ability of the organism to cause dental caries, including three glucosyltransferases, the products of the gtfB, gtfC and gtfD genes, and a fructosyltransferase, encoded by the ftf gene. To better understand the regulation of the expression of these genes under environmental conditions that more closely mimic those in dental plaque, two strains of S. mutans harbouring fusions of the gtfBC (SMS102) and ftf (SMS101) promoters to a chloramphenicol acetyltransferase (CAT) gene were examined in biofilms formed in vitro. The strains were grown in a Rototorque biofilm reactor in a tryptone-yeast extract-sucrose medium. CAT specific activity in biofilm cells was measured at quasi-steady state or following additions of 25 mM sucrose or glucose, with or without pH control. After approximately 10 generations of biofilm growth, the ftf and gtfBC genes of S. mutans were found to be expressed at levels different from those reported for planktonic cells growing under otherwise similar conditions. The expression of these genes was induced by the addition of sucrose to the quasi-steady-state cultures. Expression of the gtfBC genes was influenced by environmental pH, since CAT specific activities in quasi-steady-state biofilms of strain SMS102 grown without pH control were twice those produced by cells grown with pH control. Moreover, addition of glucose to quasi-steady-state biofilms resulted in increased expression of the gtfBC-cat fusion, although the magnitude of the induction was less than that seen with sucrose. The effect of pH on ftf expression was negligible. A modest and transient induction of ftf was observed in biofilms pulsed with excess glucose and the kinetics and level of induction of ftf by excess carbohydrate were dependent on the pH of the biofilms. This study demonstrates that the type and amount of carbohydrate and the environmental pH have a major influence on transcription of the gtfBC and ftf genes when the organisms are growing in biofilms, and provides evidence for previously undisclosed regulatory circuits for exopolysaccharide gene expression in S. mutans.

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.

Construction of a new integration vector for use in Streptococcus mutans

A new integration vector, pBGK, was constructed for delivery of heterologous DNA into the chromosome of Streptococcus mutans. A kanamycin resistance element (OmegaKm), which is flanked by transcriptional and translational terminators and which is selectable in both Escherichia coli and streptococci, was inserted into a 2.4-kb EcoRI fragment carrying the S. mutans gtfA gene. A unique SmaI site flanking OmegaKm is available for cloning of promoter:reporter gene fusions or foreign genes, which can then be integrated into the S. mutans chromosome by allelic exchange with the gtfA gene. The vector was used to analyze the activity of an S. mutans promoter by fusing it to a chloramphenicol acetyltransferase gene. The reporter fusions could readily be cloned into the vector at a unique SmaI site and the vector and passenger DNA were stable in E. coli. DNAs flanked by gtfA sequees integrated efficiently into the chromosome of S. mutans and were stably maintained in the absence of selective pressure. Expression levels of the reporter gene were consistent regardless of orientation or repeated subculturing.

Environmental conditions modulate the expression of the sspA and sspB genes in Streptococcus gordonii

The SspA and SspB surface proteins of Streptococcus gordonii are multifunctional adhesins encoded by tandemly arranged genes. The transcriptional regulation of the sspA and sspB genes was investigated by generating chromosomal promoter- cat gene fusions and measuring CAT enzyme activity. The sspA promoter was found to be three-fold more active than the sspB promoter. In addition, sspA transcriptional activity increased throughout growth, whereas sspB activity decreased in stationary phase. Promoter activity of both sspA and sspB was regulated in response to temperature, pH and osmolarity; however the two promoters showed a different pattern of regulation. Changes in promoter activity were reflected in levels of surface protein and in adherence of S. gordonii to Porphyromonas gingivalis, a phenotypic property dependent on Ssp proteins. The results show that S. gordonii strain DL1 differentially regulates sspA and sspB transcription in response to oral environmental cues, suggesting that the SspA and SspB polypeptides may have distinct functional roles in cell adherence to oral substrates.

Characterization of the fructosyltransferase gene of Actinomyces naeslundii WVU45

Oral actinomycetes produce fructosyltransferase (FTF) enzymes which convert sucrose into polymers of D-fructose, known as levans, and these polymers are thought to contribute to the persistence and virulence of the organisms. A gene encoding FTF was isolated from Actinomyces naeslundii WVU45; the deduced amino acid sequence showed significant similarity to known levansucrases of gram-negative environmental isolates but was less similar to FTFs from gram-positive bacteria. A transcriptional start site was mapped by primer extension 70 bp 5' from the putative start codon. Promoter fusions to a chloramphenicol acetyltransferase gene were used to confirm that there was a functional promoter driving ftf expression and to show that sequences located 86 to 218 bp upstream of the transcription initiation site were required for optimal ftf expression. Quantitative slot blot analysis against total RNA from cells grown on different sugars or from different growth phases revealed that ftf was constitutively transcribed. Thus, the A. naeslundii FTF is more similar in primary sequence and the regulation of expression to levansucrases of gram-negative bacteria than gram-positive bacteria.

Characterization of recombinant, ureolytic Streptococcus mutans demonstrates an inverse relationship between dental plaque ureolytic capacity and cariogenicity

Dental caries results from prolonged plaque acidification that leads to the establishment of a cariogenic microflora and demineralization of the tooth. Urease enzymes of oral bacteria hydrolyze urea to ammonia, which can neutralize plaque acids. To begin to examine the relationship between plaque ureolytic activity and the incidence of dental caries, recombinant, ureolytic strains of Streptococcus mutans were constructed. Specifically, the ureABCEFGD operon from Streptococcus salivarius 57.I was integrated into the S. mutans chromosome in such a way that the operon was transcribed from a weak, cognate promoter in S. mutans ACUS4 or a stronger promoter in S. mutans ACUS6. Both strains expressed NiCl(2)-dependent urease activity, but the maximal urease levels in ACUS6 were threefold higher than those in ACUS4. In vitro pH drop experiments demonstrated that the ability of the recombinant S. mutans strains to moderate a decrease in pH during the simultaneous metabolism of glucose and urea increased proportionately with the level of urease activity expressed. Specific-pathogen-free rats that were infected with ACUS6 and fed a cariogenic diet with drinking water containing 25 mM urea and 50 microM NiCl(2) had relatively high levels of oral urease activity, as well as dramatic decreases in the prevalence of smooth-surface caries and the severity of sulcal caries, relative to controls. Urease activity appears to influence plaque biochemistry and metabolism in a manner that reduces cariogenicity, suggesting that recombinant, ureolytic bacteria may be useful to promote dental health.

Regulation of expression of the fructan hydrolase gene of Streptococcus mutans GS-5 by induction and carbon catabolite repression

The polymers of fructose, levan and inulin, as well as sucrose and raffinose, are substrates for the product of the fruA gene of Streptococcus mutans GS-5. The purpose of this study was to characterize the DNA immediately flanking fruA, to explore the regulation of expression of fruA by the carbohydrate source, and to begin to elucidate the molecular basis for differential expression of the gene. Located 3' to fruA was an open reading frame (ORF) with similarity to beta-fructosidases which was cotranscribed with fruA. A transcriptional initiation site, located an appropriate distance from an extended -10-like promoter, was mapped at 165 bp 5' to the fruA structural gene. By the use of computer algorithms, two overlapping, stable stem-loop sequences with the potential to function as rho-independent terminators were found in the 5' untranslated region. Catabolite response elements (CREs), which have been shown to govern carbon catabolite repression (CCR) by functioning as negative cis elements in gram-positive bacteria, were located close to the promoter. The levels of production of fruA mRNA and FruA were elevated in cells growing on levan, inulin, or sucrose as the sole carbohydrate source, and repression was observed when cells were grown on readily metabolizable hexoses. Deletion derivatives containing fusions of fruA promoter regions, lacking sequences 5' or 3' to the promoter, and a promoterless chloramphenicol acetyltransferase gene were used (i) to demonstrate the functionality of the promoter mapped by primer extension, (ii) to demonstrate that CCR of the fru operon requires the CRE that is located 3' to the promoter region, and (iii) to provide preliminary evidence that supports the involvement of an antitermination mechanism in fruA induction.

Oral streptococci... products of their environment

Microbial biofilms form on oral surfaces. These biofilms usually exist in dynamic equilibria with host defenses and are compatible with maintenance of the integrity of the target tissues. Disease occurs when the composition and the metabolic activities of complex communities in biofilms are perturbed. These ecologically driven changes in oral biofilms result in increases in the proportions of pathogenic micro-organisms, which possess enzymatic and structural determinants that may render them more virulent than organisms associated with oral health. This brief review focuses on key environmental influences, and genetic and physiologic aspects of bacteria associated with the formation of dental caries, and attempts to identify some areas of oral microbiology in which interdisciplinary efforts will be essential for dissection of the molecular events controlling the development and persistence of pathogenic plaques. The focus is on strategies to enhance fundamental knowledge of oral biofilm composition, structure, and activities, with the rationale that broadly effective therapeutic strategies targeted at plaque physiology, or at biofilm development and persistence, can arise from such knowledge.

Expression of the Streptococcus mutans fructosyltransferase gene within a mammalian host

In vivo expression of the virulence-associated fructosyltransferase gene (ftf) of Streptococcus mutans has been examined. S. mutans ftf expression is affected by both the specific carbohydrate consumed and the age of the host animal.

Streptococcal adhesion and colonization

Streptococci express arrays of adhesins on their cell surfaces that facilitate adherence to substrates present in their natural environment within the mammalian host. A consequence of such promiscuous binding ability is that streptococcal cells may adhere simultaneously to a spectrum of substrates, including salivary glycoproteins, extracellular matrix and serum components, host cells, and other microbial cells. The multiplicity of streptococcal adherence interactions accounts, at least in part, for their success in colonizing the oral and epithelial surfaces of humans. Adhesion facilitates colonization and may be a precursor to tissue invasion and immune modulation, events that presage the development of disease. Many of the streptococcal adhesins and virulence-related factors are cell-wall-associated proteins containing repeated sequence blocks of amino acids. Linear sequences, both within the blocks and within non-repetitive regions of the proteins, have been implicated in substrate binding. Sequences and functions of these proteins among the streptococci have become assorted through gene duplication and horizontal transfer between bacterial populations. Several adhesins identified and characterized through in vitro binding assays have been analyzed for in vivo expression and function by means of animal models used for colonization and virulence. Information on the molecular structure of adhesins as related to their in vivo function will allow for the rational design of novel acellular vaccines, recombinant antibodies, and adhesion agonists for the future control or prevention of streptococcal colonization and streptococcal diseases.

Cloning and nucleotide sequence analysis of the Streptococcus mutans membrane-bound, proton-translocating ATPase operon

The function of the membrane-bound ATPase in S. mutans is to regulate cytoplasmic pH values for the purpose of maintaining delta pH. Previous studies have shown that as part of its acid-adaptive ability, S. mutans is able to increase H(+)-ATPase levels in response to acidification. As part of the study of ATPase regulation in S. mutans, we have cloned the ATPase operon and determined its genetic organization. The structural genes from S. mutans were found to be in the order: c, a, b, delta, alpha, gamma, beta, and epsilon; where c and a were reversed from the more typical bacterial organization. The operon contained no I gene homologue but was preceded by a 239-bp intergenic space. Deduced aa sequences from open reading frames indicated that genes encoding homologues of glycogen phosphorylase and nonphosphorylating, NADP-dependent glyceraldehyde-3-phosphate dehydrogenase flank the H(+)-ATPase operon, 5' and 3' respectively. Sequence analysis indicated the presence of three inverted-repeat nt sequences in the glgP-uncE intergenic space. Primer extension analysis of mRNAs prepared from batch-grown or steady-state cultures demonstrated that the transcriptional start site did not change as a function of culture pH value. The data suggest that potential stem-and-loop structures in the promoter region of the operon do not function to alter the starting position of ATPase-specific mRNA transcription.

Molecular and genetic analysis of multiple changes in the levels of production of virulence factors in a subcultured variant of Streptococcus mutans

We previously isolated a variant strain, Xc100L, which shows decreased production of a surface protein antigen with a molecular mass of 190 kDa (PAc), after repeated subculturing of Streptococcus mutans strain Xc [Koga, T. et al. (1989) J.Gen. Microbiol. 135, 3199-3202]. In the present study, the levels of expression of the gtfB, gtfC, gtfD and ftf genes coding for polysaccharide-synthesizing enzymes in strain Xc100L were compared with those in strain Xc. Western blot analysis revealed multiple differences in the levels of production of these enzymes between these two strains. The amounts of the gtfB and gtfC gene products responsible for water-insoluble glucan synthesis in strain Xc100L were lower than those in strain Xc, whereas the amounts of the gtfD and ftf gene products responsible for water soluble glucan synthesis and fructan synthesis, respectively, in strain Xc100L were higher than those in strain Xc. Northern blot analysis revealed that the amounts of the four enzymes and PAc produced by strain Xc100L reflected the relative amounts of mRNAs from the genes. The chloramphenicol acetyltransferase gene was fused with each of these five genes, and the transcriptional activity of each gene in strain Xc100L was quantitatively compared with that in strain Xc. The chloramphenicol acetyltransferase assay also indicated that the phenotypic differences between strain Xc and strain Xc100L were due to differences in the transcriptional activities of the virulence genes. No differences in the nucleotide sequences of the promoter regions of the gtfB, gtfC, gtfD, ftf and pac genes were found between strain Xc and strain Xc100L. It is possible that a factor(s) affecting the levels of transcription of the multiple virulence genes exists in S. mutans.

Identification of the Streptococcus mutans frp gene as a potential regulator of fructosyltransferase expression

Four putative open reading frames (ORFs) were previously identified in the regions flanking the Streptococcus mutans GS-5 fructosyltransferase (FTF) gene. One of these, ORF 3, appeared to code for a low-molecular-mass protein containing amino acid sequences sharing homology with several Gram-positive bacterial DNA-binding proteins and it was suggested that the ORF 3 gene product might be an FTF regulatory protein (FRP). In order to characterize this protein, we have purified the biotinylated tag-FRP fusion protein using the PinPoint protein purification system and this fusion protein was used in gel shift assays with DNA fragments containing the ftf promoter region. FRP bound specifically to the upstream region of the ftf promoter containing the inverted repeat structure that is present upstream of the -35 sequence. In contrast, FRP did not bind to DNA fragments lacking the inverted repeat structure. The results of these experiments suggest that FRP interacts with the inverted repeat region upstream of the ftf promoter and such interactions may regulate FTF expression.

Genetic analysis of fructan-hyperproducing strains of Streptococcus mutans

Fructan polymer, synthesized from sucrose by the extracellular fructosyltransferase of Streptococcus mutans, is thought to contribute to the progression of dental caries. It may serve as an extracellular storage polysaccharide facilitating survival and acid production. It may also have a role in adherence or accumulation of bacterial cells on the tooth surface. A number of clinical isolates of S. mutans which produce large, mucoid colonies on sucrose-containing agar as a result of increased production of fructan have been discovered. By using eight independent isolates, we sought to determine if such fructan-hyperproducing strains represented a genetically homogeneous group of organisms. Restriction fragment patterns of total cellular DNA were examined by using pulsed-field and conventional gel electrophoresis. Four genetic types which appeared to correlate with the serotype of the organism and the geographic site of isolation were evident. Southern blot analysis of several genetic loci for extracellular enzymes revealed some minor differences between the strains, but the basic genomic organizations of these loci were similar. To evaluate whether the excess fructan produced by these strains enhanced the virulence of these organisms in the oral cavity, it was of interest to create mutants deficient in fructosidase (FruA), the extracellular enzyme which degrades this polymer. The fruA gene was inactivated by allelic exchange in two fructan-hyperproducing strains as well as in S. mutans GS5, a strain which does not hyperproduce fructan. All of the fruA mutant strains were devoid of fructan hydrolase activity when levan was used as a substrate. However, the fructan-hyperproducing strains retained the ability to hydrolyze inulin, suggesting the presence of a second fructosidase with specificity for inulin in these strains.