Isolation and characterization of the Streptococcus mutans gtfD gene, coding for primer-dependent soluble glucan synthesis

Monoclonal antibodies specific to Streptococcus mutans GS-5 glucosyltransferase-C inhibit bacterial glucosyltransferase

Glucosyltransferase-C (GTFC) is a virulence factor of Streptococcus mutans. Additionally, GTFC represents an essential element required for bacterial cell coherence, allowing for the formation of dental plaque, which leads to dental caries. As such, monoclonal antibodies (MAbs) against S. mutans are believed to offer some protection against dental caries. In the current study, we amplified an approximately 1.5 kb fragment of the N-terminal half of the S. mutans gtfC gene by PCR, then induced expression of this gene. This protein was designated GTFCN. After the expressed protein was purified, it was used as an immunogen and injected into BALB/c mice. We selected and established two MAbs by producing hybridomas (HCN17 and HCN37). The anti-GTFCN antibody isotype was confirmed as IgG2a for HCN17 and IgG2b for HCN37. The anti-GTFCN antibody was found to specifically react with the GTFCN protein. The enzymatic activity of the crude glucosyltransferase of S. mutans GS-5 was significantly inhibited at a concentration of 350 ng of MAb/mL. These results suggest that the monoclonal anti-GTFCN antibodies could represent an alternative modality for passive immunization to prevent S. mutans aggregation and dental plaque.

Antimicrobial effects of herbal extracts on Streptococcus mutans and normal oral streptococci

Streptococcus mutans is associated with dental caries. A cariogenic biofilm, in particular, has been studied extensively for its role in the formation of dental caries. Herbal extracts such as Cudrania tricuspidata, Sophora flavescens, Ginkgo biloba, and Betula Schmidtii have been used as a folk remedy for treating diseases. The purpose of this study was to evaluate and compare the antibacterial activity of herbal extracts against normal oral streptococci, planktonic and biofilm of S. mutans. Streptococcus gordonii, Streptococcus oralis, Streptococcus salivarius, Streptococcus sanguinis, and S. mutans were cultivated with brain heart infusion broth and susceptibility assay for the herbal extracts was performed according to the protocol of Clinical and Laboratory Standard Institute. Also, S. mutans biofilm was formed on a polystyrene 12-well plate and 8-well chamber glass slip using BHI broth containing 2% sucrose and 1% mannose after conditioning the plate and the glass slip with unstimulated saliva. The biofilm was treated with the herbal extracts in various concentrations and inoculated on Mitis-Salivarius bacitracin agar plate for enumeration of viable S. mutans by counting colony forming units. Planktonic S. mutans showed susceptibility to all of the extracts and S. mutans biofilm exhibited the highest level of sensitivity for the extracts of S. flavescens. The normal oral streptococci exhibited a weak susceptibility in comparison to S. mutans. S. oralis, however, was resistant to all of the extracts. In conclusion, the extract of S. flavescens may be a potential candidate for prevention and management of dental caries.

Phenotypic heterogeneity of genomically-diverse isolates of Streptococcus mutans

High coverage, whole genome shotgun (WGS) sequencing of 57 geographically- and genetically-diverse isolates of Streptococcus mutans from individuals of known dental caries status was recently completed. Of the 57 sequenced strains, fifteen isolates, were selected based primarily on differences in gene content and phenotypic characteristics known to affect virulence and compared with the reference strain UA159. A high degree of variability in these properties was observed between strains, with a broad spectrum of sensitivities to low pH, oxidative stress (air and paraquat) and exposure to competence stimulating peptide (CSP). Significant differences in autolytic behavior and in biofilm development in glucose or sucrose were also observed. Natural genetic competence varied among isolates, and this was correlated to the presence or absence of competence genes, comCDE and comX, and to bacteriocins. In general strains that lacked the ability to become competent possessed fewer genes for bacteriocins and immunity proteins or contained polymorphic variants of these genes. WGS sequence analysis of the pan-genome revealed, for the first time, components of a Type VII secretion system in several S. mutans strains, as well as two putative ORFs that encode possible collagen binding proteins located upstream of the cnm gene, which is associated with host cell invasiveness. The virulence of these particular strains was assessed in a wax-worm model. This is the first study to combine a comprehensive analysis of key virulence-related phenotypes with extensive genomic analysis of a pathogen that evolved closely with humans. Our analysis highlights the phenotypic diversity of S. mutans isolates and indicates that the species has evolved a variety of adaptive strategies to persist in the human oral cavity and, when conditions are favorable, to initiate disease.

Prevalence of Streptococcus mutans and Lactobacillus fermentum and their association with caries and dietary habits in preschool Thai children

AIMS

To detect the prevalence of Streptococcus mutans and Lactobacillus fermentum in plaque obtained from severe early childhood caries (S-ECC) and caries-free (CF) groups and to assess any association between their prevalence, feeding habits and oral hygiene practices within each group.

STUDY DESIGN

Sixty-four supra-gingival plaque samples were collected from S-ECC and CF groups (CF = 30, S-ECC = 34). All participants were asked about feeding habits and oral hygiene care behaviour by direct questionnaire.

METHODS

After DNA extraction, S. mutans and L. fermentum were amplified using standard PCR. Nested PCR was performed as a second identification method for S. mutans.

RESULTS

Mean age in CF and S-ECC was 2.29 ± 0.42 and 2.49 ± 0.41 years, respectively. Mean dmft in S-ECC was 4.51 ± 0.72. By standard PCR, no S. mutans was detected in CF group, while 10 % was detected in S-ECC group. By nested PCR, S. mutans was detected for 48 and 50 % in CF and S-ECC, respectively. L. fermentum was detected for 16 and 40 % in CF and S-ECC, respectively.

CONCLUSION

Nested PCR gave a higher detection frequency than standard PCR but not significant (p ≤ 0.05). The prevalence of S. mutans was not different between the two groups, while the prevalence of L. fermentum was significantly associated with S-ECC (p ≤ 0.05).

Phylogenetic analysis of glucosyltransferases and implications for the coevolution of mutans streptococci with their mammalian hosts

Glucosyltransferases (Gtfs) catalyze the synthesis of glucans from sucrose and are produced by several species of lactic-acid bacteria. The oral bacterium Streptococcus mutans produces large amounts of glucans through the action of three Gtfs. GtfD produces water-soluble glucan (WSG), GtfB synthesizes water-insoluble glucans (WIG) and GtfC produces mainly WIG but also WSG. These enzymes, especially those synthesizing WIG, are of particular interest because of their role in the formation of dental plaque, an environment where S. mutans can thrive and produce lactic acid, promoting the formation of dental caries. We sequenced the gtfB, gtfC and gtfD genes from several mutans streptococcal strains isolated from the oral cavity of humans and searched for their homologues in strains isolated from chimpanzees and macaque monkeys. The sequence data were analyzed in conjunction with the available Gtf sequences from other bacteria in the genera Streptococcus, Lactobacillus and Leuconostoc to gain insights into the evolutionary history of this family of enzymes, with a particular emphasis on S. mutans Gtfs. Our analyses indicate that streptococcal Gtfs arose from a common ancestral progenitor gene, and that they expanded to form two clades according to the type of glucan they synthesize. We also show that the clade of streptococcal Gtfs synthesizing WIG appeared shortly after the divergence of viviparous, dentate mammals, which potentially contributed to the formation of dental plaque and the establishment of several streptococci in the oral cavity. The two S. mutans Gtfs capable of WIG synthesis, GtfB and GtfC, are likely the product of a gene duplication event. We dated this event to coincide with the divergence of the genomes of ancestral early primates. Thus, the acquisition and diversification of S. mutans Gtfs predates modern humans and is unrelated to the increase in dietary sucrose consumption.

A novel phosphotransferase system of Streptococcus mutans is responsible for transport of carbohydrates with α-1,3 linkage

The most common type of carbohydrate-transport system in Streptococcus mutans is the phosphoenolpyruvate-sugar phosphotransferase system (PTS). Fourteen PTS exist in S. mutans UA159. Several studies have shown that microorganisms growing in biofilms express different genes compared with their planktonic counterparts. In this study, we showed that one PTS of S. mutans was expressed in sucrose-grown biofilms. Furthermore, the same PTS was also responsible for the transport and metabolism of disaccharide nigerose (3-O-α-d-glucopyranosyl-d-glucose). Additionally, the results indicate that the studied PTS might be involved in the transport and metabolism of carbohydrates synthesized by glucosyltransferase B and glucosyltransferase C of S. mutans. To our knowledge, this is the first report that shows PTS transport of a disaccharide (and possibly extracellular oligosaccharides) with α-1,3 linkage.

Inhibitory effects of the phenolic fraction from the pomace of Vitis coignetiae on biofilm formation by Streptococcus mutans

OBJECTIVES

The anti-cariogenic properties of the phenolic fraction from the pomace of Vitis coignetiae (VcPP) were evaluated by in vitro assays and compared with fruit juices from V. coignetiae and common grapes and with other phenolic fractions. The effects of VcPP against the biofilm of Streptococcus mutans were investigated.

DESIGN

Sucrose-dependent biofilm formation by S. mutans cultured in the presence of VcPP was measured by crystal violet dye uptake. Inhibition of adhesion to the saliva-coated hydroxyapatite (sHA) beads was quantified using fluorescent-labelled cells. The MIC for S. mutans was determined by colony counting on agar plates containing VcPP. The ability of VcPP to inhibit glucan synthesis by three distinct recombinant glucosyltransferases (Gtfs) was assessed by quantifying the production of water-soluble and -insoluble polysaccharides in bacterial cultures. In addition, the buffering effect of VcPP in cultures of S. mutans was evaluated.

RESULTS

VcPP reduced adhesion of S. mutans to sHA and biofilm formation in a dose-dependent manner. The MIC of VcPP was 7.50mg/ml. VcPP inhibited GtfB activity associated with the synthesis of water-insoluble glucans. It also inhibited GtfD activity associated with the synthesis of water-soluble glucans at a concentration which was lower than that used for inhibition of GtfB. VcPP had no effect on acidification associated with glucose utilization by S. mutans.

CONCLUSIONS

The current study supports the potential of VcPP as a food additive for reducing caries by inhibiting adhesion to the tooth surface and GtfD-mediated soluble glucan synthesis.

Development of a monoclonal antibody against glucosyltransferase D of Streptococcus mutans GS 5

Glucosyltransferases GtfB, GtfC, and GtfD of Streptococcus mutans are virulent factors involved in dental caries. Consequently, they are considered to be target molecules in the development of vaccines against dental caries. Among them, GtfD plays a significant role in the sucrose-dependent cellular adhesion of S. mutans, and a number of studies have suggested that the N-terminus of GtfD is an important part of its role in enzymatic activity. In this study, we generated monoclonal antibodies against the N-terminus of GtfD (anti-GtfDN antibody) in an initial attempt to investigate its preventive efficacy against dental caries. To obtain anti-GtfDN monoclonal antibodies, the gene for the N-terminus of gtfD (2 kb) was cloned into an Escherichia coli expression vector, pQE30; then the expressed protein (about 75 kDa) was purified. The purified GtfDN protein was injected into BALB/c mice, and hybridoma clones were established. We obtained three hybridoma clones (HDN9, HDN11, and HDN28) capable of producing anti-GtfDN antibodies. Their binding specificity was characterized by ELISA, dot blot, and Western blot analysis after purification using affinity column chromatography. The isotype of the monoclonal antibodies was confirmed to be IgG2a.

Biology of Streptococcus mutans-derived glucosyltransferases: role in extracellular matrix formation of cariogenic biofilms

The importance of Streptococcus mutans in the etiology and pathogenesis of dental caries is certainly controversial, in part because excessive attention is paid to the numbers of S. mutans and acid production while the matrix within dental plaque has been neglected. S. mutans does not always dominate within plaque; many organisms are equally acidogenic and aciduric. It is also recognized that glucosyltransferases from S. mutans (Gtfs) play critical roles in the development of virulent dental plaque. Gtfs adsorb to enamel synthesizing glucans in situ, providing sites for avid colonization by microorganisms and an insoluble matrix for plaque. Gtfs also adsorb to surfaces of other oral microorganisms converting them to glucan producers. S. mutans expresses 3 genetically distinct Gtfs; each appears to play a different but overlapping role in the formation of virulent plaque. GtfC is adsorbed to enamel within pellicle whereas GtfB binds avidly to bacteria promoting tight cell clustering, and enhancing cohesion of plaque. GtfD forms a soluble, readily metabolizable polysaccharide and acts as a primer for GtfB. The behavior of soluble Gtfs does not mirror that observed with surface-adsorbed enzymes. Furthermore, the structure of polysaccharide matrix changes over time as a result of the action of mutanases and dextranases within plaque. Gtfs at distinct loci offer chemotherapeutic targets to prevent caries. Nevertheless, agents that inhibit Gtfs in solution frequently have a reduced or no effect on adsorbed enzymes. Clearly, conformational changes and reactions of Gtfs on surfaces are complex and modulate the pathogenesis of dental caries in situ, deserving further investigation.

Identification of different bacterial species in biofilms using confocal Raman microscopy

Confocal Raman microspectroscopy is used to discriminate between different species of bacteria grown in biofilms. Tests are performed using two bacterial species, Streptococcus sanguinis and Streptococcus mutans, which are major components of oral plaque and of particular interest due to their association with healthy and cariogenic plaque, respectively. Dehydrated biofilms of these species are studied as a simplified model of dental plaque. A prediction model based on principal component analysis and logistic regression is calibrated using pure biofilms of each species and validated on pure biofilms grown months later, achieving 96% accuracy in prospective classification. When biofilms of the two species are partially mixed together, Raman-based identifications are achieved within ∼2 μm of the boundaries between species with 97% accuracy. This combination of spatial resolution and predication accuracy should be suitable for forming images of species distributions within intact two-species biofilms.

Defect of glucosyltransferases reduces platelet aggregation activity of Streptococcus mutans: analysis of clinical strains isolated from oral cavities

OBJECTIVE

Streptococcus mutans is a major pathogen of dental caries and occasionally isolated from the blood of patients with infective endocarditis, though the association of its cell-surface glucosyltransferases (GTFB, GTFC, and GTFD) with pathogenicity for infective endocarditis remains to be elucidated. In this study, we investigated the contribution of S. mutans GTFs to platelet aggregation and analysed GTF expression profiles in a large number of clinical oral isolates.

DESIGN

The platelet aggregation properties of GTF-defective isogenic mutant strains constructed from S. mutans reference strain MT8148 were evaluated using whole blood and platelet-rich plasma (PRP) taken from mice, as well as human PRP. In addition, GTF expression profiles for 396 S. mutans strains isolated from the oral cavities of 396 subjects were analysed by western blotting using antisera specific for each GTF.

RESULTS

The platelet aggregation activities of the GTF-defective isogenic mutants were significantly lower than that of MT8148 when added to a large number of cells. Western blotting revealed no strains without GTF expression, though six strains had alterations of GTFB and GTFC as compared to MT8148. PCR analyses indicated that the gtfB-gtfC region length was approximately 4.5 kb shorter in those strains as compared to MT8148. These were designated as "GTFBC-fusion" strains and they demonstrated lower levels of platelet aggregation.

CONCLUSIONS

Our findings indicate that GTFs are associated with platelet aggregation. Although the clinical detection frequency of S. mutans strains with altered expressions is extremely low, GTFBC-fusion strains have activities similar to GTF-defective mutant strains.

Molecular characterization of Streptococcus mutans strains containing the cnm gene encoding a collagen-binding adhesin

OBJECTIVE

Streptococcus mutans, known to be a major pathogen of dental caries, is also considered to cause infective endocarditis. Its 120-kDa Cnm protein binds to type I collagen, which may be a potential virulence factor. In this study, we characterized S. mutans clinical strains focusing on the cnm gene encoding Cnm.

DESIGN

A total of 528 S. mutans strains isolated from Japanese, Finnish, and Thai subjects were investigated. Using molecular techniques, the distribution frequency of cnm-positive strains and location of the inserted cnm were analyzed. Furthermore, isogenic mutant strains were constructed by inactivation of the cnm gene, then their biological properties of collagen-binding and glucan-binding were evaluated. Southern hybridization of the genes encoding glucan-binding proteins was also performed.

RESULTS

The distribution frequency of cnm-positive strains from Thai subjects was 12%, similar to that previously reported for Japanese and Finnish subjects. Furthermore, the location of insertion of cnm was the same in all cnm-positive clinical isolates. As for the cnm-inactivated mutant strains constructed from 28 clinical isolates, their collagen-binding activity was negligible. In addition, glucan-binding activity in the cnm-positive clinical isolates was significantly reduced and corresponded to a lack of gbpA encoding glucan-binding protein A.

CONCLUSIONS

Our results indicate that strains with cnm genes, the most crucial factor for the collagen-binding property of S. mutans, are detectable at similar frequencies over several different geographic locations. In addition, the common properties of these strains are a high level of collagen-binding activity and tendency for a low level of glucan-binding activity.

Serotype classification of Streptococcus mutans and its detection outside the oral cavity

Streptococcus mutans, generally known as a major pathogen of dental caries, is also a possible causative agent of bacteremia and infective endocarditis. S. mutans is classified into serotypes c, e, f and k based on the chemical composition of serotype-specific polysaccharides, with approximately 70-80% of strains found in the oral cavity classified as serotype c, followed by e (approximately 20%), and f and k (less than 5% each). Serotype k was recently designated as a novel serotype and shown to possess unique features, the most prominent being a defect of the glucose side chain in serotype-specific rhamnose-glucose polymers, which is related to a higher incidence of detection in cardiovascular specimens, owing to phagocytosis resistance. Molecular analyses of cardiovascular specimens showed a high detection frequency for S. mutans DNA, among which the detection rate for serotype k was quite high. These findings suggest that serotype k S. mutans possibly has a high level of virulence for systemic diseases.

Molecular and clinical analyses of the gene encoding the collagen-binding adhesin of Streptococcus mutans

Streptococcus mutans is a known pathogen of dental caries and its major cell surface antigens have been widely investigated. Recently, an approximately 120 kDa Cnm protein with binding properties to type I collagen was identified, and its encoding gene (cnm) cloned and sequenced. In the present study, we sequenced cnm from 47 different clinical S. mutans strains and found that the nucleotide alignment of the collagen-binding domain was well conserved. We devised a PCR method for identifying the cnm gene, examined the prevalence of cnm-positive S. mutans strains in various mother-child groups, and assessed the significance of such strains for transmission and dental caries. The detection rate of cnm-positive strains was significantly lower in strains isolated from Japanese children in the 2000s (8.0 %) as compared to those isolated in the 1980s (15.8 %) (P<0.05). Furthermore, the presence of S. mutans possessing cnm in salivary specimens collected from 55 S. mutans-positive mother-child pairs was 40 and 32.7 % in the mothers and children, respectively. The frequency of cnm-positive children whose mothers were also positive was 72 %, which was significantly higher than that of cnm-positive children with negative mothers (P<0.0001, odds ratio 17.5). In addition, clinical parameters indicating dental caries were significantly increased in children with cnm-positive S. mutans in saliva (n=13), as compared to those with cnm-negative S. mutans (n=15) and S. mutans-negative children (n=20) (P<0.01). These results indicate that cnm-positive S. mutans strains are closely correlated with dental caries, while vertical transmission in cnm-positive mother-child pairs was also demonstrated.

Protein antigen in serotype k Streptococcus mutans clinical isolates

Streptococcus mutans, a major pathogen of dental caries and infective endocarditis, is classified into serotypes c, e, f, and k, with serotype k strains recently reported to be frequently detected in persons with infective endocarditis. Thus, we hypothesized that common properties associated with infective endocarditis are present in those strains. Fifty-six oral S. mutans strains, including 11 serotype k strains, were analyzed. Western blotting analysis revealed expression of the 3 types of glucosyltransferases in all strains, while expression of the approximately 190-kDa cell-surface protein (PA) was absent in 12 strains, among which the prevalence of serotype k (7/12) was significantly high. Furthermore, cellular hydrophobicity and phagocytosis susceptibility were lower in the group of serotype k strains. These results indicate that the absence of PA expression, low cellular hydrophobicity, and phagocytosis susceptibility are common bacterial properties associated with serotype k strains, which may be associated with virulence for infective endocarditis.

Salivary glucosyltransferase B as a possible marker for caries activity

Bacteria-derived glucosyltransferases (Gtf) (EC 2.4.1.5), through synthesizing glucan polymers from sucrose and starch hydrolysates, play an essential role in the etiology and pathogenesis of caries. We attempted to correlate the levels of Gtf in whole saliva with the prevalence of carious lesions in young children. We examined saliva from children who were either free of overt carious lesions, or had severe early childhood caries (mean dmfs = 18.72 +/- 9.0 SD), for Gtf by direct enzyme assay. The levels of GtfB, GtfC and GtfD from Streptococcus mutans in the saliva using monoclonal/specific antibodies in an enzyme-linked immunosorbent assay were determined. Multiple logistic regression analyses with model selection showed that GtfB levels correlated with dmfs values of the subjects (p = 0.006). There was no correlation between total Gtf activity as measured by direct enzyme assay and dmfs values. There was a strong correlation between mutans streptococci populations in saliva and caries activity. Collectively, these data show that GtfB levels in saliva correlate strongly with presence of clinical caries and with number of carious lesions in young children. It is also possible to measure different Gtfs, separately, in whole saliva. These observations may have important clinical implications, may lead to development of a chair side caries activity test and support the importance of GtfB in the pathogenesis of dental caries.

High level accumulation of alpha-glucan in maize kernels by expressing the gtfD gene from Streptococcus mutans

Glucosyltransferases (GTFs, EC.2.4.1.5) are bacterial enzymes that catalyze the polymerization of glucose residues from sucrose, leading to the production of high molecular weight glucan with alpha-1,3 /alpha-1,6 linkages. Such glucans, with many potential food and industrial applications, do not normally exist in higher plants. We fused a mutant form of the gtfD gene from Sreptococcus mutans with the maize (Zea mays L.) chloroplastic Brittle 1 transit peptide for amyloplast targeting. This construct, driven by the ubiquitin promoter, was introduced into maize by Agrobacterium-mediated transformation. We developed a novel HPLC-based method that enabled us differentially to distinguish transgene glucan from other endogenous polysaccharides in maize kernels. Using this method, we screened over 100 transgenic plants for the presence of GTF-produced glucan whose content varied between 0.8 and 14% of dry weight in the mature transgenic seeds. The mature transgenic plants were indistinguishable from wildtype plants in growth rate and morphology. Furthermore, starch granule size in the transgenic maize kernel was unaffected by the accumulation of the foreign polysaccharide. Mutation in Sh2, which encodes a subunit of ADP-glucose pyrophosphorylase, had no effect on glucan accumulation caused by gtfD expression. Our results indicated that high levels of novel carbohydrate polymer can be accumulated in crop plants through transgene technology.

Effects of Nidus Vespae extract and chemical fractions on glucosyltransferases, adherence and biofilm formation of Streptococcus mutans

Nidus Vespae (the honeycomb of Polistes olivaceous, P. japonicus Saussure and Parapolybiavaria fabricius) have been extensively used in traditional Chinese medicine, given their multiple pharmacological activities, including antimicrobial, anti-inflammatory, anti-virus, anti-tumor and anesthetic properties. The present study evaluated the anti-glucosyltransferases (GTFs) activity, anti-adherence and anti-biofilm properties of 95% ethanol/water extract, cyclohexane/ethyl acetate, petroleum ether/ethyl acetate and chloroform/methanol fractions of Nidus Vespae. Chloroform/methanol fraction showed a remarkable capacity for inhibiting the adherence of Streptococcus mutans ATCC 25175 to saliva-coated hydroxyapatite disc (S-HA) at sub-MC concentrations. In addition, the Nidus Vespae extract and chemical fractions significantly inhibited the activity of cell-associated and extracellular GTFs at sub-MIC concentrations, and the chloroform/methanol fraction was the most effective one. For the anti-biofilm activity assays, minimum biofilm inhibition concentrations (MBIC50) and minimum biofilm reduction concentrations (MBRC50) were determined using the microdilution method. The chloroform/methanol fraction showed the highest anti-biofilm activities with a MBIC50 of 8mg/ml and a MBRC(50) of 16mg/ml against Streptococcus mutans ATCC 25175. The significant inhibition of GTFs activity and biofilm formation demonstrated by Nidus Vespae shows it to be a promising natural product for the prevention of dental caries.

A novel signal transduction system and feedback loop regulate fructan hydrolase gene expression in Streptococcus mutans

The fruA gene of Streptococcus mutans encodes for a secreted fructan hydrolase (fructanase), an established virulence determinant required for releasing D-fructose from levan- and inulin-type fructans. Expression of fruA is under the control of carbon catabolite repression and is induced by growth in fructans. In this report, we identified an operon in S. mutans UA159 encoding a two-component system flanked by two predicted carbohydrate-binding proteins that is absolutely required for the expression of fruA. All four genes were found to be required for optimal growth of S. mutans on inulin-containing medium and for transcriptional activation of fruA. Complementation assays using a plasmid expressing the response regulator suggested that the two-component system works in concert with the sugar-binding proteins. This operon was also shown to activate a four-gene cluster located immediately downstream and encoding an Enzyme II (EII(Lev)) for a fructose/mannose sugar : phosphotransferase enzyme, which was found to negatively regulate the expression of fruA. Using transcriptional fusions, it was found that fructose could signal induction of the fruA and levD operons through the two-component system/sugar-binding protein complex. A recombinant LevR protein was shown to bind to the promoter regions of fruA and levD in gel mobility shift assays. Thus, a 'four-component signal transduction system' activates fructan catabolism and the expression of an Enzyme II complex that functions in a feedback loop to sense the accumulation of the end-product of fructan degradation.

The Streptococcus mutans vicX gene product modulates gtfB/C expression, biofilm formation, genetic competence, and oxidative stress tolerance

Streptococcus mutans is considered one of the primary etiologic agents of dental caries. Previously, we characterized the VicRK two-component signal transduction system, which regulates multiple virulence factors of S. mutans. In this study, we focused on the vicX gene of the vicRKX tricistronic operon. To characterize vicX, we constructed a nonpolar deletion mutation in the vicX coding region in S. mutans UA159. The growth kinetics of the mutant (designated SmuvicX) showed that the doubling time was longer and that there was considerable sensitivity to paraquat-induced oxidative stress. Supplementing a culture of the wild-type UA159 strain with paraquat significantly increased the expression of vicX (P < 0.05, as determined by analysis of variance [ANOVA]), confirming the role of this gene in oxidative stress tolerance in S. mutans. Examination of mutant biofilms revealed architecturally altered cell clusters that were seemingly denser than the wild-type cell clusters. Interestingly, vicX-deficient cells grown in a glucose-supplemented medium exhibited significantly increased glucosyltransferase B/C (gtfB/C) expression compared with the expression in the wild type (P < 0.05, as determined by ANOVA). Moreover, a sucrose-dependent adhesion assay performed using an S. mutans GS5-derived vicX null mutant demonstrated that the adhesiveness of this mutant was enhanced compared with that of the parent strain and isogenic mutants of the parent strain lacking gtfB and/or gtfC. Also, disruption of vicX reduced the genetic transformability of the mutant approximately 10-fold compared with that of the parent strain (P < 0.05, as determined by ANOVA). Collectively, these findings provide insight into important phenotypes controlled by the vicX gene product that can impact S. mutans pathogenicity.

The inhibitory effect of magnolol from Magnolia officinalis on glucosyltransferase

Dental caries has been an intractable disease in spite of intense dental research. Glucosyltransferase (GTF) enzyme plays the most important role in the development of dental caries. In our previous studies, magnolol, a compound from Magnolia officinalis Rehder et Wilson (Magnoliaceae), was shown to possess a strong anti-GTF activity. The purpose of the present study was to examine the effect of magnolol on the functional domains of GTF for the purpose of defining its anti-GTF activity mechanism. GTF-I which was prepared from Streptococcus milleri transformant KSB8 cells expressing the gtfB gene was used. The results demonstrated magnolol reduced total glucan synthesis, depending on the magnolol concentration. There were no significant differences in Michaelis constant (K(m)) values between the presence and absence of magnolol as determined by Lineweaver-Burk plot, and maximum velocity (V(m)) in the presence of magnolol was lower than that in its absence. Magnolol significantly inhibited both sucrose hydrolysis and glucosyl transfer to glucan by GTF-I. Free glucose in the presence of magnolol was reduced by 33-48% as compared to in its absence, while the quantity of glucan was reduced by 75-82%. These findings suggested that magnolol inhibited both two sequential reaction phases of GTF non-competitively by operating on the glucan-binding domain, but not on the catalytic domain. Magnolol could be a valuable resource for the exploration of novel bioactive compounds in natural products.

Effect of Weissella cibaria Isolates on the Formation of Streptococcus mutans Biofilm

The objective of this study was to isolate and identify lactic acid bacteria able to inhibit the in vitro formation of Streptococcus mutans biofilm as well as the in vivo formation of oral biofilm. Two strains, CMS1 and CMS3, exhibiting profound inhibitory effects on the formation of S. mutans biofilm and the proliferation of S. mutans, were isolated from children's saliva and identified as Weissella cibaria by 16S rDNA sequencing. The water-soluble polymers produced from sucrose by the W. cibaria isolates also inhibited the formation of S. mutans biofilm. According to the results of thin-layer chromatographic analysis, the hydrolysates of water-soluble polymers produced by the isolates were identical to those of dextran, forming mostly alpha-(1-6) glucose linkages. In the clinical study, the subjects mouthrinsed with a solution containing W. cibaria CMS1 evidenced plaque index reduction of approximately 20.7% (p < 0.001). These results indicate that the W. cibaria isolates possess the ability to inhibit biofilm formation, both in vitro and in vivo.

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.

Structure-function relationships of glucansucrase and fructansucrase enzymes from lactic acid bacteria

Lactic acid bacteria (LAB) employ sucrase-type enzymes to convert sucrose into homopolysaccharides consisting of either glucosyl units (glucans) or fructosyl units (fructans). The enzymes involved are labeled glucansucrases (GS) and fructansucrases (FS), respectively. The available molecular, biochemical, and structural information on sucrase genes and enzymes from various LAB and their fructan and alpha-glucan products is reviewed. The GS and FS enzymes are both glycoside hydrolase enzymes that act on the same substrate (sucrose) and catalyze (retaining) transglycosylation reactions that result in polysaccharide formation, but they possess completely different protein structures. GS enzymes (family GH70) are large multidomain proteins that occur exclusively in LAB. Their catalytic domain displays clear secondary-structure similarity with alpha-amylase enzymes (family GH13), with a predicted permuted (beta/alpha)(8) barrel structure for which detailed structural and mechanistic information is available. Emphasis now is on identification of residues and regions important for GS enzyme activity and product specificity (synthesis of alpha-glucans differing in glycosidic linkage type, degree and type of branching, glucan molecular mass, and solubility). FS enzymes (family GH68) occur in both gram-negative and gram-positive bacteria and synthesize beta-fructan polymers with either beta-(2-->6) (inulin) or beta-(2-->1) (levan) glycosidic bonds. Recently, the first high-resolution three-dimensional structures have become available for FS (levansucrase) proteins, revealing a rare five-bladed beta-propeller structure with a deep, negatively charged central pocket. Although these structures have provided detailed mechanistic insights, the structural features in FS enzymes dictating the synthesis of either beta-(2-->6) or beta-(2-->1) linkages, degree and type of branching, and fructan molecular mass remain to be identified.

Identification of a glucan-binding protein C gene homologue in Streptococcus macacae

BACKGROUND/AIMS

The past few decades have seen the isolation of certain glucosyltransferases and a number of proteins from mutans streptococci. Some of these proteins have been shown to possess glucan-binding capabilities which confer an important virulence property on mutans streptococci for the role of these bacteria play in dental caries. Among these proteins is glucan-binding protein C, which is encoded by the gbpC gene, and which we have identified as being involved in the dextran-dependent aggregation of Streptococcus mutans. However, gbpC homologues have yet to be identified in other mutans streptococci.

METHODS

We carried out polymerase chain reaction amplification of Streptococcus macacae using primers that were designed based on conserved sequences of S. mutans gbpC and identified a gbpC gene homologue. The gene of that homologue was then characterized.

RESULTS

Nucleotide sequencing of the S. macacae gbpC homologue revealed a 1854 bp open reading frame encoding a protein with an N-terminal signal peptide. The molecular mass of the processed protein was calculated to be 67 kDa. We also found an LPxTG motif, the consensus sequence for gram-positive cocci cell wall-anchored surface proteins, which was followed by a characteristic sequence at the carboxal terminal region of the putative protein. This suggests that the S. macacae GbpC homologue protein was tethered to the cell wall.

CONCLUSION

Based on these results, together with the demonstrated glucan-binding ability of the S. macacae GbpC homologue protein, we suggest that S. macacae cells are capable of binding dextran via the GbpC homologue protein, which is similar to the S. mutans GbpC protein. In addition, Southern hybridization analysis using the S. macacae gbpC homologue as a probe showed a distribution of gbpC homologues throughout the mutans streptococci.

Effects of Streptococcus mutansgtfC Deficiency on Mixed Oral Biofilms in vitro

The aim of this study was to examine the influence of glucosyltransferase-gene-negative (gtf-) Streptococcus mutans strains unable to synthesize water-insoluble or soluble glucan on the structure and macromolecular diffusion properties of in vitro grown mixed oral biofilms. Biofilms modeling supragingival plaque consisted of Actinomyces naeslundii OMZ 745, Candida albicans OMZ 110, Fusobacterium nucleatum KP-F2, Streptococcus oralis SK 248, Veillonella dispar ATCC 17748T and one of the S. mutans strains UA159, OMZ 966, OMZ 937 or OMZ 977. Biofilms were grown anaerobically on sintered hydroxyapatite disks for 64.5 h at 37 degrees C. To perform confocal laser scanning microscopy analyses, microorganisms were stained with Syto 13 and extracellular polysaccharides (EPS) with Calcofluor. Macromolecular diffusion properties were measured following timed biofilm exposure to Texas-Red-labeled 70-kDa dextran. Results showed that replacing wild-type S. mutans by a gtfC- mutant led to an increase in the volume fraction occupied by cells from 29 to 48% and a decrease of the EPS volume fraction from 51 to 33%. No such changes were observed when the S. mutans wild-type strain was replaced by a gtfB- or gtfD- mutant. The diffusion coefficient of 70-kDa dextran in biofilms containing the gtfC- S. mutans was 16-fold higher than in biofilms with the wild-type strain indicating a strong macromolecular sieving effect of GTF C-generated glucans. Our data demonstrate the influence of EPS on the structure and macromolecular diffusion properties of an oral biofilm model and uncover our still limited knowledge of the function of EPS in biofilms and plaque.

Atomic force microscopy study of the structure-function relationships of the biofilm-forming bacterium Streptococcus mutans

Atomic force microscopy (AFM) has garnered much interest in recent years for its ability to probe the structure, function and cellular nanomechanics inherent to specific biological cells. In particular, we have used AFM to probe the important structure-function relationships of the bacterium Streptococcus mutans. S. mutans is the primary aetiological agent in human dental caries (tooth decay), and is of medical importance due to the virulence properties of these cells in biofilm initiation and formation, leading to increased tolerance to antibiotics. We have used AFM to characterize the unique surface structures of distinct mutants of S. mutans. These mutations are located in specific genes that encode surface proteins, thus using AFM we have resolved characteristic surface features for mutant strains compared to the wild type. Ultimately, our characterization of surface morphology has shown distinct differences in the local properties displayed by various S. mutans strains on the nanoscale, which is imperative for understanding the collective properties of these cells in biofilm formation.

Glucosyltransferases of viridans group streptococci modulate interleukin-6 and adhesion molecule expression in endothelial cells and augment monocytic cell adherence

Recruitment of monocytes plays important roles during vegetation formation and endocardial inflammation in the pathogenesis of infective endocarditis (IE). Bacterial antigens or modulins can activate endothelial cells through the expression of cytokines or adhesion molecules and modulate the recruitment of leukocytes. We hypothesized that glucosyltransferases (GTFs), modulins of viridans group streptococci, may act directly to up-regulate the expression of adhesion molecules and also interleukin-6 (IL-6) to augment monocyte attachment to endothelial cells. Using primary cultured human umbilical vein endothelial cells (HUVECs) as an in vitro model, we demonstrated that GTFs (in the cell-bound or free form) could specifically modulate the expression of IL-6, and also adhesion molecules, in a dose- and time-dependent manner. Results of inhibition assays suggested that enhanced expression of adhesion molecules was dependent on the activation of nuclear factor kappaB (NF-kappaB) and extracellular signal-regulated kinase and that p38 mitogen-activated protein kinase pathways also contributed to the release of IL-6. Streptococcus-infected HUVECs or treatment with purified IL-6 plus soluble IL-6 receptor alpha enhanced the expression of ICAM-1 and the adherence of the monocytic cell line U937. These results suggest that streptococcal GTFs might play an important role in recruiting monocytic cells during inflammation in IE through induction of adhesion molecules and IL-6, a cytokine involved in transition from neutrophil to monocyte recruitment.

Differential regulation of Streptococcus mutans gtfBCD genes in response to copper ions

To persist in the oral cavity, bacteria must be able to tolerate environmental fluctuation, particularly in pH, nutrients, and essential elements. Glucosyltransferases B, C, and D of Streptococcus mutans synthesize glucans, and play essential roles in the sucrose-dependent adhesion of the organism to tooth surfaces. Transcriptions of gtfB, gtfC, and gtfD could be differentially regulated through independent promoters. To test the hypothesis that environmental factors frequently encountered in the dental plaque might serve as effector molecules involved in regulation, transcripts of individual gtfs were identified by reverse transcriptase-polymerase chain reaction assay and confirmed by Northern blot analysis using anti-sense RNA probes. When S. mutans was grown in different medium at low pH, differential regulation of the gtfs was observed. More specifically, the transcription and translational expression of gtfD but not gtfB and gtfC was specifically induced by copper ion (Cu(2+)). The up-regulation was independent of the Cu(2+)-transport operon copYAZ. These findings support the involvement of Cu(2+) as an effector molecule in the regulation of S. mutans gtfD. Nutrient change dominates influence of pH but not the effect of Cu(2+).

Interactions of Streptococcus mutans glucosyltransferase B with lysozyme in solution and on the surface of hydroxyapatite

Several active enzymes have been identified as components of acquired enamel pellicle. In the present study, the interactions of Streptococcus mutans glucosyltransferase B (GtfB) with lysozyme in solution and on the surface of hydroxyapatite (HA) beads were studied. Experiments were also performed to investigate whether structural differences exist between glucans formed by GtfB enzyme in the presence or absence of lysozyme in solution and on the surface of HA. Hen egg-white lysozyme (HEWL) and saliva were used as the sources of lysozyme; lysozyme-depleted saliva was used as control. Lysozyme activity was significantly reduced when adsorbed onto HA beads compared with that in solution. The GtfB enzyme did not affect the activity of lysozyme in solution or that of adsorbed lysozyme onto HA. The presence of HEWL increased GtfB activity; bovine serum albumin had an even greater enhancing effect. Depletion of lysozyme from whole saliva increased GtfB activity in solution, but not on the surface of saliva-coated HA. The presence of lysozyme affected the amount of glucan formation by GtfB, but not the structure of glucans formed in solution and on the surface. Therefore, the interaction of lysozyme and GtfB enzymes on HA surface may modulate the formation of glucan and dental plaque.

The influence of mutanase and dextranase on the production and structure of glucans synthesized by streptococcal glucosyltransferases

Glucanohydrolases, especially mutanase [alpha-(1-->3) glucanase; EC 3.2.1.59] and dextranase [alpha-(1-->6) glucanase; EC 3.2.1.11], which are present in the biofilm known as dental plaque, may affect the synthesis and structure of glucans formed by glucosyltransferases (GTFs) from sucrose within dental plaque. We examined the production and the structure of glucans synthesized by GTFs B (synthesis of alpha-(1-->3)-linked glucans) or C [synthesis of alpha-(1-->6)- and alpha-(1-->3)-linked glucans] in the presence of mutanase and dextranase, alone or in combination, in solution phase and on saliva-coated hydroxyapatite beads (surface phase). The ability of Streptococcus sobrinus 6715 to adhere to the glucan, which was formed in the presence of the glucanohydrolases was also explored. The presence of mutanase and/or dextranase during the synthesis of glucans by GTF B and C altered the proportions of soluble to insoluble glucan. The presence of either dextranase or mutanase alone had a modest effect on total amount of glucan formed, especially in the surface phase; the glucanohydrolases in combination reduced the total amount of glucan. The amount of (1-->6)-linked glucan was reduced in presence of dextranase. In contrast, mutanase enhanced the formation of soluble glucan, and reduced the percentage of 3-linked glucose of GTF B and C glucans whereas dextranase was mostly without effect. Glucan formed in the presence of dextranase provided fewer binding sites for S. sobrinus; mutanase was devoid of any effect. We also noted that the GTFs bind to dextranase and mutanase. Glucanohydrolases, even in the presence of GTFs, influence glucan synthesis, linkage remodeling, and branching, which may have an impact on the formation, maturation, physical properties, and bacterial binding sites of the polysaccharide matrix in dental plaque. Our data have relevance for the formation of polysaccharide matrix of other biofilms.

A VicRK signal transduction system in Streptococcus mutans affects gtfBCD, gbpB, and ftf expression, biofilm formation, and genetic competence development

Bacteria exposed to transient host environments can elicit adaptive responses by triggering the differential expression of genes via two-component signal transduction systems. This study describes the vicRK signal transduction system in Streptococcus mutans. A vicK (putative histidine kinase) deletion mutant (SmuvicK) was isolated. However, a vicR (putative response regulator) null mutation was apparently lethal, since the only transformants isolated after attempted mutagenesis overexpressed all three genes in the vicRKX operon (Smuvic+). Compared with the wild-type UA159 strain, both mutants formed aberrant biofilms. Moreover, the vicK mutant biofilm formed in sucrose-supplemented medium was easily detachable relative to that of the parent. The rate of total dextran formation by this mutant was remarkably reduced compared to the wild type, whereas it was increased in Smuvic+. Based on real-time PCR, Smuvic+ showed increased gtfBCD, gbpB, and ftf expression, while a recombinant VicR fusion protein was shown to bind the promoter regions of the gtfB, gtfC, and ftf genes. Also, transformation efficiency in the presence or absence of the S. mutans competence-stimulating peptide was altered for the vic mutants. In vivo studies conducted using SmuvicK in a specific-pathogen-free rat model resulted in significantly increased smooth-surface dental plaque (Pearson-Filon statistic [PF], <0.001). While the absence of vicK did not alter the incidence of caries, a significant reduction in SmuvicK CFU counts was observed in plaque samples relative to that of the parent (PF, <0.001). Taken together, these findings support involvement of the vicRK signal transduction system in regulating several important physiological processes in S. mutans.

LuxS-based signaling affects Streptococcus mutans biofilm formation

Streptococcus mutans is implicated as a major etiological agent in human dental caries, and one of the important virulence properties of this organism is its ability to form biofilms (dental plaque) on tooth surfaces. We examined the role of autoinducer-2 (AI-2) on S. mutans biofilm formation by constructing a GS-5 luxS-null mutant. Biofilm formation by the luxS mutant in 0.5% sucrose defined medium was found to be markedly attenuated compared to the wild type. Scanning electron microscopy also revealed that biofilms of the luxS mutant formed larger clumps in sucrose medium compared to the parental strain. Therefore, the expression of glucosyltransferase genes was examined and the gtfB and gtfC genes, but not the gtfD gene, in the luxS mutant were upregulated in the mid-log growth phase. Furthermore, we developed a novel two-compartment system to monitor AI-2 production by oral streptococci and periodontopathic bacteria. The biofilm defect of the luxS mutant was complemented by strains of S. gordonii, S. sobrinus, and S. anginosus; however, it was not complemented by S. oralis, S. salivarius, or S. sanguinis. Biofilm formation by the luxS mutant was also complemented by Porphyromonas gingivalis 381 and Actinobacillus actinomycetemcomitans Y4 but not by a P. gingivalis luxS mutant. These results suggest that the regulation of the glucosyltransferase genes required for sucrose-dependent biofilm formation is regulated by AI-2. Furthermore, these results provide further confirmation of previous proposals that quorum sensing via AI-2 may play a significant role in oral biofilm formation.

Glucosyltransferases of viridans streptococci are modulins of interleukin-6 induction in infective endocarditis

The glucosyltransferases (GTFs) of viridans streptococci, common pathogens of infective endocarditis, are extracellular proteins that convert sucrose into exopolysaccharides and glucans. GTFs B, C, and D of Streptococcus mutans are modulins that induce, in vitro and in vivo, the production of cytokines, in particular interleukin-6 (IL-6), from monocytes. The roles of S. mutans GTFs in infectivity and inflammation in situ were tested in a rat experimental model of endocarditis. No significant differences in infectivity, in terms of 95% infective dose and densities of bacteria inside vegetations, were observed between laboratory strain GS-5 and two clinical isolates or isogenic mutant NHS1DD, defective in the expression of GTFs. In aortic valves and surrounding tissues, IL-6 was detected by Western blots and immunostaining 24 h after GS-5 infection, was maintained over 72 h, and was followed by production of tumor necrosis factor alpha but not IL-1beta. Animals infected with NHS1DD showed markedly lower levels of IL-6 (less than 5% of that of parental GS-5-infected rats), while tumor necrosis factor alpha was unaffected. In contrast, animals infected with NHR1DD, another isogenic mutant expressing only GtfB, showed a much smaller reduction (down to 56%). These results suggest that GTFs are specific modulins that act during acute inflammation, inducing IL-6 from endothelial cells surrounding the infected valves without affecting bacterial colonization in vegetations, and that IL-6 might persist in chronic inflammation in endocarditis.

Inducible antisense RNA expression in the characterization of gene functions in Streptococcus mutans

In order to examine gene function in Streptococcus mutans, we have recently initiated an antisense RNA strategy. Toward this end, we have now constructed and evaluated three Escherichia coli-S. mutans shuttle expression vectors with the fruA and scrB promoters from S. mutans, as well as the tetR-controlled tetO promoter from Staphylococcus aureus. Among these, the tetO/tetR system proved to be the most tightly controlled promoter. By using this shuttle plasmid system, modulation of gene function by inducible antisense RNA expression was demonstrated for comC antisense fragments of different sizes as well as for distinct gtfB antisense fragments. It was demonstrated that the size, but not the relative position, of an antisense DNA fragment is important in mediating the antisense phenomenon. Furthermore, by constructing and screening random DNA libraries with the tet expression shuttle system, 78 growth-retarded transformants harboring antisense DNA fragments were also identified. Almost all of them corresponded to homologous essential genes in other bacteria. In addition, a novel essential gene, the coaE gene, encoding dephospho-coenzyme A kinase, which is involved in the final step of coenzyme A catabolism in S. mutans, was identified and characterized. These results suggest that the antisense RNA strategy can be useful for identifying novel essential genes in S. mutans bacteria as well as further characterizing the physiology (including potential virulence factors) of these organisms.

In vitro effects of crude khat extracts on the growth, colonization, and glucosyltransferases of Streptococcus mutans

Millions of Yemenites, East Africans, and immigrants to Western countries chew khat daily for its amphetamine-like effects. There is little information in the literature concerning the possible effects of the habit on oral microbiota. Our objective was to study in vitro crude khat extract effects on Streptococcus mutans growth and sucrose-dependent colonization, and on its glucosyltransferase (GTF) activity and production. Three khat cultivars were used. Lyophilized crude aqueous khat extracts were applied to the different assays at concentrations of 0-1% (w/v). Sucrose-dependent colonization was assessed as the ability of Streptococcus mutans UA159 to form adherent biofilms in glass culture tubes. Colony forming units (CFUs) in the planktonic phase served as a measure of bacterial growth, while CFUs in the biofilm phase were used to quantify viability in the biofilms. GTFs activity was tested by incubating a crude GTFs preparation with sucrose and determining the amount of water-soluble and water-insoluble glucans formed. GTFs production was assayed by comparing intensities of GTF bands in Western blots of extracts from control and khat-containing cultures. The khat extracts effectively inhibited biofilm formation. The minimum biofilm inhibitory concentration (MBIC) varied among the cultivars (0.25-1%). The extracts also inhibited synthesis of both glucan types, particularly insoluble glucans (average 85% inhibition at 1%), with significant differences among the cultivars. However, khat increased bacterial growth and at sub-MBIC also viability within biofilms; there were no inter-cultivar differences. It is shown that khat leaves contain water-soluble constituents that inhibit some cariogenic properties of S. mutans in vitro.

Development of a rapid, quantitative glucosyltransferase assay based on a screen-printed fructose enzyme electrode and application to optimization studies ongtfD expression in recombinantEscherichia coli

A biosensor for fructose determination was used as basis of an assay for the determination of glucosyltransferase (GTF) activities and applied to monitoring recombinant enzyme production. GTFs catalyze the synthesis of glucans from sucrose leading to the release of fructose. Specific fructose determinations in the microM concentration range were achieved with a fructose electrode based on fructose dehydrogenase, which was immobilized on a screen-printed platinum electrode. This electrode was used as basis of the new assay for GTF activity determinations. Depending on the amount of enzyme, the assay was completed within 15-30 min compared to 1-2 h for the traditional photometric assay. From the amount of fructose released in a given reaction time, GTF activities were determined down to approx. 20 U/L. Even unpurified samples from a recombinant GTF-S production process could be analyzed without any problems, and a good correlation was obtained to data obtained from the photometric assay. Analysis of samples from cultures of various rGTF-S-producing recombinant E. coli strains grown on different media with SDS-PAGE and with the new assay identified the same strain and culture medium as optimum for recombinant GTF-S production.

Effects of Magnolol and Honokiol on the activities of streptococcal glucosyltransferases both in solution and adsorbed on an experimental pellicle

AIMS

To investigate the inhibitory effects of Magnolol and Honokiol on the activity of streptococcal glucosyltransferases (Gtfs).

METHODS AND RESULTS

The effect of Magnolol and Honokiol that inhibits the activities of streptococcal GtfB, GtfC, GtfD and GtfS was explored with standard assays. The results showed that both samples can efficiently inhibit the activity of all Gtfs in solution (66.4-96.3%) and adsorbed on the surface of saliva-coated hydroxyapatite (sHA) beads (65.5-92.7%) at concentrations between 1.25 and 5.0 mg ml(-1). Furthermore, Magnolol had a stronger inhibition of four kinds of Gtfs than Honokiol both in solution and adsorbed on the surface of sHA beads at concentrations between 0.04 and 0.63 mg ml(-1) (P < 0.05).

CONCLUSIONS

Magnolol had significant effects on the activities of streptococcal Gtfs.

SIGNIFICANCE AND IMPACT OF THE STUDY

Magnolol as a natural herb can be developed into a new oral hygiene product to prevent plaque formation.

Molecular analysis of the inhibitory effects of oolong tea polyphenols on glucan-binding domain of recombinant glucosyltransferases fromStreptococcus mutansMT8148

An oolong tea polyphenol (OTF6) has been shown to possess a strong anti-glucosyltransferase (GTF) activity and inhibit experimental dental caries in rats infected with mutans streptococci. The effects of OTF6 on the functional domains of GTFs of Streptococcus mutans, an N-terminal catalytic domain (CAT), and a C-terminal glucan-binding domain (GBD), were examined. The maximum velocity of glucan synthesis by recombinant GTFB (rGTFB) and GTFD (rGTFD) became significantly slower in the presence of OTF6, however, Km values remained stable when compared in their absence. These results suggest that OTF6 reduces glucan synthesis by non-competitively inhibiting the GBD of S. mutans GTFB and GTFD. Further, the recombinant proteins of CAT (rCAT) and GBD (rGBD) were expressed using Escherichia coli, and purified by affinity column chromatography. rGBD but not rCAT was found to possess dextran-binding activity, which was shown to be inhibited by OTF6. These results indicate that OTF6, a polymeric polyphenol specific for oolong tea is able to reduce glucan synthesis by inhibiting the GBD of S. mutans GTFB.

Effect of an orphan response regulator on Streptococcus mutans sucrose-dependent adherence and cariogenesis

Streptococcus mutans is the principal acidogenic component of dental plaque that demineralizes tooth enamel, leading to dental decay. Cell-associated glucosyltransferases catalyze the sucrose-dependent synthesis of sticky glucan polymers that, together with glucan binding proteins, promote S. mutans adherence to teeth and cell aggregation. We generated an S. mutans Tn916 transposon mutant, GMS315, which is defective in sucrose-dependent adherence and significantly less cariogenic than the UA130 wild-type progenitor in germfree rats. The results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis, Western blotting, and N-terminal sequence analysis confirmed the absence of a 155-kDa glucosyltransferase S (Gtf-S) from GMS315 protein profiles. Mapping of the unique transposon insertion in GMS315 revealed disruption of a putative regulatory region located upstream of gcrR, a gene previously described by Sato et al. that shares significant amino acid identity with other bacterial response regulators (Y. Sato, Y. Yamamoto, and H. Kizaki, FEMS Microbiol. Lett. 186: 187-191, 2000). The gcrR regulator, which we call "tarC," does not align with any of the 13 proposed two-component signal transduction systems derived from in silico analysis of the S. mutans genome, but rather represents one of several orphan response regulators in the genome. The results of Northern hybridization and/or real-time reverse transcription-PCR experiments reveal increased expression of both Gtf-S and glucan binding protein C (GbpC) in a tarC knockout mutant (GMS900), thereby supporting the notion that TarC acts as a negative transcriptional regulator. In addition, we noted that GMS900 has altered biofilm architecture relative to the wild type and is hypocariogenic in germfree rats. Taken collectively, these findings support a role for signal transduction in S. mutans sucrose-dependent adherence and aggregation and implicate TarC as a potential target for controlling S. mutans-induced cariogenesis.

Structural analysis of the functional influence of the surface peptide Gtf-P1 on Streptococcus mutans glucosyltransferase C activity

Glucosyltransferases (GtfB/C/D) in Streptococcus mutans are responsible for synthesizing water-insoluble and water-soluble glucans from sucrose and play very crucial roles in the formation of dental plaque. A monoclonal antibody against a 19-mer peptide fragment named Gtf-P1 was found in GtfC to reduce the enzyme activity to 50%. However, a similar experiment suggested almost unchanged activity in GtfD, despite of the very high sequence homology between the two enzymes. No further details are yet available to elucidate the biochemical mechanism responsible for such discrimination. For a better understanding of the catalytic behavior of these glucosyltransferases, structural and functional analyses were performed. First, the exact epitope was identified to specify the residue(s) required for monoclonal antibody recognition. The results suggest that the discrimination is determined solely by single residue substitution. Second, based on a combined sequence and secondary structure alignment against known crystal structure of segments from closely related proteins, a three-dimensional homology model for GtfC was built. Structural analysis for the region communicating between Gtf-P1 and the catalytic triad revealed the possibility for an "en bloc" movement of hydrophobic residues, which may transduce the functional influence on enzyme activity from the surface of molecule into the proximity of the active site. Figure Side chain interactions between Gtf-P1 and catalytic Asp-477 in GtfC. Calpha-tracing of GtfC with the two crucial peptides (Gtf-P1, orange; Gtf-P2, blue) and the catalytic triad residues ( red) highlighted to show their relative spatial organization. Side chains for the residues are also depicted according to their atom types. The structure is viewed with the barrel opening facing down