The VicRK System of Streptococcus mutans Responds to Oxidative Stress

Nanoceria Aggregate Formulation Promotes Buffer Stability, Cell Clustering, and Reduction of Adherent Biofilm in Streptococcus mutans

Streptococcus mutans is one of the key etiological factors in tooth-borne biofilm development that leads to dental caries in the presence of fermentable sugars. We previously reported on the ability of acid-stabilized nanoceria (CeO2-NP) produced by the hydrolysis of ceric salts to limit biofilm adherence of S. mutans via non-bactericidal mechanism(s). Herein, we report a chondroitin sulfate A (CSA) formulation (CeO2-NP-CSA) comprising nanoceria aggregates that promotes resistance to bulk precipitation under a range of conditions with retention of the biofilm-inhibiting activity, allowing for a more thorough mechanistic study of its bioactivity. The principal mechanism of reduced in vitro biofilm adherence of S. mutans by CeO2-NP-CSA is the production of nonadherent cell clusters. Additionally, dose-dependent in vitro human cell toxicity studies demonstrated no additional toxicity beyond that of equimolar doses of sodium fluoride, currently utilized in many oral health products. This study represents a unique approach and use of a nanoceria aggregate formulation with implications for promoting oral health and dental caries prevention as an adjunctive treatment.

vicR overexpression in Streptococcus mutans causes aggregation and affects interspecies competition

Streptococcus mutans is considered to be a major causative agent of dental caries. VicRK is a two-component signal transduction system (TCSTS) of S. mutans, which can regulate the virulence of S. mutans, such as biofilm formation, exopolysaccharide production, acid production, and acid resistance. Meanwhile, it can also regulate the production of mutacins (nlmC) through the TCSTS ComDE. In this study, we found that the vicR-overexpressing strain was more likely to aggregate to form cell clusters, leading to the formation of abnormal biofilm; the overexpression of vicR increased the length of the chain of S. mutans. Furthermore, the expression of the mutacins in the vicR overexpression strain was increased under aerobic conditions. Compared with the control strain and the parental strain, the vicR overexpression strain was more competitive against Streptococcus gordonii. But there was no significant difference against Streptococcus sanguinis. In clinical strains, the expression level of vicR was positively correlated with their competitive ability against S. gordonii. Transcriptional profiling revealed 24 significantly upregulated genes in the vicR-overexpressing strain, including nlmA, nlmB, nlmC, and nlmD encoding mutacins. Electrophoretic mobility shift assays and DNase I footprinting assays confirmed that VicR can directly bind to the promoter sequence of nlmD. Taken together, our findings further demonstrate that VicRK, an important TCSTS of S. mutans, is involved in S. mutans cell morphology and biofilm formation. VicRK regulates the production of more mutacins in S. mutans in response to oxygen stimulation. VicR can bind to the promoter sequence of nlmD, thereby directly regulating the production of mutacins NlmD.

Mechanisms regulating the CRISPR-Cas systems

The CRISPR-Cas (Clustered Regularly Interspaced Short Palindromic Repeats- CRISPR associated proteins) is a prokaryotic system that enables sequence specific recognition and cleavage of nucleic acids. This is possible due to cooperation between CRISPR array which contains short fragments of DNA called spacers that are complimentary to the targeted nucleic acid and Cas proteins, which take part in processes of: acquisition of new spacers, processing them into their functional form as well as recognition and cleavage of targeted nucleic acids. The primary role of CRISPR-Cas systems is to provide their host with an adaptive and hereditary immunity against exogenous nucleic acids. This system is present in many variants in both Bacteria and Archea. Due to its modular structure, and programmability CRISPR-Cas system become attractive tool for modern molecular biology. Since their discovery and implementation, the CRISPR-Cas systems revolutionized areas of gene editing and regulation of gene expression. Although our knowledge on how CRISPR-Cas systems work has increased rapidly in recent years, there is still little information on how these systems are controlled and how they interact with other cellular mechanisms. Such regulation can be the result of both auto-regulatory mechanisms as well as exogenous proteins of phage origin. Better understanding of these interaction networks would be beneficial for optimization of current and development of new CRISPR-Cas-based tools. In this review we summarize current knowledge on the various molecular mechanisms that affect activity of CRISPR-Cas systems.

Antisense vicR-Loaded Dendritic Mesoporous Silica Nanoparticles Regulate the Biofilm Organization and Cariogenicity of Streptococcus mutans

Purpose

VicR is the essential response regulator related to the synthesis of exopolysaccharide (EPS) – one of the main cariogenic factors of S. mutans. An antisense vicR RNA (ASvicR) could bind to vicR mRNA, hindering the transcription and translation of the vicR gene. We had constructed a recombinant plasmid containing the ASvicR sequence (plasmid-ASvicR) and proved that it could reduce EPS synthesis, biofilm formation, and cariogenicity. However, the recombinant plasmids are supposed to be protected from enzymatic degradation and possess higher transformation efficiency. The principal objective of the present research was to construct an appropriate vector that can carry and protect the plasmid-ASvicR and investigate the effects of the carried plasmids on the cariogenicity of the S. mutans.

Methods

Aminated dendritic mesoporous silica nanoparticles (DMSNs-NH₂) were synthesized and characterized. The ability of DMSNs-NH₂ to carry and preserve the plasmid-ASvicR (DMSNs-NH₂-ASvicR) was proved by the loading curve, agarose electrophoresis, DNase I digestion assays, and energy-dispersive spectrometry (EDS) mapping. Transformation assays demonstrated whether the plasmid could enter S. mutans. The effect of DMSNs-NH₂-ASvicR on the 12-hour and 24-hour biofilms of S. mutans was evaluated by biofilm formation experiments and quantitative reverse transcription polymerase chain reaction (qRT-PCR). The cytotoxicity of DMSNs-NH₂-ASvicR was assessed by CCK-8 and live/dead staining assays. The regulation of DMSNs-NH₂-ASvicR on the cariogenicity of S. mutans was also evaluated in vivo.

Results

DMSNs-NH₂ could load approximately 92% of plasmid-ASvicR at a mass ratio of 80 and protect most of plasmid-ASvicR from degradation by DNase I. The plasmid-ASvicR loaded on DMSNs-NH₂ could be transformed into S. mutans, which down-regulated the expression of the vicR gene, reducing EPS synthesis and biofilm organization of S. mutans. DMSNs-NH₂-ASvicR exhibited favorable biocompatibility, laying a foundation for its subsequent biomedical application. In addition, DMSNs-NH₂-ASvicR led to decreased caries in vivo.

Conclusion

DMSNs-NH₂ is a suitable vector of plasmid-ASvicR, and DMSNs-NH₂-ASvicR can inhibit biofilm formation, reducing the cariogenicity of S. mutans. These findings reveal that DMSNs-NH₂-ASvicR is a promising agent for preventing and treating dental caries.

The vicK gene of Streptococcus mutans mediates its cariogenicity via exopolysaccharides metabolism

Streptococcus mutans (S. mutans) is generally regarded as a major contributor to dental caries because of its ability to synthesize extracellular polysaccharides (EPS) that aid in the formation of plaque biofilm. The VicRKX system of S. mutans plays an important role in biofilm formation. The aim of this study was to investigate the effects of vicK gene on specific characteristics of EPS in S. mutans biofilm. We constructed single-species biofilms formed by different mutants of vicK gene. Production and distribution of EPS were detected through atomic force microscopy, scanning electron microscopy and confocal laser scanning microscopy. Microcosmic structures of EPS were analyzed by gel permeation chromatography and gas chromatography-mass spectrometry. Cariogenicity of the vicK mutant was assessed in a specific pathogen-free rat model. Transcriptional levels of cariogenicity-associated genes were confirmed by quantitative real-time polymerase chain reaction. The results showed that deletion of vicK gene suppressed biofilm formation as well as EPS production, and EPS were synthesized mostly around the cells. Molecular weight and monosaccharide components underwent evident alterations. Biofilms formed in vivo were sparse and contributed a decreased degree of caries. Moreover, expressional levels of genes related to EPS synthesis were down-regulated, except for gtfB. Our report demonstrates that vicK gene enhances biofilm formation and subsequent caries development. And this may due to its regulations on EPS metabolism, like synthesis or microcosmic features of EPS. This study suggests that vicK gene and EPS can be considered as promising targets to modulate dental caries.

3, 5-Di-tert-butylphenol combat against Streptococcus mutans by impeding acidogenicity, acidurance and biofilm formation

Streptococcus mutans is a common pathogen present in the oral cavity and it causes dental caries for all aged groups of people, in particular, children. S. mutans have several virulence factors such as acidogenecity, aciduricity, adhesion and biofilm formation. These virulence factors are working together and lead to the development of caries in the tooth surface. The present study aimed to investigate the anticariogenic potential of 3, 5-di-tert-butylphenol (3, 5-DTBP) against S. mutans. 3, 5-DTBP biofilm inhibitory concentration (BIC) was found at 100 µg/ml concentration without any lethal effect on the growth. Moreover, 3, 5-DTBP significantly reduced water soluble and water insoluble glucans production, in concurrence with downregulation of gtfBC genes. Moreover, acidogenicity associated virulence factors such as lactate dehydrogenase and enolase enzymatic production was arrested upon 3, 5-DTBP treatment. In addition, 3, 5-DTBP greatly reduced acidtolerance ability through impedes of F₁F₀-ATPase. Gene expression analysis unveiled the downregulation of gtfB, gtfC, gtfD, vicRK, comDE, gbpB, smu0630 and relA upon 3, 5-DTBP treatment. The present study paves the way for exhibiting 3, 5-DTBP as a promising therapeutic agent to control S. mutans infections.

Dissecting the Role of VicK Phosphatase in Aggregation and Biofilm Formation of Streptococcus mutans

VicRK (WalRK or YycFG) is a conserved 2-component regulatory system (TCS) that regulates cell division, cell wall biosynthesis, and homeostasis in low-GC Gram-positive bacteria. VicRK is also associated with biofilm formation of Streptococcus mutans on the tooth surface as it directly regulates the extracellular polysaccharide (EPS) synthesis. Of the 2 components, VicK possesses both autokinase and phosphatase activities, which regulate the phosphorylation and dephosphorylation of the regulator VicR in response to environmental cues. However, the dual mechanism of VicK as the autokinase/phosphatase in regulating S. mutans' responses is not well elucidated. Previously, it has been shown that the phosphatase activity depends on the PAS domain and residues in the DHp domain of VicK in S. mutans. Specifically, mutating proline at 222 in the PAS domain inhibits VicK phosphatase activity. We generated a VicK^P222A mutant to determine the level of VicR-P in the cytoplasm by Phos-tag sodium dodecyl sulfate polyacrylamide gel electrophoresis. We show that in VicK^P222A phosphatase, attenuation increased phosphorylated VicR (VicR-P) that downregulated glucosyltransferases, gtfBC, thereby reducing the synthesis of water-insoluble polysaccharides (WIS-EPS) in the biofilm. In addition, VicK^P222A presented as long-rod cells, reduced growth, and displayed asymmetrical division. A major adhesin of S. mutans, SpaP was downregulated in VicK^P222A, making it unable to agglutinate in saliva. In summary, we have confirmed that VicK phosphatase activity is critical to maintain optimal phosphorylation status of VicR in S. mutans, which is important for cell growth, cell division, EPS synthesis, and bacterial agglutination in saliva. Hence, VicK phosphatase activity may represent a promising target to modulate S. mutans' pathogenicity.

Usnic acid deteriorates acidogenicity, acidurance and glucose metabolism of Streptococcus mutans through downregulation of two-component signal transduction systems

The principal etiological agent of human dental caries, Streptococcus mutans is a multi-virulent pathogen that can transform commensal oral microbial community to plaque biofilms. Major virulence factors that are associated with the cariogenicity of S. mutans include adhesion, acidogenicity and acidurity. All these pathogenic traits coordinate and alter the dental plaque ecology which provide room for interaction with other similar acidogenic and aciduric bacteria. This cariogenic flora increases the possibility of enamel demineralization which headway to caries development. The present study was aimed at evaluating the antimicrobial and antiinfective potential of a lichen secondary metabolite usnic acid (UA) against S. mutans. Minimum inhibitory concentration (MIC), Minimum bactericidal concentration (MBC) and growth kinetics were evaluated to determine the antimicrobial potential of UA against S. mutans. UA at 5 µg mL-1 and 10 µg mL-1 concentration were considered as MIC and MBC respectively. Effect on biofilm formation was microscopically assessed and found to be reduced in a concentration dependent manner. Gene expression of gtfB, gtfC, gtfD, vicR, ComDE and smu0630 was found to be downregulated upon treatment with sub-MIC of UA. Acidogenicity, acidurity, eDNA synthesis and response to oxidative stress were found to be attenuated by the influence of UA. It was also demonstrated to act on preformed mature biofilm of S. mutans. Moreover, UA was shown to possess very low frequency to acquire spontaneous resistance development in S. mutans. Besides, no morphological aberrations or toxic effect was instigated by UA in the human buccal epithelial cells as well as to the oral commensals. Altogether, these results demonstrate the therapeutic potential of usnic acid in the treatment of S. mutans infection.

Amino Sugars Reshape Interactions between Streptococcus mutans and Streptococcus gordonii

Amino sugars, particularly glucosamine (GlcN) and N-acetylglucosamine (GlcNAc), are abundant carbon and nitrogen sources supplied in host secretions and in the diet to the biofilms colonizing the human oral cavity. Evidence is emerging that these amino sugars provide ecological advantages to beneficial commensals over oral pathogens and pathobionts. Here, we performed transcriptome analysis on Streptococcus mutans and Streptococcus gordonii growing in single-species or dual-species cultures with glucose, GlcN, or GlcNAc as the primary carbohydrate source. Compared to glucose, GlcN caused drastic transcriptomic shifts in each species of bacteria when it was cultured alone. Likewise, cocultivation in the presence of GlcN yielded transcriptomic profiles that were dramatically different from the single-species results from GlcN-grown cells. In contrast, GlcNAc elicited only minor changes in the transcriptome of either organism in single- and dual-species cultures. Interestingly, genes involved in pyruvate metabolism were among the most significantly affected by GlcN in both species, and these changes were consistent with measurements of pyruvate in culture supernatants. Differing from what was found in a previous report, growth of S. mutans alone with GlcN inhibited the expression of multiple operons required for mutacin production. Cocultivation with S. gordonii consistently increased the expression of two manganese transporter operons (slo and mntH) and decreased expression of mutacin genes in S. mutans Conversely, S. gordonii appeared to be less affected by the presence of S. mutans but did show increases in genes for biosynthetic processes in the cocultures. In conclusion, amino sugars profoundly alter the interactions between pathogenic and commensal streptococci by reprogramming central metabolism.IMPORTANCE Carbohydrate metabolism is central to the development of dental caries. A variety of sugars available to dental microorganisms influence the development of caries by affecting the physiology, ecology, and pathogenic potential of tooth biofilms. Using two well-characterized oral bacteria, one pathogen (Streptococcus mutans) and one commensal (Streptococcus gordonii), in an RNA deep-sequencing analysis, we studied the impact of two abundant amino sugars on bacterial gene expression and interspecies interactions. The results indicated large-scale remodeling of gene expression induced by GlcN in particular, affecting bacterial energy generation, acid production, protein synthesis, and release of antimicrobial molecules. Our study provides novel insights into how amino sugars modify bacterial behavior, information that will be valuable in the design of new technologies to detect and prevent oral infectious diseases.

Effect of Rubusoside, a Natural Sucrose Substitute, on Streptococcus mutans Biofilm Cariogenic Potential and Virulence Gene Expression In Vitro

Dental caries is a biofilm-mediated disease in which Streptococcus mutans is the main pathogenic microorganism, and its incidence is closely related to sucrose. Rubusoside is a natural nonnutritive sweetener isolated from Rubus suavissimus S. Lee. This study was designed to determine the effect of this sucrose substitute on the cariogenic properties and virulence gene expression of S. mutans biofilms. S. mutans was exposed to brain heart infusion (BHI) medium (as a control), 1% sucrose-supplemented medium, 1% rubusoside-supplemented medium, and 1% xylitol-supplemented medium. The growth curve of the biofilm was monitored by crystal violet staining, and the pH was measured every 24 h. After 5 days, the biofilms formed on the glass coverslips were recovered to determine the biomass (dry weight and total amount of soluble proteins), numbers of CFU, and amounts of intra- and extracellular polysaccharides. Biofilm structural imaging was performed using a scanning electron microscope (SEM). Virulence gene expression (gtfB, gtfC, gtfD, ftf, spaP, gbpB, ldh, atpF, vicR, and comD) was determined by reverse transcription-quantitative PCR. Growth in rubusoside resulted in lower levels of acid production than observed during growth in sucrose, xylitol, and the control, while it also reduced the level of biofilm accumulation and bacterial viability and even reduced the level of production of extracellular polysaccharides. By SEM, the levels of biofilm formation and extracellular matrix during growth in rubusoside were lower than these levels during growth in sucrose and xylitol. From the perspective of virulence genes, growth in rubusoside and xylitol significantly inhibited the expression of virulence genes compared with their levels of expression after growth in sucrose. Among these genes, gtfB, gtfC, gbpB, ldh, and comD downregulation was found with growth in rubusoside compared with their expression with growth in xylitol. Therefore, rubusoside appears to be less potentially cariogenic than sucrose and xylitol and may become an effective sucrose substitute for caries prevention. Further studies are needed to deepen these findings.IMPORTANCE Dental caries is a major public health challenge and places heavy biological, social, and financial burdens on individuals and health care systems. To palliate the deleterious effect of sucrose on the virulence factors of S. mutans, massive commercial efforts have been oriented toward developing products that may act as sucrose substitutes. Rubusoside, a natural sucrose substitute, is a plant extract with a high level of sweetness. Although some studies have shown that rubusoside does not produce acids or inhibit the growth of S. mutans, little attention has been paid to its effect on dental biofilm and the underlying mechanisms. Our study focuses on the effect of rubusoside on the formation and structure of biofilms and the expression of virulence genes. The results confirm that rubusoside can inhibit accumulation, bacterial viability, polysaccharide production by the biofilm, and related gene expression. These results provide further insight into the cariogenicity of S. mutans biofilms and demonstrate a new perspective for studying the impact of sucrose substitutes on caries.

Guardian genes ensuring subsistence of oral Streptococcus mutans

Despite the substantial research advancements on oral diseases, dental caries remains a major healthcare burden. A disease of microbial dysbiosis, dental caries is characterised by the formation of biofilms that assist demineralisation and destruction of the dental hard tissues. While it is well understood that this is a multi-kingdom biofilm-mediated disease, it has been elucidated that acid producing and acid tolerant bacteria play pioneering roles in the process. Specifically, Streptococcus mutans houses major virulence pathways that enable it to thrive in the oral cavity and cause caries. This pathogen adheres to the tooth substrate, forms biofilms, resists external stress, produces acids, kills closely related species, and survives the acid as well as the host clearance mechanisms. For an organism to be able to confer such virulence, it requires a large and complex gene network which synergise to establish disease. In this review, we have charted how these multi-faceted genes control several caries-related functions of Streptococcus mutans. In a futuristic thinking approach, we also briefly discuss the potential roles of omics and machine learning, to ease the study of non-functional genes that may play a major role and enable the integration of experimental data.

Physiological function analysis of Lactobacillus plantarum Y44 based on genotypic and phenotypic characteristics

In our previous studies, Lactobacillus plantarum Y44 showed antioxidant activity and favorable gastric and intestinal transit tolerance. In the current study, we investigated the physiological function of L. plantarum Y44 based on an analysis of its genotype and phenotype. The complete genome of L. plantarum Y44 contained a single circular chromosome of 3,255,555 bp, with a GC content of 44.6%, and a single circular plasmid of 51,167 bp, with a GC content of 38.8%. The L. plantarum Y44 genome contained 3,293 genes including 3,112 protein coding sequences, 16 rRNAs, 66 tRNAs, 4 small (s)RNAs, and 95 pseudo genes. Lactobacillus plantarum Y44 could metabolize 24 different carbohydrate sources. Nineteen complete phosphoenolpyruvate-dependent sugar phosphotransferase system complex genes and intact Embden-Meyerhof-Parnas pathway and hexose monophosphate pathway enzyme genes, as well as abundant carbohydrate active enzyme genes, were identified in the L. plantarum Y44 genome. We also identified genes related to the biosynthesis of exopolysaccharide and surface proteins. Surface proteins played an important role in the L. plantarum Y44 adhesion to HT-29 cell monolayers, as evidenced by the removal of cell surface proteins leading to decreased adhesion capacity. The L. plantarum Y44 genome contained genes encoding chaperones, intracellular proteases, and 2-component systems, which were associated with the general stress response. Genes encoding bile salt hydrolase, F₀F₁-ATPase, Na⁺/H⁺-antiporter, H⁺/Cl^- exchange transporter, cyclopropane-fatty acyl-phospholipid synthase, and alkaline shock protein were identified in the L. plantarum Y44 genome, which might explain the strain's favorable gastric and intestinal transit tolerance. Some genes associated with encoding the NADH system, glutathione system, and thioredoxin system were predicted via in silico analysis and might account for the strain's ability to scavenge reactive oxygen species. Lactobacillus plantarum Y44 was susceptive to 7 antibiotics and did not produce biogenic amines, likely due to the absence of acquired antibiotic resistance genes and amino acid decarboxylase genes. The phenotype profile of L. plantarum Y44 was associated with its genetic characteristics, indicating that strains with certain physiological functions can be screened by analyzing their phenotypic and genotypic characteristics. Lactobacillus plantarum Y44 has the potential to be used as a starter culture in fermented dairy products.

Environmental Triggers of lrgA Expression in Streptococcus mutans

The cidAB and lrgAB operons of Streptococcus mutans encode proteins that are structurally similar to the bacteriophage lambda family of holin-antiholin proteins, which are believed to facilitate cell death in other bacterial species. Although their precise function is not known, cidAB and lrgAB are linked to multiple virulence traits of S. mutans, including oxidative stress tolerance, biofilm formation, and autolysis. Here we investigate the regulation of lrgAB which in S. mutans shows a complex dependence on growth conditions that is not fully understood. By combining single-cell imaging of a fluorescent gene reporter with microfluidic control of the extracellular environment, we identify specific environmental cues that trigger lrgA expression and characterize cell-to-cell heterogeneity in lrgA activity. We find that the very abrupt activation of lrgA at stationary phase is tightly synchronized across the population. This activation is controlled by a small number of inputs that are sensitive to growth phase: extracellular pyruvate, glucose, and molecular oxygen. Activation of lrgA appears to be self-limiting, so that strong expression of lrgA is confined to a short interval of time. lrgA is programmed to switch on briefly at the end of exponential growth, as glucose and molecular oxygen are exhausted and extracellular pyruvate is available. Our findings are consistent with studies of other bacteria showing that homologs of lrgAB participate, with input from lytST, in the reimport of pyruvate for anaerobic fermentative growth.

Complex Responses to Hydrogen Peroxide and Hypochlorous Acid by the Probiotic Bacterium Lactobacillus reuteri

Inflammatory diseases of the gut are associated with increased intestinal oxygen concentrations and high levels of inflammatory oxidants, including hydrogen peroxide (H₂O₂) and hypochlorous acid (HOCl), which are antimicrobial compounds produced by the innate immune system. This contributes to dysbiotic changes in the gut microbiome, including increased populations of proinflammatory enterobacteria (Escherichia coli and related species) and decreased levels of health-associated anaerobic Firmicutes and Bacteroidetes The pathways for H₂O₂ and HOCl resistance in E. coli have been well studied, but little is known about how commensal and probiotic bacteria respond to inflammatory oxidants. In this work, we have characterized the transcriptomic response of the anti-inflammatory, gut-colonizing Gram-positive probiotic Lactobacillus reuteri to both H₂O₂ and HOCl. L. reuteri mounts distinct but overlapping responses to each of these stressors, and both gene expression and survival were strongly affected by the presence or absence of oxygen. Oxidative stress response in L. reuteri required several factors not found in enterobacteria, including the small heat shock protein Lo18, polyphosphate kinase 2, and RsiR, an L. reuteri-specific regulator of anti-inflammatory mechanisms.IMPORTANCE Reactive oxidants, including hydrogen peroxide and hypochlorous acid, are antimicrobial compounds produced by the immune system during inflammation. Little is known, however, about how many important types of bacteria present in the human microbiome respond to these oxidants, especially commensal and other health-associated species. We have now mapped the stress response to both H₂O₂ and HOCl in the intestinal lactic acid bacterium Lactobacillus reuteri.

Role of two-component regulatory systems in the virulence of Streptococcus suis

Streptococcus suis is an important zoonotic pathogen that causes severe infections and great economic losses worldwide. Understanding how this pathogen senses and responds to environmental signals during the infectious process can offer insight into its pathogenesis and may be helpful in the development of drug targets. Two-component regulatory systems (TCSs) play an essential role in this environmental response. In S. suis, at least 15 groups of TCSs have been predicted. Among them, several have been demonstrated to be involved in virulence and/or stress response. In this review, we discuss the progress in the study of TCSs in S. suis, focusing on the role of these systems in the virulence of this bacterium.

Relationship between the genetic polymorphisms of vicR and vicK Streptococcus mutans genes and early childhood caries in two-year-old children

Background

The VicRK two-component signalling system regulates virulence and cariogenicity in Streptococcus mutans (S. mutans). The purpose of this study was to explore the genetic polymorphisms of the vicR and vicK genes, which are associated with dental caries in children with S. mutans.

Methods

In this study, 121 (from each group) clinical S. mutans strains were isolated from caries-free children and children with high-severity caries to sequence the vicR and vicK genes. Genomic DNA was extracted from S. mutans strains and amplified using PCR. The PCR products were purified and sequenced. A chi-squared test and ABI Variant Reporter software were used to analyse the sequencing results.

Results

The 242 clinically isolated S. mutans strains contained the full-length vicR and vicK genes. No nucleotide sequence insertions or deletions were observed in the two genes. Four silent point mutations were identified in the vicR genes, and no missense mutations could be detected. Forty-one mutations were identified in the vicK genes. In addition to 32 silent mutations, 9 missense mutations at the 173, 337, 470, 1051, 1132, 1258, 1260, 1277, and 1348 bp positions were found. The distribution frequencies of the missense mutations were not significantly different between the groups, except for the C470T mutation. The frequency of the C470T missense mutation was higher in the high-severity caries group than in the caries-free group.

Conclusions

vicR sequences are highly conserved in S. mutans clinical isolates. The locus 470 missense mutation of the vicK gene may be related to caries in children with S. mutans.

Oxidative Stressors Modify the Response of Streptococcus mutans to Its Competence Signal Peptides

The dental caries pathogen Streptococcus mutans is continually exposed to several types of stress in the oral biofilm environment. Oxidative stress generated by reactive oxygen species has a major impact on the establishment, persistence, and virulence of S. mutans Here, we combined fluorescent reporter-promoter fusions with single-cell imaging to study the effects of reactive oxygen species on activation of genetic competence in S. mutans Exposure to paraquat, which generates superoxide anion, produced a qualitatively different effect on activation of expression of the gene for the master competence regulator, ComX, than did treatment with hydrogen peroxide (H₂O₂), which can yield hydroxyl radical. Paraquat suppressed peptide-mediated induction of comX in a progressive and cumulative fashion, whereas the response to H₂O₂ displayed a strong threshold behavior. Low concentrations of H₂O₂ had little effect on induction of comX or the bacteriocin gene cipB, but expression of these genes declined sharply if extracellular H₂O₂ exceeded a threshold concentration. These effects were not due to decreased reporter gene fluorescence. Two different threshold concentrations were observed in the response to H₂O₂, depending on the gene promoter that was analyzed and the pathway by which the competence regulon was stimulated. The results show that paraquat and H₂O₂ affect the S. mutans competence signaling pathway differently, and that some portions of the competence signaling pathway are more sensitive to oxidative stress than others.IMPORTANCEStreptococcus mutans inhabits the oral biofilm, where it plays an important role in the development of dental caries. Environmental stresses such as oxidative stress influence the growth of S. mutans and its important virulence-associated behaviors, such as genetic competence. S. mutans competence development is a complex behavior that involves two different signaling peptides and can exhibit cell-to-cell heterogeneity. Although oxidative stress is known to influence S. mutans competence, it is not understood how oxidative stress interacts with the peptide signaling or affects heterogeneity. In this study, we used fluorescent reporters to probe the effect of reactive oxygen species on competence signaling at the single-cell level. Our data show that different reactive oxygen species have different effects on S. mutans competence, and that some portions of the signaling pathway are more acutely sensitive to oxidative stress than others.

AtlA Mediates Extracellular DNA Release, Which Contributes to Streptococcus mutans Biofilm Formation in an Experimental Rat Model of Infective Endocarditis

Host factors, such as platelets, have been shown to enhance biofilm formation by oral commensal streptococci, inducing infective endocarditis (IE), but how bacterial components contribute to biofilm formation in vivo is still not clear. We demonstrated previously that an isogenic mutant strain of Streptococcus mutans deficient in autolysin AtlA (ΔatlA) showed a reduced ability to cause vegetation in a rat model of bacterial endocarditis. However, the role of AtlA in bacterial biofilm formation is unclear. In this study, confocal laser scanning microscopy analysis showed that extracellular DNA (eDNA) was embedded in S. mutans GS5 floes during biofilm formation on damaged heart valves, but an ΔatlA strain could not form bacterial aggregates. Semiquantification of eDNA by PCR with bacterial 16S rRNA primers demonstrated that the ΔatlA mutant strain produced dramatically less eDNA than the wild type. Similar results were observed with in vitro biofilm models. The addition of polyanethol sulfonate, a chemical lysis inhibitor, revealed that eDNA release mediated by bacterial cell lysis is required for biofilm initiation and maturation in the wild-type strain. Supplementation of cultures with calcium ions reduced wild-type growth but increased eDNA release and biofilm mass. The effect of calcium ions on biofilm formation was abolished in ΔatlA cultures and by the addition of polyanethol sulfonate. The VicK sensor, but not CiaH, was found to be required for the induction of eDNA release or the stimulation of biofilm formation by calcium ions. These data suggest that calcium ion-regulated AtlA maturation mediates the release of eDNA by S. mutans, which contributes to biofilm formation in infective endocarditis.

The two-component system VicRK regulates functions associated with Streptococcus mutans resistance to complement immunity

Streptococcus mutans, a dental caries pathogen, can promote systemic infections upon reaching the bloodstream. The two-component system (TCS) VicRK_Sm of S. mutans regulates the synthesis of and interaction with sucrose-derived exopolysaccharides (EPS), processes associated with oral and systemic virulence. In this study, we investigated the mechanisms by which VicRK_Sm affects S. mutans susceptibility to blood-mediated immunity. Compared with parent strain UA159, the vicK_Sm isogenic mutant (UAvic) showed reduced susceptibility to deposition of C3b of complement, low binding to serum immunoglobulin G (IgG), and low frequency of C3b/IgG-mediated opsonophagocytosis by polymorphonuclear cells in a sucrose-independent way (P<.05). Reverse transcriptase quantitative polymerase chain reaction analysis comparing gene expression in UA159 and UAvic revealed that genes encoding putative peptidases of the complement (pepO and smu.399) were upregulated in UAvic in the presence of serum, although genes encoding murein hydrolases (SmaA and Smu.2146c) or metabolic/surface proteins involved in bacterial interactions with host components (enolase, GAPDH) were mostly affected in a serum-independent way. Among vicK_Sm -downstream genes (smaA, smu.2146c, lysM, atlA, pepO, smu.399), only pepO and smu.399 were associated with UAvic phenotypes; deletion of both genes in UA159 significantly enhanced levels of C3b deposition and opsonophagocytosis (P<.05). Moreover, consistent with the fibronectin-binding function of PepO orthologues, UAvic showed increased binding to fibronectin. Reduced susceptibility to opsonophagocytosis was insufficient to enhance ex vivo persistence of UAvic in blood, which was associated with growth defects of this mutant under limited nutrient conditions. Our findings revealed that S. mutans employs mechanisms of complement evasion through peptidases, which are controlled by VicRK_Sm.

Pleiotropic Regulation of Virulence Genes in Streptococcus mutans by the Conserved Small Protein SprV

Streptococcus mutans, an oral pathogen associated with dental caries, colonizes tooth surfaces as polymicrobial biofilms known as dental plaque. S. mutans expresses several virulence factors that allow the organism to tolerate environmental fluctuations and compete with other microorganisms. We recently identified a small hypothetical protein (90 amino acids) essential for the normal growth of the bacterium. Inactivation of the gene, SMU.2137, encoding this protein caused a significant growth defect and loss of various virulence-associated functions. An S. mutans strain lacking this gene was more sensitive to acid, temperature, osmotic, oxidative, and DNA damage-inducing stresses. In addition, we observed an altered protein profile and defects in biofilm formation, bacteriocin production, and natural competence development, possibly due to the fitness defect associated with SMU.2137 deletion. Transcriptome sequencing revealed that nearly 20% of the S. mutans genes were differentially expressed upon SMU.2137 deletion, thereby suggesting a pleiotropic effect. Therefore, we have renamed this hitherto uncharacterized gene as sprV (streptococcal pleiotropic regulator of virulence). The transcript levels of several relevant genes in the sprV mutant corroborated the phenotypes observed upon sprV deletion. Owing to its highly conserved nature, inactivation of the sprV ortholog in Streptococcus gordonii also resulted in poor growth and defective UV tolerance and competence development as in the case of S. mutans Our experiments suggest that SprV is functionally distinct from its homologs identified by structure and sequence homology. Nonetheless, our current work is aimed at understanding the importance of SprV in the S. mutans biology.IMPORTANCEStreptococcus mutans employs several virulence factors and stress resistance mechanisms to colonize tooth surfaces and cause dental caries. Bacterial pathogenesis is generally controlled by regulators of fitness that are critical for successful disease establishment. Sometimes these regulators, which are potential targets for antimicrobials, are lost in the genomic context due to the lack of annotated homologs. This work outlines the regulatory impact of a small, highly conserved hypothetical protein, SprV, encoded by S. mutans We show that SprV affects the transcript levels of various virulence factors required for normal growth, biofilm formation, stress tolerance, genetic competence, and bacteriocin production.

The Staphylococcus aureus SrrAB Regulatory System Modulates Hydrogen Peroxide Resistance Factors, Which Imparts Protection to Aconitase during Aerobic Growth

The SrrAB two-component regulatory system (TCRS) positively influences the transcription of genes involved in aerobic respiration in response to changes in respiratory flux. Hydrogen peroxide (H₂O₂) can arise as a byproduct of spontaneous interactions between dioxygen and components of respiratory pathways. H₂O₂ damages cellular factors including protein associated iron-sulfur cluster prosthetic groups. We found that a Staphylococcus aureus strain lacking the SrrAB two-component regulatory system (TCRS) is sensitive to H₂O₂ intoxication. We tested the hypothesis that SrrAB manages the mutually inclusive expression of genes required for aerobic respiration and H₂O₂ resistance. Consistent with our hypothesis, a ΔsrrAB strain had decreased transcription of genes encoding for H₂O₂ resistance factors (kat, ahpC, dps). SrrAB was not required for the inducing the transcription of these genes in cells challenged with H₂O₂. Purified SrrA bound to the promoter region for dps suggesting that SrrA directly influences dps transcription. The H₂O₂ sensitivity of the ΔsrrAB strain was alleviated by iron chelation or deletion of the gene encoding for the peroxide regulon repressor (PerR). The positive influence of SrrAB upon H₂O₂ metabolism bestowed protection upon the solvent accessible iron-sulfur (FeS) cluster of aconitase from H₂O₂ poisoning. SrrAB also positively influenced transcription of scdA (ytfE), which encodes for a FeS cluster repair protein. Finally, we found that SrrAB positively influences H₂O₂ resistance only during periods of high dioxygen-dependent respiratory activity. SrrAB did not influence H₂O₂ resistance when cellular respiration was diminished as a result of decreased dioxygen availability, and negatively influenced it in the absence of respiration (fermentative growth). We propose a model whereby SrrAB-dependent regulatory patterns facilitate the adaptation of cells to changes in dioxygen concentrations, and thereby aids in the prevention of H₂O₂ intoxication during respiratory growth upon dixoygen.

Role of Streptococcus mutans two-component systems in antimicrobial peptide resistance in the oral cavity

Approximately 100 trillion microorganisms exist in the oral cavity. For the commensal bacteria of the oral cavity, it is important to adapt to environmental stimuli, including human- or bacteria-derived antimicrobial agents. Recently, bacterial-specific signal transduction regulatory systems, called two-component systems (TCSs), which appear to be focused on sensing and adapting to the environment, were discovered. Streptococcus mutans is an oral commensal bacteria and is also known as a cariogenic bacteria. Although the virulence factors of S. mutans have been well demonstrated, the mechanism underlying the adaptation of the species to the oral cavity is poorly understood. S. mutans UA159 has 15 sets of TCSs. Among them, several have been demonstrated to be involved in acid tolerance, competence and biofilm formation. Recently, together with our findings, it was demonstrated that 5 TCSs were involved in resistance to antimicrobial agents. Furthermore, another TCS was associated with the production of bacteriocin. Six of 15 TCSs are associated with antimicrobial agents, implying that S. mutans can survive in the oral cavity by resisting various antimicrobial peptides.

In this review, we highlight the role of antimicrobial peptides in the oral cavity.

Understanding the Streptococcus mutans Cid/Lrg System through CidB Function

The Streptococcus mutans lrgAB and cidAB operons have been previously described as a potential model system to dissect the complexity of biofilm development and virulence of S. mutans Herein, we have attempted to further characterize the Cid/Lrg system by focusing on CidB, which has been shown to be critical for the ability of S. mutans to survive and persist in a nonpreferred oxygen-enriched condition. We have found that the expression level of cidB is critical to oxidative stress tolerance of S. mutans, most likely by impacting lrg expression. Intriguingly, the impaired aerobic growth phenotype of the cidB mutant could be restored by the additional loss of either CidA or LrgA. Growth-dependent expression of cid and lrg was demonstrated to be tightly under the control of both CcpA and the VicKR two-component system (TCS), regulators known to play an essential role in controlling major catabolic pathways and cell envelope homeostasis, respectively. RNA sequencing (RNA-Seq) analysis revealed that mutation of cidB resulted in global gene expression changes, comprising major domains of central metabolism and virulence processes, particularly in those involved with oxidative stress resistance. Loss of CidB also significantly changed the expression of genes related to genomic islands (GI) TnSmu1 and TnSmu2, the CRISPR (clustered regularly interspaced short palindromic repeats)-Cas system, and toxin-antitoxin (T/A) modules. Taken together, these data show that CidB impinges on the stress response, as well as the fundamental cellular physiology of S. mutans, and further suggest a potential link between Cid/Lrg-mediated cellular processes, S. mutans pathogenicity, and possible programmed growth arrest and cell death mechanisms.

IMPORTANCE

The ability of Streptococcus mutans to survive a variety of harmful or stressful conditions and to emerge as a numerically significant member of stable oral biofilm communities are essential elements for its persistence and cariogenicity. In this study, the homologous cidAB and lrgAB operons, previously identified as being highly balanced and coordinated during S. mutans aerobic growth, were further characterized through the functional and transcriptomic analysis of CidB. Precise control of CidB levels is shown to impact the expression of lrg, oxidative stress tolerance, major metabolic domains, and the molecular modules linked to cell death and lysis. This study advances our understanding of the Cid/Lrg system as a key player in the integration of complex environmental signals (such as oxidative stress) into the regulatory networks that modulate S. mutans virulence and cell homeostasis.

Regulation of oxidative response and extracellular polysaccharide synthesis by a diadenylate cyclase in Streptococcus mutans

Cyclic diadenosine monophosphate (c-di-AMP) has been implicated in the control of many important bacterial activities. However, the function of this molecule in Streptococcus mutans, the primary aetiological agent of human dental caries, is unknown. In this study, we identified and characterized a diadenylate cyclase, named CdaA, in S. mutans. Furthermore, we showed that in-frame deletion of the cdaA gene in S. mutans causes decreased c-di-AMP levels, increased sensitivity to hydrogen peroxide and increased production of extracellular polysaccharides. Global gene expression profiling revealed that more than 200 genes were significantly upregulated or downregulated (> 2.0-fold) in the cdaA mutant. Interestingly, genes with increased or decreased expression were clustered in cellular polysaccharide biosynthetic processes and oxidoreductase activity respectively. Notably, the expression of several genomic islands, such as GTF-B/C, TnSmu, CRISPR1-Cas and CRISPR2-Cas, was found to be altered in the cdaA mutant, indicating a possible link between these genomic islands and c-di-AMP signalling. Collectively, the results reported here show that CdaA is an important global modulator in S. mutans and is required for optimal growth and environmental adaption. This report also paves the way to unveil further the roles of c-di-AMP signalling networks in the biology and pathogenicity of S. mutans.

Complete genome sequences and comparative genome analysis of Lactobacillus plantarum strain 5-2 isolated from fermented soybean

Lactobacillus plantarum is an important probiotic and is mostly isolated from fermented foods. We sequenced the genome of L. plantarum strain 5-2, which was derived from fermented soybean isolated from Yunnan province, China. The strain was determined to contain 3114 genes. Fourteen complete insertion sequence (IS) elements were found in 5-2 chromosome. There were 24 DNA replication proteins and 76 DNA repair proteins in the 5-2 genome. Consistent with the classification of L. plantarum as a facultative heterofermentative lactobacillus, the 5-2 genome encodes key enzymes required for the EMP (Embden-Meyerhof-Parnas) and phosphoketolase (PK) pathways. Several components of the secretion machinery are found in the 5-2 genome, which was compared with L. plantarum ST-III, JDM1 and WCFS1. Most of the specific proteins in the four genomes appeared to be related to their prophage elements.

In vitro manganese-dependent cross-talk between Streptococcus mutans VicK and GcrR: implications for overlapping stress response pathways

Streptococcus mutans, a major acidogenic component of the dental plaque biofilm, has a key role in caries etiology. Previously, we demonstrated that the VicRK two-component signal transduction system modulates biofilm formation, oxidative stress and acid tolerance responses in S. mutans. Using in vitro phosphorylation assays, here we demonstrate for the first time, that in addition to activating its cognate response regulator protein, the sensor kinase, VicK can transphosphorylate a non-cognate stress regulatory response regulator, GcrR, in the presence of manganese. Manganese is an important micronutrient that has been previously correlated with caries incidence, and which serves as an effector of SloR-mediated metalloregulation in S. mutans. Our findings supporting regulatory effects of manganese on the VicRK, GcrR and SloR, and the cross-regulatory networks formed by these components are more complex than previously appreciated. Using DNaseI footprinting we observed overlapping DNA binding specificities for VicR and GcrR in native promoters, consistent with these proteins being part of the same transcriptional regulon. Our results also support a role for SloR as a positive regulator of the vicRK two component signaling system, since its transcription was drastically reduced in a SloR-deficient mutant. These findings demonstrate the regulatory complexities observed with the S. mutans manganese-dependent response, which involves cross-talk between non-cognate signal transduction systems (VicRK and GcrR) to modulate stress response pathways.

A biochemical characterization of the DNA binding activity of the response regulator VicR from Streptococcus mutans

Two-component systems (TCSs) are ubiquitous among bacteria and are among the most elegant and effective sensing systems in nature. They allow for efficient adaptive responses to rapidly changing environmental conditions. In this study, we investigated the biochemical characteristics of the Streptococcus mutans protein VicR, an essential response regulator that is part of the VicRK TCS. We dissected the DNA binding requirements of the recognition sequences for VicR in its phosphorylated and unphosphorylated forms. In doing so, we were able to make predictions for the expansion of the VicR regulon within S. mutans. With the ever increasing number of bacteria that are rapidly becoming resistant to even the antibiotics of last resort, TCSs such as the VicRK provide promising targets for a new class of antimicrobials.

Intercellular communications in multispecies oral microbial communities

The oral cavity contains more than 700 microbial species that are engaged in extensive cell–cell interactions. These interactions contribute to the formation of highly structured multispecies communities, allow them to perform physiological functions, and induce synergistic pathogenesis. Co-adhesion between oral microbial species influences their colonization of oral cavity and effectuates, to a large extent, the temporal and spatial formation of highly organized polymicrobial community architecture. Individual species also compete and collaborate with other neighboring species through metabolic interactions, which not only modify the local microenvironment such as pH and the amount of oxygen, making it more suitable for the growth of other species, but also provide a metabolic framework for the participating microorganisms by maximizing their potential to extract energy from limited substrates. Direct physical contact of bacterial species with its neighboring co-habitants within microbial community could initiate signaling cascade and achieve modulation of gene expression in accordance with different species it is in contact with. In addition to communication through cell–cell contact, quorum sensing (QS) mediated by small signaling molecules such as competence-stimulating peptides (CSPs) and autoinducer-2 (AI-2), plays essential roles in bacterial physiology and ecology. This review will summarize the evidence that oral microbes participate in intercellular communications with co-inhabitants through cell contact-dependent physical interactions, metabolic interdependencies, as well as coordinative signaling systems to establish and maintain balanced microbial communities.

Construction and verification of the transcriptional regulatory response network of Streptococcus mutans upon treatment with the biofilm inhibitor carolacton

Background

Carolacton is a newly identified secondary metabolite causing altered cell morphology and death of Streptococcus mutans biofilm cells. To unravel key regulators mediating these effects, the transcriptional regulatory response network of S. mutans biofilms upon carolacton treatment was constructed and analyzed. A systems biological approach integrating time-resolved transcriptomic data, reverse engineering, transcription factor binding sites, and experimental validation was carried out.

Results

The co-expression response network constructed from transcriptomic data using the reverse engineering algorithm called the Trend Correlation method consisted of 8284 gene pairs. The regulatory response network inferred by superimposing transcription factor binding site information into the co-expression network comprised 329 putative transcriptional regulatory interactions and could be classified into 27 sub-networks each co-regulated by a transcription factor. These sub-networks were significantly enriched with genes sharing common functions. The regulatory response network displayed global hierarchy and network motifs as observed in model organisms. The sub-networks modulated by the pyrimidine biosynthesis regulator PyrR, the glutamine synthetase repressor GlnR, the cysteine metabolism regulator CysR, global regulators CcpA and CodY and the two component system response regulators VicR and MbrC among others could putatively be related to the physiological effect of carolacton. The predicted interactions from the regulatory network between MbrC, known to be involved in cell envelope stress response, and the murMN-SMU_718c genes encoding peptidoglycan biosynthetic enzymes were experimentally confirmed using Electro Mobility Shift Assays. Furthermore, gene deletion mutants of five predicted key regulators from the response networks were constructed and their sensitivities towards carolacton were investigated. Deletion of cysR, the node having the highest connectivity among the regulators chosen from the regulatory network, resulted in a mutant which was insensitive to carolacton thus demonstrating not only the essentiality of cysR for the response of S. mutans biofilms to carolacton but also the relevance of the predicted network.

Conclusion

The network approach used in this study revealed important regulators and interactions as part of the response mechanisms of S. mutans biofilm cells to carolacton. It also opens a door for further studies into novel drug targets against streptococci.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-362) contains supplementary material, which is available to authorized users.

Influence of S. mutans on Base-metal Dental Casting Alloy Toxicity

We have highlighted that exposure of base-metal dental casting alloys to the acidogenic bacterium Streptococcus mutans significantly increases cellular toxicity following exposure to immortalized human TR146 oral keratinocytes. With Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), S. mutans-treated nickel-based (Ni-based) and cobalt-chromium-based (Co-Cr-based) dental casting alloys were shown to leach elevated levels of metal ions compared with untreated dental casting alloys. We targeted several biological parameters: cell morphology, viable cell counts, cell metabolic activity, cell toxicity, and inflammatory cytokine expression. S. mutans-treated dental casting alloys disrupted cell morphology, elicited significantly decreased viable cell counts (p < 0.0001) and cell metabolic activity (p < 0.0001), and significantly increased cell toxicity (p < 0.0001) and inflammatory cytokine expression (p < 0.0001). S. mutans-treated Ni-based dental casting alloys induced elevated levels of cellular toxicity compared with S. mutans-treated Co-Cr-based dental casting alloys. While our findings indicated that the exacerbated release of metal ions from S. mutans-treated base-metal dental casting alloys was the likely result of the pH reduction during S. mutans growth, the exact nature of mechanisms leading to accelerated dissolution of alloy-discs is not yet fully understood. Given the predominance of S. mutans oral carriage and the exacerbated cytotoxicity observed in TR146 cells following exposure to S. mutans-treated base-metal dental casting alloys, the implications for the long-term stability of base-metal dental restorations in the oral cavity are a cause for concern.

Identification of the Streptococcus mutans LytST two-component regulon reveals its contribution to oxidative stress tolerance

Background

The S. mutans LrgA/B holin-like proteins have been shown to affect biofilm formation and oxidative stress tolerance, and are regulated by oxygenation, glucose levels, and by the LytST two-component system. In this study, we sought to determine if LytST was involved in regulating lrgAB expression in response to glucose and oxygenation in S. mutans.

Results

Real-time PCR revealed that growth phase-dependent regulation of lrgAB expression in response to glucose metabolism is mediated by LytST under low-oxygen conditions. However, the effect of LytST on lrgAB expression was less pronounced when cells were grown with aeration. RNA expression profiles in the wild-type and lytS mutant strains were compared using microarrays in early exponential and late exponential phase cells. The expression of 40 and 136 genes in early-exponential and late exponential phase, respectively, was altered in the lytS mutant. Although expression of comYB, encoding a DNA binding-uptake protein, was substantially increased in the lytS mutant, this did not translate to an effect on competence. However, a lrgA mutant displayed a substantial decrease in transformation efficiency, suggestive of a previously-unknown link between LrgA and S. mutans competence development. Finally, increased expression of genes encoding antioxidant and DNA recombination/repair enzymes was observed in the lytS mutant, suggesting that the mutant may be subjected to increased oxidative stress during normal growth. Although the intracellular levels of reaction oxygen species (ROS) appeared similar between wild-type and lytS mutant strains after overnight growth, challenge of these strains with hydrogen peroxide (H₂O₂) resulted in increased intracellular ROS in the lytS mutant.

Conclusions

Overall, these results: (1) Reinforce the importance of LytST in governing lrgAB expression in response to glucose and oxygen, (2) Define a new role for LytST in global gene regulation and resistance to H₂O₂, and (3) Uncover a potential link between LrgAB and competence development in S. mutans.

A genome-wide study of two-component signal transduction systems in eight newly sequenced mutans streptococci strains

BACKGROUND

Mutans streptococci are a group of gram-positive bacteria including the primary cariogenic dental pathogen Streptococcus mutans and closely related species. Two component systems (TCSs) composed of a signal sensing histidine kinase (HK) and a response regulator (RR) play key roles in pathogenicity, but have not been comparatively studied for these oral bacterial pathogens.

RESULTS

HKs and RRs of 8 newly sequenced mutans streptococci strains, including S. sobrinus DSM20742, S. ratti DSM20564 and six S. mutans strains, were identified and compared to the TCSs of S. mutans UA159 and NN2025, two previously genome sequenced S. mutans strains. Ortholog analysis revealed 18 TCS clusters (HK-RR pairs), 2 orphan HKs and 2 orphan RRs, of which 8 TCS clusters were common to all 10 strains, 6 were absent in one or more strains, and the other 4 were exclusive to individual strains. Further classification of the predicted HKs and RRs revealed interesting aspects of their putative functions. While TCS complements were comparable within the six S. mutans strains, S. sobrinus DSM20742 lacked TCSs possibly involved in acid tolerance and fructan catabolism, and S. ratti DSM20564 possessed 3 unique TCSs but lacked the quorum-sensing related TCS (ComDE). Selected computational predictions were verified by PCR experiments.

CONCLUSIONS

Differences in the TCS repertoires of mutans streptococci strains, especially those of S. sobrinus and S. ratti in comparison to S. mutans, imply differences in their response mechanisms for survival in the dynamic oral environment. This genomic level study of TCSs should help in understanding the pathogenicity of these mutans streptococci strains.

Regulation of bacteriocin production and cell death by the VicRK signaling system in Streptococcus mutans

The VicRK two-component signaling system modulates biofilm formation, genetic competence, and stress tolerance in Streptococcus mutans. We show here that the VicRK modulates bacteriocin production and cell viability, in part by direct modulation of competence-stimulating peptide (CSP) production in S. mutans. Global transcriptome and real-time transcriptional analysis of the VicK-deficient mutant (SmuvicK) revealed significant modulation of several bacteriocin-related loci, including nlmAB, nlmC, and nlmD (P < 0.001), suggesting a role for the VicRK in producing mutacins IV, V, and VI. Bacteriocin overlay assays revealed an altered ability of the vic mutants to kill related species. Since a well-conserved VicR binding site (TGTWAH-N(5)-TGTWAH) was identified within the comC coding region, we confirmed VicR binding to this sequence using DNA footprinting. Overexpression of the vic operon caused growth-phase-dependent repression of comC, comDE, and comX. In the vic mutants, transcription of nlmC/cipB encoding mutacin V, previously linked to CSP-dependent cell lysis, as well as expression of its putative immunity factor encoded by immB, were significantly affected relative to the wild type (P < 0.05). In contrast to previous reports that proposed a hyper-resistant phenotype for the VicK mutant in cell viability, the release of extracellular genomic DNA was significantly enhanced in SmuvicK (P < 0.05), likely as a result of increased autolysis compared with the parent. The drastic influence of VicRK on cell viability was also demonstrated using vic mutant biofilms. Taken together, we have identified a novel regulatory link between the VicRK and ComDE systems to modulate bacteriocin production and cell viability of S. mutans.

Dental caries pathogenicity: a genomic and metagenomic perspective

In this review we address the subject of dental caries pathogenicity from a genomic and metagenomic perspective. The application of genomic technologies is certain to yield novel insights into the relationship between the bacterial flora, dental health and disease. Three primary attributes of bacterial species are thought to have direct impact on caries development, these include: adherence on tooth surfaces (biofilm formation), acid production and acid tolerance. Attempts to define the specific aetiological agents of dental caries have proven to be elusive, supporting the notion that caries aetiology is perhaps complex and multi-faceted. The recently introduced Human Microbiome Project (HMP) that endeavors to characterise the micro-organisms living in and on the human body is likely to shed new light on these questions and improve our understanding of polymicrobial disease, microbial ecology in the oral cavity and provide new avenues for therapeutic and molecular diagnostics developments.

Characterization of Streptococcus thermophilus two-component systems: In silico analysis, functional analysis and expression of response regulator genes in pure or mixed culture with its yogurt partner, Lactobacillus delbrueckii subsp. bulgaricus

The lactic acid bacterium Streptococcus thermophilus (S. thermophilus) is widely used in the dairy industry. As a food bacterium, it has to cope with changing environments such as milk, yogurt, as well as the digestive tract, after the product has been ingested. In bacteria, two-component systems (TCS) are one of the most prevalent mechanisms to sense and respond appropriately to a wide range of signals. They are typically composed of a sensor kinase (HK) that detects a stimulus and a response regulator (RR) which acts as a transcriptional regulator. Our objective was to make an inventory of the TCS present in S. thermophilus LMD-9 and investigate the contribution of each TCS to LMD-9 growth in milk. For that purpose, we performed in silico, transcriptomic as well as functional analysis. The LMD-9 genome presented 6 complete TCS with both HK and RR (TCS 2, 4, 5, 6, 7, and 9) and 2 orphan RRs (RR01 and 08) with truncated HK. Our in silico analysis revealed that for 5 TCS out of the 8, orthologs with known functions were found in other bacterial species whereas for TCS02, 4 and 6 the function of the orthologs are unidentified. Transcriptomic studies (using quantitative PCR) revealed that all S. thermophilus LMD-9 response regulator genes were expressed in milk; they were expressed at different levels and with different profiles during growth. In mixed culture with Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus), the S. thermophilus partner in yogurt, the expression of four S. thermophilus LMD-9 response regulator increased; two of them, rr02 and rr09, increased by a factor of 6. These results indicate that the presence of L. bulgaricus induces regulatory changes in S. thermophilus. We also demonstrated that a response regulator (rr02) can exert its regulatory function on its target genes even when expressed at very low levels. We showed that RR05-an ortholog of Bacillus subtilis YycF or Staphylococcus aureus WalR-was essential for the growth of S. thermophilus. For the 7 other RRs, the absence of a single response regulator gene was insufficient to notably impact the growth of LMD-9 in milk, with or without supplementation with purines, formate, or stress agents (lactate, H₂O₂). We demonstrated here that the 8 response regulators of LMD-9 are expressed--and thus potentially active--during growth in milk and suggested that the response regulators have possibly overlapping regulons and/or functions not essential under the conditions tested.

Gene regulation in S. mutans: complex control in a complex environment

Dental caries is a chronic infectious disease of multifactorial etiology that derives from the interplay among cariogenic bacteria on the dentition, the host diet, and other environmental exposures. Streptococcus mutans proliferates as a biofilm on the tooth surface, where it obtains nutrients and metabolizes fermentable dietary carbohydrates. The accumulation of lactic acid as a by-product of fermentation results in acidification of the plaque biofilm and demineralization of tooth enamel, marking the onset of decay. The ability of S. mutans to respond to environmental stresses presented by salivary flow, acid pH, oxidative stress, and changes in carbohydrate source and availability is essential for its survival and predominance in caries lesions. Importantly, S. mutans has evolved a network of regulators to integrate its cellular response to environmental change. Herein we describe the latest insights into global gene regulation in S. mutans, including mechanisms of signal transduction, carbon catabolite repression, and quorum-sensing. An improved understanding of these regulatory networks can provide a basis for novel therapeutic applications aimed at treating and/or preventing caries.

The putative hydrolase YycJ (WalJ) affects the coordination of cell division with DNA replication in Bacillus subtilis and may play a conserved role in cell wall metabolism

Bacteria must accurately replicate and segregate their genetic information to ensure the production of viable daughter cells. The high fidelity of chromosome partitioning is achieved through mechanisms that coordinate cell division with DNA replication. We report that YycJ (WalJ), a predicted member of the metallo-β-lactamase superfamily found in most low-G+C Gram-positive bacteria, contributes to the fidelity of cell division in Bacillus subtilis. B. subtilis ΔwalJ (ΔwalJ(Bsu)) mutants divide over unsegregated chromosomes more frequently than wild-type cells, and this phenotype is exacerbated when DNA replication is inhibited. Two lines of evidence suggest that WalJ(Bsu) and its ortholog in the Gram-positive pathogen Streptococcus pneumoniae, WalJ(Spn) (VicX), play a role in cell wall metabolism: (i) strains of B. subtilis and S. pneumoniae lacking walJ exhibit increased sensitivity to a narrow spectrum of cephalosporin antibiotics, and (ii) reducing the expression of a two-component system that regulates genes involved in cell wall metabolism, WalRK (YycFG), renders walJ essential for growth in B. subtilis, as observed previously with S. pneumoniae. Together, these results suggest that the enzymatic activity of WalJ directly or indirectly affects cell wall metabolism and is required for accurate coordination of cell division with DNA replication.

Downregulation of GbpB, a component of the VicRK regulon, affects biofilm formation and cell surface characteristics of Streptococcus mutans

The virulence of the dental caries pathogen Streptococcus mutans relies in part on the sucrose-dependent synthesis of and interaction with glucan, a major component of the extracellular matrix of tooth biofilms. However, the mechanisms by which secreted and/or cell-associated glucan-binding proteins (Gbps) produced by S. mutans participate in biofilm growth remain to be elucidated. In this study, we further investigate GbpB, an essential immunodominant protein with similarity to murein hydrolases. A conditional knockdown mutant that expressed gbpB antisense RNA under the control of a tetracycline-inducible promoter was constructed in strain UA159 (UACA2) and used to investigate the effects of GbpB depletion on biofilm formation and cell surface-associated characteristics. Additionally, regulation of gbpB by the two-component system VicRK was investigated, and phenotypic analysis of a vicK mutant (UAvicK) was performed. GbpB was directly regulated by VicR, and several phenotypic changes were comparable between UACA2 and UAvicK, although differences between these strains existed. It was established that GbpB depletion impaired initial phases of sucrose-dependent biofilm formation, while exogenous native GbpB partially restored the biofilm phenotype. Several cellular traits were significantly affected by GbpB depletion, including altered cell shape, decreased autolysis, increased cell hydrophobicity, and sensitivity to antibiotics and osmotic and oxidative stresses. These data provide the first experimental evidence for GbpB participation in sucrose-dependent biofilm formation and in cell surface properties.

Use of tray-applied 10 percent carbamide peroxide gels for improving oral health in patients with special-care needs

BACKGROUND

Plaque accumulation and resulting caries or periodontal disease is a frequent problem in patients with special-care needs. Tray-applied 10 percent carbamide peroxide (CP) is a tooth-bleaching agent that has positive effects on plaque, gingival health and caries.

METHODS

The authors review the antibacterial properties of CP and the effects of CP on saliva, plaque, caries and gingival health. They also review tray fabrication options and techniques, application methods, safety and side effects. Finally, they address the challenges involved in and research needed regarding use of tray-applied CP materials in special-care patients.

RESULTS

In their literature review and clinical experience, the authors found 10 percent CP delivered in a custom-fitted tray to be an effective treatment for caries in patients with compromised oral hygiene. Plaque suppression and caries control result from a CP-induced increase in salivary and plaque pH caused by CP's urea component, and from possible antimicrobial action via physical debridement and the direct chemical effect of hydrogen peroxide.

CONCLUSIONS

Tray-applied 10 percent CP may hold great promise for improving the oral health of many special-care patients, including elderly patients, patients with cancer and patients with dry mouth. Further research is needed to verify the potential benefits, specifics of treatment times and protocols and most cost-effective products for use in various patient groups.

CLINICAL IMPLICATIONS

Application of 10 percent CP in a custom-fitted tray may reduce caries by elevating the pH above the level at which the caries process can occur, in addition to debriding the teeth and improving gingival health.

Identification of Streptococcus suis serotype 2 genes preferentially expressed in the natural host

Streptococcus suis serotype 2 (SS2) is an important zoonotic pathogen for swine and humans. Previous research about the mechanism of SS2 infection was largely established on in vitro or ex vivo models. In this study, we focused on the identification of SS2 genes preferentially expressed in vivo during natural infection in pigs. Eighty SS2 genes were identified to be up-regulated in the porcine brains and lungs by selective capture of transcribed sequences (SCOTS) and comparative dot blot analysis, followed by quantitative RT-PCR validation. These genes could be classified into 5 functional categories: metabolism, cell wall associated proteins, transporters, cell replication, and function unknown. Some of these genes may contribute to the survival and pathogenesis of SS2 in the host via the following strategies. First, SS2 evades the host innate immune clearance through modifying its metabolism and cell wall composition as indicated by the up-regulation of the corresponding gene ldh and pbp2A, respectively. Secondly, SS2 adapts to the in vivo conditions by inducing the expression of the two-component signal transduction system VicKR which may function on the target genes such as pcsB involved in stress response and cell wall biosynthesis. Thirdly, SS2 enhances its virulence in vivo by up-regulating the virulence genes, such as sly, pdgA, ssp, gidA, gcp and hp1311. Further study of these in vivo up-regulated genes will contribute to understanding the in vivo survival mechanism and pathogenesis of SS2.

The Streptococcus mutans serine/threonine kinase, PknB, regulates competence development, bacteriocin production, and cell wall metabolism

Bacteria can detect, transmit, and react to signals from the outside world by using two-component systems (TCS) and serine-threonine kinases and phosphatases. Streptococcus mutans contains one serine-threonine kinase, encoded by pknB. A gene encoding a serine-threonine phosphatase, pppL, is located upstream of pknB. In this study, the phenotypes of pknB and pppL single mutants and a pknB pppL double mutant were characterized. All mutants exhibited a reduction in genetic transformability and biofilm formation, showed abnormal cell shapes, grew slower than the wild-type strain in several complex media, and exhibited reduced acid tolerance. The mutants had reduced cariogenic capacity but no significant defects in colonization in a rat caries model. Whole-genome transcriptome analysis revealed that a pknB mutant showed reduced expression of genes involved in bacteriocin production and genetic competence. Among the genes that were differentially regulated in the pknB mutant, several were likely to be involved in cell wall metabolism. One such gene, SMU.2146c, and two genes encoding bacteriocins were shown to also be downregulated in a vicK mutant, which encodes a sensor kinase involved in the response to oxidative stress. Collectively, the results lead us to speculate that PknB may modulate the activity of the two-component signal transduction systems VicKR and ComDE. Real-time reverse transcriptase PCR (RT-PCR) showed that genes downregulated in the pknB mutant were upregulated in the pppL mutant, indicating that PppL serves to counteract PknB.