The two-component system ScnRK of Streptococcus mutans affects hydrogen peroxide resistance and murine macrophage killing

Mechanistic insights into the inhibitory effect of theaflavins on virulence factors production in Streptococcus mutans

Streptococcus mutans is the primary etiological agent associated with cariogenic process. The present study aimed to investigate the antibacterial and anti-virulence activities of theaflavins (TFs) to Streptococcus mutans UA159 as well as the underlying mechanisms. The results showed that TFs were capable of suppressing the acid production, cell adherence, water-insoluble exopolysaccharides production, and biofilm formation by S. mutans UA159 with a dosage-dependent manner while without influencing the cell growth. By a genome-wide transcriptome analysis (RNA-seq), we found that TFs attenuated the biofilm formation of S. mutans UA159 by inhibiting glucosyltransferases activity and the production of glucan-binding proteins (GbpB and GbpC) instead of directly blocking the expression of genes coding for glucosyltransferases. Further, TFs inhibited the expression of genes implicated in peptidoglycan synthesis, glycolysis, lipid synthesis, two-component system, signaling peptide transport (comA), oxidative stress response, and DNA replication and repair, suggesting that TFs suppressed the virulence factors of S. mutans UA159 by affecting the signal transduction and cell envelope stability, and weakening the ability of cells on oxidative stress resistance. In addition, an upregulated expression of the genes involved in protein biosynthesis, amino acid metabolism, and transport system upon TFs treatment indicated that cells increase the protein synthesis and nutrients uptake as one self-protective mechanism to cope with stress caused by TFs. The results of this study increase our current understanding of the anti-virulence activity of TFs on S. mutans and provide clues for the use of TFs in the prevention of dental 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.

Macrophage Polarization Alters Postphagocytosis Survivability of the Commensal Streptococcus gordonii

Oral streptococci are generally considered commensal organisms; however, they are becoming recognized as important associate pathogens during the development of periodontal disease as well as being associated with several systemic diseases, including as a causative agent of infective endocarditis. An important virulence determinant of these bacteria is an ability to evade destruction by phagocytic cells, yet how this subversion occurs is mostly unknown. Using Streptococcus gordonii as a model commensal oral streptococcus that is also associated with disease, we find that resistance to reactive oxygen species (ROS) with an active ability to damage phagosomes allows the bacterium to avoid destruction within macrophages. This ability to survive relies not only on the ROS resistance capabilities of the bacterium but also on ROS production by macrophages, with both being required for maximal survival of internalized bacteria. Importantly, we also show that this dependence on ROS production by macrophages for resistance has functional significance: S. gordonii intracellular survival increases when macrophages are polarized toward an activated (M1) profile, which is known to result in prolonged phagosomal ROS production compared to that of alternatively (M2) polarized macrophages. We additionally find evidence of the bacterium being capable of both delaying the maturation of and damaging phagosomes. Taken together, these results provide essential insights regarding the mechanisms through which normally commensal oral bacteria can contribute to both local and systemic inflammatory disease.

Role of Streptococcus mutans surface proteins for biofilm formation

Streptococcus mutans has been implicated as a primary causative agent of dental caries in humans. An important virulence property of the bacterium is its ability to form biofilm known as dental plaque on tooth surfaces. In addition, this organism also produces glucosyltransferases, multiple glucan-binding proteins, protein antigen c, and collagen-binding protein, surface proteins that coordinate to produce dental plaque, thus inducing dental caries. Bacteria utilize quorum-sensing systems to modulate environmental stress responses. A major mechanism of response to signals is represented by the so called two-component signal transduction system, which enables bacteria to regulate their gene expression and coordinate activities in response to environmental stress. As for S. mutans, a signal peptide-mediated quorum-sensing system encoded by comCDE has been found to be a regulatory system that responds to cell density and certain environmental stresses by excreting a peptide signal molecule termed CSP (competence-stimulating peptide). One of its principal virulence factors is production of bacteriocins (peptide antibiotics) referred to as mutacins. Two-component signal transduction systems are commonly utilized by bacteria to regulate bacteriocin gene expression and are also related to biofilm formation by 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.

Identification of an Efflux Transporter LmrB Regulating Stress Response and Extracellular Polysaccharide Synthesis in Streptococcus mutans

Efflux transporters have been implicated in regulating bacterial virulence properties such as resistance to antibiotics, biofilm formation and colonization. The pathogenicity of Streptococcus mutans, the primary etiologic agent of human dental caries, relies on the bacterium's ability to form biofilms on tooth surface. However, the studies on efflux transporters in S. mutans are scare and the function of these transporters remained to be clarified. In this study, we identified an efflux transporter (LmrB) in S. mutans through cloning the lmrB gene into Escherichia coli. Introducing lmrB into E. coli conferred a multidrug-resistant phenotype and resulted in higher EtBr efflux activity which could be suppressed by efflux inhibitor. To explore whether LmrB was involved in S. mutans virulence properties regulation, we constructed the lmrB inactivation mutant and examined the phenotypes of the mutant. It was found that LmrB deficiency resulted in increased IPS storage and prolonged acid production. Enhanced biofilm formation characterized by increased extracellular polysaccharides (EPS) production and elevated resistance to hydrogen peroxide and antimicrobials were also observed in lmrB mutant. To gain a better understanding of the global role of LmrB, a transcriptome analysis was performed using lmrB mutant strain. The expression of 107 genes was up- or down-regulated in the lmrB mutant compared with the wild type. Notably, expression of genes in several genomic islands was differentially modulated, such as stress-related GroELS and scnRK, sugar metabolism associated glg operons and msmREFGK transporter. The results presented here indicate that LmrB plays a vital global role in the regulation of several important virulence properties in S. mutans.

Collagen-binding proteins of Streptococcus mutans and related streptococci

The ability of Streptococcus mutans to interact with collagen through the expression of collagen-binding proteins (CBPs) bestows this oral pathogen with an alternative to the sucrose-dependent mechanism of colonization classically attributed to caries development. Based on the abundance and distribution of collagen throughout the human body, stringent adherence to this molecule grants S. mutans with the opportunity to establish infection at different host sites. Surface proteins, such as SpaP, WapA, Cnm and Cbm, have been shown to bind collagen in vitro, and it has been suggested that these molecules play a role in colonization of oral and extra-oral tissues. However, robust collagen binding is not achieved by all strains of S. mutans, particularly those that lack Cnm or Cbm. These observations merit careful dissection of the contribution from these different CBPs towards tissue colonization and virulence. In this review, we will discuss the current understanding of mechanisms used by S. mutans and related streptococci to colonize collagenous tissues, and the possible contribution of CBPs to infections in different sites of the host.

Comparing the cariogenic species Streptococcus sobrinus and S. mutans on whole genome level

Background

Two closely related species of mutans streptococci, namely Streptococcus mutans and Streptococcus sobrinus, are associated with dental caries in humans. Their acidogenic and aciduric capacity is directly associated with the cariogenic potential of these bacteria. To survive acidic and temporarily harsh conditions in the human oral cavity with hundreds of other microbial co-colonizers as competitors, both species have developed numerous mechanisms for adaptation.

Objectives

The recently published novel genome information for both species is used to elucidate genetic similarities but especially differences and to discuss the impact on cariogenicity of the corresponding phenotypic properties including adhesion, carbohydrate uptake and fermentation, acid tolerance, signaling by two component systems, competence, and oxidative stress resistance.

Conclusions

S. sobrinus can down-regulate the SpaA-mediated adherence to the pellicle. It has a smaller number of two-component signaling systems and bacteriocin-related genes than S. mutans, but all or even more immunity proteins. It lacks the central competence genes comC, comS, and comR. There are more genes coding for glucosyltransferases and a novel energy production pathway formed by lactate oxidase, which is not found in S. mutans. Both species show considerable differences in the regulation of fructan catabolism. However, both S. mutans and S. sobrinus share most of these traits and should therefore be considered as equally virulent with regard to dental caries.

Core-gene-encoded peptide regulating virulence-associated traits in Streptococcus mutans

Recently, high-coverage genome sequence of 57 isolates of Streptococcus mutans, the primary etiological agent of human dental caries, was completed. The SMU.1147 gene, encoding a 61-amino-acid (61-aa) peptide, was present in all sequenced strains of S. mutans but absent in all bacteria in current databases. Reverse transcription-PCR revealed that SMU.1147 is cotranscribed with scnK and scnR, which encode the histidine kinase and response regulator, respectively, of a two-component system (TCS). The C terminus of the SMU.1147 gene product was tagged with a FLAG epitope and shown to be expressed in S. mutans by Western blotting with an anti-FLAG antibody. A nonpolar mutant of SMU.1147 formed less biofilm in glucose-containing medium and grew slower than did the wild-type strain under aerobic and anaerobic conditions, at low pH, or in the presence of H2O2. Mutation of SMU.1147 dramatically reduced genetic competence and expression of comX and comY, compared to S. mutans UA159. The competence defect of the SMU.1147 mutant could not be overcome by addition of sigX-inducing peptide (XIP) in defined medium or by competence-stimulating peptide (CSP) in complex medium. Complementation with SMU.1147 on a plasmid restored all phenotypes. Interestingly, mutants lacking either one of the TCS components and a mutant lacking all three genes behaved like the wild-type strain for all phenotypes mentioned above, but all mutant strains grew slower than UA159 in medium supplemented with 0.3 M NaCl. Thus, the SMU.1147-encoded peptide affects virulence-related traits and dominantly controls quorum-sensing pathways for development of genetic competence in S. mutans.

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.

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.

Cell surface of Lactococcus lactis is covered by a protective polysaccharide pellicle

In Gram-positive bacteria, the functional role of surface polysaccharides (PS) that are not of capsular nature remains poorly understood. Here, we report the presence of a novel cell wall PS pellicle on the surface of Lactococcus lactis. Spontaneous PS-negative mutants were selected using semi-liquid growth conditions, and all mutations were mapped in a single chromosomal locus coding for PS biosynthesis. PS molecules were shown to be composed of hexasaccharide phosphate repeating units that are distinct from other bacterial PS. Using complementary atomic force and transmission electron microscopy techniques, we showed that the PS layer forms an outer pellicle surrounding the cell. Notably, we found that this cell wall layer confers a protective barrier against host phagocytosis by murine macrophages. Altogether, our results suggest that the PS pellicle could represent a new cell envelope structural component of Gram-positive bacteria.

Streptococcus mutans autolysin AtlA is a fibronectin-binding protein and contributes to bacterial survival in the bloodstream and virulence for infective endocarditis

Streptococcus mutans, a commensal of the human oral cavity, can survive in the bloodstream and cause infective endocarditis (IE). However, the virulence factors associated with this manifestation of disease are not known. Here, we demonstrate that AtlA, an autolysin of S. mutans is a newly identified fibronectin (Fn) binding protein and contributes to bacterial resistance to phagocytosis and survival in the bloodstream. Interestingly, prior exposure to plasma at low concentrations was sufficient to enhance bacterial survival in the circulation. Calcium ions at physiological plasma concentrations induced maturation of AtlA from the 104-90 kDa isoform resulting in increased Fn binding and resistance to phagocytosis. An isogenic mutant strain defective in AtlA expression exhibited reduced survival and virulence when tested in a rat model of IE compared with the wild-type and complemented strains. The data presented suggest that plasma components utilized by S. mutans enhanced survival in the circulation and AtlA is a virulence factor associated with infective endocarditis.

3'-Phosphoadenosine-5'-phosphate phosphatase activity is required for superoxide stress tolerance in Streptococcus mutans

Aerobic microorganisms have evolved different strategies to withstand environmental oxidative stresses generated by various reactive oxygen species (ROS). For the facultative anaerobic human oral pathogen Streptococcus mutans, the mechanisms used to protect against ROS are not fully understood, since it does not possess catalase, an enzyme that degrades hydrogen peroxide. In order to elucidate the genes that are essential for superoxide stress response, methyl viologen (MV)-sensitive mutants of S. mutans were generated via ISS1 mutagenesis. Screening of approximately 2,500 mutants revealed six MV-sensitive mutants, each containing an insertion in one of five genes, including a highly conserved hypothetical gene, SMU.1297. Sequence analysis suggests that SMU.1297 encodes a hypothetical protein with a high degree of homology to the Bacillus subtilis YtqI protein, which possesses an oligoribonuclease activity that cleaves nano-RNAs and a phosphatase activity that degrades 3'-phosphoadenosine-5'-phosphate (pAp) and 3'-phosphoadenosine-5'-phosphosulfate (pApS) to produce AMP; the latter activity is similar to the activity of the Escherichia coli CysQ protein, which is required for sulfur assimilation. SMU.1297 was deleted using a markerless Cre-loxP-based strategy; the SMU.1297 deletion mutant was just as sensitive to MV as the ISS1 insertion mutant. Complementation of the deletion mutant with wild-type SMU.1297, in trans, restored the parental phenotype. Biochemical analyses with purified SMU.1297 protein demonstrated that it has pAp phosphatase activity similar to that of YtqI but apparently lacks an oligoribonuclease activity. The ability of SMU.1297 to dephosphorylate pApS in vivo was confirmed by complementation of an E. coli cysQ mutant with SMU.1297 in trans. Thus, our results suggest that SMU.1297 is involved in superoxide stress tolerance in S. mutans. Furthermore, the distribution of homologs of SMU.1297 in streptococci indicates that this protein is essential for superoxide stress tolerance in these organisms.

Additive attenuation of virulence and cariogenic potential of Streptococcus mutans by simultaneous inactivation of the ComCDE quorum-sensing system and HK/RR11 two-component regulatory system

The genome of Streptococcus mutans harbours 13 two-component signal transduction systems (TCSTSs). Of these, a peptide-mediated quorum-sensing system, ComCDE, and the HK/RR11 two-component system are well known to regulate several virulence-associated traits in in vitro experiments, including genetic competence, bacteriocin production, biofilm formation and stress responses. In this study, we investigated the hypothesis that inactivation of ComCDE, HK/RR11 or both systems would attenuate the virulence and cariogenicity of S. mutans. The results showed that simultaneous inactivation of both signal transduction systems additively attenuated S. mutans virulence and cariogenicity, since inactivation of either of these systems alone did not result in the same degree of effect. The double deletion mutant SMcde-hk11 was defective in genetic competence, had a reduced acid production, was unable to grow at pH 5.0 and formed an abnormal biofilm with reduced biomass. Animal studies showed that this mutant had reduced capabilities for oral colonization, succession and initiation of dental caries. A competitive index (CI) analysis using a mixed-infection animal model revealed that all the mutants, particularly SMcde-hk11, had reduced fitness in their ecological niches and were unable to compete with the wild-type strain for persistence in dental biofilms. The evidence from this study suggests that the ComCDE and HK/RR11 signal transduction systems can be considered to be novel targets for the development of strategies in the prevention and treatment of S. mutans infections.

A model of efficiency: stress tolerance by Streptococcus mutans

The complete genome sequence of Streptococcus mutans, a bacterial pathogen commonly associated with human dental caries, was published in 2002. The streamlined genome (2.03 Mb) revealed an organism that is well adapted to its obligately host-associated existence in multispecies biofilms on tooth surfaces: a dynamic environment that undergoes rapid and substantial fluctuations. However, S. mutans lacks many of the sensing systems and alternative sigma factors that bacteria often use to coordinate gene expression in response to stress and changes in their environment. Over the past 7 years, functional genomics and proteomics have enhanced our understanding of how S. mutans has integrated the stress regulon and global transcriptional regulators to coordinate responses to environmental fluctuations with modulation of virulence in a way that ensures persistence in the oral cavity and capitalizes on conditions that are favourable for the development of dental caries. Here, we highlight advances in dissection of the stress regulon of S. mutans and its intimate interrelationship with pathogenesis.