Global transcriptional analysis of Streptococcus mutans sugar transporters using microarrays

Human saliva modifies growth, biofilm architecture, and competitive behaviors of oral streptococci

The bacteria within supragingival biofilms participate in complex exchanges with other microbes inhabiting the same niche. One example is the mutans group streptococci (Streptococcus mutans), implicated in the development of tooth decay, and other health-associated commensal streptococci species. Previously, our group transcriptomically characterized intermicrobial interactions between S. mutans and several species of oral bacteria. However, these experiments were carried out in a medium without human saliva. To better mimic their natural environment, we first evaluated how inclusion of saliva affected growth and biofilm formation of eight Streptococcus species individually and found saliva to positively benefit growth rates while negatively influencing biofilm biomass accumulation and altering spatial arrangement. These results carried over during evaluation of 29 saliva-derived isolates of various species. Surprisingly, we also found that addition of saliva increased the competitive behaviors of S. mutans in coculture competitions against commensal streptococci that led to increases in biofilm microcolony volumes. Through transcriptomically characterizing mono- and cocultures of S. mutans and Streptococcus oralis with and without saliva, we determined that each species developed a nutritional niche under mixed-species growth, with S. mutans upregulating carbohydrate uptake and utilization pathways while S. oralis upregulated genome features related to peptide uptake and glycan foraging. S. mutans also upregulated genes involved in oxidative stress tolerance, particularly manganese uptake, which we could artificially manipulate by supplementing in manganese leading to an advantage over its opponent. Our report highlights observable changes in microbial behaviors through leveraging environmental- and host-supplied resources over their competitors.

IMPORTANCE

Dental caries (tooth decay) is the most prevalent disease for both children and adults nationwide. Caries are initiated from demineralization of the enamel due to organic acid production through the metabolic activity of oral bacteria growing in biofilm communities attached to the tooth's surface. Mutans group streptococci are closely associated with caries development and initiation of the cariogenic cycle, which decreases the amount of acid-sensitive, health-associated commensal bacteria while selecting for aciduric and acidogenic species that then further drives the disease process. Defining the exchanges that occur between mutans group streptococci and oral commensals in a condition that closely mimics their natural environment is of critical need toward identifying factors that can influence odontopathogen establishment, persistence, and outgrowth. The goal of our research is to develop strategies, potentially through manipulation of microbial interactions characterized here, that prevent the emergence of mutans group streptococci while keeping the protective flora intact.

Human Saliva Modifies Growth, Biofilm Architecture and Competitive Behaviors of Oral Streptococci

The bacteria within supragingival biofilms participate in complex exchanges with other microbes inhabiting the same niche. One example are the mutans group streptococci (Streptococcus mutans), implicated in the development of tooth decay, and other health-associated commensal streptococci species. Previously, our group transcriptomically characterized intermicrobial interactions between S. mutans and several species of oral bacteria. However, these experiments were carried out in a medium that was absent of human saliva. To better mimic their natural environment, we first evaluated how inclusion of saliva affected growth and biofilm formation of eight streptococci species individually, and found saliva to positively benefit growth rates while negatively influencing biomass accumulation and altering spatial arrangement. These results carried over during evaluation of 29 saliva-derived isolates of various species. Surprisingly, we also found that addition of saliva increased the competitive behaviors of S. mutans in coculture competitions against commensal streptococci that led to increases in biofilm microcolony volumes. Through transcriptomically characterizing mono- and cocultures of S. mutans and Streptococcus oralis with and without saliva, we determined that each species developed a nutritional niche under mixed-species growth, with S. mutans upregulating carbohydrate uptake and utilization pathways while S. oralis upregulated genome features related to peptide uptake and glycan foraging. S. mutans also upregulated genes involved in oxidative stress tolerance, particularly manganese uptake, which we could artificially manipulate by supplementing in manganese to give it an advantage over its opponent. Our report highlights observable changes in microbial behaviors via leveraging environmental- and host-supplied resources over their competitors.

Cnm of Streptococcus mutans is important for cell surface structure and membrane permeability

Streptococcus mutans, a Gram-positive facultative anaerobic bacterium, is a major pathogen of dental caries. The protein Cnm of S. mutans is involved in collagen binding, but its other biological functions are unknown. In this study, a Cnm-deficient isogenic mutant and a complementation strain were generated from a Cnm-positive S. mutans strain to help determine the properties of Cnm. Initially, comparison of the cell surface structure was performed by electron microscopy, which demonstrated that Cnm appears to be localized on the cell surface and associated with a protruding cell surface structure. Deep RNA sequencing of the strains revealed that the defect in Cnm caused upregulated expression of many genes related to ABC transporters and cell-surface proteins, while a few genes were downregulated. The amount of biofilm formed by the Cnm-defective strain increased compared with the parental and complemented strains, but the biofilm structure was thinner because of elevated expression of genes encoding glucan synthesis enzymes, leading to increased production of extracellular polysaccharides. Particular antibiotics, including bacitracin and chloramphenicol, had a lower minimum inhibitory concentration for the Cnm-defective strain than particular antibiotics, including bacitracin and chloramphenicol, compared with the parental and complemented strains. Our results suggest that S. mutans Cnm is located on the cell surface, gives rise to the observed protruding cell surface, and is associated with several biological properties related to membrane permeability.

Streptococcus mutans gene expression and functional profile in root caries: an RNA-seq study

The literature is still scarce on studies describing S. mutans global gene expression under clinical conditions such as those found on complex biofilms from sound root surfaces (SRS) and carious root surfaces (RC). This study aimed to investigate the S. mutans gene expression and functional profile within the metatranscriptome of biofilms from SRS and from RC in an attempt to identify enriched functional signatures potentially associated with healthy to disease transitioning process. Total RNA was extracted, and prepared libraries (SRS=10 and RC=9) were paired-end sequenced using the Illumina HiSeq2500. Read count assigned to each gene of the S. mutans UA159 strain were obtained. Differentially expressed genes (DEG) between SRS and RC were identified using the DESeq2 R package and weighted gene co-expression network analysis (WGCNA) was performed to explore and identify functional modules related to SRS and RC. We found seventeen DEG between SRS and RC samples, with three overexpressed in RC and related to membrane protein, alanyl-tRNA synthetase and GTP-binding protein with the remaining ones overexpressed in SRS samples and related to hypothetical protein, transposon integrase, histidine kinase, putative transporter, bacteriocin immunity protein, response regulator, 6-phospho-beta-galactosidase, purine metabolism and to transcriptional regulator. Key-functional modules were identified for SRS and RC conditions based on WCGNA, being 139 hub genes found on SRS key-module and 17 genes on RC key-module. Functional analysis of S. mutans within the metatranscriptome of biofilms from sound root and from carious root revealed a similar pattern of gene expression, and only a few genes have been differentially expressed between biofilms from sound root surfaces and from root carious lesions. However, S. mutans presented a greater functional abundance in the lesion samples. Some functional patterns related to sugar (starch, sucrose, fructose, mannose and lactose) and heterofermentative metabolisms, to cell-wall biosynthesis and to acid tolerance stress seem to be enriched on carious root surfaces conferring ecological advantages to S. mutans. Altogether, the present data suggest that a functional signature may be associated with carious root lesions.

Production and in vitro evaluation of prebiotic manno-oligosaccharides prepared with a recombinant Aspergillus niger endo-mannanase, Man26A

In this study, a GH26 endo-mannanase (Man26A) from an Aspergillus niger ATCC 10864 strain, with a molecular mass of 47.8 kDa, was cloned in a yBBH1 vector and expressed in Saccharomyces cerevisiae Y294 strain cells. Upon fractionation by ultra-filtration, the substrate specificity and substrate degradation pattern of the endo-mannanase (Man26A) were investigated using ivory nut linear mannan and two galactomannan substrates with varying amounts of galactosyl substitutions, guar gum and locust bean gum. Man26A exhibited substrate specificity in the order: locust bean gum ≥ ivory nut mannan > guar gum; however, the enzyme generated more manno-oligosaccharides (MOS) from the galactomannans than from linear mannan during extended periods of mannan hydrolysis. MOS with a DP of 2-4 were the major products from mannan substrate hydrolysis, while guar gum also generated higher DP length MOS. All the Man26A generated MOS significantly improved the growth (approximately 3-fold) of the probiotic bacterial strains Streptococcus thermophilus and Bacillus subtilis in M9 minimal medium. Ivory nut mannan and locust bean gum derived MOS did not influence the auto-aggregation ability of the bacteria, while the guar gum derived MOS led to a 50 % reduction in bacterial auto-aggregation. On the other hand, all the MOS significantly improved bacterial biofilm formation (approximately 3-fold). This study suggests that the prebiotic characteristics exhibited by MOS may be dependent on their primary structure, i.e. galactose substitution and DP. Furthermore, the data suggests that the enzyme-generated MOS may be useful as potent additives to dietary foods.

Transcriptional Profiling Reveals the Importance of RcrR in the Regulation of Multiple Sugar Transportation and Biofilm Formation in Streptococcus mutans

The ability of Streptococcus mutans to survive and cause dental caries is dependent on its ability to metabolize various carbohydrates, accompanied by extracellular polysaccharide synthesis and biofilm formation. Here, the role of an rel competence-related regulator (RcrR) in the regulation of multiple sugar transportation and biofilm formation is reported. The deletion of the rcrR gene in S. mutans caused delayed growth, decreased biofilm formation ability, and affected the expression level of its multiple sugar transportation-related genes. Transcriptional profiling revealed 17 differentially expressed genes in the rcrR mutant. Five were downregulated and clustered with the sugar phosphotransferase (PTS) systems (mannitol- and trehalose-specific PTS systems). The conserved sites bound by the rcrR promoter were then determined by electrophoretic mobility shift assays (EMSAs) and DNase I footprinting assays. Furthermore, a potential binding motif in the promoters of the two PTS operons was predicted using MEME Suite 5.1.1. RcrR could bind to the promoter regions of the two operons in vitro, and the sugar transporter-related genes of the two operons were upregulated in an rcrR-overexpressing strain. In addition, when RcrR-binding sites were deleted, the growth rates and final yield of S. mutans were significantly decreased in tryptone-vitamin (TV) medium supplemented with different sugars, but not in absolute TV medium. These results revealed that RcrR acted as a transcription activator to regulate the two PTS systems, accompanied by multiple sugar transportation and biofilm formation. Collectively, these results indicate that RcrR is a critical transcription factor in S. mutans that regulates bacterial growth, biofilm formation, and multiple sugar transportation.

IMPORTANCE The human oral cavity is a constantly changing environment. Tooth decay is a commonly prevalent chronic disease mainly caused by the cariogenic bacterium Streptococcus mutans. S. mutans is an oral pathogen that metabolizes various carbohydrates into extracellular polysaccharides (EPSs), biofilm, and tooth-destroying lactic acid. The host diet strongly influences the availability of multiple carbohydrates. Here, we showed that the RcrR transcription regulator plays a significant role in the regulation of biofilm formation and multiple sugar transportation. Further systematic evaluation of how RcrR regulates the transportation of various sugars and biofilm formation was also conducted. Notably, this study decrypts the physiological functions of RcrR as a potential target for the better prevention of dental caries.

Comparative transcriptomics analysis of Streptococcus mutans with disruption of LuxS/AI-2 quorum sensing and recovery of methyl cycle

OBJECTIVE

The LuxS/AI-2 quorum sensing (QS) system has critical roles in Streptococcus mutans cariogenicity. Whereas the molecular and cellular mechanisms of the LuxS/AI-2 QS system are not thoroughly understood. Given that LuxS has roles in QS and methyl cycle, its mutation can cause QS deficiency and methyl cycle disruption. The aim of this study was to investigate effects of the LuxS/AI-2 QS system on gene expression in Streptococcus mutans when methyl cycle was recovered with exogenous sahH gene.

METHODS

Our previous study introduced the exogenous sahH gene from Pseudomonas aeruginosa into an S. mutans luxS-null strain to restore the disrupted methyl cycle, and this produced the solely the LuxS/AI-2 QS system deficient strain. Here, we analyzed the transcriptomics of this strain to get insights into the molecular mechanisms of the LuxS/AI-2 QS system applying RNA-seq.

RESULTS

With recovery of methyl cycle, 84 genes didn't change in expression trends in S. mutans luxS-null strain. These genes mainly encode the ABC transporters, sugar transporter EII and enzymes of carbohydrate metabolism, and are rich in the Phosphotransferase system, Fructose and mannose metabolism, Amino sugar and nucleotide sugar metabolism, Galactose metabolism, Glycolysis/Gluconeogenesis, RNA degradation, Lysine biosynthesis, and Glycine, serine and threonine metabolism.

CONCLUSIONS

The LuxS/AI-2 QS system may mainly affect ABC transporters and carbohydrate transport, transformation and metabolism via EII subunits and enzymes to influence virulence-associated traits without effects of methyl cycle inStreptococcus mutans.

Quorum Sensing in Streptococcus mutans Regulates Production of Tryglysin, a Novel RaS-RiPP Antimicrobial Compound

Bacteria interact and compete with a large community of organisms in their natural environment. Streptococcus mutans is one such organism, and it is an important member of the oral microbiota. We found that S. mutans uses a quorum-sensing system to regulate production of a novel posttranslationally modified peptide capable of inhibiting growth of several streptococcal species.

The genus Streptococcus encompasses a large bacterial taxon that commonly colonizes mucosal surfaces of vertebrates and is capable of disease etiologies originating from diverse body sites, including the respiratory, digestive, and reproductive tracts. Identifying new modes of treating infections is of increasing importance, as antibiotic resistance has escalated. Streptococcus mutans is an important opportunistic pathogen that is an agent of dental caries and is capable of systemic diseases such as endocarditis. As such, understanding how it regulates virulence and competes in the oral niche is a priority in developing strategies to defend from these pathogens. We determined that S. mutans UA159 possesses a bona fide short hydrophobic peptide (SHP)/Rgg quorum-sensing system that regulates a specialized biosynthetic operon featuring a radical-SAM (S-adenosyl-l-methionine) (RaS) enzyme and produces a ribosomally synthesized and posttranslationally modified peptide (RiPP). The pairing of SHP/Rgg regulatory systems with RaS biosynthetic operons is conserved across streptococci, and a locus similar to that in S. mutans is found in Streptococcus ferus, an oral streptococcus isolated from wild rats. We identified the RaS-RiPP product from this operon and solved its structure using a combination of analytical methods; we term these RiPPs tryglysin A and B for the unusual Trp-Gly-Lys linkage. We report that tryglysins specifically inhibit the growth of other streptococci, but not other Gram-positive bacteria such as Enterococcus faecalis or Lactococcus lactis. We predict that tryglysin is produced by S. mutans in its oral niche, thus inhibiting the growth of competing species, including several medically relevant streptococci.

Repression of the TreR transcriptional regulator in Streptococcus mutans by the global regulator, CcpA

Streptococcus mutans, the etiologic agent of dental caries in humans, is considered a dominating force in the oral microbiome due to its highly-evolved propensity for survival. The oral pathogen encodes an elaborate array of regulatory elements, including the carbon catabolite-responsive regulator, CcpA, a global regulator key in the control of sugar metabolism and in stress tolerance response mechanisms. The recently characterized trehalose utilization operon, integral for the catabolism of the disaccharide trehalose, is controlled by a local regulator, TreR, which has been implicated in a number of cellular functions outside of trehalose catabolism. Electrophoretic mobility shift assays demonstrated that CcpA bound a putative cre site in the treR promoter. Loss of ccpA resulted in elevated expression of treR in cultures of the organism grown in glucose or trehalose, indicating that CcpA not only acts as a repressor of trehalose catabolism genes, but also the local regulator. The loss of both CcpA and TreR in S. mutans resulted in an impaired growth rate and fitness response, supporting the hypothesis that these regulators are involved in carbon catabolism control and in induction of components of the organism's stress response.

Manganese Depletion Leads to Multisystem Changes in the Transcriptome of the Opportunistic Pathogen Streptococcus sanguinis

Streptococcus sanguinis is a primary colonizer of teeth and is typically considered beneficial due to its antagonistic relationship with the cariogenic pathogen Streptococcus mutans. However, S. sanguinis can also act as an opportunistic pathogen should it enter the bloodstream and colonize a damaged heart valve, leading to infective endocarditis. Studies have implicated manganese acquisition as an important virulence determinant in streptococcal endocarditis. A knockout mutant lacking the primary manganese import system in S. sanguinis, SsaACB, is severely attenuated for virulence in an in vivo rabbit model. Manganese is a known cofactor for several important enzymes in S. sanguinis, including superoxide dismutase, SodA, and the aerobic ribonucleotide reductase, NrdEF. To determine the effect of manganese depletion on S. sanguinis, we performed transcriptomic analysis on a ΔssaACB mutant grown in aerobic fermentor conditions after the addition of the metal chelator EDTA. Despite the broad specificity of EDTA, analysis of cellular metal content revealed a decrease in manganese, but not in other metals, that coincided with a drop in growth rate. Subsequent supplementation with manganese, but not iron, zinc, or magnesium, restored growth in the fermentor post-EDTA. Reduced activity of Mn-dependent SodA and NrdEF likely contributed to the decreased growth rate post-EDTA, but did not appear entirely responsible. With the exception of the Dps-like peroxide resistance gene, dpr, manganese depletion did not induce stress response systems. By comparing the transcriptome of ΔssaACB cells pre- and post-EDTA, we determined that manganese deprivation led to altered expression of diverse systems. Manganese depletion also led to an apparent induction of carbon catabolite repression in a glucose-independent manner. The combined results suggest that manganese limitation produces effects in S. sanguinis that are diverse and complex, with no single protein or system appearing entirely responsible for the observed growth rate decrease. This study provides further evidence for the importance of this trace element in streptococcal biology. Future studies will focus on determining mechanisms for regulation, as the multitude of changes observed in this study indicate that multiple regulators may respond to manganese levels.

Influence of 2'-fucosyllactose and galacto-oligosaccharides on the growth and adhesion of Streptococcus mutans

Human milk oligosaccharides, such as 2'-fucosyllactose (2'-FL), and galacto-oligosaccharides (GOS), a prebiotic carbohydrate mixture, are being increasingly added to infant formulas, necessitating the understanding of their impact on the oral microbiota. Here, for the first time, the effects of 2'-FL and GOS on the planktonic growth and adhesion characteristics of the caries-associated oral pathogen Streptococcus mutans were assessed, and the results were compared against the effects of xylitol, lactose and glucose. There were differences in S. mutans growth between 2'-FL and GOS. None of the three S. mutans strains grew with 2'-FL, while they all grew with GOS as well as lactose and glucose. Xylitol inhibited S. mutans growth. The adhesion of S. mutans CI 2366 to saliva-coated hydroxyapatite was reduced by 2'-FL and GOS. Exopolysaccharide-mediated adhesion of S. mutans DSM 20523 to a glass surface was decreased with 2'-FL, GOS and lactose, and the adhesion of strain CI 2366 strain was reduced only by GOS. Unlike GOS, 2'-FL did not support the growth of any S. mutans strain. Neither 2'-FL nor GOS enhanced the adhesive properties of the S. mutans strains, but they inhibited some of the tested strains. Thus, the cariogenic tendency may vary between infant formulas containing different types of oligosaccharides.

Disruption of l-Rhamnose Biosynthesis Results in Severe Growth Defects in Streptococcus mutans

The rhamnose-glucose cell wall polysaccharide (RGP) of Streptococcus mutans plays a significant role in cell division, virulence, and stress protection. Prior studies examined function of the RGP using strains carrying deletions in the machinery involved in RGP assembly. In this study, we explored loss of the substrate for RGP, l-rhamnose, via deletion of rmlD (encoding the protein responsible for the terminal step in l-rhamnose biosynthesis). We demonstrate that loss of rhamnose biosynthesis causes a phenotype similar to strains with disrupted RGP assembly (ΔrgpG and ΔrgpF strains). Deletion of rmlD not only caused a severe growth defect under nonstress growth conditions but also elevated susceptibility of the strain to acid and oxidative stress, common conditions found in the oral cavity. A genetic complement of the ΔrmlD strain completely restored wild-type levels of growth, whereas addition of exogenous rhamnose did not. The loss of rhamnose production also significantly disrupted biofilm formation, an important aspect of S. mutans growth in the oral cavity. Further, we demonstrate that loss of either rmlD or rgpG results in ablation of rhamnose content in the S. mutans cell wall. Taken together, these results highlight the importance of rhamnose production in both the fitness and the ability of S. mutans to overcome environmental stresses.IMPORTANCE Streptococcus mutans is a pathogenic bacterium that is the primary etiologic agent of dental caries, a disease that affects billions yearly. Rhamnose biosynthesis is conserved not only in streptococcal species but in other Gram-positive, as well as Gram-negative, organisms. This study highlights the importance of rhamnose biosynthesis in RGP production for protection of the organism against acid and oxidative stresses, the two major stressors that the organism encounters in the oral cavity. Loss of RGP also severely impacts biofilm formation, the first step in the onset of dental caries. The high conservation of the rhamnose synthesis enzymes, as well as their importance in S. mutans and other organisms, makes them favorable antibiotic targets for the treatment of disease.

Quantitative Proteomics Uncovers the Interaction between a Virulence Factor and Mutanobactin Synthetases in Streptococcus mutans

Streptococcus mutans is the major bacterium associated with dental caries. In order to thrive on the highly populated tooth surface and cause disease, S. mutans must be able to protect itself from hydrogen peroxide-producing commensal bacteria and compete effectively against the neighboring microbes. S. mutans produces mutacins, small antimicrobial peptides which help control the population of competing bacterial species. In addition, S. mutans produces a peptide called mutanobactin, which offers S. mutans protection against oxidative stress. Here, we uncover a new link between the putative glycosyltransferase SMU_833 and the mutanobactin-synthesizing protein complex through quantitative proteomic analysis and a tandem-affinity protein purification scheme. Furthermore, we show that SMU_833 mediates bacterial sensitivity to oxidative stress and bacterial ability to compete with commensal streptococci. This study has revealed a previously unknown association between SMU_833 and mutanobactin and demonstrated the importance of SMU_833 in the fitness of S. mutans.

Streptococcus mutans, the primary etiological agent of tooth decay, has developed multiple adhesion and virulence factors which enable it to colonize and compete with other bacteria. The putative glycosyltransferase SMU_833 is important for the virulence of S. mutans by altering the biofilm matrix composition and cariogenicity. In this study, we further characterized the smu_833 mutant by evaluating its effects on bacterial fitness. Loss of SMU_833 led to extracellular DNA-dependent bacterial aggregation. In addition, the mutant was more susceptible to oxidative stress and less competitive against H₂O₂ producing oral streptococci. Quantitative proteomics analysis revealed that SMU_833 deficiency resulted in the significant downregulation of 10 proteins encoded by a biosynthetic gene cluster responsible for the production of mutanobactin, a compound produced by S. mutans which helps it survive oxidative stress. Tandem affinity purification demonstrated that SMU_833 interacts with the synthetic enzymes responsible for the production of mutanobactin. Similar to the smu_833 mutant, the deletion of the mutanobactin gene cluster rendered the mutant less competitive against H₂O₂-producing streptococci. Our studies revealed a new link between SMU_833 virulence and mutanobactin, suggesting that SMU_833 represents a new virulent target that can be used to develop potential anticaries therapeutics.

IMPORTANCEStreptococcus mutans is the major bacterium associated with dental caries. In order to thrive on the highly populated tooth surface and cause disease, S. mutans must be able to protect itself from hydrogen peroxide-producing commensal bacteria and compete effectively against the neighboring microbes. S. mutans produces mutacins, small antimicrobial peptides which help control the population of competing bacterial species. In addition, S. mutans produces a peptide called mutanobactin, which offers S. mutans protection against oxidative stress. Here, we uncover a new link between the putative glycosyltransferase SMU_833 and the mutanobactin-synthesizing protein complex through quantitative proteomic analysis and a tandem-affinity protein purification scheme. Furthermore, we show that SMU_833 mediates bacterial sensitivity to oxidative stress and bacterial ability to compete with commensal streptococci. This study has revealed a previously unknown association between SMU_833 and mutanobactin and demonstrated the importance of SMU_833 in the fitness of S. mutans.

Streptococcus mutans requires mature rhamnose-glucose polysaccharides for proper pathophysiology, morphogenesis and cellular division

The cell wall of Gram-positive bacteria has been shown to mediate environmental stress tolerance, antibiotic susceptibility, host immune evasion and overall virulence. The majority of these traits have been demonstrated for the well-studied system of wall teichoic acid (WTA) synthesis, a common cell wall polysaccharide among Gram-positive organisms. Streptococcus mutans, a Gram-positive odontopathogen that contributes to the enamel-destructive disease dental caries, lacks the capabilities to generate WTA. Instead, the cell wall of S. mutans is highly decorated with rhamnose-glucose polysaccharides (RGP), for which functional roles are poorly defined. Here, we demonstrate that the RGP has a distinct role in protecting S. mutans from a variety of stress conditions pertinent to pathogenic capability. Mutant strains with disrupted RGP synthesis failed to properly localize cell division complexes, suffered from aberrant septum formation and exhibited enhanced cellular autolysis. Surprisingly, mutant strains of S. mutans with impairment in RGP side chain modification grew into elongated chains and also failed to properly localize the presumed cell wall hydrolase, GbpB. Our results indicate that fully mature RGP has distinct protective and morphogenic roles for S. mutans, and these structures are functionally homologous to the WTA of other Gram-positive bacteria.

Rational design of peptides with enhanced antimicrobial and anti-biofilm activities against cariogenic bacterium Streptococcus mutans

Streptococcus mutans (S. mutans) is known to be a leading cariogenic pathogen in the oral cavity. Antimicrobial peptides possess excellent properties to combat such pathogens. In this study, we compared the antimicrobial activity of novel linear reutericin 6- and/or gassericin A-inspired peptides and identified LR-10 as the leading peptide. Antibacterial assays demonstrate that LR-10 is more active against S. mutans (3.3 μM) than many peptide-based agents without resistance selection, capable of killing many oral pathogens, and tolerant of physiological conditions. LR-10 also presented a faster killing rate than chlorhexidine and erythromycin, and appeared to display selective activity against S. mutans within 10 s. S. mutans is usually encased in plaque biofilms. Biofilm inhibitory assays indicated that LR-10 had excellent inhibitory effect on the biofilm formation of S. mutans and biofilm-encased cells in vitro at low concentrations (6.5 μM). Consistent with most peptides, LR-10 kills S. mutans mainly by disrupting the cell membranes. Notably, both hemolytic activity assays and cytotoxicity tests indicated that LR-10 could keep biocompatible at the effective concentrations. Hence, LR-10 could be a good candidate for clinical treatment of dental caries.

Streptococcus mutans transcriptome in the presence of sodium fluoride and sucrose

OBJECTIVE

Considering the diverse mechanisms by which fluoride could affect oral bacteria, this study evaluated the effect of sodium fluoride onStreptococcus mutans transcriptome in the presence of sucrose.

METHODS

S. mutans UA159 was cultured in 3 different types of media: medium control[TY], sucrose control[TY_S], and sodium fluoride sucrose test[TY_S_NaF]. Triplicates of each group were sampled at exponential phase 3 h after inoculation, total RNA was isolated, mRNA enriched and cDNA paired-end sequenced (Illumina Hi-Seq2500).

RESULTS

Genes related toS. mutans adhesion(gtfB and gtfC), acidogenicity and sugar transport were up-regulated in the presence of sucrose(TY_S) and sucrose/fluoride(TY_S_NaF), whereas gene dltA, D-alanine-activating enzyme, which is related to regulation of non-PTS sugar internalization was down-regulated. Up-regulation of the scrA gene and the PTS fructose-and mannose system, as well as functions such as those involved in stress and defence responses and peptidases; and down-regulation of lacACDG and pyruvate formate-lyase were observed in the TY_S_NaF group, as compared to TY_S group.

CONCLUSIONS

The presence of NaF has decreased the overall gene expression level inS. mutans. However, its major effect seems to be the inducing of expression of genes involved in some PEP:PTS systems and other metabolic transporters which imply specific cellular internalisation of sugars.

Genome-wide screening of potential RNase Y-processed mRNAs in the M49 serotype Streptococcus pyogenes NZ131

RNase Y is a major endoribonuclease in Group A streptococcus (GAS) and other Gram-positive bacteria. Our previous study showed that RNase Y was involved in mRNA degradation and processing in GAS. We hypothesized that mRNA processing regulated the expression of important GAS virulence factors via altering their mRNA stabilities and that RNase Y mediated at least some of the mRNA-processing events. The aims of this study were to (1) identify mRNAs that were processed by RNase Y and (2) confirm the mRNA-processing events. The transcriptomes of Streptococcus pyogenes NZ131 wild type and its RNase Y mutant (Δrny) were examined with RNA-seq. The data were further analyzed to define GAS operons. The mRNA stabilities of the wild type and Δrny at subgene level were determined with tiling array analysis. Operons displaying segmental stability in the wild type but not in the Δrny were predicted to be RNase Y processed. Overall 865 operons were defined and their boundaries predicted. Further analysis narrowed down 15 mRNAs potentially processed by RNase Y. A selection of four candidates including folC1 (folylpolyglutamate synthetase), prtF (fibronectin-binding protein), speG (streptococcal exotoxin G), ropB (transcriptional regulator of speB), and ypaA (riboflavin transporter) mRNAs was examined with Northern blot analysis. However, only folC1 was confirmed to be processed, but it is unlikely that RNase Y is responsible. We conclude that GAS use RNase Y to selectively process mRNA, but the overall impact is confined to selected virulence factors.

Characterization of the Trehalose Utilization Operon in Streptococcus mutans Reveals that the TreR Transcriptional Regulator Is Involved in Stress Response Pathways and Toxin Production

Streptococcus mutans, the organism most frequently associated with the development of dental caries, is able to utilize a diverse array of carbohydrates for energy metabolism. One such molecule is trehalose, a disaccharide common in human foods, which has been recently implicated in enhancing the virulence of epidemic strains of the pathogen Clostridium difficile In this study, mutants with deletions of all three genes in the putative S. mutans trehalose utilization operon were characterized, and the genes were shown to be required for wild-type levels of growth when trehalose was the only carbohydrate source provided. Interestingly, the TreR transcriptional regulator appeared to be critical for responding to oxidative stress and for mounting a protective stress tolerance response following growth at moderately acidic pH. mRNA sequencing (RNA-seq) of a treR deletion mutant suggested that in S. mutans, TreR acts as a trehalose-sensing activator of transcription of the tre operon, rather than as a repressor, as described in other species. In addition, deletion of treR caused the downregulation of a number of genes involved in genetic competence and bacteriocin production, supporting the results of a recent study linking trehalose and the S. mutans competence pathways. Finally, deletion of treR compromised the ability of S. mutans to inhibit the growth of the competing species Streptococcus gordonii and Lactococcus lactis Taking the results together, this study solidifies the role of the S. mutans tre operon in trehalose utilization and suggests novel functions for the TreR regulator, including roles in the stress response and competitive fitness.IMPORTANCES. mutans is the primary etiologic agent of dental caries, which globally is the most common chronic disease. S. mutans must be able to outcompete commensal organisms in its dental plaque niche in order to establish persistence and pathogenesis. To that end, S. mutans metabolizes a diverse array of carbohydrates to generate acid and impede its acid-sensitive neighbors. Additionally, S. mutans utilizes quorum signaling through genetic competence-associated pathways to induce production of toxins to kill its rivals. This study definitively shows that the S. mutans trehalose utilization operon is required for growth in trehalose. Furthermore, this study suggests that the S. mutans TreR transcriptional regulator has a novel role in virulence through regulation of genes involved in genetic competence and toxin production.

Antibacterial Activity of Azadirachta indica, Pongamia pinnata, Psidium guajava, and Mangifera indica and their mechanism of action against Streptococcus mutans

Background:

Curative plants have reportedly been used to make chewing sticks/toothbrushes intended for the treatment of oral diseases.

Objective:

The in vitro antibacterial activities of Azadirachta indica, Pongamia pinnata, Psidium guajava, and Mangifera indica were evaluated against Streptococcus mutans, along with the cytotoxicity and antioxidant and synergistic potentials. The effect of M. indica on the expression of crucial virulence genes spaP and gtfB of S. mutans was determined.

Materials and Methods:

The antibacterial activity was determined using a modified microdilution method. The antioxidant potential was evaluated using diphenyl picrylhydrazyl (DPPH), Griess reagent, and nitroblue tetrazolium calorimetric assays. The synergistic activity was investigated using a modified checkerboard method, while the cytotoxicity was determined according to a cell proliferation 2,3-Bis-(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide salt assay. Reverse transcription was the chosen method for determining the difference in expression of the spaP and gtfB genes after treatment with the plant sample.

Results:

M. indica and A. indica had the highest antibacterial activity at concentrations of 0.3 mg/ml and 6.25 mg/ml, respectively. A. indica had the best free radical scavenging of DPPH, exhibiting 50% inhibition at 28.72 μg/ml; while M. indica showed better superoxide scavenging potential than the positive control quercetin. Both M. indica and A. indica had adequate activity against the nitric oxide-free radical (12.87 and 18.89 μg/ml, respectively). M. indica selectively reduced the expression of the gtfB gene, indicating a mechanism involving Glucotranferases, specifically targeting bacterial attachment.

SUMMARY

Mangifera indica and Azadirachta indica had very good antibacterial activity against Streptococcus mutans and moderate toxicity against Vero cells

M. indica had the best antioxidant capacity overall

M. indica reduced the expression of gtfB gene at 0.5 mg/ml.

Abbreviations used: AA: Ascorbic acid; BHI: Brain–heart infusion; CHX: Chlorhexidine; DPPH: Diphenyl picrylhydrazyl; DMSO: Dimethlysulfoxide; NBT: Nitroblue tetrazolium; NO: Nitric oxide;

Coordinated Regulation of the EIIMan and fruRKI Operons of Streptococcus mutans by Global and Fructose-Specific Pathways

The glucose/mannose-phosphotransferase system (PTS) permease EII^Man encoded by manLMN in the dental caries pathogen Streptococcus mutans has a dominant influence on sugar-specific, CcpA-independent catabolite repression (CR). Mutations in manL affect energy metabolism and virulence-associated traits, including biofilm formation, acid tolerance, and competence. Using promoter::reporter fusions, expression of the manLMN and the fruRKI operons, encoding a transcriptional regulator, a fructose-1-phosphate kinase and a fructose-PTS permease EII^Fru, respectively, was monitored in response to carbohydrate source and in mutants lacking CcpA, FruR, and components of EII^Man Expression of genes for EII^Man and EII^Fru was directly regulated by CcpA and CR, as evinced by in vivo and in vitro methods. Unexpectedly, not only was the fruRKI operon negatively regulated by FruR, but also so was manLMN Carbohydrate transport by EII^Man had a negative influence on expression of manLMN but not fruRKI In agreement with the proposed role of FruR in regulating these PTS operons, loss of fruR or fruK substantially altered growth on a number of carbohydrates, including fructose. RNA deep sequencing revealed profound changes in gene regulation caused by deletion of fruK or fruR Collectively, these findings demonstrate intimate interconnection of the regulation of two major PTS permeases in S. mutans and reveal novel and important contributions of fructose metabolism to global regulation of gene expression.IMPORTANCE The ability of Streptococcus mutans and other streptococcal pathogens to survive and cause human diseases is directly dependent upon their capacity to metabolize a variety of carbohydrates, including glucose and fructose. Our research reveals that metabolism of fructose has broad influences on the regulation of utilization of glucose and other sugars, and mutants with changes in certain genes involved in fructose metabolism display profoundly different abilities to grow and express virulence-related traits. Mutants lacking the FruR regulator or a particular phosphofructokinase, FruK, display changes in expression of a large number of genes encoding transcriptional regulators, enzymes required for energy metabolism, biofilm development, biosynthetic and degradative processes, and tolerance of a spectrum of environmental stressors. Since fructose is a major component of the modern human diet, the results have substantial significance in the context of oral health and the development of dental caries.

Interference of a speB 5' untranslated region partial deletion with mRNA degradation in Streptococcus pyogenes

The 5' untranslated region (5' UTR) of an mRNA molecule embeds important determinants that modify its stability and translation efficiency. In Streptococcus pyogenes, a strict human pathogen, a gene encoding a secreted protease (speB) has a large 5' UTR with unknown functions. Here we describe that a partial deletion of the speB 5' UTR caused a general accumulation of mRNA in the stationary phase, and that the mRNA accumulation was due to retarded mRNA degradation. The phenotype was observed in several M serotypes harboring the partial deletion of the speB 5' UTR. The phenotype was triggered by the production of the truncated speB 5' UTR, but not by the disruption of the intact speB 5' UTR. RNase Y, a major endoribonuclease, was previously shown to play a central role in bulk mRNA turnover in stationary phase. However, in contrast to our expectations, we observed a weaker interaction between the truncated speB 5' UTR and RNase Y compared with the wild-type, which suggests that other unidentified RNA degrading components are required for the pleiotropic effects observed from the speB UTR truncation. Our study demonstrates how S. pyogenes uses distinct mRNA degradation schemes in exponential and stationary growth phases.

Antibacterial and Antibiofilm Activities of a Novel Synthetic Cyclic Lipopeptide against Cariogenic Streptococcus mutans UA159

Despite continuous efforts to control cariogenic dental biofilms, very few effective antimicrobial treatments exist. In this study, we characterized the activity of the novel synthetic cyclic lipopeptide 4 (CLP-4), derived from fusaricidin, against the cariogenic pathogen Streptococcus mutans UA159. We determined CLP-4's MIC, minimum bactericidal concentration (MBC), and spontaneous resistance frequency, and we performed time-kill assays. Additionally, we assessed CLP-4's potential to inhibit biofilm formation and eradicate preformed biofilms. Our results demonstrate that CLP-4 has strong antibacterial activity in vitro and is a potent bactericidal agent with low spontaneous resistance frequency. At a low concentration of 5 μg/ml, CLP-4 completely inhibited S. mutans UA159 biofilm formation, and at 50 μg/ml, it reduced the viability of established biofilms by >99.99%. We also assessed CLP-4's cytotoxicity and stability against proteolytic digestion. CLP-4 withstood trypsin or chymotrypsin digestion even after treatment for 24 h, and our toxicity studies showed that CLP-4 effective concentrations had negligible effects on hemolysis and the viability of human oral fibroblasts. In summary, our findings showed that CLP-4 is a potent antibacterial and antibiofilm agent with remarkable stability and low nonspecific cytotoxicity. Hence, CLP-4 is a promising novel antimicrobial peptide with potential for clinical application in the prevention and treatment of dental caries.

K+ modulates genetic competence and the stress regulon of Streptococcus mutans

Potassium (K+) is the most abundant cation in dental plaque fluid. Previously, we reported the link between K+ transport via Trk2 in Streptococcus mutans and its two critical virulence attributes: acid tolerance and surface adhesion. Herein, we build further on the intimate link between K+ levels and S. mutans biology. High (>25 mM) versus low (≤5 mM) K+ concentrations in the growth medium affected conformational epitopes of cell surface-localized adhesin P1. At low K+, the expression of stress response elements gcrR and codY, cell-adhesion-associated genes such as spaP and metabolism-associated genes such as bglP was induced at stationary phase (P<0.05), suggesting that K+-mediated regulation is growth phase-dependent and stress-sensitive. Production of the newly discovered secretory protein encoded by SMU_63c was strongly dependent on the availability of K+ and growth phase. This protein is a newly discovered regulator of genetic competence and biofilm cell density. Thus, the influence of K+ on DNA transformation efficiency was also examined. Compared with 25 mM K+ concentration, the presence of low K+ reduced the transformation frequency by 100-fold. Genetic transformation was abolished in a strain lacking a Trk2 system under all K+ concentrations tested. Consistent with these findings, repression of competence-associated genes, comS and comX, was observed under low environmental K+ conditions and in the strain lacking Trk2. Taken together, these results highlight a pivotal role for environmental K+ as a regulatory cation that modulates stress responses and genetic transformation in S. mutans.

Is the oral fungal pathogen Candida albicans a cariogen?

Pathobiology of dental caries is complex. Data from recent molecular microbiologic studies have further redefined the role of the oral microbiome in the etiology of dental caries. This new information challenges the conventional view on the hegemony of classic cariogenic prokaryotes such as Streptococcus mutans in caries etiology, and raises the intriguing possibility of the participation of the eukaryotic oral fungal pathogen Candida in the caries process. The virulence attributes of Candida species such as their acidogenicity and aciduric nature, the ability to develop profuse biofilms, ferment and assimilate dietary sugars, and produce collagenolytic proteinases are all indicative of their latent cariogenic potential. Based on the above, oral candidal counts have been used by some as a caries risk indicator. On the contrary, other studies suggest that Candida is merely a passenger extant in an acidic cariogenic milieu, and not a true pathogen. In this review, we critically examine the varying roles of Candida, and traditionally accepted cariogens such as the mutans group of streptococci in the pathobiology of dental caries. The weight of available data tends to imply that Candida may play a pivotal role as a secondary agent perpetuating the carious process, especially in dentinal caries.

RNA-Seq Reveals Enhanced Sugar Metabolism in Streptococcus mutans Co-cultured with Candida albicans within Mixed-Species Biofilms

Early childhood caries (ECC), which can lead to rampant tooth-decay that is painful and costly to treat, is one of the most prevalent infectious diseases affecting children worldwide. Previous studies support that interactions between Streptococcus mutans and Candida albicans are associated with the pathogenesis of ECC. The presence of Candida enhances S. mutans growth, fitness and accumulation within biofilms in vitro, although the molecular basis for these behaviors is undefined. Using an established co-cultivation biofilm model and RNA-Seq, we investigated how C. albicans influences the transcriptome of S. mutans. The presence of C. albicans dramatically altered gene expression in S. mutans in the dual-species biofilm, resulting in 393 genes differentially expressed, compared to mono-species biofilms of S. mutans. By Gene Ontology analysis, the majority of up-regulated genes were related to carbohydrate transport and metabolic/catabolic processes. KEGG pathway impact analysis showed elevated pyruvate and galactose metabolism, suggesting that co-cultivation with C. albicans influences carbohydrate utilization by S. mutans. Analysis of metabolites confirmed the increases in carbohydrate metabolism, with elevated amounts of formate in the culture medium of co-cultured biofilms. Moreover, co-cultivation with C. albicans altered transcription of S. mutans signal transduction (comC and ciaRH) genes associated with fitness and virulence. Interestingly, the expression of genes for mutacins (bacteriocins) and CRISPR were down-regulated. Collectively, the data provide a comprehensive insight into S. mutans transcriptomic changes induced by C. albicans, and offer novel insights into how bacterial–fungal interactions may enhance the severity of dental caries.

Antimicrobial photodynamic therapy on Streptococcus mutans is altered by glucose in the presence of methylene blue and red LED

BACKGROUND

Dental caries are a multifactorial disease that progressively produces tooth destruction as a result of bacterial colonization of enamel surface, especially Streptococcus mutans. The objective of this work was to investigate the role of glucose in antimicrobial photodynamic therapy (aPDT) on S. mutans.

METHODS

S. mutans ATCC 25175 were cultured on microaerophilia at 37°C for 48h, and we tested aPDT in the presence of 50mM glucose. Bacterial suspension was used to investigate aPDT with 100μM methylene blue (MB) under LED emitting radiation at ʎ=660nm and parameters as following (P=473 mW; I=166.8 mW/cm², and doses of 5, 10 and 20J/cm²). A seventy-two hours biofilm was grown on 96 flat buttoned well-plate and irradiation was performed from 10 to 80J/cm² at similar conditions.

RESULTS

There was no dark toxicity nor bacterial death regarding LED irradiation on suspension and on biofilm. Nevertheless, aPDT presented expressive bacterial inactivation following 1 and 2min of irradiation on cell suspension. On the other hand, there was no inactivation in the presence of glucose under the same conditions. Biofilm was completely inactivated by MB-mediated aPDT after 6min of irradiation. However, the presence of glucose delayed the complete inactivation of the biofilm.

CONCLUSION

The presence of glucose in the suspension drastically delayed the effect of aPDT on S. mutans and this effect is more pronounced in bacterial suspension than on biofilm.

Sugar Allocation to Metabolic Pathways is Tightly Regulated and Affects the Virulence of Streptococcus mutans

Bacteria take up and metabolize sugar as a carbohydrate source for survival. Most bacteria can utilize many sugars, including glucose, sucrose, and galactose, as well as amino sugars, such as glucosamine and N-acetylglucosamine. After entering the cytoplasm, the sugars are mainly allocated to the glycolysis pathway (energy production) and to various bacterial component biosynthesis pathways, including the cell wall, nucleic acids and amino acids. Sugars are also utilized to produce several virulence factors, such as capsule and lipoteichoic acid. Glutamine-fructose-6-phosphate aminotransferase (GlmS) and glucosamine-6-phosphate deaminase (NagB) have crucial roles in sugar distribution to the glycolysis pathway and to cell wall biosynthesis. In Streptococcus mutans, a cariogenic pathogen, the expression levels of glmS and nagB are coordinately regulated in response to the presence or absence of amino sugars. In addition, the disruption of this regulation affects the virulence of S. mutans. The expression of nagB and glmS is regulated by NagR in S. mutans, but the precise mechanism underlying glmS regulation is not clear. In Staphylococcus aureus and Bacillus subtilis, the mRNA of glmS has ribozyme activity and undergoes self-degradation at the mRNA level. However, there is no ribozyme activity region on glmS mRNA in S. mutans. In this review article, we summarize the sugar distribution, particularly the coordinated regulation of GlmS and NagB expression, and its relationship with the virulence of S. mutans.

[Effect of ptxA and ptxB genes of phosphotransferase system on growth of Streptococcus mutans]

OBJECTIVE

This study aims to evaluate the effect of ptxA and ptxB genes, which are important genes in the L-ascorbate phosphotransferase system (PTS) of Streptococcus mutans (S. mutans).

METHODS

The ptxA-, ptxB-, and ptxAB-double deficient mutant as well as ptxAB-complemented strain were constructed. Quantitative real-time polymerase chain reaction analysis was performed to evaluate the expression of the target genes of wild-type S. mutans when L-ascorbate was used as the sole carbohydrate source. The OD₆₀₀ values of the wild type, deficient, and complemented strains were continuously monitored, and their growth curves were constructed to compare growth capacity.

RESULTS

Polymerase chain reaction and sequencing analyses suggested that deficient and complemented strains were successfully constructed. The expression levelsof ptxA and ptxB significantly increased (P < 0.01) when L-ascorbate was used as the sole carbohydrate source. The growth capacity of the deficient mutants decreased compared with that of the wild-type strain. However, the wild-type phenotype could be restored in the complemented strain.

CONCLUSION

ptxA and ptxB genes are associated with L-ascorbate metabolism of S. mutans. The construction of deficient strains and complemented strain lay a foundation for further mechanism study on L-ascorbate metabolism in S. mutans.

Differential proteome and cellular adhesion analyses of the probiotic bacterium Lactobacillus acidophilus NCFM grown on raffinose - an emerging prebiotic

Whole cell and surface proteomes were analyzed together with adhesive properties of the probiotic bacterium Lactobacillus acidophilus NCFM (NCFM) grown on the emerging prebiotic raffinose, exemplifying a synbiotic. Adhesion of NCFM to mucin and intestinal HT-29 cells increased three-fold after culture with raffinose versus glucose, as also visualized by scanning electron microscopy. Comparative proteomics using 2D-DIGE showed 43 unique proteins to change in relative abundance in whole cell lysates from NCFM grown on raffinose compared to glucose. Furthermore, 14 unique proteins in 18 spots of the surface subproteome underwent changes identified by differential 2DE, including elongation factor G, thermostable pullulanase, and phosphate starvation inducible stress-related protein increasing in a range of +2.1 - +4.7 fold. By contrast five known moonlighting proteins decreased in relative abundance by up to -2.4 fold. Enzymes involved in raffinose catabolism were elevated in the whole cell proteome; α-galactosidase (+13.9 fold); sucrose phosphorylase (+5.4 fold) together with metabolic enzymes from the Leloir pathway for galactose utilization and the glycolysis; β-galactosidase (+5.7 fold); galactose (+2.9/+3.1 fold) and fructose (+2.8 fold) kinases. The insights at the molecular and cellular levels contributed to the understanding of the interplay of a synbiotic composed of NCFM and raffinose with the host.

The fruRBA Operon Is Necessary for Group A Streptococcal Growth in Fructose and for Resistance to Neutrophil Killing during Growth in Whole Human Blood

Bacterial pathogens rely on the availability of nutrients for survival in the host environment. The phosphoenolpyruvate-phosphotransferase system (PTS) is a global regulatory network connecting sugar uptake with signal transduction. Since the fructose PTS has been shown to impact virulence in several streptococci, including the human pathogen Streptococcus pyogenes(the group A Streptococcus[GAS]), we characterized its role in carbon metabolism and pathogenesis in the M1T1 strain 5448. Growth in fructose as a sole carbon source resulted in 103 genes affected transcriptionally, where the frulocus (fruRBA) was the most induced. Reverse transcriptase PCR showed that fruRBA formed an operon which was repressed by FruR in the absence of fructose, in addition to being under carbon catabolic repression. Growth assays and carbon utilization profiles revealed that although the entire fruoperon was required for growth in fructose, FruA was the main transporter for fructose and also was involved in the utilization of three additional PTS sugars: cellobiose, mannitol, and N-acetyl-D-galactosamine. The inactivation of sloR, a fruA homolog that also was upregulated in the presence of fructose, failed to reveal a role as a secondary fructose transporter. Whereas the ability of both ΔfruR and ΔfruB mutants to survive in the presence of whole human blood or neutrophils was impaired, the phenotype was not reproduced in murine whole blood, and those mutants were not attenuated in a mouse intraperitoneal infection. Since the ΔfruA mutant exhibited no phenotype in the human or mouse assays, we propose that FruR and FruB are important for GAS survival in a human-specific environment.

Identification and functional analysis of the L-ascorbate-specific enzyme II complex of the phosphotransferase system in Streptococcus mutans

Background

Streptococcus mutans is the primary etiological agent of human dental caries. It can metabolize a wide variety of carbohydrates and produce large amounts of organic acids that cause enamel demineralization. Phosphoenolpyruvate-dependent sugar phosphotransferase system (PTS) plays an important role in carbohydrates uptake of S. mutans. The ptxA and ptxB genes in S. mutans encode putative enzyme IIA and enzyme IIB of the L-ascorbate-specific PTS. The aim of this study was to analyze the function of these proteins and understand the transcriptional regulatory mechanism.

Results

ptxA⁻, ptxB⁻, as well as ptxA⁻, ptxB⁻ double-deletion mutants all had more extended lag phase and lower growth yield than wild-type strain UA159 when grown in the medium using L-ascorbate as the sole carbon source. Acid production and acid killing assays showed that the absence of the ptxA and ptxB genes resulted in a reduction in the capacity for acidogenesis, and all three mutant strains did not survive an acid shock. According to biofilm and extracellular polysaccharides (EPS) formation analysis, all the mutant strains formed much less prolific biofilms with small amounts of EPS than wild-type UA159 when using L-ascorbate as the sole carbon source. Moreover, PCR analysis and quantitative real-time PCR revealed that sgaT, ptxA, ptxB, SMU.273, SMU.274 and SMU.275 appear to be parts of the same operon. The transcription levels of these genes were all elevated in the presence of L-ascorbate, and the expression of ptxA gene decreased significantly once ptxB gene was knockout.

Conclusions

The ptxA and ptxB genes are involved in the growth, aciduricity, acidogenesis, and formation of biofilms and EPS of S. mutans when L-ascorbate is the sole carbon source. In addition, the expression of ptxA is regulated by ptxB. ptxA, ptxB, and the upstream gene sgaT, the downstream genes SMU.273, SMU.274 and SMU.275 appear to be parts of the same operon, and L-ascorbate is a potential inducer of the operon.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0668-9) contains supplementary material, which is available to authorized users.

Trk2 Potassium Transport System in Streptococcus mutans and Its Role in Potassium Homeostasis, Biofilm Formation, and Stress Tolerance

Potassium (K(+)) is the most abundant cation in the fluids of dental biofilm. The biochemical and biophysical functions of K(+) and a variety of K(+) transport systems have been studied for most pathogenic bacteria but not for oral pathogens. In this study, we establish the modes of K(+) acquisition in Streptococcus mutans and the importance of K(+) homeostasis for its virulence attributes. The S. mutans genome harbors four putative K(+) transport systems that included two Trk-like transporters (designated Trk1 and Trk2), one glutamate/K(+) cotransporter (GlnQHMP), and a channel-like K(+) transport system (Kch). Mutants lacking Trk2 had significantly impaired growth, acidogenicity, aciduricity, and biofilm formation. [K(+)] less than 5 mM eliminated biofilm formation in S. mutans. The functionality of the Trk2 system was confirmed by complementing an Escherichia coli TK2420 mutant strain, which resulted in significant K(+) accumulation, improved growth, and survival under stress. Taken together, these results suggest that Trk2 is the main facet of the K(+)-dependent cellular response of S. mutans to environment stresses.

IMPORTANCE

Biofilm formation and stress tolerance are important virulence properties of caries-causing Streptococcus mutans. To limit these properties of this bacterium, it is imperative to understand its survival mechanisms. Potassium is the most abundant cation in dental plaque, the natural environment of S. mutans. K(+) is known to function in stress tolerance, and bacteria have specialized mechanisms for its uptake. However, there are no reports to identify or characterize specific K(+) transporters in S. mutans. We identified the most important system for K(+) homeostasis and its role in the biofilm formation, stress tolerance, and growth. We also show the requirement of environmental K(+) for the activity of biofilm-forming enzymes, which explains why such high levels of K(+) would favor biofilm formation.

Sucrose- and Fructose-Specific Effects on the Transcriptome of Streptococcus mutans, as Determined by RNA Sequencing

Recent genome-scale studies have begun to establish the scope and magnitude of the impacts of carbohydrate source and availability on the regulation of gene expression in bacteria. The effects of sugars on gene expression are particularly profound in a group of lactic acid bacteria that rely almost entirely on their saccharolytic activities for energy production and growth. For Streptococcus mutans, the major etiologic agent of human dental caries, sucrose is the carbohydrate that contributes in the most significant manner to establishment, persistence, and virulence of the organism. However, because this organism produces multiple extracellular sucrolytic enzymes that can release hexoses from sucrose, it has not been possible to study the specific effects of sucrose transport and metabolism on gene expression in the absence of carbohydrates that by themselves can elicit catabolite repression and induce expression of multiple genes. By employing RNA deep-sequencing (RNA-Seq) technology and mutants that lacked particular sucrose-metabolizing enzymes, we compared the transcriptomes of S. mutans bacteria growing on glucose, fructose, or sucrose as the sole carbohydrate source. The results provide a variety of new insights into the impact of sucrose transport and metabolism by S. mutans, including the likely expulsion of fructose after sucrose internalization and hydrolysis, and identify a set of genes that are differentially regulated by sucrose versus fructose. The findings significantly enhance our understanding of the genetics and physiology of this cariogenic pathogen.

Transcriptome analysis of Streptococcus gordonii Challis DL1 indicates a role for the biofilm-associated fruRBA operon in response to Candida albicans

Multiple levels of interkingdom signaling have been implicated in maintaining the ecological balance between Candida albicans and commensal streptococci to assure a state of oral health. To better understand the molecular mechanisms involved in the initial streptococcal response to the presence of C. albicans that can initiate oral surface colonization and biofilm formation, hypha-forming cells were incubated with Streptococcus gordonii cells for 30 min to assess the streptococcal transcriptome response. A genome-wide microarray analysis and quantitative polymerase chain reaction validation of S. gordonii transcripts identified a number of genes, the majority of which were involved in metabolic functions that were differentially expressed in the presence of hyphae. The fruR, fruB, and fruA genes encoding the transcriptional regulator, fructose-1-phosphate kinase, and fructose-specific permease, respectively, of the phosphoenolpyruvate-dependent fructose phosphotransferase system, were consistently upregulated. An S. gordonii mutant in which these genes were deleted by allelic replacement formed an architecturally distinct, less robust biofilm with C. albicans than did parental strain cells. Complementing the mutant with plasmid borne fruR, fruB, and fruA genes caused phenotype reversion, indicating that the genes in this operon played a role in dual-species biofilm formation. This genome-wide analysis of the S. gordonii transcriptional response to C. albicans has identified several genes that have potential roles in interkingdom signaling and responses.

Carbon catabolite repression by seryl phosphorylated HPr is essential to Streptococcus pneumoniae in carbohydrate-rich environments

Carbon catabolite repression (CCR) is a regulatory phenomenon implemented by bacteria to hierarchically organize carbohydrate utilization in order to achieve maximal growth. CCR is likely of great importance to Streptococcus pneumoniae because the human host sites inhabited by this pathogen represent complex carbohydrate environments. In this species, inactivation of the prototypical Gram-positive CCR master regulator, ccpA, attenuates virulence in mice but does not relieve CCR of most metabolic enzymes, suggesting CcpA-independent CCR mechanisms predominate. Here we show the activities of three transcriptional regulators constitute the majority of transcriptional CCR of galactose metabolism operons. We determined seryl-phosphorylated histidine phosphocarrier protein (HPr-Ser∼P)-mediated regulation is a major CCR mechanism and an essential activity in the pneumococcus, as an HPr point mutation abolishing HPrK/P-dependent phosphorylation was not tolerated nor was deletion of hprk/p. The HPr-Ser∼P phosphomimetic mutant HPr S46D had reduced phosphotransferase system transport rates and limited induction of CCR-repressed genes. These results support a model of pneumococcal CCR in which HPr-Ser∼P directly affects the activity of CcpA while indirectly affecting the activity of pathway-specific transactional regulators. This report describes the first CcpA-independent CCR mechanism identified in the pneumococcus and the first example of lethality from loss of HPr-Ser∼P-mediated CCR in any species.

Cloning, characterization and anion inhibition study of a β-class carbonic anhydrase from the caries producing pathogen Streptococcus mutans

The oral pathogenic bacterium involved in human dental caries formation Streptococcus mutans, encodes for two carbonic anhydrase (CA, EC 4.2.1.1) one belonging to the α- and the other one to the β-class. This last enzyme (SmuCA) has been cloned, characterized and investigated for its inhibition profile with a major class of CA inhibitors, the inorganic anions. Here we show that SmuCA has a good catalytic activity for the CO2 hydration reaction, with kcat 4.2×10(5)s(-1) and kcat/Km of 5.8×10(7)M(-1)×s(-1), being inhibited by cyanate, carbonate, stannate, divannadate and diethyldithiocarbamate in the submillimolar range (KIs of 0.30-0.64mM) and more efficiently by sulfamide, sulfamate, phenylboronic acid and phenylarsonic acid (KIs of 15-46μM). The anion inhibition profile of the S. mutans enzyme is very different from other α- and β-CAs investigated earlier. Identification of effective inhibitors of this new enzyme may lead to pharmacological tools useful for understanding the role of S. mutans CAs in dental caries formation, and eventually the development of pharmacological agents with a new mechanism of antibacterial action.

Sulfonamide inhibition study of the β-class carbonic anhydrase from the caries producing pathogen Streptococcus mutans

Streptococcus mutans, the oral pathogenic bacterium provoking dental caries formation, encodes for a β-class carbonic anhydrase (CA, EC 4.2.1.1), SmuCA. This enzyme was cloned, characterized and investigated for its inhibition profile with the major class of CA inhibitors, the primary sulfonamides. SmuCA has a good catalytic activity for the CO2 hydration reaction, with a kcat of 4.2×10(5) s(-1) and kcat/Km of 5.8×10(7) M(-1)×s(-1), and is efficiently inhibited by most sulfonamides (KIs of 246 nM-13.5 μM). The best SmuCA inhibitors were bromosulfanilamide, deacetylated acetazolamide, 4-hydroxymethylbenzenesulfonamide, a pyrimidine-substituted sulfanilamide derivative, aminobenzolamide and compounds structurally similar to it, as well as acetazolamide, methazolamide, indisulam and valdecoxib. These compounds showed inhibition constants ranging between 246 and 468 nM. Identification of effective inhibitors of this enzyme may lead to pharmacological tools useful for understanding the role of S. mutans CAs in dental caries formation, and eventually the development of pharmacological agents with a new mechanism of antibacterial action.

Regulation of Streptococcus mutans PTS Bio by the transcriptional repressor NigR

Streptococcus mutans is implicated in human dental caries, and the carbohydrate metabolism of this organism plays an important role in the formation of this disease. Carbohydrate transport and metabolism are essential for the survival of S. mutans in the oral cavity. It is known that a unique phosphoenolpyruvate-sugar phosphotransferase system PTS(B) (io) of S. mutans UA159 is expressed in sucrose-grown biofilms (Mol Oral Microbiol 28: 2013; 114). In this study we analyzed the transcriptional regulation of the operon (O(B) (io) ) encoding the PTS(B) (io) and showed that it was repressed by NigR, a LacI-like transcriptional regulator. Using electro-mobility shift assay, we described two operators to which NigR bound with different affinities. We also identified the transcriptional start site and showed that one of the operators overlaps with the promoter and presumably represses initiation of transcription. Mutational analyses revealed the key nucleotides in the operators required for high-affinity binding of NigR. PTS(B) (io) is expressed in S. mutans biofilms so understanding its regulation may provide improved strategies for caries treatment and prevention.

The formation of Streptococcus mutans persisters induced by the quorum-sensing peptide pheromone is affected by the LexA regulator

The presence of multidrug-tolerant persister cells within microbial populations has been implicated in the resiliency of bacterial survival against antibiotic treatments and is a major contributing factor in chronic infections. The mechanisms by which these phenotypic variants are formed have been linked to stress response pathways in various bacterial species, but many of these mechanisms remain unclear. We have previously shown that in the cariogenic organism Streptococcus mutans, the quorum-sensing peptide CSP (competence-stimulating peptide) pheromone was a stress-inducible alarmone that triggered an increased formation of multidrug-tolerant persisters. In this study, we characterized SMU.2027, a CSP-inducible gene encoding a LexA ortholog. We showed that in addition to exogenous CSP exposure, stressors, including heat shock, oxidative stress, and ofloxacin antibiotic, were capable of triggering expression of lexA in an autoregulatory manner akin to that of LexA-like transcriptional regulators. We demonstrated the role of LexA and its importance in regulating tolerance toward DNA damage in a noncanonical SOS mechanism. We showed its involvement and regulatory role in the formation of persisters induced by the CSP-ComDE quorum-sensing regulatory system. We further identified key genes involved in sugar and amino acid metabolism, the clustered regularly interspaced short palindromic repeat (CRISPR) system, and autolysin from transcriptomic analyses that contribute to the formation of quorum-sensing-induced persister cells.

Cross-feeding and interkingdom communication in dual-species biofilms of Streptococcus mutans and Candida albicans

Polymicrobial biofilms are of large medical importance, but relatively little is known about the role of interspecies interactions for their physiology and virulence. Here, we studied two human pathogens co-occuring in the oral cavity, the opportunistic fungus Candida albicans and the caries-promoting bacterium Streptococcus mutans. Dual-species biofilms reached higher biomass and cell numbers than mono-species biofilms, and the production of extracellular polymeric substances (EPSs) by S. mutans was strongly suppressed, which was confirmed by scanning electron microscopy, gas chromatography-mass spectrometry and transcriptome analysis. To detect interkingdom communication, C. albicans was co-cultivated with a strain of S. mutans carrying a transcriptional fusion between a green fluorescent protein-encoding gene and the promoter for sigX, the alternative sigma factor of S. mutans, which is induced by quorum sensing signals. Strong induction of sigX was observed in dual-species biofilms, but not in single-species biofilms. Conditioned media from mixed biofilms but not from C. albicans or S. mutans cultivated alone activated sigX in the reporter strain. Deletion of comS encoding the synthesis of the sigX-inducing peptide precursor abolished this activity, whereas deletion of comC encoding the competence-stimulating peptide precursor had no effect. Transcriptome analysis of S. mutans confirmed induction of comS, sigX, bacteriocins and the downstream late competence genes, including fratricins, in dual-species biofilms. We show here for the first time the stimulation of the complete quorum sensing system of S. mutans by a species from another kingdom, namely the fungus C. albicans, resulting in fundamentally changed virulence properties of the caries pathogen.

Fueling the caries process: carbohydrate metabolism and gene regulation by Streptococcus mutans

The nature of the oral cavity and host behaviors has mandated that the oral microbiota evolve mechanisms for coping with environmental fluctuations, especially changes in the type and availability of carbohydrates. In the case of human dental caries, the presence of excess carbohydrates is often responsible for altering the local environment to be more favorable for species associated with the initiation and progression of disease, including Streptococcus mutans. Some of the earliest endeavors to understand how cariogenic species respond to environmental perturbations were carried out using chemostat cultivation, which provides fine control over culture conditions and bacterial behaviors. The development of genome-scale methodologies has allowed for the combination of sophisticated cultivation technologies with genome-level analysis to more thoroughly probe how bacterial pathogens respond to environmental stimuli. Recent investigations in S. mutans and other closely related streptococci have begun to reveal that carbohydrate metabolism can drastically impact pathogenic potential and highlight the important influence that nutrient acquisition has on the success of pathogens; inside and outside of the oral cavity. Collectively, research into pathogenic streptococci, which have evolved in close association with the human host, has begun to unveil the essential nature of careful orchestration of carbohydrate acquisition and catabolism to allow the organisms to persist and, when conditions allow, initiate or worsen disease.

Uptake and metabolism of N-acetylglucosamine and glucosamine by Streptococcus mutans

Glucosamine and N-acetylglucosamine are among the most abundant sugars on the planet, and their introduction into the oral cavity via the diet and host secretions, and through bacterial biosynthesis, provides oral biofilm bacteria with a source of carbon, nitrogen, and energy. In this study, we demonstrated that the dental caries pathogen Streptococcus mutans possesses an inducible system for the metabolism of N-acetylglucosamine and glucosamine. These amino sugars are transported by the phosphoenolpyruvate:sugar phosphotransferase system (PTS), with the glucose/mannose enzyme II permease encoded by manLMN playing a dominant role. Additionally, a previously uncharacterized gene product encoded downstream of the manLMN operon, ManO, was shown to influence the efficiency of uptake and growth on N-acetylglucosamine and, to a lesser extent, glucosamine. A transcriptional regulator, designated NagR, was able to bind the promoter regions in vitro, and repress the expression in vivo, of the nagA and nagB genes, encoding N-acetylglucosamine-6-phosphate deacetylase and glucosamine-6-phosphate deaminase, respectively. The binding activity of NagR could be inhibited by glucosamine-6-phosphate in vitro. Importantly, in contrast to the case with certain other Firmicutes, the gene for de novo synthesis of glucosamine-6-phosphate in S. mutans, glmS, was also shown to be regulated by NagR, and NagR could bind the glmS promoter region in vitro. Finally, metabolism of these amino sugars by S. mutans resulted in the production of significant quantities of ammonia, which can neutralize cytoplasmic pH and increase acid tolerance, thus contributing to enhanced persistence and pathogenic potential.

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.

Effect of high-fructose corn syrup on the acidogenicity, adherence and biofilm formation of Streptococcus mutans

BACKGROUND

Although high-fructose corn syrup (HFCS) as a kind of sugar has been widely used in manufactured foods recently, there is little information available regarding its cariogenicity. The aim of this study was to evaluate the cariogenic potential of HFCS.

METHODS

Streptococcus mutans UA159 was inoculated into HFCS media and cultivated. The pH of each culture was measured to assess acidogenicity. Spectrophotometric turbidity was measured to determine the percentage of adherence. Confocal laser scanning microscopy and SYTO-9 staining were employed to observe biofilm formation. Sucrose media was used as a positive control.

RESULTS

The ΔpH in HFCS media was significantly larger than that in sucrose media and the pH in HFCS media decreased faster (p < 0.05). The percentage of adherence of S. mutans in HFCS media was significantly lower than that in sucrose media (p < 0.05). The biofilm formed in sucrose media was significantly thicker than that in HFCS media (p < 0.05).

CONCLUSIONS

The results of this study suggest that the cariogenicity of S. mutans in the presence of HFCS may differ compared to its cariogenicity in the presence of sucrose. Further in vivo studies need to be undertaken to resolve this uncertainty.

YlxM is a newly identified accessory protein that influences the function of signal recognition particle pathway components in Streptococcus mutans

Streptococcus mutans is a cariogenic oral pathogen whose virulence is determined largely by its membrane composition. The signal recognition particle (SRP) protein-targeting pathway plays a pivotal role in membrane biogenesis. S. mutans SRP pathway mutants demonstrate growth defects, cannot contend with environmental stress, and exhibit multiple changes in membrane composition. This study sought to define a role for ylxM, which in S. mutans and numerous other bacteria resides directly upstream of the ffh gene, encoding a major functional element of the bacterial SRP. YlxM was observed as a produced protein in S. mutans. Its predicted helix-turn-helix motif suggested that it has a role as a transcriptional regulator of components within the SRP pathway; however, no evidence of transcriptional regulation was found. Instead, capture enzyme-linked immunosorbent assay (ELISA), affinity chromatography, and bio-layer interferometry (BLI) demonstrated that S. mutans YlxM interacts with the SRP components Ffh and small cytoplasmic RNA (scRNA) but not with the SRP receptor FtsY. In the absence of FtsY, YlxM increased the GTP hydrolysis activity of Ffh alone and in complex with scRNA. However, in the presence of FtsY, YlxM caused an overall diminution of net GTPase activity. Thus, YlxM appears to modulate GTP hydrolysis, a process necessary for proper recycling of SRP pathway components. The presence of YlxM conferred a significant competitive growth advantage under nonstress and acid stress conditions when wild-type and ylxM mutant strains were cultured together. Our results identify YlxM as a component of the S. mutans SRP and suggest a regulatory function affecting GTPase activity.

β-Phosphoglucomutase contributes to aciduricity in Streptococcus mutans

Streptococcus mutans encounters an array of sugar moieties within the oral cavity due to a varied human diet. One such sugar is β-d-glucose 1-phosphate (βDG1P), which must be converted to glucose 6-phosphate (G6P) before further metabolism to lactic acid. The conversion of βDG1P to G6P is mediated by β-phosphoglucomutase, which has not been previously observed in any oral streptococci, but has been extensively characterized and the gene designated pgmB in Lactococcus lactis. An orthologue was identified in S. mutans, SMU.1747c, and deletion of the gene resulted in the inability of the deletion strain to convert βDG1P to G6P, indicating that SMU.1747c is a β-phosphoglucomutase and should be designated pgmB. In this study, we sought to characterize how deletion of pgmB affected known virulence factors of S. mutans, specifically acid tolerance. The ΔpgmB strain showed a decreased ability to survive acid challenge. Additionally, the strain lacking β-phosphoglucomutase had a diminished glycolytic profile compared with the parental strain. Deletion of pgmB had a negative impact on the virulence of S. mutans in the Galleria mellonella (greater wax worm) animal model. Our results indicate that pgmB plays a role at the juncture of carbohydrate metabolism and virulence.