[Effect of Lipoteichoic Acid Synthesis-Related Gene dltD on Acid Tolerance of Highly Cariogenic Strains of Streptococcus mutans]

Objective

To study the role and possible mechanism of dltD in the acid tolerance of Streptococcus mutans 593 (SM593), and to provide a theoretical basis for the ecological prevention and control of dental caries by constructing the dltD gene deletion strain of SM593 (SM593-ΔdltD).

Methods

1) SM593-Δ dltD was constructed by homologous recombination. 2) The growth curve of SM593 dltD and SM593-Δ dltD under different pH culture conditions was drawn by the automatic growth curve analyzer to compare their acid tolerance. Colony forming unit (CFU) at different time points was used to calculate the survival rate and to compare the acid tolerance response (ATR) of SM593 and SM593-Δ dltD. 3) Under different pH conditions, glycolysis experiments, proton permeability test and H +-ATPase activity test were conducted to make preliminary exploration into the mechanisms of how dltD gene deletion may affect acid tolerance.

Results

1) PCR and sequencing results showed that the SM593-Δ dltD was constructed successfully. 2) With decreasing pH value of the culture medium, the growth of SM593-Δ dltD slowed down. When the pH value of the culture medium was 5.0, SM593-Δ dltD was not allowed to grow, and its acid tolerance was lower than that of SM593. Compared with SM593, the ATR capability of SM593-Δ dltD was decreased. 3) SM593 dltD and SM593-Δ dltD did not show obvious difference in their glycolysis ability under different pH conditions. Compared with SM593 dltD, the proton permeability of SM593-Δ dltD under different pH conditions was increased significantly (P<0.05), and H +-ATPase activity decreased significantly (P<0.05).

Conclusion

Compared with SM593 dltD, SM593-Δ dltD showed obvious decrease in acid tolerance, which may be caused by the significant increase in proton permeability and significant decrease in the H +-ATPase activity induced by the deletion of the dltD gene, hence reducing its ability to maintain intracellular pH homeostasis.

Influence of Fluoride-Resistant Streptococcus mutans Within Antagonistic Dual-Species Biofilms Under Fluoride In Vitro

The widespread application of fluoride, an extremely effective caries prevention agent, induces the generation of fluoride-resistant strains of opportunistic cariogenic bacteria such as fluoride-resistant Streptococcus mutans (S. mutans). However, the influence of this fluoride-resistant strain on oral microecological homeostasis under fluoride remains unknown. In this study, an antagonistic dual-species biofilm model composed of S. mutans and Streptococcus sanguinis (S. sanguinis) was used to investigate the influence of fluoride-resistant S. mutans on dual-species biofilm formation and pre-formed biofilms under fluoride to further elucidate whether fluoride-resistant strains would influence the anti-caries effect of fluoride from the point of biofilm control. The ratio of bacteria within dual-species biofilms was investigated using quantitative real-time PCR and fluorescence in situ hybridization. Cristal violet staining, scanning electron microscopy imaging, and 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide assay were used to evaluate biofilm biomass, biofilm structure, and metabolic activity, respectively. Biofilm acidogenicity was determined using lactic acid and pH measurements. The anthrone method and exopolysaccharide (EPS) staining were used to study the EPS production of biofilms. We found that, in biofilm formation, fluoride-resistant S. mutans occupied an overwhelming advantage in dual-species biofilms under fluoride, thus showing more biofilm biomass, more robust biofilm structure, and stronger metabolic activity (except for 0.275 g/L sodium fluoride [NaF]), EPS production, and acidogenicity within dual-species biofilms. However, in pre-formed biofilms, the advantage of fluoride-resistant S. mutans could not be fully highlighted for biofilm formation. Therefore, fluoride-resistant S. mutans could influence the anti-caries effect of fluoride on antagonistic dual-species biofilm formation while being heavily discounted in pre-formed biofilms from the perspective of biofilm control.

Can the Acid-formation Potential of Saliva Detect a Caries-related Shift in the Oral Microbiome?

PURPOSE

To determine acid-formation potential of saliva and evaluate whether this method corresponds with microbiome composition of individuals with and without caries.

MATERIALS AND METHODS

A clinical, controlled pilot study was performed with two groups: individuals without caries (n = 25; DMFT = 0) and individuals with at least one active carious lesion (n = 25; DMFT>0). A detailed intraoral examination was performed, and the gingival bleeding index (GBI) and plaque index (PI) were recorded. The acid-formation potential was measured (ΔpH) after 1 h. Streptococcus mutans (SM) and lactobacilli (LB) were also quantified. Intergroup comparisons were made using the Mann-Whitney U-test. The diagnostic value was evaluated using the receiver operating characteristics (ROC) method and area under the curve (AUC) values were calculated. The saliva microbiome was analysed by 16S rDNA next-generation sequencing.

RESULTS

A statistically significant difference was found in ΔpH, with the 'caries' group showing a higher mean value after 1 h ('healthy' = 1.1,'caries' = 1.4; p = 0.035). The AUC values were moderate to good (ΔpH = 0.67; SM = 0.83; LB = 0.83;1 = ideal). Streptococcus mutans and Lactobacilli were more frequently detected in the 'caries' group (p < 0.001), as were statistically significantly higher GBI (p = 0.006) and PI (p = 0.001). The saliva microbiome had a higher α-diversity and greater richness in individuals with active caries. The incidence of the genera Alloprevotella, Prevotella, Campylobacter and Veillonella was statistically significantly higher in the 'healthy' group. The incidence of the genera Fretibacterium, Lactobacillus, and Leptotrichia, as well as the phyla Spirochaetes and Synergistetes, was statistically significantly higher in the 'caries' group.

CONCLUSION

Further studies must be carried out to determine the extent to which both tests are suitable for predicting future caries development.

[Effect of csn2 gene deficiency on the starvation tolerance and the synthesis of extracellular polysaccharides in oligotrophic environment of Streptococcus mutans]

Objective: To explore the effect of csn2 gene deficiency on starvation tolerance and extracellular polysaccharides (EPS) synthesis in an oligotrophic environment of Streptococcus mutans (Sm). Methods: The csn2 gene deletion strains and complementary strains of Sm were cultivated and then an oligotrophic growth environment for Sm growth by setting different concentration gradient media were created. Cell growth in oligotrophic environment was detected by growth curve. Biofilm volume was measured by crystalline violet staining. Scanning electron microscopy (SEM) and laser confocal microscope were performed to observe the biofilm structure of Sm. The synthesis of EPS was measured by the anthrone-sulfuric acid method. The expression of genes related to EPS synthesis was evaluated by quantitative real-time PCR (qRT-PCR). Results: The growth curve results showed that the deletion of csn2 gene inhibited the growth of Sm under starvation stress. Furthermore, the results of laser confocal microscope showed that the biofilm EPS/bacteria ratios produced by the wild-type strain, csn2 gene-deficient strain and complement strains under nutrient sufficient culture conditions were 0.44±0.07, 1.05±0.13 and 0.57±0.08 respectively, while the ratios of EPS/bacteria in an oligotrophic environment were 0.93±0.24, 3.05±0.21 and 1.32±0.46 respectively, indicating that the deletion of csn2 gene enhanced the ability of extracellular polysaccharide synthesis of Sm in the oligotrophic environment. The expression levels of EPS synthesis-related genes gtfB and gtfC were up-regulated by 2.5 fold and 1.8 fold respectively and the expression level of gtfD was down-regulated by two-thirds. Conclusions: The csn2 gene deficiency showed multiple effects on the physiological functions and virulence characteristics of Sm, including starvation tolerance and EPS synthesis. These changes might be related to the shift of the complex regulative network caused by csn2 gene deletion.

Identification and Analysis of Essential Genes in Streptococcus mutans with Transposon Sequencing

Transposon sequencing (Tn-seq) has greatly accelerated the rate at which gene function can be profiled in microbial organisms. This technique has been applied to the study of the dental caries pathogen Streptococcus mutans where it has been used to generate large transposon mutant libraries. Coupled with high-throughput sequencing and bioinformatics tools, culture of these transposon mutant libraries has facilitated the identification of essential and conditional essential genes. In this chapter, we describe a procedure for performing Tn-seq studies in S. mutans that covers pooled transposon mutant construction, in vitro culture, and DNA library sequencing and data analysis.

Involvement of ClpE ATPase in Physiology of Streptococcus mutans

Streptococcus mutans, a dental pathogen, harbors at least three Clp ATPases (ClpC, ClpE, and ClpX) that form complexes with ClpP protease and participate in regulated proteolysis. Among these, the function of ClpE ATPase is poorly understood. We have utilized an isogenic clpE-deficient strain derived from S. mutans UA159 and evaluated the role of ClpE in cellular physiology. We found that loss of ClpE leads to increased susceptibility against thiol stress but not to oxidative and thermal stress. Furthermore, we found that the mutant displays altered tolerance against some antibiotics and altered biofilm formation. We performed a label-free proteomic analysis by comparing the mutant with the wild-type UA159 strain under nonstressed conditions and found that ClpE modulates a relatively limited proteome in the cell compared to the proteomes modulated by ClpX and ClpP. Nevertheless, we found that ClpE deficiency leads to an overabundance of some cell wall synthesis enzymes, ribosomal proteins, and an unknown protease encoded by SMU.2153. Our proteomic data strongly support some of the stress-related phenotypes that we observed. Our study emphasizes the significance of ClpE in the physiology of S. mutans. IMPORTANCE When bacteria encounter environmental stresses, the expression of various proteins collectively known as heat shock proteins is induced. These heat shock proteins are necessary for cell survival specifically under conditions that induce protein denaturation. A subset of heat shock proteins known as the Clp proteolytic complex is required for the degradation of the misfolded proteins in the cell. The Clp proteolytic complex contains an ATPase and a protease. A specific Clp ATPase, ClpE, is uniquely present in Gram-positive bacteria, including streptococci. Here, we have studied the functional role of the ClpE protein in Streptococcus mutans, a dental pathogen. Our results suggest that ClpE is required for survival under certain antibiotic exposure and stress conditions but not others. Our results demonstrate that loss of ClpE leads to a significantly altered cellular proteome, and the analysis of those changes suggests that ClpE's functions in S. mutans are different from its functions in other Gram-positive bacteria.

[Regulatory role of small RNA srn821978 in mutacin IV expression in Streptococcus mutans]

OBJECTIVE

To analyze the role of small RNA srn821798 in posttranscriptional regulation of mutacin IV expression in Streptococcus mutans.

METHODS

The potential target genes of srn821978 were predicted using RNAhybrid, RNAPredator and IntaRNA. We collected 10 Streptococcus mutans (S.muans) strains with high expression of mutacin IV and another 10 S.muans strains that did not express mutacin IV screened by inhibition zone test, and the expression levels of srn821798 and the candidate target genes in these strains were detected by qPCR. Using synthesized mimics and inhibitors of srn821798, we constructed S.muans strains with high or low srn821798 expression via electroporation based on the standard strain of S.muans UA159, and analyzed the expression levels of srn821798 and its candidate target genes in these strains. We also examined the binding ability of srn821798 to its target gene sepM using electrophoresis and a dual- luciferase reporter system.

RESULTS

The expression levels of the candidate target genes of srn821798 including sepM, comD, comE, nlmA and nlmB were significantly higher while the expression level of srn821798 was significantly lower in clinical S.muans strains with high expression of mutacin IV than in those without mutacin IV expression (P < 0.05). Although the expression levels of the candidate target genes in strains with up- regulated or down- regulated srn821798 expression did not differ significantly from those in the standard strain, the expression level of sepM showed a trend of differential distribution, and srn821798 was predicted to have a strong binding ability to sepM action site.

CONCLUSION

srn821798 may play a regulatory role in the expression of mutacin IV in S.muans, but the underlying mechanism remains to be explored.

Effect of epigallocatechin gallate on dental biofilm of Streptococcus mutans: An in vitro study

BACKGROUND

Streptococcus mutans (S. mutans) plays a major role in the formation of dental caries. The aim of this study was to examine the effect of the green tea polyphenol, epigallocatechin gallate (EGCG), on biofilm formation of S. mutans.

METHODS

Following exposure to increasing concentrations of EGCG, the planktonic growth was measured by optical density and the biofilm biomass was quantified by crystal violet staining. Exopolysaccharides (EPS) production was visualized by confocal scanning laser microscopy, and the bacterial DNA content was determined by quantitative polymerase chain reaction (qPCR). Gene expression of selected genes was analyzed by real time (RT)-qPCR and membrane potential was examined by flow cytometry.

RESULTS

We observed that EGCG inhibited in a dose-dependent manner both the planktonic growth and the biofilm formation of S. mutans. Significant reduction of S. mutans biofilm formation, DNA content, and EPS production was observed at 2.2-4.4 mg/ml EGCG. EGCG reduced the expression of gtfB, gtfC and ftf genes involved in EPS production, and the nox and sodA genes involved in the protection against oxidative stress. Moreover, EGCG caused an immediate change in membrane potential.

CONCLUSIONS

EGCG, a natural polyphenol, has a significant inhibitory effect on S. mutans dental biofilm formation and EPS production, and thus might be a potential drug in preventing dental caries.

Cariogenic and oral health taxa in the oral cavity among children and adults: A scoping review

OBJECTIVE

To review published oral microbiome studies and create a comprehensive list of bacterial species found in saliva and dental plaque among healthy children and adults associated with presence of carious lesions and caries-free state (oral health).

DESIGN

This review followed PRISMA-ScR guidelines. We searched published studies querying PUBMED and EMBASE using the following keywords: (plaque OR saliva) AND caries AND (next generation sequencing OR checkerboard OR 16s rRNA or qPCR). Studies were limited to human studies published in English between January 1, 2010 and June 24, 2020 that included > 10 caries-active and > 10 caries-free participants, and assessed the entire bacterial community.

RESULTS

Our search strategy identified 298 articles. After exclusion criteria, 22 articles remained; we considered 2 studies that examined saliva and plaque as separate studies, for a total of 24 studies. Species associated with caries or oral health varied widely among studies reviewed, with notable differences by age and biologic sample type. No bacterial species was associated with caries in all studies. Streptococcus mutans was found more frequently among those with caries (14/24 (58.3 %)) and Fusobacterium periodonticum was found more frequently among those that were caries-free (5/24 (20.8 %)).

CONCLUSION

No bacterial species was associated with caries or oral health across all studies supporting multiple pathways to cariogenesis. However, the variation may be due to sampling at different time points during caries development, varying methods of specimen sampling, storage, sequencing or analysis or differences in host factors such as age.

[CARIES GENESIS IN THE ERA OF "OMICS"]

The search for an understanding of the genesis of tooth decay spans millennia. Streptococcus mutans have been proposed as the main etiological agents of dental cariessince the species discovery in 1924. However "Omics" era has provided a unique opportunity to unravel contemporary paradigms in the etiology of dental caries. Novel approaches using OMICS techniques has uncovered an extraordinarily diverse ecosystem of carious lesions where S. mutans accounts only a tiny fraction of the bacterial community. This supports the concept that caries has polymicrobial nature that do not follow classical Koch's postulates of infectious diseases and that the microbial causative agents are better described as pathobionts. Understanding the etiology of dental caries is not a mere academic exercise; it provides the basis for preventive, diagnostic, and treatment strategies and gives the dentist a theoretical framework to become a better professional.

Anticariogenic activities of Libidibia ferrea, gallic acid and ethyl gallate against Streptococcus mutans in biofilm model

ETHNOPHARMACOLOGICAL RELEVANCE

In Brazil, ethnopharmacological studies show that Libidibia ferrea (Mart. ex Tul.) L. P. Queiroz is commonly used in folk medicine as an antifungal, antimicrobial and anti-inflammatory. In the Amazon region, the dried fruit powder of L. ferrea are widely used empirically by the population in an alcoholic tincture as an antimicrobial mouthwash in oral infections and the infusion is also recommended for healing oral wounds. However, there are few articles that have evaluated the antimicrobial activity against oral pathogens in a biofilm model, identifying active compounds and mechanisms of action.

AIM OF THE STUDY

The aim of this study was to evaluate the antimicrobial and anti-adherence activities of the ethanolic extract, fractions and isolated compounds (gallic acid and ethyl gallate) of the fruit and seed of L. ferrea against Streptococcus mutans. The inhibition of acidicity/acidogenicity and the expression of the S. mutans GTF genes in biofilms were also evaluated.

MATERIALS AND METHODS

Minimal Inhibitory Concentration (MIC), Minimum Bactericidal Concentration (MBC) and Minimum Inhibitory Concentration of Cell Adhesion (MICA) were evaluated with ethanolic extract (EELF), fractions, gallic acid (GA) and ethyl gallate (EG) against S. mutans. Inhibition of biofilm formation, pH drop and proton permeability tests were conducted with EELF, GA and EG, and also evaluated the expression of the GTF genes in biofilms. The compounds of dichloromethane fraction were identified by GC-MS.

RESULTS

This is the first report of shikimic, pyroglutamic, malic and protocatechuic acids identified in L. ferrea. EELF, GA and EG showed MIC at 250 μg/mL, and MBC at 1000 μg/mL by EELF. EELF biofilms showed reduced dry weight and acidogenicity of S. mutans in biofilms. GA and EG reduced viable cells, glucans soluble in alkali, acidogenicity, aciduricity and downregulated expression of gtfB, gtfC and gtfD genes in biofilms. SEM images of GA and EG biofilms showed a reduction of biomass, exopolysaccharide and microcolonies of S. mutans.

CONCLUSIONS

The ethanolic extract of fruit and seed of L. ferrea, gallic acid and ethyl gallate showed great antimicrobial activity and inhibition of adhesion, reduction of acidogenicity and aciduricity in S. mutans biofilms. The results obtained in vitro validate the use of this plant in ethnopharmacology, and open opportunities for the development of new oral anticariogenic agents, originated by plants that can inhibit pathogenic biofilm that leads to the development of caries.

Zinc import mediated by AdcABC is critical for colonization of the dental biofilm by Streptococcus mutans in an animal model

Trace metals are essential to all domains of life but toxic when found at high concentrations. Although the importance of iron in host-pathogen interactions is firmly established, contemporary studies indicate that other trace metals, including manganese and zinc, are also critical to the infectious process. In this study, we sought to identify and characterize the zinc uptake system(s) of Streptococcus mutans, a keystone pathogen in dental caries and a causative agent of bacterial endocarditis. Different than other pathogenic bacteria, including several streptococci, that encode multiple zinc import systems, bioinformatic analysis indicated that the S. mutans core genome encodes a single, highly conserved, zinc importer commonly known as AdcABC. Inactivation of the genes coding for the metal-binding AdcA (ΔadcA) or both AdcC ATPase and AdcB permease (ΔadcCB) severely impaired the ability of S. mutans to grow under zinc-depleted conditions. Intracellular metal quantifications revealed that both mutants accumulated less zinc when grown in the presence of a subinhibitory concentration of a zinc-specific chelator. Notably, the ΔadcCB strain displayed a severe colonization defect in a rat oral infection model. Both Δadc strains were hypersensitive to high concentrations of manganese, showed reduced peroxide tolerance, and formed less biofilm in sucrose-containing media when cultivated in the presence of the lowest amount of zinc that support their growth, but not when zinc was supplied in excess. Collectively, this study identifies AdcABC as the major high affinity zinc importer of S. mutans and provides preliminary evidence that zinc is a growth-limiting factor within the dental biofilm.

Detection of Streptococcus mutans in symptomatic and asymptomatic infected root canals

OBJECTIVES

To investigate the presence of Streptococcus mutans in root canals of symptomatic necrotic teeth (SNT) and their associated acute apical abscesses (AAA) and in the root canals of asymptomatic necrotic teeth (ANT). It also aimed to investigate the presence of the cnm and cbm genes in specimens that harbored S. mutans.

MATERIALS AND METHODS

DNA was extracted from samples collected from 10 patients presenting pulpal necrosis associated with radiographic evidence of apical periodontitis (ANT) and from 10 patients in need of endodontic therapy due to the presence of pulpal necrosis (SNT) and AAA. The control group consisted of 10 patients with teeth with normal vital pulp and requiring endodontic treatment for prosthetic reasons. The presence of S. mutans was detected by quantitative real-time-PCR (qPCR) using species-specific primers. Samples harboring S. mutans were further evaluated for the presence of CBP genes by qPCR as well.

RESULTS

All studied sites showed a high prevalence of S. mutans, except the control group. Specifically, 60% of ANT and 70% of AAA/SNT paired samples were positive for S. mutans. The cnm gene was detected positive for S. mutans only in ANT samples (66.6%). The cbm gene was not detected in any of the investigated sites.

CONCLUSIONS

S. mutans was found in high prevalence in both asymptomatic and symptomatic endodontic infections, including in abscesses, but it was not detected in the root canals of teeth with normal vital pulp. Interestingly, cnm+ S. mutans was only detected in asymptomatic/chronic primary endodontic infections associated with apical lesion. Therefore, it appears that cnm, and possibly other CBPs, may play an underestimated role in chronic endodontic infections.

CLINICAL RELEVANCE

A high prevalence of Streptococcus mutans cnm+ gene was detected only in asymptomatic primary endodontic infections associated with apical lesion. Therefore, it appears that this collagen-binding protein gene plays an underestimated role in asymptomatic/chronic endodontic infections.

Ursolic acid inhibits multi-species biofilms developed by Streptococcus mutans, Streptococcus sanguinis, and Streptococcus gordonii

OBJECTIVE

The current study aimed to assess the antimicrobial activity of ursolic acid (UA) against multi-species biofilms formed by Streptococcus mutans, Streptococcus sanguinis, and Streptococcus gordonii, as well as to measure its biocompatibility.

METHODS

Crystal violet staining, CFU counting, CCK-8 assays and scanning electron microscopy (SEM) were applied to investigate the effect of UA on multi-species biofilms. UA's effect on exopolysaccharides (EPS) production was measured using confocal laser scanning microscopy (CLSM) and the anthrone-sulfuric acid method. Fluorescent in situ hybridization (FISH) was applied to visualize and quantify the microbial composition of multi-species biofilms. Quantitative real-time PCR (qRT-PCR) was used to measure the expression of virulence genes of S. mutans, S. sanguinis, and S. gordonii under UA treatment. Moreover, CCK-8 assays were performed to evaluate its cytotoxicity against human oral keratinocytes (HOKs) and human gingival epithelial cells (HGEs).

RESULTS

The results showed that UA had significant antimicrobial activity against common oral streptococci. UA also reduced the EPS synthesis of oral streptococci and suppressed gtf genes' expression. In addition, UA reduced the proportion of S. mutans in multi-species biofilms. Besides, UA had low cytotoxicity against HOKs and HGEs.

CONCLUSIONS

UA exhibited antibiofilm activity against oral pathogenic bacteria and had the potential to be used in dental caries treatment.

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.

Exploring the anti-caries properties of baicalin against Streptococcus mutans: an in vitro study

Plant-derived molecules are excellent alternatives to antibiotics as anti-infective agents owing to their minimal cytotoxicity. Herein, the anti-infective property of the hydroxyflavone baicalin, was investigated against biofilms of the key dental caries pathogen Streptococcus mutans. Baicalin inhibited sucrose-dependent biofilm formation at a concentration of 500 µg ml^-1 without affecting bacterial growth. It significantly inhibited acid production for an extended period of 8 h. Microscopic analysis revealed a 6-fold reduction in the number of adhered cells with baicalin treatment. Transcriptomic analysis of the mid-log phase and biofilm cells showed marked downregulation of the virulence genes required for biofilm formation and acid production. This study sheds significant new light on the potential for baicalin to be developed into an anti-caries agent.

PspC domain-containing protein (PCP) determines Streptococcus mutans biofilm formation through bacterial extracellular DNA release and platelet adhesion in experimental endocarditis

Bacterial extracellular DNA (eDNA) and activated platelets have been found to contribute to biofilm formation by Streptococcus mutans on injured heart valves to induce infective endocarditis (IE), yet the bacterial component directly responsible for biofilm formation or platelet adhesion remains unclear. Using in vivo survival assays coupled with microarray analysis, the present study identified a LiaR-regulated PspC domain-containing protein (PCP) in S. mutans that mediates bacterial biofilm formation in vivo. Reverse transcriptase- and chromatin immunoprecipitation-polymerase chain reaction assays confirmed the regulation of pcp by LiaR, while PCP is well-preserved among streptococcal pathogens. Deficiency of pcp reduced in vitro and in vivo biofilm formation and released the eDNA inside bacteria floe along with reduced bacterial platelet adhesion capacity in a fibrinogen-dependent manner. Therefore, LiaR-regulated PCP alone could determine release of bacterial eDNA and binding to platelets, thus contributing to biofilm formation in S. mutans-induced IE.

Direct interactions with commensal streptococci modify intercellular communication behaviors of Streptococcus mutans

The formation of dental caries is a complex process that ultimately leads to damage of the tooth enamel from acids produced by microbes in attached biofilms. The bacterial interactions occurring within these biofilms between cariogenic bacteria, such as the mutans streptococci, and health-associated commensal streptococci, are thought to be critical determinants of health and disease. To better understand these interactions, a Streptococcus mutans reporter strain that actively monitors cell-cell communication via peptide signaling was cocultured with different commensal streptococci. Signaling by S. mutans, normally highly active in monoculture, was completely inhibited by several species of commensals, but only when the bacteria were in direct contact with S. mutans. We identified a novel gene expression pattern that occurred in S. mutans when cultured directly with these commensals. Finally, mutant derivatives of commensals lacking previously shown antagonistic gene products displayed wild-type levels of signal inhibition in cocultures. Collectively, these results reveal a novel pathway(s) in multiple health-associated commensal streptococci that blocks peptide signaling and induces a common contact-dependent pattern of differential gene expression in S. mutans. Understanding the molecular basis for this inhibition will assist in the rational design of new risk assessments, diagnostics, and treatments for the most pervasive oral infectious diseases.

Horizontal Transmission of Streptococcus mutans in Children and its Association with Dental Caries: A Systematic Review and Meta-Analysis

Purpose: To systematically evaluate the horizontal transmission of Streptococcus mutans in children and analyze its relationship with dental caries. Methods: Seven databases were searched for observational studies that have determined the transmission of S. mutans among children younger than seven years. Selection of included studies, data extraction, and quality assessment using Downs and Black's (1998) scoring system were performed. The inverse variance random-effect approach was used to pool the results, and statistical heterogeneity was evaluated using I-squared statistics. Results: Fifteen studies were included for qualitative synthesis, five of which were pooled for quantitative analysis. The risk ratio (RR) of sharing only one genotype in caries-free children versus children with caries was found to be 0.60 (95 percent confidence interval [95% CI] equals 0.45 to 0.80; P ≤ 0.001). The RR of sharing more than one genotype was 1.46 (95% CI equals 1.13 to 1.89; P=0.004) in children with caries versus caries-free children. These findings imply that children sharing only one genotype have a 40 percent lesser risk, and children sharing more than one genotype have a 46 percent higher risk of having dental caries. Conclusions: The systematic review provides evidence of the horizontal transmission of S. mutans and its association with dental caries.

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

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

[Transcriptomic Analysis of csn2 Gene Mutant Strains of Streptococcus mutans CRISPR-Cas9 System]

OBJECTIVE

To explore the differences in transcriptional levels between mutant strains of csn2 gene of CRISPR-Cas9 system of Streptococcus mutans( S. mutans) and wild-type strains.

METHODS

The S. mutans UA159, csn2-gene-deleted strains (Δ csn2) and csn2-gene-covering strains (Δ csn2/pDL278- csn2) of S. mutans were cultivated. Total RNA was extracted, and high-throughput sequencing technology was used for transcriptome sequencing. Based on the GO analysis and the KEGG analysis of the differentially expressed genes, the biological processes involved were thoroughly examined. The qRT-PCR method was used to verify the transcriptome sequencing results.

RESULTS

The transcriptome results showed that, compared with UA159, there were 176 genes in Δ csn2 whose gene expression changed more than one fold ( P<0.05), of which 72 were up-regulated and 104 were down-regulated. The GO enrichment analysis and the KEGG enrichment analysis revealed that both the up-regulated and down-regulated differentially expressed genes (DEG) were involved in amino acid transport and metabolism. In addition, the biological processes that up-regulated DEGs participated in were mainly related to carbohydrate metabolism, energy production and conversion, and transcription; down-regulated DEGs were mainly related to lipid metabolism, DNA replication, recombination and repair, signal transduction mechanisms, nucleotide transport and metabolism. The functions of some DEGs were still unclear. Results of qRT-PCR verified that the expressions of leuA, leuC and leuD(genes related to the formation of branched-chain amino acids) were significantly down-regulated in Δ csn2 when compared with UA159 and Δ csn2/pDL278- csn2.

CONCLUSION

Through transcriptome sequencing and qRT-PCR verification, it was found that the expression of genes related to branched-chain amino acid synthesis and cell membrane permeability in Δ csn2 changed significantly.

Characterization of the Streptococcus mutans SMU.1703c-SMU.1702c Operon Reveals Its Role in Riboflavin Import and Response to Acid Stress

Streptococcus mutans utilizes numerous metabolite transporters to obtain essential nutrients in the "feast or famine" environment of the human mouth. S. mutans and most other streptococci are considered auxotrophic for several essential vitamins including riboflavin (vitamin B2), which is used to generate key cofactors and to perform numerous cellular redox reactions. Despite the well-known contributions of this vitamin to central metabolism, little is known about how S. mutans obtains and metabolizes B2 The uncharacterized protein SMU.1703c displays high sequence homology to the riboflavin transporter RibU. Deletion of SMU.1703c hindered S. mutans growth in complex and defined medium in the absence of saturating levels of exogenous riboflavin, whereas deletion of cotranscribed SMU.1702c alone had no apparent effect on growth. Expression of SMU.1703c in a Bacillus subtilis riboflavin auxotroph functionally complemented growth in nonsaturating riboflavin conditions. S. mutans was also able to grow on flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN) in an SMU.1703c-dependent manner. Deletion of SMU.1703c and/or SMU.1702c impacted S. mutans acid stress tolerance, as all mutants showed improved growth at pH 5.5 compared to that of the wild type when medium was supplemented with saturating riboflavin. Cooccurrence of SMU.1703c and SMU.1702c, a hypothetical PAP2 family acid phosphatase gene, appears unique to the streptococci and may suggest a connection of SMU.1702c to the acquisition or metabolism of flavins within this genus. Identification of SMU.1703c as a RibU-like riboflavin transporter furthers our understanding of how S. mutans acquires essential micronutrients within the oral cavity and how this pathogen successfully competes within nutrient-starved oral biofilms.IMPORTANCE Dental caries form when acid produced by oral bacteria erodes tooth enamel. This process is driven by the fermentative metabolism of cariogenic bacteria, most notably Streptococcus mutans Nutrient acquisition is key in the competitive oral cavity, and many organisms have evolved various strategies to procure carbon sources or necessary biomolecules. B vitamins, such as riboflavin, which many oral streptococci must scavenge from the oral environment, are necessary for survival within the competitive oral cavity. However, the primary mechanism and proteins involved in this process remain uncharacterized. This study is important because it identifies a key step in S. mutans riboflavin acquisition and cofactor generation, which may enable the development of novel anticaries treatment strategies via selective targeting of metabolite transporters.

Phenotypic and Genotypic Characterization of Streptococcus mutans Strains Isolated from Endodontic Infections

Streptococcus mutans plays an important role in caries etiology and eventually in systemic infections. However, it is often found in infected root canals, but the pathophysiological characteristics of strains residing in this site are largely unknown. Here, we characterized strains of S. mutans isolated from root canals of primary (PI) and secondary/persistent (SI) endodontic infections in relation to serotype and genotype; presence of genes coding for collagen binding proteins (CBPs); collagen binding activity and biofilm formation capacity; ability to withstand environmental stresses; systemic virulence in Galleria mellonella; and invasion of human coronary artery endothelial cells and human dental pupal fibroblasts. Samples from 10 patients with PI and 10 patients with SI were collected, and a total of 14 S. mutans isolates, belonging to 3 genotypes, were obtained. Of these, 13 were serotype c, and 1 was serotype k. When compared with the reference strains, the clinical isolates were hypersensitive to hydrogen peroxide. Remarkably, all 14 strains harbored and expressed the CBP-encoding gene cbm, showing increased binding to collagen, enhanced systemic virulence in G. mellonella, and ability to invade human coronary artery endothelial cells and human dental pupal fibroblasts when compared with CBP-negative strains. Whole genome sequence analysis of PI and SI isolates revealed that these strains are phylogenetically related but genetically distinct from each other. Our findings highlight the importance of CBPs in facilitating colonization and persistence of S. mutans in collagenous substrates such as root canals and their potential role in the pathogenesis of endodontic infections.

PepO is a target of the two-component systems VicRK and CovR required for systemic virulence of Streptococcus mutans

Streptococcus mutans, a cariogenic species, is often associated with cardiovascular infections. Systemic virulence of specific S. mutans serotypes has been associated with the expression of the collagen- and laminin-binding protein Cnm, which is transcriptionally regulated by VicRK and CovR. In this study, we characterized a VicRK- and CovR-regulated gene, pepO, coding for a conserved endopeptidase. Transcriptional and protein analyses revealed that pepO is highly expressed in S. mutans strains resistant to complement immunity (blood isolates) compared to oral isolates. Gel mobility assay, transcriptional, and Western blot analyses revealed that pepO is repressed by VicR and induced by CovR. Deletion of pepO in the Cnm+ strain OMZ175 (OMZpepO) or in the Cnm- UA159 (UApepO) led to an increased susceptibility to C3b deposition, and to low binding to complement proteins C1q and C4BP. Additionally, pepO mutants showed diminished ex vivo survival in human blood and impaired capacity to kill G. mellonella larvae. Inactivation of cnm in OMZ175 (OMZcnm) resulted in increased resistance to C3b deposition and unaltered blood survival, although both pepO and cnm mutants displayed attenuated virulence in G. mellonella. Unlike OMZcnm, OMZpepO could invade HCAEC endothelial cells. Supporting these phenotypes, recombinant proteins rPepO and rCnmA showed specific profiles of binding to C1q, C4BP, and to other plasma (plasminogen, fibronectin) and extracellular matrix proteins (type I collagen, laminin). Therefore this study identifies a novel VicRK/CovR-target required for immune evasion and host persistence, pepO, expanding the roles of VicRK and CovR in regulating S. mutans virulence.