Caries Prevention by Arginine Metabolism in Oral Biofilms: Translating Science into Clinical Success

Natural Antibacterial Compounds with Potential for Incorporation into Dental Adhesives: A Systematic Review

Dental adhesives are essential in modern restorative dentistry and are constantly evolving. However, challenges like secondary caries from bacterial infiltration at the adhesive-tooth interface persist. While synthetic antibacterial agents in adhesives show promise, safety concerns have shifted interest toward natural options that are biocompatible, sustainable, and effective. Therefore, this study evaluated whether natural antibacterial compounds in dental adhesives can provide effective antimicrobial activity without compromising their integrity. This systematic review followed PRISMA 2020 statement guidelines. Four databases were screened, PubMed, Scopus, EMBASE, and Web of Science, without language or publication date restrictions until July 2024. The selection criteria were in vitro studies in which natural antimicrobial substances were incorporated into dental adhesives and the resulting composites were tested for their antibacterial and physicochemical properties. A quality assessment was conducted on the selected studies. Most of the studies reviewed reported significant antibacterial activity while retaining the adhesive's integrity, generally achieved with lower concentrations of the natural agents. Higher concentrations increase the antimicrobial effectiveness but negatively impact the adhesive's properties. This review highlights the promising role of natural antibacterial compounds in enhancing the functionality of dental adhesives while also pointing to the need for continued research to address current challenges.

Anti-caries Streptococcus spp.: A potential preventive tool for special needs patients

INTRODUCTION

Probiotics are living microorganisms that act on the host-microbiome interface to restore the microbiota's physiological homeostasis. Numerous probiotics have been marketed with inhibitory activity against Streptococcus mutans and consequently with a potential anti-caries effect, mainly of the genera Lactobacillus and Bifidobacterium, whose main disadvantage is their limited ability to settle in the oral cavity.

METHODS

This narrative review describes the main Streptococcus spp. with probiotic anti-Streptococcus mutans activity, whose substantivity is greater than that of Lactobacillus spp. and consequently with anti-caries potentiality. We performed a literature review in the PubMed, Science Direct and Google Scholar databases of articles published in English (without time restriction) related to caries and probiotics.

RESULTS

The potential identified anti-caries probiotics included Streptococcus spp. A12, Streptococcus oralis (AJ3), Streptococcus oligofermentans, Streptococcus salivarius (K12, M18, JH, LAB813, 24SMB), Streptococcus spp. with arginolytic activity (S. sanguinis, S. gordonii, S. ratti, S. parasanguinis, S. intermedius, S. australis, and S. cristatus), Streptococcus rattus (JH145), Streptococcus dentisani and Streptococcus downii.

CONCLUSIONS

The possibility of using these Streptococcus spp. as probiotics that inhibit the growth of dental plaque and the development of carious lesions represents a potential tool of particular interest for individuals with physical or intellectual disabilities that impede the routine and effective application of mechanical dental plaque removal techniques.

Heterogeneous lineage-specific arginine deiminase expression within dental microbiome species

Arginine catabolism by the bacterial arginine deiminase system (ADS) has anticariogenic properties through the production of ammonia, which modulates the pH of the oral environment. Given the potential protective capacity of the ADS pathway, the exploitation of ADS-competent oral microbes through pre- or probiotic applications is a promising therapeutic target to prevent tooth decay. To date, most investigations of the ADS in the oral cavity and its relation to caries have focused on indirect measures of activity or on specific bacterial groups, yet the pervasiveness and rate of expression of the ADS operon in diverse mixed microbial communities in oral health and disease remain an open question. Here, we use a multivariate approach, combining ultra-deep metatranscriptomic sequencing with paired metataxonomic and in vitro citrulline quantification to characterize the microbial community and ADS operon expression in healthy and late-stage cavitated teeth. While ADS activity is higher in healthy teeth, we identify multiple bacterial lineages with upregulated ADS activity on cavitated teeth that are distinct from those found on healthy teeth using both reference-based mapping and de novo assembly methods. Our dual metataxonomic and metatranscriptomic approach demonstrates the importance of species abundance for gene expression data interpretation and that patterns of differential expression can be skewed by low-abundance groups. Finally, we identify several potential candidate probiotic bacterial lineages within species that may be useful therapeutic targets for the prevention of tooth decay and propose that the development of a strain-specific, mixed-microbial probiotic may be a beneficial approach given the heterogeneity of taxa identified here across health groups.

IMPORTANCE

Tooth decay is the most common preventable chronic disease, affecting more than two billion people globally. The development of caries on teeth is primarily a consequence of acid production by cariogenic bacteria that inhabit the plaque microbiome. Other bacterial strains in the oral cavity may suppress or prevent tooth decay by producing ammonia as a byproduct of the arginine deiminase metabolic pathway, increasing the pH of the plaque biofilm. While the benefits of arginine metabolism on oral health have been extensively documented in specific bacterial groups, the prevalence and consistency of arginine deiminase system (ADS) activity among oral bacteria in a community context remain an open question. In the current study, we use a multi-omics approach to document the pervasiveness of the expression of the ADS operon in both health and disease to better understand the conditions in which ADS activity may prevent tooth decay.

Manipulating the diseased oral microbiome: the power of probiotics and prebiotics

Dental caries and periodontal disease are amongst the most prevalent global disorders. Their aetiology is rooted in microbial activity within the oral cavity, through the generation of detrimental metabolites and the instigation of potentially adverse host immune responses. Due to the increasing threat of antimicrobial resistance, alternative approaches to readdress the balance are necessary. Advances in sequencing technologies have established relationships between disease and oral dysbiosis, and commercial enterprises seek to identify probiotic and prebiotic formulations to tackle preventable oral disorders through colonisation with, or promotion of, beneficial microbes. It is the metabolic characteristics and immunomodulatory capabilities of resident species which underlie health status. Research emphasis on the metabolic environment of the oral cavity has elucidated relationships between commensal and pathogenic organisms, for example, the sequential metabolism of fermentable carbohydrates deemed central to acid production in cariogenicity. Therefore, a focus on the preservation of an ecological homeostasis in the oral environment may be the most appropriate approach to health conservation. In this review we discuss an ecological approach to the maintenance of a healthy oral environment and debate the potential use of probiotic and prebiotic supplementation, specifically targeted at sustaining oral niches to preserve the delicately balanced microbiome.

Spatiotemporal monitoring of a periodontal multispecies biofilm model: demonstration of prebiotic treatment responses

Biofilms are complex polymicrobial communities which are often associated with human infections such as the oral disease periodontitis. Studying these complex communities under controlled conditions requires in vitro biofilm model systems that mimic the natural environment as close as possible. This study established a multispecies periodontal model in the drip flow biofilm reactor in order to mimic the continuous flow of nutrients at the air-liquid interface in the oral cavity. The design is engineered to enable real-time characterization. A community of five bacteria, Streptococcus gordonii-GFPmut3*, Streptococcus oralis-GFPmut3*, Streptococcus sanguinis-pVMCherry, Fusobacterium nucleatum, and Porphyromonas gingivalis-SNAP26 is visualized using two distinct fluorescent proteins and the SNAP-tag. The biofilm in the reactor develops into a heterogeneous, spatially uniform, dense, and metabolically active biofilm with relative cell abundances similar to those in a healthy individual. Metabolic activity, structural features, and bacterial composition of the biofilm remain stable from 3 to 6 days. As a proof of concept for our periodontal model, the 3 days developed biofilm is exposed to a prebiotic treatment with L-arginine. Multifaceted effects of L-arginine on the oral biofilm were validated by this model setup. L-arginine showed to inhibit growth and incorporation of the pathogenic species and to reduce biofilm thickness and volume. Additionally, L-arginine is metabolized by Streptococcus gordonii-GFPmut3* and Streptococcus sanguinis-pVMCherry, producing high levels of ornithine and ammonium in the biofilm. In conclusion, our drip flow reactor setup is promising in studying spatiotemporal behavior of a multispecies periodontal community.ImportancePeriodontitis is a multifactorial chronic inflammatory disease in the oral cavity associated with the accumulation of microorganisms in a biofilm. Not the presence of the biofilm as such, but changes in the microbiota (i.e., dysbiosis) drive the development of periodontitis, resulting in the destruction of tooth-supporting tissues. In this respect, novel treatment approaches focus on maintaining the health-associated homeostasis of the resident oral microbiota. To get insight in dynamic biofilm responses, our research presents the establishment of a periodontal biofilm model including Streptococcus gordonii, Streptococcus oralis, Streptococcus sanguinis, Fusobacterium nucleatum, and Porphyromonas gingivalis. The added value of the model setup is the combination of simulating continuously changing natural mouth conditions with spatiotemporal biofilm profiling using non-destructive characterization tools. These applications are limited for periodontal biofilm research and would contribute in understanding treatment mechanisms, short- or long-term exposure effects, the adaptation potential of the biofilm and thus treatment strategies.

Incorporation of Arginine to Commercial Orthodontic Light-Cured Resin Cements-Physical, Adhesive, and Antibacterial Properties

(1) Background: The amino acid arginine is now receiving great attention due to its potential anti-caries benefits. The purpose of this in vitro study was to evaluate the shear bond strength (SBS), ultimate tensile strength (UTS), and antimicrobial potential (CFU) of two arginine-containing orthodontic resin cements. (2) Methods: Forty bovine incisors were separated into four groups (n = 10): Orthocem, Orthocem + arginine (2.5 wt%), Transbond XT, and Transbond XT + arginine (2.5 wt%). The brackets were fixed to the flat surface of the enamel, and after 24 h the SBS was evaluated using the universal testing machine (Instron). For the UTS test, hourglass samples (n = 10) were made and tested in a mini-testing machine (OM-100, Odeme). For the antibacterial test (colony forming unit-CFU), six cement discs from each group were made and exposed to Streptococcus mutans UA159 biofilm for 7 days. The microbiological experiment was performed by serial and triplicate dilutions. The data from each test were statistically analyzed using a two-way ANOVA, followed by Tukey's test (α = 0.05). (3) Results: The enamel SBS mean values of Transbond XT were statistically higher than those of Orthocem, both with and without arginine (p = 0.02033). There was no significant difference in the SBS mean values between the orthodontic resin cements, either with or without arginine (p = 0.29869). The UTS of the Transbond XT was statistically higher than the Orthocem, but the addition of arginine at 2.5 wt% did not influence the UTS for either resin cement. The Orthocem + arginine orthodontic resin cement was able to significantly reduce S. mutans growth, but no difference was observed for the Transbond XT (p = 0.03439). (4) Conclusion: The incorporation of arginine to commercial orthodontic resin cements may be an efficient preventive strategy to reduce bacterial growth without compromising their adhesive and mechanical properties.

The role of sugar-rich diet and salivary proteins in dental plaque formation and oral health

BACKGROUND

Dental plaque is a complex colorless film of bacteria that develops on the surfaces of teeth. Different mechanisms of microbial adhesion to tooth surfaces exist. Both non-specific and specific types of adherence have been anticipated.

HIGHLIGHT

The present review evaluated the effect of sugar-rich diet and salivary proteins on oral hygiene and dental plaque development.

CONCLUSION

The oral microbiota is essential for maintaining and reestablishing a healthy oral cavity. Different types of sugars have different effects on the inhibition and formation of dental plaque. The peptides, proteins, and amino acids secreted by parotid glands in the oral cavity facilitate neutralizing the acidity in dental plaque and preventing dental caries. A properly balanced diet is crucial for both a healthy oral cavity and the oral microbiome.

Threats to adhesive/dentin interfacial integrity and next generation bio-enabled multifunctional adhesives

Nearly 100 million of the 170 million composite and amalgam restorations placed annually in the United States are replacements for failed restorations. The primary reason both composite and amalgam restorations fail is recurrent decay, for which composite restorations experience a 2.0-3.5-fold increase compared to amalgam. Recurrent decay is a pernicious problem-the standard treatment is replacement of defective composites with larger restorations that will also fail, initiating a cycle of ever-larger restorations that can lead to root canals, and eventually, to tooth loss. Unlike amalgam, composite lacks the inherent capability to seal discrepancies at the restorative material/tooth interface. The low-viscosity adhesive that bonds the composite to the tooth is intended to seal the interface, but the adhesive degrades, which can breach the composite/tooth margin. Bacteria and bacterial by-products such as acids and enzymes infiltrate the marginal gaps and the composite's inability to increase the interfacial pH facilitates cariogenic and aciduric bacterial outgrowth. Together, these characteristics encourage recurrent decay, pulpal damage, and composite failure. This review article examines key biological and physicochemical interactions involved in the failure of composite restorations and discusses innovative strategies to mitigate the negative effects of pathogens at the adhesive/dentin interface. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B:2466-2475, 2019.

Approaches to Modulate Biofilm Ecology

Dental caries is closely related to a dysbiosis of the microbial consortia of supragingival oral biofilms driven by a sugar-frequent and acidic-pH environment. The pH is a key factor affecting the homeostasis of supragingival biofilms seen in health. There is increasing interest on the ecological dynamics of the oral microbiome and how a dysbiotic microbiota can be successfully replaced by health-beneficial flora. The concept of preventing the microbial dysbiosis related to caries through modulation of sugar intake and pH has fully emerged.

Evaluation of Different Dentifrice Compositions for Increasing the Hardness of Demineralized Enamel: An in Vitro Study

This study aimed to evaluate microhardness of a dentifrice containing fluoride and arginine compared to a positive control (fluoride only) and a negative control (no fluoride) on sound and demineralized bovine enamel surfaces. Specimens were randomly assigned to different treatments that included daily pH cycling and brushing three times a day with one of the following dentifrices (n = 8): Neutraçucar (arginine and fluoride), Colgate Total 12 (fluoride) and My First Colgate (no fluoride). Enamel carious lesions were artificially created one week before the beginning of these treatments (demineralized bovine enamel (DE) groups). The same groups were also tested in sound enamel (sound bovine enamel (SE) groups). Microhardness was measured at baseline and after one, two, and five weeks of treatment using a Knoop indenter. Statistical analysis involved two-way Analysis of Variance (ANOVA) and Tukey’s test. After five weeks, both Total 12 and Neutraçucar had increased the microhardness of DE specimens (p < 0.05). Only Neutraçucar had increased the microhardness of the sound enamel after five weeks of treatment. Thus, it could be concluded that arginine-based dentifrices increase the microhardness of sound and demineralized bovine enamel surfaces.

Modulating pH through lysine integrated dental adhesives

OBJECTIVES

The objective of this study was to explore the effect of lysine integration to dental adhesives with respect to the polymerization kinetics, neutralization capacities in the acidic microenvironment, dynamic mechanical properties, and thermal properties.

MATERIALS AND METHOD

Lysine was incorporated into liquid resin formulations at 2.5 and 5.0wt % with additional water/ethanol co-solvents. The co-monomer system contained 2-hydroxyethyl-methacrylate (HEMA) and Bisphenol A glycerolate dimethacrylate (BisGMA) with a mass ratio of 45/55. The kinetics of photopolymerization, neutralization capacities, lysine-leaching, dynamic mechanical properties and thermal properties of the control and experimental adhesives were analyzed.

RESULTS

The degree of conversion of the experimental adhesive was increased substantially at 2.5wt% lysine as compared to the control. The experimental polymers provided acute neutralization of the acidic microenvironment. Approximately half of the lysine was released from the polymer network within one month. Under dry conditions and physiologic temperatures, the incorporation of lysine did not compromise the storage modulus. Comparison of the thermal properties suggests that the more compact structure of the control adhesive inhibits movement of the polymer chains resulting in increased T_g.

SIGNIFICANCE

Incorporating lysine in the adhesive formulations led to promising results regarding modulating pH, which may serve as one aspect of a multi-spectrum approach for enhancing the durability of composite restorations. The results provide insight and lay a foundation for incorporating amino acids or peptides into adhesive formulations for pH modulation or desired bioactivity at the interfacial margin between the composite and tooth.

Saliva Dysfunction and Oral Microbial Changes among Systemic Lupus Erythematosus Patients with Dental Caries

Systemic lupus erythematosus (SLE) is a chronic autoimmune and inflammatory disease affecting multiorgans of human body. Independent studies show that SLE patients had higher caries prevalence compared to non-SLE individuals. However, the underlying mechanisms remain unclear. In present study, we enrolled SLE patients to explore potential factors contributing to the susceptibility of SLE patients to dental caries (such as oral hygiene, salivary function, and oral microbial community). Dental examination confirmed SLE patients were more vulnerable to caries. Although subjects in both groups announced similar oral hygiene habits, more dental plaque was found on tooth surfaces of SLE patents as revealed by plaque index. In addition, the salivary function was impaired in SLE group as salivary flow rate, buffering capacity, and pH were lower among SLE subjects compared to healthy controls. Importantly, disturbed microbial community with lower richness and diversity was observed in SLE group, as well as disequilibrium between acidogenic/aciduric pathogens and alkali-generating commensal bacteria. Our data suggest that SLE increases patients' sensitivity to dental caries through imposing stress to both host and oral microbes.

Potential Uses of Arginine in Dentistry

Carious lesions develop in tooth surfaces where there is an imbalance of the processes of acid and alkali production by supragingival biofilms. Since low pH is the main driving factor in the development of carious lesions, most efforts to identify an effective anticaries therapy have focused on targeting the acid-producing bacteria and their mechanisms of acid production. An expanding area of oral microbiology has now been devoted to explore microbial metabolic activities that help to neutralize biofilm pH and thus inhibit the caries process. Arginine metabolism via the arginine deiminase pathway (ADS) produces alkali in the form of ammonia that counteracts the effects of biofilm acidification from bacterial glycolysis. ADS also functions as an adaptive strategy used by certain bacteria to thrive in oral biofilms. Substantial evidence accumulated from laboratory and clinical observations supports the hypotheses that measurements of arginine metabolism via ADS may serve as an important caries risk assessment criterion and that providing arginine regularly to supragingival biofilms can be an effective therapy for caries intervention. This article reviews the potential of arginine-based therapies such as the use of arginine as prebiotic, ADS+ strains as probiotics, and oral care formulations containing arginine for prevention and management of dental caries.

Taxonomic and Functional Analyses of the Supragingival Microbiome from Caries-Affected and Caries-Free Hosts

Caries is one of the most prevalent and costly infectious diseases affecting humans of all ages. It is initiated by cariogenic supragingival dental plaques forming on saliva-coated tooth surfaces, yet the etiology remains elusive. To determine which microbial populations may predispose a patient to caries, we report here an in-depth and comprehensive view of the microbial community associated with supragingival dental plaque collected from the healthy teeth of caries patients and healthy adults. We found that microbial communities from caries patients had a higher evenness and inter-individual variations but simpler ecological networks compared to healthy controls despite the overall taxonomic structure being similar. Genera including Selenomonas, Treponema, Atopobium, and Bergeriella were distributed differently between the caries and healthy groups with disturbed co-occurrence patterns. In addition, caries and healthy subjects carried different Treponema, Atopobium, and Prevotella species. Moreover, distinct populations of 13 function genes involved in organic acid synthesis, glycan biosynthesis, complex carbohydrate degradation, amino acid synthesis and metabolism, purine and pyrimidine metabolism, isoprenoid biosynthesis, lipid metabolism, and co-factor biosynthesis were present in each of the healthy and caries groups. Our results suggested that the fundamental differences in dental plaque ecology partially explained the patients' susceptibility to caries, and could be used for caries risk prediction in the future.

A new arginine-based dental adhesive system: formulation, mechanical and anti-caries properties

Secondary caries at the margins of composite restorations has been attributed to adhesive failure and consequent accumulation of cariogenic biofilms.

OBJECTIVES

To develop and evaluate an etch-and-rinse adhesive system containing arginine for sustainable release and recharge without affecting its mechanical properties. Arginine metabolism by oral bacteria generates ammonia, which neutralizes glycolytic acids and creates a neutral environmental pH that is less favorable to the growth of caries pathogens, thus reducing the caries risk at the tooth-composite interface.

METHODS

Experimental adhesives were formulated with methacrylate monomers and arginine at 5%, 7%, and 10% or no arginine (control). Adhesives were tested for: (i) mechanical properties of true stress (FS and UTS), modulus of elasticity (E), degree of conversion (DC), Knoop hardness number (KHN) and dentin microtensile bond strength (μ-TBS), (ii) arginine release and recharge, and (iii) antibacterial activities. Data was analyzed by t-test, one-way ANOVA and Tukey's tests.

RESULTS

FS and UTS results showed no statistically significant differences between the 7% arginine-adhesive and control, while the results for E, DC, KHN and μ-TBS showed no difference among all groups. The 7% arginine-adhesive showed a high release rate of arginine (75.0μmol/cm²) at 2h, and a more sustainable, controlled release rate (up to 0.2μmol/cm²) at 30days.

CONCLUSIONS

Incorporation of 7% arginine did not affect the physical and mechanical properties of the adhesive. Arginine was released from the adhesive at a rate and concentration that exhibited antibacterial effects, regardless of shifts in biofilm conditions such as sugar availability and pH.

CLINICAL SIGNIFICANCE

Secondary caries is recognized as the main reason for failure of dental restorations. The development of an arginine-based adhesive system has the potential to dramatically reduce the incidence and severity of secondary caries in adhesive restorations in a very economical fashion.

Arginine Improves pH Homeostasis via Metabolism and Microbiome Modulation

Dental caries can be described as a dysbiosis of the oral microbial community, in which acidogenic, aciduric, and acid-adapted bacterial species promote a pathogenic environment, leading to demineralization. Alkali generation by oral microbes, specifically via arginine catabolic pathways, is an essential factor in maintaining plaque pH homeostasis. There is evidence that the use of arginine in dentifrices helps protect against caries. The aim of the current study was to investigate the mechanistic and ecological effect of arginine treatment on the oral microbiome and its regulation of pH dynamics, using an in vitro multispecies oral biofilm model that was previously shown to be highly reflective of the in vivo oral microbiome. Pooled saliva from 6 healthy subjects was used to generate overnight biofilms, reflecting early stages of biofilm maturation. First, we investigated the uptake of arginine by the cells of the biofilm as well as the metabolites generated. We next explored the effect of arginine on pH dynamics by pretreating biofilms with 75 mM arginine, followed by the addition of sucrose (15 mM) after 0, 6, 20, or 48 h. pH was measured at each time point and biofilms were collected for 16S sequencing and targeted arginine quantification, and supernatants were prepared for metabolomic analysis. Treatment with only sucrose led to a sustained pH drop from 7 to 4.5, while biofilms treated with sucrose after 6, 20, or 48 h of preincubation with arginine exhibited a recovery to higher pH. Arginine was detected within the cells of the biofilms, indicating active uptake, and arginine catabolites citrulline, ornithine, and putrescine were detected in supernatants, indicating active metabolism. Sequencing analysis revealed a shift in the microbial community structure in arginine-treated biofilms as well as increased species diversity. Overall, we show that arginine improved pH homeostasis through a remodeling of the oral microbial community.

l-Arginine Modifies the Exopolysaccharide Matrix and Thwarts Streptococcus mutans Outgrowth within Mixed-Species Oral Biofilms

l-Arginine, a ubiquitous amino acid in human saliva, serves as a substrate for alkali production by arginolytic bacteria. Recently, exogenous l-arginine has been shown to enhance the alkalinogenic potential of oral biofilm and destabilize its microbial community, which might help control dental caries. However, l-arginine exposure may inflict additional changes in the biofilm milieu when bacteria are growing under cariogenic conditions. Here, we investigated how exogenous l-arginine modulates biofilm development using a mixed-species model containing both cariogenic (Streptococcus mutans) and arginolytic (Streptococcus gordonii) bacteria in the presence of sucrose. We observed that 1.5% (wt/vol) l-arginine (also a clinically effective concentration) exposure suppressed the outgrowth of S. mutans, favored S. gordonii dominance, and maintained Actinomyces naeslundii growth within biofilms (versus vehicle control). In parallel, topical l-arginine treatments substantially reduced the amounts of insoluble exopolysaccharides (EPS) by >3-fold, which significantly altered the three-dimensional (3D) architecture of the biofilm. Intriguingly, l-arginine repressed S. mutans genes associated with insoluble EPS (gtfB) and bacteriocin (SMU.150) production, while spxB expression (H2O2 production) by S. gordonii increased sharply during biofilm development, which resulted in higher H2O2 levels in arginine-treated biofilms. These modifications resulted in a markedly defective EPS matrix and areas devoid of any bacterial clusters (microcolonies) on the apatitic surface, while the in situ pH values at the biofilm-apatite interface were nearly one unit higher in arginine-treated biofilms (versus the vehicle control). Our data reveal new biological properties of l-arginine that impact biofilm matrix assembly and the dynamic microbial interactions associated with pathogenic biofilm development, indicating the multiaction potency of this promising biofilm disruptor.

IMPORTANCE

Dental caries is one of the most prevalent and costly infectious diseases worldwide, caused by a biofilm formed on tooth surfaces. Novel strategies that compromise the ability of virulent species to assemble and maintain pathogenic biofilms could be an effective alternative to conventional antimicrobials that indiscriminately kill other oral species, including commensal bacteria. l-Arginine at 1.5% has been shown to be clinically effective in modulating cariogenic biofilms via alkali production by arginolytic bacteria. Using a mixed-species ecological model, we show new mechanisms by which l-arginine disrupts the process of biofilm matrix assembly and the dynamic microbial interactions that are associated with cariogenic biofilm development, without impacting the bacterial viability. These results may aid in the development of enhanced methods to control biofilms using l-arginine.

Insights into the Virulence Traits of Streptococcus mutans in Dentine Carious Lesions of Children with Early Childhood Caries

Streptococcus mutans is an oral bacterium considered to play a major role in the development of dental caries. This study aimed to investigate the prevalence of S. mutans in active and arrested dentine carious lesions of children with early childhood caries and to examine the expression profile of selected S. mutans genes associated with survival and virulence, within the same carious lesions. Dentine samples were collected from 29 active and 16 arrested carious lesions that were diagnosed in preschool children aged 2-5 years. Total RNA was extracted from the dentine samples, and reverse transcription quantitative real-time PCR analyses were performed for the quantification of S. mutans and for analyses of the expression of S. mutans genes associated with bacterial survival (atpD, nox, pdhA) and virulence (fabM and aguD). There was no statistically significant difference in the prevalence of S. mutans between active and arrested carious lesions. Expression of the tested genes was detected in both types of carious dentine. The pdhA (p = 0.04) and aguD (p = 0.05) genes were expressed at higher levels in arrested as compared to active lesions. Our findings revealed that S. mutans is part of the viable microbial community in active and arrested dentine carious lesions. The increase in expression of the pdhA and aguD genes in arrested lesions is likely due to the unfavourable environmental conditions for microbial growth, inherent to this type of lesions.

A Highly Arginolytic Streptococcus Species That Potently Antagonizes Streptococcus mutans

The ability of certain oral biofilm bacteria to moderate pH through arginine metabolism by the arginine deiminase system (ADS) is a deterrent to the development of dental caries. Here, we characterize a novel Streptococcus strain, designated strain A12, isolated from supragingival dental plaque of a caries-free individual. A12 not only expressed the ADS pathway at high levels under a variety of conditions but also effectively inhibited growth and two intercellular signaling pathways of the dental caries pathogen Streptococcus mutans. A12 produced copious amounts of H2O2 via the pyruvate oxidase enzyme that were sufficient to arrest the growth of S. mutans. A12 also produced a protease similar to challisin (Sgc) of Streptococcus gordonii that was able to block the competence-stimulating peptide (CSP)-ComDE signaling system, which is essential for bacteriocin production by S. mutans. Wild-type A12, but not an sgc mutant derivative, could protect the sensitive indicator strain Streptococcus sanguinis SK150 from killing by the bacteriocins of S. mutans. A12, but not S. gordonii, could also block the XIP (comX-inducing peptide) signaling pathway, which is the proximal regulator of genetic competence in S. mutans, but Sgc was not required for this activity. The complete genome sequence of A12 was determined, and phylogenomic analyses compared A12 to streptococcal reference genomes. A12 was most similar to Streptococcus australis and Streptococcus parasanguinis but sufficiently different that it may represent a new species. A12-like organisms may play crucial roles in the promotion of stable, health-associated oral biofilm communities by moderating plaque pH and interfering with the growth and virulence of caries pathogens.