Cell death in Streptococcus mutans biofilms: a link between CSP and extracellular DNA

Cocktail Approach with Polyserotonin Nanoparticles and Peptides for Treatment of Streptococcus mutans

Dental plaque, formed by a Streptococcus mutans biofilm, is a major contributor to cavity formation. While antimicrobial strategies exist, the growing risk of antibiotic resistance necessitates alternative therapeutic solutions. Polyserotonin nanoparticles (PSeNPs), recently recognized for their photothermal property and promising biomedical applications, open up a new avenue for antimicrobial use. Here, we introduced a UV-initiated synthetic route for PSeNPs with improved yield. Using these PSeNPs, a cocktail treatment to reduce the viability of this cavity-causing bacteria was developed. This cocktail comprises an S. mutans-targeting antimicrobial peptide (GH12), an intraspecies competence-stimulating peptide that triggers altruistic cell death in S. mutans, and laser-activated heating of PSeNPs. The "peptide + PSeNP + laser" combination effectively inhibits S. mutans growth in both planktonic and biofilm states. Moreover, the cocktail approach remains effective in reducing the viability of S. mutans in a more virulent dual-species biofilm with Candida albicans. Overall, our results reinforce the utility of a multipronged therapeutic strategy to reduce cariogenic bacteria in the complex model oral biofilm.

Examination of the Structure and Formation Streptococcus mutans Biofilm Induced by Glucose, Lactose, Soy Protein, and Iron

OBJECTIVE

 Streptococcus mutans, the main causative agent of caries, have the ability to form biofilms on the surface of teeth. The availability of nutrients such as glucose, lactose, soy protein, and iron can influence S. mutans in biofilm formation. All four sources of nutrients have been shown to increase the formation of S. mutans biofilms. The purpose of this study was to determine the structure and thickness of S. mutans biofilms induced by glucose, lactose, soy protein, and iron.

MATERIALS AND METHODS

 This experimental laboratory study aimed to examine the formation of biofilm structures (chemical elements) and determine the thickness of S. mutans biofilms induced by glucose, lactose, soy protein, and iron. The structures (chemical elements) were examined using scanning electron microscopy-energy-dispersive X-ray (SEM-EDX) analysis. Confocal laser scanning microscopy (CLSM) was used to determine the thickness of S. mutans biofilms with an Olympus FV1000 microscope, and the findings were analyzed using Olympus Fluoview Ver. 4.2a software.

RESULTS

 It was established that the results of SEM-EDX examination of the structure of S. mutans biofilms induced by glucose had oxygen (O) as the dominant chemical element (30.24 w%); lactose reported oxygen (O) as the dominant element (29.65 w%); soy protein had carbon (C) as the dominant element (34.31 w%); and iron showed oxygen (O) as the dominant element (32.51 w%). The thickness (measured by the CLSM examination) of biofilms induced by glucose, lactose, soy protein, and iron were 17,666, 12,666, 18,000, and 15,666 nm, respectively.

CONCLUSION

 The structure of S. mutans biofilms induced by glucose, lactose, and iron contain the following elements in amounts from the highest to lowest: O, C, N, P, and S; the biofilm produced by S. mutans induced by soy protein in amounts from the highest to lowest comprised the elements: C, O, N, S, and P. The S. mutans biofilms induced by soy protein had the maximum thickness, followed by those induced by glucose, iron, and lactose.

Comprehensive in vitro and in vivo evaluation of therapeutic potential of Bacopa-derived asiatic acid against a human oral pathogen Streptococcus mutans

Dental caries is a common human oral disease worldwide, caused by an acid-producing bacteria Streptococcus mutans. The use of synthetic drugs and antibiotics to prevent dental caries has been increasing, but this can lead to severe side effects. To solve this issue, developing and developed countries have resorted to herbal medicines as an alternative to synthetic drugs for the treatment and prevention of dental caries. Therefore, there is an urgent need for plant-derived products to treat such diseases. Bacopa monnieri, a well-documented medicinal plant, contains 52 phytocompounds, including the pentacyclic triterpenoid metabolite known as asiatic acid (ASTA). Hence, this study aimed to demonstrate, for the first time, the antibacterial activity of phytocompound ASTA against S. mutans. The findings revealed that ASTA significantly inhibited the growth of S. mutans and the production of virulence factors such as acidurity, acidogenicity, and eDNA synthesis. Molecular docking analysis evaluated the potential activity of ASTA against S. mutans virulence genes, including VicR and GtfC. Furthermore, toxicity assessment of ASTA in human buccal epithelial cells was performed, and no morphological changes were observed. An in vivo analysis using Danio rerio (zebrafish) confirmed that the ASTA treatment significantly increased the survival rates of infected fish by hindering the intestinal colonization of S. mutans. Furthermore, the disease protection potential of ASTA against the pathognomonic symptom of S. mutans infection was proven by the histopathological examination of the gills, gut, and kidney. Overall, these findings suggest that ASTA may be a promising therapeutic and alternative drug for the treatment and prevention of oral infection imposed by S. mutans.

N-acyl homoserine lactone signaling modulates bacterial community associated with human dental plaque

N-acyl homoserine lactones (AHLs) are small diffusible signaling molecules that mediate a cell density-dependent bacterial communication system known as quorum sensing (QS). AHL-mediated QS regulates gene expression to control many critical bacterial behaviors including biofilm formation, pathogenicity, and antimicrobial resistance. Dental plaque is a complex multispecies oral biofilm formed by successive colonization of the tooth surface by groups of commensal, symbiotic, and pathogenic bacteria, which can contribute to tooth decay and periodontal diseases. While the existence and roles of AHL-mediated QS in oral microbiota have been debated, recent evidence indicates that AHLs play significant roles in oral biofilm development and community dysbiosis. The underlying mechanisms, however, remain poorly characterized. To better understand the importance of AHL signaling in dental plaque formation, we manipulated AHL signaling by adding AHL lactonases or exogenous AHL signaling molecules. We find that AHLs can be detected in dental plaque grown under 5% CO2 conditions, but not when grown under anaerobic conditions, and yet anaerobic cultures are still responsive to AHLs. QS signal disruption using lactonases leads to changes in microbial population structures in both planktonic and biofilm states, changes that are dependent on the substrate preference of the used lactonase but mainly result in the increase in the abundance of commensal and pioneer colonizer species. Remarkably, the opposite manipulation, that is the addition of exogenous AHLs increases the abundance of late colonizer bacterial species. Hence, this work highlights the importance of AHL-mediated QS in dental plaque communities, its potential different roles in anaerobic and aerobic parts of dental plaque, and underscores the potential of QS interference in the control of periodontal diseases.

Interspecies competition in oral biofilms mediated by Streptococcus gordonii extracellular deoxyribonuclease SsnA

Extracellular DNA (eDNA) is a key component of many microbial biofilms including dental plaque. However, the roles of extracellular deoxyribonuclease (DNase) enzymes within biofilms are poorly understood. Streptococcus gordonii is a pioneer colonizer of dental plaque. Here, we identified and characterised SsnA, a cell wall-associated protein responsible for extracellular DNase activity of S. gordonii. The SsnA-mediated extracellular DNase activity of S. gordonii was suppressed following growth in sugars. SsnA was purified as a recombinant protein and shown to be inactive below pH 6.5. SsnA inhibited biofilm formation by Streptococcus mutans in a pH-dependent manner. Further, SsnA inhibited the growth of oral microcosm biofilms in human saliva. However, inhibition was ameliorated by the addition of sucrose. Together, these data indicate that S. gordonii SsnA plays a key role in interspecies competition within oral biofilms. Acidification of the medium through sugar catabolism could be a strategy for cariogenic species such as S. mutans to prevent SsnA-mediated exclusion from biofilms.

Antimicrobial Efficiency of Chitosan and Its Methylated Derivative against Lentilactobacillus parabuchneri Biofilms

Antimicrobial materials are considered potential alternatives to prevent the development of biofilm-associated contaminations. Concerns regarding synthetic preservatives necessitate the development of innovative and safe natural antimicrobials. In the present study, we discuss the in situ infrared attenuated total reflection spectroscopy (IR-ATR) investigations of the selective antimicrobial efficiency of chitosan in controlling the growth of Lentilactobacillus parabuchneri biofilms. The protonated charges of chitosan were additionally amplified by structural modification via methylation, yielding quaternized derivative TMC (i.e., N, N, N-trimethyl chitosan). To evaluate antimicrobial effectiveness against L. parab. biofilms, IR-ATR spectroscopy provided information on molecular mechanisms and insights into chemical changes during real-time biofilm inhibition studies. The integrated fiberoptic oxygen microsensors enabled monitoring oxygen (O₂) concentration gradients within biofilms, thereby confirming the metabolic oxygen depletion dropping from 4.5 to 0.7 mg L^-1. IR studies revealed strong electrostatic interactions between chitosan/its water-soluble derivative and bacteria, indicating that a few hours were sufficient to affect biofilm disruption. The significant decrease in the IR bands is related to the characteristic spectral information of amide I, II, III, nucleic acid, and extracellular polymeric matrix (EPS) produced by L. parabuchneri biofilms. Cell clusters of biofilms, microcolonies, and destabilization of the EPS matrix after the addition of biopolymers were visualized using optical microscopy. In addition, scanning electron microscopy (SEM) of biofilms grown on polystyrene and stainless-steel surfaces was used to examine morphological changes, indicating the disintegration of the biofilm matrix into individual cells. Quantification of the total biofilm formation correlated with the CV assay results, indicating cell death and lysis. The electrostatic interactions between chitosan and the bacterial cell wall typically occur between protonated amino groups and negatively charged phospholipids, which promote permeabilization. Biofilm growth inhibition was assessed by a viability assay for a period of 72 h and in the range of low MIC values (varying 0.01-2%). These results support the potential of chitosan and TMC for bacterial growth prevention of the foodborne contaminant L. parabuchneri in the dairy industry and for further implementation in food packaging.

Streptococcus mutans Proteases Degrade Dentinal Collagen

Here, we explored the role of S. mutans’s whole cell and discrete fractions in the degradation of type I collagen and dentinal collagen. Type I collagen gels and human demineralized dentin slabs (DS) were incubated in media alone or with one of the following: overnight (O/N) or newly inoculated (NEW) cultures of S. mutans UA159; intracellular proteins, supernatant or bacterial membranes of O/N cultures. Media from all groups were analyzed for protease-mediated release of the collagen-specific imino acid hydroxyproline. Images of type I collagen and DS were analyzed, respectively. Type I collagen degradation was highest for the supernatant (p < 0.05) fractions, followed by intracellular components and O/N cultures. Collagen degradation for DS samples was highest for O/N samples, followed by supernatant, and intracellular components (p < 0.05). There was lower detectable degradation for both type I collagen and DS from NEW culture samples (p < 0.05), and there was no type I collagen or DS degradation detected for bacterial membrane samples. Structural changes to type I collagen gel and dentinal collagen were observed, respectively, following incubation with S. mutans cultures (O/N and NEW), intracellular components, and supernatant. This study demonstrates that intracellular and extracellular proteolytic activities from S. mutans enable this cariogenic bacterium to degrade type I and dentinal collagen in a growth-phase dependent manner, potentially contributing to the progression of dental caries.

Distinct Agents Induce Streptococcus mutans Cells with Altered Biofilm Formation Capacity

Dental caries is a multifactorial biofilm- and sugar-dependent disease. This study investigated the influence of different agents on the induction of surviving Streptococcus mutans cells after successive treatment cycles and characterized the biofilms formed by these cells recovered posttreatment. The agents (with their main targets listed in parentheses) were compound 1771 (lipoteichoic acids), 4′ hydroxychalcone (exopolysaccharides), myricetin (exopolysaccharides), tt-farnesol (cytoplasmatic membrane), sodium fluoride (enolase—glycolysis), chlorhexidine (antimicrobial), and vehicle. Recovered cells from biofilms were generated from exposure to each agent during 10 cycles of consecutive treatments (modeled on a polystyrene plate bottom). The recovered cell counting was different for each agent. The recovered cells from each group were grown as biofilms on saliva-coated hydroxyapatite discs (culture medium with sucrose/starch). In S. mutans biofilms formed by cells recovered from biofilms previously exposed to compound 1771, 4′ hydroxychalcone, or myricetin, cells presented higher expression of the 16S rRNA, gyrA (DNA replication and transcription), gtfB (insoluble exopolysaccharides), and eno (enolase—glycolysis) genes and lower quantities of insoluble dry weight and insoluble exopolysaccharides than those derived from other agents. These findings were confirmed by the smaller biovolume of bacteria and/or exopolysaccharides and the biofilm distribution (coverage area). Moreover, preexposure to chlorhexidine increased exopolysaccharide production. Therefore, agents with different targets induce cells with distinct biofilm formation capacities, which is critical for developing formulations for biofilm control.

IMPORTANCE This article addresses the effect of distinct agents with distinct targets in the bacterial cell (cytoplasmatic membrane and glycolysis), the cell’s extracellular synthesis of exopolysaccharides that are important for cariogenic extracellular matrix construction and biofilm buildup in the generation of cells that persisted after treatment, and how these cells form biofilms in vitro. For example, if preexposure to an agent augments the production of virulence determinants, such as exopolysaccharides, its clinical value may be inadequate. Modification of biofilm formation capacity after exposure to agents is critical for the development of formulations for biofilm control to prevent caries, a ubiquitous disease associated with biofilm and diet.

Mutations of the brpR and brpS genes affect biofilm formation in Staphylococcus aureus

BACKGROUND

In the United States, Staphylococcus aureus (S. aureus) kills tens of thousands of individuals each year and the formation of a biofilm contributes to lethality. Biofilm-associated infections are hard to treat once the biofilm has formed. A new stilbene drug, labeled SK-03-92, was shown to kill S. aureus and affected transcription of two genes tied to a putative two-component system (TCS) we have named brpR (biofilm regulating protein regulator) and brpS (biofilm regulating protein sensor).

AIM

To determine if BrpR and BrpS regulate biofilm formation, brpR and brpS mutants were assessed using biofilm assays compared to wild-type S. aureus.

METHODS

A combination of biofilm and quantitative real-time-polymerase chain reaction assays were used. In addition, bioinformatic software tools were also utilized.

RESULTS

Significantly more biofilm was created in the brpR and brpS mutants vs wild-type cells. Quantitative real-time polymerase chain reactions showed the brpS mutant had differences in transcription of biofilm associated genes that were eight-fold higher for srtA, two-fold lower for lrgA, and 1.6-fold higher for cidA compared to wild-type. Bioinformatic analysis demonstrated that the S. aureus brpR/brpS TCS had homology to streptococcal late-stage competence proteins involved in cell-death, increased biofilm production, and the development of persister cells.

CONCLUSION

Our study suggests that brpR/brpS is a TCS that may repress S. aureus biofilm production and be linked to late-stage competence in S. aureus.

Understanding the Matrix: The Role of Extracellular DNA in Oral Biofilms

Dental plaque is the key etiological agent in caries formation and the development of the prevalent chronic oral inflammatory disease, periodontitis. The dental plaque biofilm comprises a diverse range of microbial species encased within a rich extracellular matrix, of which extracellular DNA (eDNA) has been identified as an important component. The molecular mechanisms of eDNA release and the structure of eDNA have yet to be fully characterized. Nonetheless, key functions that have been proposed for eDNA include maintaining biofilm structural integrity, initiating adhesion to dental surfaces, acting as a nutrient source, and facilitating horizontal gene transfer. Thus, eDNA is a potential therapeutic target for the management of oral disease–associated biofilm. This review aims to summarize advances in the understanding of the mechanisms of eDNA release from oral microorganisms and in the methods of eDNA detection and quantification within oral biofilms.

Inhibitory Effect of Asplenium incisum on Bacterial Growth, Inflammation, and Osteoclastogenesis

Background and Objectives:Asplenium incisum, a natural plant, is known to possess numerous pharmacological and biochemical properties. However, the inhibitory effect of A. incisum against Porphyromonas gingivalis and other factors related to periodontal disease have not yet been demonstrated. This study aimed to investigate the potential of A. incisum extract as a phytotherapeutic candidate for improving periodontal diseases by assessing its antibacterial, anti-inflammatory, and anti-osteoclastogenic activities. Materials and Methods: The inhibition of proliferation of P. gingivalis by A. incisum and the sustainability of its antibacterial activity were evaluated in this study. The production of inflammatory cytokines (tumor necrosis factor-α (TNF-α) and interleukin-6 (IL-6)) and nitric oxide (NO) from lipopolysaccharide-stimulated RAW 264.7 cells was assessed using an enzyme-linked immunosorbent assay. To identify the anti-osteoclastogenic activity, tartrate-resistant acid phosphatase (TRAP) staining and TRAP activity analyses were performed on bone marrow macrophages. Results: The proliferation of P. gingivalis was significantly inhibited by A. incisum (p < 0.001), and the antibacterial activity was sustained for up to 3 days. A. incisum showed anti-inflammatory activities by significantly decreasing the release of TNF-α, IL-6 (p < 0.05), and NO (p < 0.01). In addition, A. incisum significantly suppressed TRAP-positive cells and TRAP activity (at 30 μg/mL, p < 0.01) without causing any cytotoxicity (p > 0.05). Conclusions:A. incisum showed antibacterial, anti-inflammatory, and anti-osteoclastogenic activities, suggesting it has strong therapeutic potential against periodontal diseases.

Mucin O-glycans suppress quorum-sensing pathways and genetic transformation in Streptococcus mutans

Mucus barriers accommodate trillions of microorganisms throughout the human body while preventing pathogenic colonization¹. In the oral cavity, saliva containing the mucins MUC5B and MUC7 forms a pellicle that coats the soft tissue and teeth to prevent infection by oral pathogens, such as Streptococcus mutans². Salivary mucin can interact directly with microorganisms through selective agglutinin activity and bacterial binding^2-4, but the extent and basis of the protective functions of saliva are not well understood. Here, using an ex vivo saliva model, we identify that MUC5B is an inhibitor of microbial virulence. Specifically, we find that natively purified MUC5B downregulates the expression of quorum-sensing pathways activated by the competence stimulating peptide and the sigX-inducing peptide⁵. Furthermore, MUC5B prevents the acquisition of antimicrobial resistance through natural genetic transformation, a process that is activated through quorum sensing. Our data reveal that the effect of MUC5B is mediated by its associated O-linked glycans, which are potent suppressors of quorum sensing and genetic transformation, even when removed from the mucin backbone. Together, these results present mucin O-glycans as a host strategy for domesticating potentially pathogenic microorganisms without killing them.

Biofilm Formation by Streptococcus mutans is Enhanced by Indole via the Quorum Sensing Pathway

Interspecies interactions among oral microorganisms in the pathogenic biofilms causing dental caries have not yet been elucidated in detail. We herein demonstrated that indole and its derivatives induced biofilm formation by Streptococcus mutans. Indole is an intercellular signaling molecule that is produced by oral bacteria other than S. mutans. The amounts of biofilm and extracellular DNA were significantly increased by the addition of indole and 4-hydroxyindole (4-HI). An examination with quorum sensing mutants showed that the induction of biofilm formation by indole and 4-HI required a quorum sensing system. These results suggest that this intercellular signaling molecule plays a role in pathogenic biofilm formation.

Rapid Isolation and Purification of Secreted Bacteriocins from Streptococcus mutans and Other Lactic Acid Bacteria

Bacteriocins are small ribosomally synthesized antimicrobial peptides produced by some microorganisms including lactic acid bacteria (LAB), a group of Gram-positive bacteria (cocci, rods) expressing high tolerance for low pH. Bacteriocins kill bacteria rapidly and are biologically active at very low concentrations. Bacteriocins produced by LAB are primarily active against closely related bacterial species. Many bacteriocins have been investigated with respect to their potential use in promoting human, plant, and animal health, and as food biopreservatives. Bacteriocins produced by LAB are particularly interesting since several LAB have been granted GRAS (Generally Recognized as Safe) status. Because it is not always possible to extract active bacteriocins secreted from cells grown in liquid medium, we developed a simple and inexpensive peptide extraction procedure using a semi-solid nutrient-rich agar medium. We hereby present a detailed procedure that leads to the rapid extraction of secreted bioactive bacteriocin peptides from the oral species Streptococcus mutans, a prolific bacteriocin-producing species, and its potential application for bacteriocin extraction from other LAB (e.g., Streptococcus, Lactococcus, Enterococcus). We also present a simple method for the detection of bacteriocin activity from the purified extracellular peptide extract.

Peptides encoded in the Streptococcus mutans RcrRPQ operon are essential for thermotolerance

The MarR-like transcriptional regulator and two ABC transporters encoded by the rcrRPQ operon in the dental caries pathogen Streptococcus mutans have important regulatory roles related to oxidative stress tolerance, genetic competence and (p)ppGpp metabolism. A unique feature of the rcrRPQ operon, when compared to other bacteria, is the presence of two peptides, designated Pep1 and Pep2, encoded in alternative reading frames at the 3' end of rcrQ. Here, we show that the rcrRPQ operon, including Pep1 and 2, is essential for S. mutans to survive and maintain viability at elevated temperatures. No major changes in the levels of the heat shock proteins DnaK or GroEL that could account for the thermosensitivity of rcrRPQ mutants were observed. By introducing a single amino acid substitution into the comX gene that deletes an internally encoded peptide, XrpA, we found that XrpA is a contributing factor to the thermosensitive phenotype of a ΔrcrR strain. Overexpression of XrpA on a plasmid also caused a significant growth defect at 42 °C. Interestingly, loss of the gene for the RelA/SpoT homologue (RSH) enzyme, relA, restored growth of the ΔrcrR strain at 42 °C. During heat stress and when a stringent response was induced, levels of (p)ppGpp were elevated in the ΔrcrR strain. Deletion of relA in the ΔrcrR strain lowered the basal levels of (p)ppGpp to those observed in wild-type S. mutans. Thus, (p)ppGpp pools are dysregulated in ΔrcrR, which likely leads to aberrant control of transcriptional/translational processes and the thermosensitive phenotype. In summary, the genes and peptides encoded in the rcrRPQ operon are critical for thermotolerance, and in some strains these phenotypes are related to altered (p)ppGpp metabolism and increased production of the XrpA peptide.

Antimicrobial antidegradative dental adhesive preserves restoration-tooth bond

OBJECTIVE

Assess the ability of an antimicrobial drug-releasing resin adhesive, containing octenidine dihydrochloride (OCT)-silica co-assembled particles (DSPs), to enhance the biostability and preserve the interfacial fracture toughness (FT) of composite restorations bonded to dentin. Enzyme-catalyzed degradation compromises the dental restoration-tooth interface, increasing cariogenic bacterial infiltration. In addition to bacterial ingress inhibition, antimicrobial-releasing adhesives may exhibit direct interfacial biodegradation inhibition as an additional benefit.

METHODS

Mini short-rod restoration bonding specimens with total-etch adhesive with/without 10% wt. DSPs were made. Interfacial fracture toughness (FT) was measured as-manufactured or post-incubation in simulated human salivary esterase (SHSE) for up to 6-months. Effect of OCT on SHSE and whole saliva/bacterial enzyme activity was assessed. Release of OCT outside the restoration interface was assessed.

RESULTS

No deleterious effect of DSPs on initial bonding capacity was observed. Aging specimens in SHSE reduced FT of control but not DSP-adhesive-bonded specimens. OCT inhibited SHSE degradation of adhesive monomer and may inhibit endogenous proteases. OCT inhibited bacterial esterase and collagenase. No endogenous collagen breakdown was detected in the present study. OCT increased human saliva degradative esterase activity below its minimum inhibitory concentration towards S. mutans (MIC), but inhibited degradation above MIC. OCT release outside restoration margins was below detection.

SIGNIFICANCE

DSP-adhesive preserves the restoration bond through a secondary enzyme-inhibitory effect of released OCT, which is virtually confined to the restoration interface microgap. Enzyme activity modulation may produce a positive-to-negative feedback switch, by increasing OCT concentration via biodegradation-triggered release to an effective dose, then subsequently slowing degradation and degradation-triggered release.

Competence-Stimulating-Peptide-Dependent Localized Cell Death and Extracellular DNA Production in Streptococcus mutans Biofilms

Extracellular DNA (eDNA) is a biofilm component that contributes to the formation and structural stability of biofilms. Streptococcus mutans, a major cariogenic bacterium, induces eDNA-dependent biofilm formation under specific conditions. Since cell death can result in the release and accumulation of DNA, the dead cells in biofilms are a source of eDNA. However, it remains unknown how eDNA is released from dead cells and is localized within S. mutans biofilms. We focused on cell death induced by the extracellular signaling peptide called competence-stimulating peptide (CSP). We demonstrate that nucleic acid release into the extracellular environment occurs in a subpopulation of dead cells. eDNA production induced by CSP was highly dependent on the lytF gene, which encodes an autolysin. Although lytF expression was induced bimodally by CSP, lytF-expressing cells further divided into surviving cells and eDNA-producing dead cells. Moreover, we found that lytF-expressing cells were abundant near the bottom of the biofilm, even when all cells in the biofilm received the CSP signal. Dead cells and eDNA were also abundantly present near the bottom of the biofilm. The number of lytF-expressing cells in biofilms was significantly higher than that in planktonic cultures, which suggests that adhesion to the substratum surface is important for the induction of lytF expression. The deletion of lytF resulted in reduced adherence to a polystyrene surface. These results suggest that lytF expression and eDNA production induced near the bottom of the biofilm contribute to a firmly attached and structurally stable biofilm.IMPORTANCE Bacterial communities encased by self-produced extracellular polymeric substances (EPSs), known as biofilms, have a wide influence on human health and environmental problems. The importance of biofilm research has increased, as biofilms are the preferred bacterial lifestyle in nature. Furthermore, in recent years it has been noted that the contribution of phenotypic heterogeneity within biofilms requires analysis at the single-cell or subpopulation level to understand bacterial life strategies. In Streptococcus mutans, a cariogenic bacterium, extracellular DNA (eDNA) contributes to biofilm formation. However, it remains unclear how and where the cells produce eDNA within the biofilm. We focused on LytF, an autolysin that is induced by extracellular peptide signals. We used single-cell level imaging techniques to analyze lytF expression in the biofilm population. Here, we show that S. mutans generates eDNA by inducing lytF expression near the bottom of the biofilm, thereby enhancing biofilm adhesion and structural stability.

Modulation of Lipoteichoic Acids and Exopolysaccharides Prevents Streptococcus mutans Biofilm Accumulation

Dental caries is a diet–biofilm-dependent disease. Streptococcus mutans contributes to cariogenic biofilms by producing an extracellular matrix rich in exopolysaccharides and acids. The study aimed to determine the effect of topical treatments with compound 1771 (modulates lipoteichoic acid (LTA) metabolism) and myricetin (affects the synthesis of exopolysaccharides) on S. mutans biofilms. In vitro S. mutans UA159 biofilms were grown on saliva-coated hydroxyapatite discs, alternating 0.1% sucrose and 0.5% sucrose plus 1% starch. Twice-daily topical treatments were performed with both agents alone and combined with and without fluoride: compound 1771 (2.6 µg/mL), myricetin (500 µg/mL), 1771 + myricetin, fluoride (250 ppm), 1771 + fluoride, myricetin + fluoride, 1771 + myricetin + fluoride, and vehicle. Biofilms were evaluated via microbiological, biochemical, imaging, and gene expression methods. Compound 1771 alone yielded less viable counts, biomass, exopolysaccharides, and extracellular LTA. Moreover, the combination 1771 + myricetin + fluoride decreased three logs of bacterium counts, 60% biomass, >74% exopolysaccharides, and 20% LTA. The effect of treatments on extracellular DNA was not pronounced. The combination strategy affected the size of microcolonies and exopolysaccharides distribution and inhibited the expression of genes linked to insoluble exopolysaccharides synthesis. Therefore, compound 1771 prevented the accumulation of S. mutans biofilm; however, the effect was more pronounced when it was associated with fluoride and myricetin.

Proteolytic Activity of Bacillus subtilis upon κ-Casein Undermines Its "Caries-Safe" Effect

Milk is believed to be a relatively “caries-safe” food. This belief relies on the fact that caseins, which constitute around 80% of milk’s protein content, were found to inhibit the adhesion of Streptococcus mutans to enamel and, therefore, decrease biofilm formation. While S. mutans is considered a leading cause of dental disorders, Bacillus subtilis is a non-pathogenic foodborne bacterium, frequently contaminating milk and its products. This study aimed to investigate the effects of dairy-associated foodborne bacteria such as B. subtilis on biofilm formation by S. mutans in the presence of casein proteins. Our results indicate that there is a significant decrease in total biofilm formation by S. mutans exposed to a casein protein mixture in a mono-species culture, whereas, in the co-culture with B. subtilis, an inhibitory effect of the caseins mixture on S. mutans biofilm formation was observed. Proteolytic activity analysis suggested that B. subtilis is capable of breaking down milk proteins, especially κ-casein, which enables biofilm formation by S. mutans in the presence of milk caseins. Therefore, these findings may challenge the assumption that milk is “caries-safe”, especially in a complex microbial environment.

Short-Chain N-Acylhomoserine Lactone Quorum-Sensing Molecules Promote Periodontal Pathogens in In Vitro Oral Biofilms

Acylhomoserine lactones (AHLs), the quorum-sensing (QS) signals produced by a range of Gram-negative bacteria, are involved in biofilm formation in many pathogenic and environmental bacteria. Nevertheless, the current paradigm excludes a role of AHLs in dental plaque formation, while other QS signals, such as AI-2 and autoinducer peptides, have been demonstrated to play an important role in biofilm formation and virulence-related gene expression in oral pathogens. In the present work, we have explored the effect of externally added AHLs on in vitro oral biofilm models for commensal, cariogenic, and periodontal dental plaque. While little effect on bacterial growth was observed, some AHLs specifically affected the lactic acid production and protease activity of the biofilms. Most importantly, the analysis of bacterial diversity in the biofilms showed that the addition of C₆-homoserine lactone (C₆-HSL) results in a shift toward a periodontal bacterial composition profile by increasing the relative presence of the orange-complex bacteria Peptostreptococcus and Prevotella These results point to a relevant role of AHL-mediated QS in dental plaque formation and might be involved in the development of dysbiosis, the mechanism of which should be further investigated. This finding potentially opens new opportunities for the prevention or treatment of the periodontal disease.IMPORTANCE Dental plaque is omnipresent in healthy oral cavities and part of our commensal microbial colonization. At the same time, dental plaque is the cause of the most common human diseases, caries and gum disease. Dental plaque consists of billions of microbes attached to the surface of your teeth. Communication among these microbes is pivotal for development of these complex communities yet poorly studied in dental plaque. In the present study, we show that a specific communication molecule induces changes within the community related to the development of gum disease. This finding suggests that interfering with microbial communication may represent an interesting novel strategy to prevent gum disease that should be further investigated.

Transcriptome, Phenotypic, and Virulence Analysis of Streptococcus sanguinis SK36 Wild Type and Its CcpA-Null Derivative (ΔCcpA)

Catabolic control protein (CcpA) is linked to complex carbohydrate utilization and virulence factor in many bacteria species, influences the transcription of target genes by many mechanisms. To characterize the activity and regulatory mechanisms of CcpA in Streptococcus sanguinis, here, we analyzed the transcriptome of Streptococcus sanguinis SK36 and its CcpA-null derivative (ΔCcpA) using RNA-seq. Compared to the regulon of CcpA in SK36 in the RegPrecise database, we found that only minority of differentially expressed genes (DEGs) contained putative catabolite response element (cre) in their regulatory regions, indicating that many genes could have been affected indirectly by the loss of CcpA and analyzing the sequence of the promoter region using prediction tools is not a desirable method to recognize potential target genes of global regulator CcpA. Gene ontology and pathway analysis of DEGs revealed that CcpA exerts an influence predominantly involved in carbon catabolite metabolism and some amino acid catabolite pathways, which has been linked to expression of virulence genes in many pathogens and coordinately regulate the disease progression in vivo studies. However, in some scenarios, differences observed at the transcript level could not reflect the real differences at the protein level. Therefore, to confirm the differences in phenotype and virulence of SK36 and ΔCcpA, we characterized the role of CcpA in the regulation of biofilm development, EPS production and the virulence of Streptococcus sanguinis. Results showed CcpA inactivation impaired biofilm and EPS formation, and CcpA also involved in virulence in rabbit infective endocarditis model. These findings will undoubtedly contribute to investigate the mechanistic links between the global regulator CcpA and the virulence of Streptococcus sanguinis, further broaden our understanding of the relationship between basic metabolic processes and virulence.

Genomic instability of TnSMU2 contributes to Streptococcus mutans biofilm development and competence in a cidB mutant

Streptococcus mutans is a key pathogenic bacterium in the oral cavity and a primary contributor to dental caries. The S. mutans Cid/Lrg system likely contributes to tolerating stresses encountered in this environment as cid and/or lrg mutants exhibit altered oxidative stress sensitivity, genetic competence, and biofilm phenotypes. It was recently noted that the cidB mutant had two stable colony morphologies: a “rough” phenotype (similar to wild type) and a “smooth” phenotype. In our previously published work, the cidB rough mutant exhibited increased sensitivity to oxidative stress, and RNAseq identified widespread transcriptomic changes in central carbon metabolism and oxidative stress response genes. In this current report, we conducted Illumina‐based genome resequencing of wild type, cidB rough, and cidB smooth mutants and compared their resistance to oxidative and acid stress, biofilm formation, and competence phenotypes. Both cidB mutants exhibited comparable aerobic growth inhibition on agar plates, during planktonic growth, and in the presence of 1 mM hydrogen peroxide. The cidB smooth mutant displayed a significant competence defect in BHI, which was rescuable by synthetic CSP. Both cidB mutants also displayed reduced XIP‐mediated competence, although this reduction was more pronounced in the cidB smooth mutant. Anaerobic biofilms of the cidB smooth mutant displayed increased propidium iodide staining, but corresponding biofilm CFU data suggest this phenotype is due to cell damage and not increased cell death. The cidB rough anaerobic biofilms showed altered structure relative to wild type (reduced biomass and average thickness) which correlated with decreased CFU counts. Sequencing data revealed that the cidB smooth mutant has a unique “loss of read coverage” of ~78 kb of DNA, corresponding to the genomic island TnSMU2 and genes flanking its 3′ end. It is therefore likely that the unique biofilm and competence phenotypes of the cidB smooth mutant are related to its genomic changes in this region.

Effects of Complex DNA and MVs with GTF Extracted from Streptococcus mutans on the Oral Biofilm

Streptococcus mutans is one of the principal pathogens for the development of dental caries. Oral biofilms formed by S. mutans are constructed of insoluble glucan formation induced by the principal enzymes, GTF-I and GTF-SI, in sucrose-containing conditions. However, as another means of biofilm formation, extracellular DNA (eDNA) and membrane vesicles (MVs) are also contributors. To explore the roles of eDNA and MVs for biofilm formation, short and whole size pure DNAs, two types of sub-purified DNAs and MVs were extracted from S. mutans by beads destruction, treatment of proteinase K, and ultracentrifugation of culture supernatant, and applied into the biofilm formation assay using the S. mutans UA159 gtfBC mutant, which lost GTF-I and GTF-SI, on a human saliva-coated 96 well microtiter plate in sucrose-containing conditions. Sub-purified DNAs after cell lysis by beads destruction for total 90 and 180 s showed a complex form of short-size DNA with various proteins and MVs associated with GTF-I and GTF-SI, and induced significantly higher biofilm formation of the S. mutans UA159.gtfBC mutant than no sample (p < 0.05). Short-size pure DNA without proteins induced biofilm formation but whole-size pure DNA did not. Moreover, the complex form of MV associated with GTFs and short-size DNA showed significantly higher biofilm formation of initial colonizers on the human tooth surface such as Streptococcus mitis than no sample (p < 0.05). The short-size DNAs associated with MVs and GTFs are important contributors to the biofilm formation and may be one of additional targets for the prevention of oral biofilm-associated diseases.

Suicidal selection: Programmed cell death can evolve in unicellular organisms due solely to kin selection

ABSTRACT

Unicellular organisms can engage in a process by which a cell purposefully destroys itself, termed programmed cell death (PCD). While it is clear that the death of specific cells within a multicellular organism could increase inclusive fitness (e.g., during development), the origin of PCD in unicellular organisms is less obvious. Kin selection has been shown to help maintain instances of PCD in existing populations of unicellular organisms; however, competing hypotheses exist about whether additional factors are necessary to explain its origin. Those factors could include an environmental shift that causes latent PCD to be expressed, PCD hitchhiking on a large beneficial mutation, and PCD being simply a common pathology. Here, we present results using an artificial life model to demonstrate that kin selection can, in fact, be sufficient to give rise to PCD in unicellular organisms. Furthermore, when benefits to kin are direct-that is, resources provided to nearby kin-PCD is more beneficial than when benefits are indirect-that is, nonkin are injured, thus increasing the relative amount of resources for kin. Finally, when considering how strict organisms are in determining kin or nonkin (in terms of mutations), direct benefits are viable in a narrower range than indirect benefits.

OPEN RESEARCH BADGES

This article has been awarded Open Data and Open Materials Badges. All materials and data are publicly accessible via the Open Science Framework at https://github.com/anyaevostinar/SuicidalAltruismDissertation/tree/master/LongTerm.

Theaflavin-3,3'-Digallate Suppresses Biofilm Formation, Acid Production, and Acid Tolerance in Streptococcus mutans by Targeting Virulence Factors

As one of the most important cariogenic pathogens, Streptococcus mutans has strong abilities to form biofilms, produce acid and tolerate acid. In present study, we found that theaflavin-3,3′-digallate (TF3) had an inhibitory effect on S. mutans UA159 in vitro. Visualized by field emission-scanning electron microscopy, the suppressed formation of S. mutans biofilms grown with TF3 at sub-inhibitory concentrations could be attributed to the reduced biofilm matrix, which was proven to contain glucans and extracellular DNA (eDNA). Glucan-reduced effect of TF3 was achieved by down-regulating expression levels of gtfB, gtfC, and gtfD encoding glucosyltransferases. Besides, TF3 reduced eDNA formation of S. mutans by negatively regulating lrgA, lrgB, and srtA, which govern cell autolysis and membrane vesicle components. Furthermore, TF3 also played vital roles in antagonizing preformed biofilms of S. mutans. Bactericidal effects of TF3 became significant when its concentrations increased more than twofold of minimum inhibitory concentration (MIC). Moreover, the capacities of S. mutans biofilms to produce acid and tolerate acid were significantly weakened by TF3 at MIC. Based on real-time PCR (RT-PCR) analysis, the mechanistic effects of TF3 were speculated to comprise the inhibition of enolase, lactate dehydrogenase, F-type ATPase and the agmatine deiminase system. Moreover, TF3 has been found to downregulate LytST, VicRK, and ComDE two component systems in S. mutans, which play critical roles in the regulatory network of virulence factors. Our present study found that TF3 could suppress the formation and cariogenic capacities of S. mutans biofilms, which will provide new strategies for anti-caries in the future.

Effects of 7S globulin 3 derived from the adzuki bean [Vigna angularis] on the CSP- and eDNA- dependent biofilm formation of Streptococcus mutans

OBJECTIVE

Streptococcus mutans is a principal bacterium that forms pathogenic biofilm involved in the development of dental caries. S. mutans possesses a quorum sensing system (QS) stimulated by competence stimulating peptide (CSP), which is associated with bacteriocin production, genetic competency and biofilm formation. Inhibiting CSP-dependent QS is one of the aims leading to the inhibition of biofilm formation and is useful for establishing new prevention systems for dental caries.

DESIGN

In this study, we selected adzuki bean [Vigna angularis] extract as a candidate component to inhibit CSP-dependent biofilm formation among various foods. To purify an inhibitory component from the adzuki extracts, we performed the salting-out method, two rounds of ion-exchange chromatography, and SDS and native PAGE.

RESULTS

A primary protein band that inhibits CSP-dependent biofilm formation appeared at approximately 50 kDa and was identified as 7S globulin 3 (7S3), a major seed storage protein in adzuki bean. To determine the characteristics of 7S3 as an inhibitory component, aggregated proteins were extracted from the adzuki crude extracts at pH values lower than 6. The aggregated proteins inhibited CSP- and eDNA-dependent biofilm formation and showed 50 kDa band, which is identical with 7S3 in the purified sample. Moreover, 7S globulin 3 in the adzuki bean extract directly interacted with CSP at low pH conditions but not at neutral conditions, and inhibited CSP-dependent bacteriocin production.

CONCLUSION

It was suggested that 7S3 might be a safe and useful material to prevent pathogenic activities in the biofilm formation of S. mutans.

Extracellular DNA enhances the formation and stability of symplasmata in Pantoea agglomerans YS19

Extracellular DNA (eDNA) is an important polymeric substance that plays essential roles in cell aggregation and nutrient provision for the sessile bacteria. eDNA in bacterial biofilms was extensively studied. Here we found that eDNA also exists in symplasmata, a bacterial cell aggregate, which is different to a biofilm, in the rice enophyte Pantoea agglomerans YS19. We found that exogenous eDNA enhanced the formation and stability of symplasmata significantly, and that, exogenous eDNA also improved the stress resistance and colonization ability of the bacterium on host rice. These results strongly indicate novel roles of the eDNA in Pantoea agglomerans YS19, showing its special relation to the stress-resistance and endophyte-host association of the strain.

Glycosyltransferase-Mediated Biofilm Matrix Dynamics and Virulence of Streptococcus mutans

Streptococcus mutans is a key cariogenic bacterium responsible for the initiation of tooth decay. Biofilm formation is a crucial virulence property. We discovered a putative glycosyltransferase, SMU_833, in S. mutans capable of modulating dynamic interactions between two key biofilm matrix components, glucan and extracellular DNA (eDNA). The deletion of smu_833 decreases glucan and increases eDNA but maintains the overall biofilm biomass. The decrease in glucan is caused by a reduction in GtfB and GtfC, two key enzymes responsible for the synthesis of glucan. The increase in eDNA was accompanied by an elevated production of membrane vesicles, suggesting that SMU_833 modulates the release of eDNA via the membrane vesicles, thereby altering biofilm matrix constituents. Furthermore, glucan and eDNA were colocalized. The complete deletion of gtfBC from the smu_833 mutant significantly reduced the biofilm biomass despite the elevated eDNA, suggesting the requirement of minimal glucans as a binding substrate for eDNA within the biofilm. Despite no changes in overall biofilm biomass, the mutant biofilm was altered in biofilm architecture and was less acidic in vitro Concurrently, the mutant was less virulent in an in vivo rat model of dental caries, demonstrating that SMU_833 is a new virulence factor. Taken together, we conclude that SMU_833 is required for optimal biofilm development and virulence of S. mutans by modulating extracellular matrix components. Our study of SMU_833-modulated biofilm matrix dynamics uncovered a new target that can be used to develop potential therapeutics that prevent and treat dental caries.IMPORTANCE Tooth decay, a costly and painful disease affecting the vast majority of people worldwide, is caused by the bacterium Streptococcus mutans The bacteria utilize dietary sugars to build and strengthen biofilms, trapping acids onto the tooth's surface and causing demineralization and decay of teeth. As knowledge of our body's microbiomes increases, the need for developing therapeutics targeted to disease-causing bacteria has arisen. The significance of our research is in studying and identifying a novel therapeutic target, a dynamic biofilm matrix that is mediated by a new virulence factor and membrane vesicles. The study increases our understanding of S. mutans virulence and also offers a new opportunity to develop effective therapeutics targeting S. mutans In addition, the mechanisms of membrane vesicle-mediated biofilm matrix dynamics are also applicable to other biofilm-driven infectious diseases.

Hidden Gems in the Transcriptome Maps of Competent Streptococci

Natural transformation is regarded as an important mechanism in bacteria that allows for adaptation to different environmental stressors by ensuring genome plasticity. Since the discovery of this phenomenon in Streptococcus pneumoniae, remarkable progress has been made in the understanding of the molecular mechanisms and pathways coordinating this process. Recently, the advent of high-throughput sequencing allows the posing of questions that address the system at a larger scale but also allow for the creation of high-resolution maps of transcription. Thus, while much is already known about genetic competence in streptococci, recent studies continue to reveal intricate novel regulation pathways and components. In this perspective article, we highlight the use of transcriptional profiling and mapping as a valuable resource in the identification and characterization of “hidden gems” pertinent to the natural transformation system. Such strategies have recently been employed in a variety of different species. In S. mutans, for example, genome editing combined with the power of promoter mapping and RNA-Seq allowed for the identification of a link between the ComCDE and the ComRS systems, a ComR positive feedback loop mediated by SigX, and the XrpA peptide, encoded within sigX, which inhibits competence. In S. pneumoniae, a novel member of the competence regulon termed BriC was found to be directly under control of ComE and to promote biofilm formation and nasopharyngeal colonization but not competence. Together these new technologies enable us to discover new links and to revisit old pathways in the compelling study of natural genetic transformation.

Intracellular Signaling by the comRS System in Streptococcus mutans Genetic Competence

Entry into genetic competence in streptococci is controlled by ComX, an alternative sigma factor for genes that enable the import of exogenous DNA. In Streptococcus mutans, the immediate activator of comX is the ComRS quorum system. ComS is the precursor of XIP, a seven-residue peptide that is imported into the cell and interacts with the cytosolic receptor ComR to form a transcriptional activator for both comX and comS Although intercellular quorum signaling by ComRS has been demonstrated, observations of bimodal expression of comX suggest that comRS may also function as an intracellular feedback loop, activating comX without export or detection of extracellular XIP. Here we used microfluidic and single-cell methods to test whether ComRS induction of comX requires extracellular XIP or ComS. We found that individual comS-overexpressing cells activate their own comX, independently of the rate at which their growth medium is replaced. However, in the absence of lysis they do not activate comS-deficient mutants growing in coculture. We also found that induction of comR and comS genes introduced into Escherichia coli cells leads to activation of a comX reporter. Therefore, ComRS control of comX does not require either the import or extracellular accumulation of ComS or XIP or specific processing of ComS to XIP. We also found that endogenously and exogenously produced ComS and XIP have inequivalent effects on comX activation. These data are fully consistent with identification of intracellular positive feedback in comS transcription as the origin of bimodal comX expression in S. mutans IMPORTANCE The ComRS system can function as a quorum sensing trigger for genetic competence in S. mutans The signal peptide XIP, which is derived from the precursor ComS, enters the cell and interacts with the Rgg-type cytosolic receptor ComR to activate comX, which encodes the alternative sigma factor for the late competence genes. Previous studies have demonstrated intercellular signaling via ComRS, although release of the ComS or XIP peptide to the extracellular medium appears to require lysis of the producing cells. Here we tested the complementary hypothesis that ComRS can drive comX through a purely intracellular mechanism that does not depend on extracellular accumulation or import of ComS or XIP. By combining single-cell, coculture, and microfluidic approaches, we demonstrated that endogenously produced ComS can enable ComRS to activate comX without requiring processing, export, or import. These data provide insight into intracellular mechanisms that generate noise and heterogeneity in S. mutans competence.

Threshold regulation and stochasticity from the MecA/ClpCP proteolytic system in Streptococcus mutans competence

Many bacterial species use the MecA/ClpCP proteolytic system to block entry into genetic competence. In Streptococcus mutans, MecA/ClpCP degrades ComX (also called SigX), an alternative sigma factor for the comY operon and other late competence genes. Although the mechanism of MecA/ClpCP has been studied in multiple Streptococcus species, its role within noisy competence pathways is poorly understood. S. mutans competence can be triggered by two different peptides, CSP and XIP, but it is not known whether MecA/ClpCP acts similarly for both stimuli, how it affects competence heterogeneity, and how its regulation is overcome. We have studied the effect of MecA/ClpCP on the activation of comY in individual S. mutans cells. Our data show that MecA/ClpCP is active under both XIP and CSP stimulation, that it provides threshold control of comY, and that it adds noise in comY expression. Our data agree quantitatively with a model in which MecA/ClpCP prevents adventitious entry into competence by sequestering or intercepting low levels of ComX. Competence is permitted when ComX levels exceed a threshold, but cell‐to‐cell heterogeneity in MecA levels creates variability in that threshold. Therefore, MecA/ClpCP provides a stochastic switch, located downstream of the already noisy comX, that enhances phenotypic diversity.

Esterase from a cariogenic bacterium hydrolyzes dental resins

OBJECTIVES

To identify and characterize specific esterases from S. mutans with degradative activity toward methacrylate-based resin monomers.

METHODS

Out of several putative esterases, an esterase encoded in an Open Reading Frame as SMU_118c (The National Center for Biotechnology Information, NCBI), was found to have true hydrolase activities. SMU_118c was cloned, expressed, purified and further characterized for its respective hydrolytic activity towards ester-containing nitrophenyl substrates and the universal resin monomers bis-phenyl-glycidyl-dimethacrylate (bisGMA) and triethyleneglycol dimethacrylate (TEGDMA) at neutral (7.0) or cariogenic (5.5) pH. Mass spectrometry (MS) was used to verify the expression of SMU_118c protein in S. mutans UA159.

RESULTS

Similar to the whole cell activity of S. mutans, SMU_118c showed the highest affinity toward para-nitrophenyl acetate (pNPA) and para-nitrophenyl butyrate (pNPB) vs. ortho-nitrophenyl butyrate (oNPB) and butyrylthiocholine iodide (BTC) (p < 0.05). The esterase retained 60% of its activity after 21 days and hydrolyzed bisGMA at a higher rate than TEGDMA at both neutral and cariogenic pH (p < 0.001), similarly to the predominant human salivary esterase degradative activity. MS confirmed that SMU_118c is an intracellular protein in S. mutans UA159 and expressed under pathogenic (pH 5.5) growth conditions.

SIGNIFICANCE

The similarity in the activity profile to the whole S. mutans bacterial cell, the stability over time at cariogenic pH, the preference to hydrolyze bisGMA and confirmed expression profile suggest that SMU_118c could be a significant contributor to the whole bacterial degradative activity of S. mutans toward the degradation of resin composites, adhesives and the restoration-tooth interface, potentially accelerating restoration's failure.

STATEMENT OF SIGNIFICANCE

The current study builds upon our highly-cited previous study by Bourbia et al., (JDR, 2013) that reported on that the cariogenic bacterium, S. mutans has esterase-like activities that enable the bacterium to degrade dental composites and adhesives. The current submission is the first to report on the isolation and characterization of the specific esterase activity (SMU_118c) from S. mutans that is a significant contributor to the whole bacterial degradative activity toward the hydrolysis of dental resins. This activity compromises the restoration-tooth interface, increases interfacial bacterial microleakage (Kermanshahi et al., JDR 2010), potentially contributing to the pathogenesis of recurrent caries around resin composite restorations. This represent a significant contribution to the field of biomaterials and their clinical performance.

Genome-Wide Screens Reveal New Gene Products That Influence Genetic Competence in Streptococcus mutans

A network of genes and at least two peptide signaling molecules tightly control when Streptococcus mutans becomes competent to take up DNA from its environment. Widespread changes in the expression of genes occur when S. mutans is presented with competence signal peptides in vitro, including the increased production of the alternative sigma factor, ComX, which activates late competence genes. Still, the way that gene products that are regulated by competence peptides influence DNA uptake and cellular physiology are not well understood. Here, we developed and employed comprehensive transposon mutagenesis of the S. mutans genome, with a screen to identify mutants that aberrantly expressed comX, coupled with transposon sequencing (Tn-seq) to gain a more thorough understanding of the factors modulating comX expression and progression to the competent state. The screens effectively identified genes known to affect competence, e.g., comR, comS, comD, comE, cipB, clpX, rcrR, and ciaH, but disclosed an additional 20 genes that were not previously competence associated. The competence phenotypes of mutants were characterized, including by fluorescence microscopy to determine at which stage the mutants were impaired for comX activation. Among the novel genes studied were those implicated in cell division, the sensing of cell envelope stress, cell envelope biogenesis, and RNA stability. Our results provide a platform for determining the specific chemical and physical cues that are required for genetic competence in S. mutans, while highlighting the effectiveness of using Tn-seq in S. mutans to discover and study novel biological processes.IMPORTANCE Streptococcus mutans acquires DNA from its environment by becoming genetically competent, a physiologic state triggered by cell-cell communication using secreted peptides. Competence is important for acquiring novel genetic traits and has a strong influence on the expression of virulence-associated traits of S. mutans Here, we used transposon mutagenesis and genomic technologies to identify novel genes involved in competence development. In addition to identifying genes previously known to be required for comX expression, 20 additional genes were identified and characterized. The findings create opportunities to diminish the pathogenic potential of S. mutans, while validating technologies that can rapidly advance our understanding of the physiology, biology, and genetics of S. mutans and related pathogens.

ciaR impacts biofilm formation by regulating an arginine biosynthesis pathway in Streptococcus sanguinis SK36

Streptococcus sanguinis is an early colonizer of the tooth surface and competes with oral pathogens such as Streptococcus mutans to maintain oral health. However, little is known about its mechanism of biofilm formation. Here, we show that mutation of the ciaR gene, encoding the response regulator of the CiaRH two-component system in S. sanguinis SK36, produced a fragile biofilm. Cell aggregation, gtfP gene expression and water-insoluble glucan production were all reduced, which suggested polysaccharide production was decreased in ΔciaR. RNA sequencing and qRT-PCR revealed that arginine biosynthesis genes (argR, argB, argC, argG, argH and argJ) and two arginine/histidine permease genes (SSA_1568 and SSA_1569) were upregulated in ΔciaR. In contrast to ΔciaR, most of strains constructed to contain deletions in each of these genes produced more biofilm and water-insoluble glucan than SK36. A ΔciaRΔargB double mutant was completely restored for the gtfP gene expression, glucan production and biofilm formation ability that was lost in ΔciaR, indicating that argB was essential for ciaR to regulate biofilm formation. We conclude that by promoting the expression of arginine biosynthetic genes, especially argB gene, the ciaR mutation reduced polysaccharide production, resulting in the formation of a fragile biofilm in Streptococcus sanguinis.

For the greater good: Programmed cell death in bacterial communities

For a long a time programmed cell death was thought to be a unique characteristic of higher eukaryotes, but evidence has accumulated showing that programmed cell death is a universal phenomenon in all life forms. Many different types of bacterial programmed cell death systems have been identified, rivalling the eukaryotic systems in diversity. Bacteria are singular, seemingly independently living organisms, however they are part of complex communities. Being part of a community seems indispensable for survival in different environments. This review is focussed on the mechanism of and reasons for bacterial programmed cell death in the context of bacterial communities.

SMU.940 regulates dextran-dependent aggregation and biofilm formation in Streptococcus mutans

The oral bacterium Streptococcus mutans is the principal agent in the development of dental caries. Biofilm formation by S. mutans requires bacterial attachment, aggregation, and glucan formation on the tooth surface under sucrose supplementation conditions. Our previous microarray analysis of clinical strains identified 74 genes in S. mutans that were related to biofilm morphology; however, the roles of almost all of these genes in biofilm formation are poorly understood. We investigated the effects of 21 genes randomly selected from our previous study regarding S. mutans biofilm formation, regulation by the complement pathway, and responses to competence-stimulating peptide. Eight competence-stimulating peptide-dependent genes were identified, and their roles in biofilm formation and aggregation were examined by mutational analyses of the S. mutansUA159 strain. Of these eight genes, the inactivation of the putative hemolysin III family SMU.940 gene of S. mutansUA159 promoted rapid dextran-dependent aggregation and biofilm formation in tryptic soy broth without dextrose (TSB) with 0.25% glucose and slightly reduced biofilm formation in TSB with 0.25% sucrose. The SMU.940 mutant showed higher expression of GbpC and gbpC gene than wild-type. GbpC is known to be involved in the dextran-dependent aggregation of S. mutans. An SMU.940-gbpC double mutant strain was constructed in the SMU.940 mutant background. The gbpC mutation completely abolished the dextran-dependent aggregation of the SMU.940 mutant. In addition, the aggregation of the mutant was abrogated by dextranase. These findings suggest that SMU.940 controls GbpC expression, and contributes to the regulation of dextran-dependent aggregation and biofilm formation.

Raffinose Induces Biofilm Formation by Streptococcus mutans in Low Concentrations of Sucrose by Increasing Production of Extracellular DNA and Fructan

Streptococcus mutans is the primary etiological agent of dental caries and causes tooth decay by forming a firmly attached biofilm on tooth surfaces. Biofilm formation is induced by the presence of sucrose, which is a substrate for the synthesis of extracellular polysaccharides but not in the presence of oligosaccharides. Nonetheless, in this study, we found that raffinose, which is an oligosaccharide with an intestinal regulatory function and antiallergic effect, induced biofilm formation by S. mutans in a mixed culture with sucrose, which was at concentrations less than those required to induce biofilm formation directly. We analyzed the possible mechanism behind the small requirement for sucrose for biofilm formation in the presence of raffinose. Our results suggested that sucrose contributed to an increase in bacterial cell surface hydrophobicity and biofilm formation. Next, we examined how the effects of raffinose interacted with the effects of sucrose for biofilm formation. We showed that the presence of raffinose induced fructan synthesis by fructosyltransferase and aggregated extracellular DNA (eDNA, which is probably genomic DNA released from dead cells) into the biofilm. eDNA seemed to be important for biofilm formation, because the degradation of DNA by DNase I resulted in a significant reduction in biofilm formation. When assessing the role of fructan in biofilm formation, we found that fructan enhanced eDNA-dependent cell aggregation. Therefore, our results show that raffinose and sucrose have cooperative effects and that this induction of biofilm formation depends on supportive elements that mainly consist of eDNA and fructan.IMPORTANCE The sucrose-dependent mechanism of biofilm formation in Streptococcus mutans has been studied extensively. Nonetheless, the effects of carbohydrates other than sucrose are inadequately understood. Our findings concerning raffinose advance the understanding of the mechanism underlying the joint effects of sucrose and other carbohydrates on biofilm formation. Since raffinose has been reported to have positive effects on enterobacterial flora, research on the effects of raffinose on the oral flora are required prior to its use as a beneficial sugar for human health. Here, we showed that raffinose induced biofilm formation by S. mutans in low concentrations of sucrose. The induction of biofilm formation generally generates negative effects on the oral flora. Therefore, we believe that this finding will aid in the development of more effective oral care techniques to maintain oral flora health.

Agriculturally important microbial biofilms: Present status and future prospects

Microbial biofilms are a fascinating subject, due to their significant roles in the environment, industry, and health. Advances in biochemical and molecular techniques have helped in enhancing our understanding of biofilm structure and development. In the past, research on biofilms primarily focussed on health and industrial sectors; however, lately, biofilms in agriculture are gaining attention due to their immense potential in crop production, protection, and improvement. Biofilms play an important role in colonization of surfaces - soil, roots, or shoots of plants and enable proliferation in the desired niche, besides enhancing soil fertility. Although reports are available on microbial biofilms in general; scanty information is published on biofilm formation by agriculturally important microorganisms (bacteria, fungi, bacterial-fungal) and their interactions in the ecosystem. Better understanding of agriculturally important bacterial-fungal communities and their interactions can have several implications on climate change, soil quality, plant nutrition, plant protection, bioremediation, etc. Understanding the factors and genes involved in biofilm formation will help to develop more effective strategies for sustainable and environment-friendly agriculture. The present review brings together fundamental aspects of biofilms, in relation to their formation, regulatory mechanisms, genes involved, and their application in different fields, with special emphasis on agriculturally important microbial biofilms.

A Nuclease from Streptococcus mutans Facilitates Biofilm Dispersal and Escape from Killing by Neutrophil Extracellular Traps

Streptococcus mutans is the primary etiologic agent of dental caries and occasionally infective endocarditis, with the ability to form biofilms and disperse cells into distal sites to exacerbate and spread infection. In this study, we identified a nuclease (DeoC) as a S. mutans biofilm dispersal modulating factor through microarray analysis. In vitro assays revealed a dispersal defect of a deoC deletion mutant, and functional studies with purified protein were indicative of the biofilm dispersal activity of DeoC. Neutrophils are a key host response factor restraining bacterial spreading through the formation of neutrophil extracellular traps (NETs), which consist of a nuclear DNA backbone associated with antimicrobial peptides. Therefore, we hypothesized that the dispersed S. mutans might utilize DeoC to degrade NETs and escape killing by the immune system. It was found that S. mutans induced NET formation upon contact with neutrophils, while the presence of NETs in turn enhanced the deoC expression of S. mutans. Fluorescence microscopy inspection showed that deoC deletion resulted in a decreased NET degradation ability of S. mutans and enhanced susceptibility to neutrophil killing. Data obtained from this study assigned two important roles for DeoC in S. mutans: contributing to the spread of infection through mediating biofilm dispersal, and facilitating the escape of S. mutans from neutrophil killing through NET degradation.

Transcriptional Profiling of the Oral Pathogen Streptococcus mutans in Response to Competence Signaling Peptide XIP

In the cariogenic Streptococcus mutans, competence development is regulated by the ComRS signaling system comprised of the ComR regulator and the ComS prepeptide to the competence signaling peptide XIP (ComX-inducing peptide). Aside from competence development, XIP signaling has been demonstrated to regulate cell lysis, and recently, the expression of bacteriocins, small antimicrobial peptides used by bacteria to inhibit closely related species. Our study further explores the effect of XIP signaling on the S. mutans transcriptome. RNA sequencing revealed that XIP induction resulted in a global change in gene expression that was consistent with a stress response. An increase in several membrane-bound regulators, including HdrRM and BrsRM, involved in bacteriocin production, and the VicRKX system, involved in acid tolerance and biofilm formation, was observed. Furthermore, global changes in gene expression corresponded to changes observed during the stringent response to amino acid starvation. Effects were also observed on genes involved in sugar transport and carbon catabolite repression and included the levQRST and levDEFG operons. Finally, our work identified a novel heat shock-responsive intergenic region, encoding a small RNA, with a potential role in competence shutoff. IMPORTANCE Genetic competence provides bacteria with an opportunity to increase genetic diversity or acquire novel traits conferring a survival advantage. In the cariogenic pathogen Streptococcus mutans, DNA transformation is regulated by the competence stimulating peptide XIP (ComX-inducing peptide). The present study utilizes high-throughput RNA sequencing (RNAseq) to provide a greater understanding of how global gene expression patterns change in response to XIP. Overall, our work demonstrates that in S. mutans, XIP signaling induces a response that resembles the stringent response to amino acid starvation. We further identify a novel heat shock-responsive intergenic region with a potential role in competence shutoff. Together, our results provide further evidence that multiple stress response mechanisms are linked through the genetic competence signaling pathway in S. mutans.

Human cathelicidin LL-37 enhance the antibiofilm effect of EGCG on Streptococcus mutans

BACKGROUND

Streptococcus mutans forms biofilms as a resistance mechanism against antimicrobial agents in the human oral cavity. We recently showed that human cathelicidin LL-37 exhibits inhibitory effects on biofilm formation of S. mutans through interaction with lipoteichoic acid (LTA), but without antibacterial or biofilm dispersal abilities. (-)-Epigallocatechin gallate (EGCG) is the most abundant constituent of tea catechins that has the greatest anti-infective potential to inhibit the growth of various microorganisms and biofilm formation. Therefore, in this study, we evaluated whether LL-37 interacts with EGCG to enhance the antibiofilm effect of EGCG on S. mutans biofilm formation.

METHODS

Clinical S. mutans strains (n = 10) isolated from children's saliva were tested in a biofilm formation assay. The antibiofilm effect of EGCG with and without LL-37 was analyzed by the minimum biofilm eradication concentration assay and confirmed using field emission-scanning electron microscopy. In addition, the interaction among EGCG, LL-37, and LTA of S. mutans was determined using quartz crystal microbalance analysis.

RESULTS

EGCG killed 100 % of planktonic S. mutans within 5 h, inhibited biofilm formation within 24 h, and reduced bacteria cells in preformed biofilms within 3 h at a concentration of 0.2 mg/mL. However, EGCG did not appear to interact with LTA. LL-37 effectively enhanced the bactericidal activity of EGCG against biofilm formation and preformed biofilms as determined by quantitative crystal violet staining and field emission-scanning electron microscopy. In addition, quartz crystal microbalance analysis revealed that LL-37 interacted with EGCG and promoted binding between EGCG and LTA of S. mutans.

CONCLUSIONS

We show that LL-37 enhances the antibiofilm effect of EGCG on S. mutans. This finding provides new knowledge for dental treatment by using LL-37 as a potential antibiofilm compound.

Inactivation of the spxA1 or spxA2 gene of Streptococcus mutans decreases virulence in the rat caries model

In oral biofilms, the major environmental challenges encountered by Streptococcus mutans are acid and oxidative stresses. Previously, we showed that the transcriptional regulators SpxA1 and SpxA2 are involved in general stress survival of S. mutans with SpxA1 playing a primary role in activation of antioxidant and detoxification strategies whereas SpxA2 serves as a back up activator of oxidative stress genes. We have also found that spxA1 mutant strains (∆spxA1 and ∆spxA1∆spxA2) are outcompeted by peroxigenic oral streptococci in vitro and have impaired abilities to colonize the teeth of rats fed a highly cariogenic diet. Here, we show that the Spx proteins can also exert regulatory roles in the expression of additional virulence attributes of S. mutans. Competence activation is significantly impaired in Δspx strains and the production of mutacin IV and V is virtually abolished in ΔspxA1 strains. Unexpectedly, the ∆spxA2 strain showed increased production of glucans from sucrose, without affecting the total amount of bacteria within biofilms when compared with the parent strain. By using the rat caries model, we showed that the capacity of the ΔspxA1 and ΔspxA2 strains to cause caries on smooth tooth surfaces is significantly impaired. The ∆spxA2 strain also formed fewer lesions on sulcal surfaces. This report reveals that global regulation via Spx contributes to the cariogenic potential of S. mutans and highlights that animal models are essential in the characterization of bacterial traits implicated in virulence.

SpxB-mediated H2 O2 induces programmed cell death in Streptococcus sanguinis

Streptococcus sanguinis (S. sanguinis) is a commensal oral streptococci that produces hydrogen peroxide (H2 O2 ), and this production is dependent on pyruvate oxidase (SpxB) activity. In addition to its well-known role in intraspecies or interspecies competitions, recent studies have shown that H2 O2 produced by S. sanguinis under aerobic conditions not only upregulates biofilm formation and eDNA release but also regulates cell death without obvious cell lysis. Here, we report that S. sanguinis exhibits characteristic hallmarks of eukaryotic apoptosis when it encounters endogenous and exogenous H2 O2 . As the most common mode of programmed cell death (PCD), apoptosis is accompanied by a series of biochemical and morphological events, including DNA fragmentation, chromosome condensation, membrane potential depolarization, phosphatidylserine (PS) exposure, and caspase substrate binding protein activity changes. In addition, we also provide genetic evidence that there is decreased expression of the related DNA repair genes comEA, recA, dnaC, dinG, and pcrA in the wild-type compared to the isogenic spxB mutant in S. sanguinis. Our data suggest that endogenous H2 O2 is the most important agent in this development process in S. sanguinis.

Death and survival in Streptococcus mutans: differing outcomes of a quorum-sensing signaling peptide

Bacteria are considered "social" organisms able to communicate with one another using small hormone-like molecules (pheromones) in a process called quorum-sensing (QS). These signaling molecules increase in concentration as a function of bacterial cell density. For most human pathogens, QS is critical for virulence and biofilm formation, and the opportunity to interfere with bacterial QS could provide a sophisticated means for manipulating the composition of pathogenic biofilms, and possibly eradicating the infection. Streptococcus mutans is a well-characterized resident of the dental plaque biofilm, and is the major pathogen of dental caries (cavities). In S. mutans, its CSP QS signaling peptide does not act as a classical QS signal by accumulating passively in proportion to cell density. In fact, particular stresses such as those encountered in the oral cavity, induce the production of the CSP pheromone, suggesting that the pheromone most probably functions as a stress-inducible alarmone by triggering the signaling to the bacterial population to initiate an adaptive response that results in different phenotypic outcomes. This mini-review discusses two different CSP-induced phenotypes, bacterial "suicide" and dormancy, and the underlying mechanisms by which S. mutans utilizes the same QS signaling peptide to regulate two opposite phenotypes.

Bidirectional signaling in the competence regulatory pathway of Streptococcus mutans

Streptococcus mutans expresses comX (also known as sigX), which encodes a sigma factor that is required for development of genetic competence, in response to the peptide signals XIP and CSP and environmental factors. XIP (sigX inducing peptide) is derived from ComS and activates comX unimodally in chemically defined media via the ComRS system. CSP (competence stimulating peptide) activates comX bimodally in peptide-rich media through the ComDE two-component system. However, CSP-ComDE activation of comX is indirect and involves ComRS. Therefore, the bimodality of CSP-dependent activation of comX may arise from either ComRS or ComDE. Here we study, at the single-cell level, how genes in the CSP signaling pathway respond to CSP, XIP and media. Our data indicate that activation of comX stimulates expression of comE. In addition, activation of comE requires intact comR and comS genes. Therefore, not only does CSP-ComDE stimulate the ComRS pathway to activate comX expression, but ComRS activation of comX also stimulates expression of the CSP-ComDE pathway and its regulon. The results demonstrate the mutual interconnection of the signaling pathways that control bacteriocin expression (ComDE) and genetic competence (ComRS), both of which are linked to lytic and virulence behaviors.