Characteristics of biofilm formation by Streptococcus mutans in the presence of saliva

The antimicrobial effects of deglycyrrhizinated licorice root extract on Streptococcus mutans UA159 in both planktonic and biofilm cultures

The objective of the study was to investigate the antimicrobial effects of deglycyrrhizinated licorice root extracts (DG-LRE) against Streptococcus mutans UA159 in both the planktonic and biofilm phases by determining the minimum inhibitory concentration and minimum bactericidal concentration, and by performing time-kill kinetic, growth, adhesion, and biofilm assays. The cell toxicity of DG-LRE on normal human gingival fibroblast (NHGF) cells was tested using a methyl thiazolyl tetrazolium assay. This study showed that DG-LRE had strong antimicrobial activity against S. mutans in the planktonic phase with little cytotoxic effect on NHGF cells. In addition, DG-LRE significantly inhibited biofilm formation by S. mutans UA159 at concentrations over 4 μg/ml for glucose or 16 μg/ml for sucrose, respectively, regardless of the presence of saliva-coating. To the best of our knowledge, this is the first report to provide evidence that DG-LRE demonstrates antimicrobial activity against S. mutans. These results suggest that DG-LRE can be used in developing oral hygiene products, such as gargling solution and dentifrice to prevent human dental caries.

Functional amyloid formation by Streptococcus mutans

Dental caries is a common infectious disease associated with acidogenic and aciduric bacteria, including Streptococcus mutans. Organisms that cause cavities form recalcitrant biofilms, generate acids from dietary sugars and tolerate acid end products. It has recently been recognized that micro-organisms can produce functional amyloids that are integral to biofilm development. We now show that the S. mutans cell-surface-localized adhesin P1 (antigen I/II, PAc) is an amyloid-forming protein. This conclusion is based on the defining properties of amyloids, including binding by the amyloidophilic dyes Congo red (CR) and Thioflavin T (ThT), visualization of amyloid fibres by transmission electron microscopy and the green birefringent properties of CR-stained protein aggregates when viewed under cross-polarized light. We provide evidence that amyloid is present in human dental plaque and is produced by both laboratory strains and clinical isolates of S. mutans. We provide further evidence that amyloid formation is not limited to P1, since bacterial colonies without this adhesin demonstrate residual green birefringence. However, S. mutans lacking sortase, the transpeptidase enzyme that mediates the covalent linkage of its substrates to the cell-wall peptidoglycan, including P1 and five other proteins, is not birefringent when stained with CR and does not form biofilms. Biofilm formation is inhibited when S. mutans is cultured in the presence of known inhibitors of amyloid fibrillization, including CR, Thioflavin S and epigallocatechin-3-gallate, which also inhibited ThT uptake by S. mutans extracellular proteins. Taken together, these results indicate that S. mutans is an amyloid-forming organism and suggest that amyloidogenesis contributes to biofilm formation by this oral microbe.

The effects of surface roughness of composite resin on biofilm formation of Streptococcus mutans in the presence of saliva

The purpose of this study was to investigate the effects of surface roughness of resin composite on biofilm formation of Streptococcus mutans in the presence of saliva. To provide uniform surface roughness on composites, disks were prepared by curing composite against 400-grit silicon carbide paper (SR400), 800-grit silicon carbide paper (SR800), or a glass slide (SRGlass). The surface roughness was examined using confocal laser microscopy. For biofilm formation, S. mutans was grown for 24 hours with each disk in a biofilm medium with either glucose or sucrose in the presence of fluid-phase or surface-adsorbed saliva. The adherent bacteria were quantified via enumeration of the total viable counts of bacteria. Biofilms were examined using scanning electron microscopy. This study showed that SR400 had deeper and larger, but fewer depressions than SR800. Compared to SRGlass and SR800, biofilm formation was significantly increased on SR400. In addition, the differences in the effect of surface roughness on the amount of biofilm formation were not significantly influenced by either the presence of saliva or the carbohydrate source. Considering that similar differences in surface roughness were observed between SR400 and SR800 and between SR800 and SRGlass, this study suggests that surface topography (size and depth of depressions) may play a more important role than surface roughness in biofilm formation of S. mutans .

Host and bacterial phenotype variation in adhesion of Streptococcus mutans to matched human hosts

The commensal pathogen Streptococcus mutans uses AgI/II adhesins to adhere to gp340 adsorbed on teeth. Here we analyzed isolates of S. mutans (n = 70 isolates) from caries and caries-free human extremes (n = 19 subjects) by multilocus sequence typing (MLST), AgI/II full-length gene sequencing, and adhesion to parotid saliva matched from the strain donors (nested from a case-control sample of defined gp340 and acidic proline-rich protein [PRP] profiles). The concatenated MLST as well as AgI/II gene sequences showed unique sequence types between, and identical types within, the subjects. The matched adhesion levels ranged widely (40% adhesion range), from low to moderate to high, between subjects but were similar within subjects (or sequence types). In contrast, the adhesion avidity of the strains was narrow, normally distributed for high, moderate, or low adhesion reference saliva or pure gp340 regardless of the sequence type. The adhesion of S. mutans Ingbritt and matched isolates and saliva samples correlated (r = 0.929), suggesting that the host specify about four-fifths (r(2) = 0.86) of the variation in matched adhesion. Half of the variation in S. mutans Ingbritt adhesion to saliva from the caries cases-controls (n = 218) was explained by the primary gp340 receptor and PRP coreceptor composition. The isolates also varied, although less so, in adhesion to standardized saliva (18% adhesion range) and clustered into three major AgI/II groups (groups A, B(1), and B(2)) due to two variable V-region segments and diverse AgI/II sequence types due to a set of single-amino-acid substitutions. Isolates with AgI/II type A versus types B(1) and B(2) tended to differ in gp340 binding avidity and qualitative adhesion profiles for saliva gp340 phenotypes. In conclusion, the host saliva phenotype plays a more prominent role in S. mutans adhesion than anticipated previously.

The effect of the flavonol morin on adhesion and aggregation of Streptococcus pyogenes

The effect of the flavonol morin on Streptococcus pyogenes biofilm growth was determined using a static biofilm model, in which reduced biofilm biomass was observed in the presence of morin, suggesting that morin inhibited biofilm development. Morin at concentrations exceeding 225 μM had the greatest impact on biofilm biomass causing reductions of up to 65%, which was found to be statistically significant. Morin was also shown to induce rapid bacterial aggregation. Approximately 55% of S. pyogenes in liquid suspension aggregated when incubated with morin at concentrations of 275 and 300 μM for 120 min, compared to the control group in which only 10% of the cells aggregated, this was also shown to be statistically significant.

BrpA is involved in regulation of cell envelope stress responses in Streptococcus mutans

Previous studies have shown that BrpA plays a major role in acid and oxidative stress tolerance and biofilm formation by Streptococcus mutans. Mutant strains lacking BrpA also display increased autolysis and decreased viability, suggesting a role for BrpA in cell envelope integrity. In this study, we examined the impact of BrpA deficiency on cell envelope stresses induced by envelope-active antimicrobials. Compared to the wild-type strain UA159, the BrpA-deficient mutant (TW14D) was significantly more susceptible to antimicrobial agents, especially lipid II inhibitors. Several genes involved in peptidoglycan synthesis were identified by DNA microarray analysis as downregulated in TW14D. Luciferase reporter gene fusion assays also revealed that expression of brpA is regulated in response to environmental conditions and stresses induced by exposure to subinhibitory concentrations of cell envelope antimicrobials. In a Galleria mellonella (wax worm) model, BrpA deficiency was shown to diminish the virulence of S. mutans OMZ175, which, unlike S. mutans UA159, efficiently kills the worms. Collectively, these results suggest that BrpA plays a role in the regulation of cell envelope integrity and that deficiency of BrpA adversely affects the fitness and diminishes the virulence of OMZ175, a highly invasive strain of S. mutans.

Effect of a novel antimicrobial peptide chrysophsin-1 on oral pathogens and Streptococcus mutans biofilms

Dental caries and pulpal diseases are common oral bacterial infectious diseases. Controlling and reducing the causative pathogens, such as Streptococcus mutans and Enterococcus faecalis, is a key step toward prevention and treatment of the two diseases. Chrysophsin-1 is a cationic antimicrobial peptide having broad-spectrum bactericidal activity against both Gram-positive and Gram-negative bacteria. In this study, we investigated the antibacterial activity of chrysophsin-1 against several oral pathogens and S. mutans biofilms and performed a preliminary study of the antimicrobial mechanism. Cytotoxic activity of chrysophsin-1 against human gingival fibroblasts (HGFs) was investigated. Minimal inhibitory concentration (MIC), minimal bactericidal concentration (MBC) and time-kill assay were used to evaluate the killing effect of chrysophsin-1. Scanning electron microscopy (SEM) was used to analyze morphological and membrane change in oral pathogens. Live/Dead staining, in conjunction with confocal scanning laser microscopy (CSLM), was used to observe and analyze S. mutans biofilms. MIC and MBC results demonstrated that chrysophsin-1 had different antimicrobial activities against the tested oral microbes. Lysis and pore formation of the cytomembrane were observed following treatment of the bacteria with chrysophsin-1 for 4h or 24h by SEM. Furthermore, CLSM images showed that chrysophsin-1 remarkably reduced the viability of cells within biofilms and had a significantly lethal effect against S. mutans biofilms. Toxicity studies showed that chrysophsin-1 at concentration between 8 μg/ml and 32 μg/ml had little effect on viability of HGFs in 5 min. Our findings suggest that chrysophsin-1 may have potential clinical applications in the prevention and treatment of dental caries and pulpal diseases.

Antiviral activities in human saliva

In this review, the authors survey the large number of antibacterial and antiviral proteins present in human saliva. Of interest, most of these antibacterial proteins display antiviral activity, typically against specific viral pathogens. The review focuses on one protein that interacts with both bacteria and viruses-gp340, originally referred to as salivary agglutinin. In the oral cavity, soluble gp340 binds to and aggregates a variety of bacteria, and this is thought to increase bacterial clearance from the mouth. However, when bound to the tooth surface, gp340 promotes bacterial adherence. In the oral cavity, most gp340 is found soluble in saliva and can function as a specific inhibitor of infectivity of HIV-1 and influenza A. In contrast, in the female reproductive track, most gp340 is bound to the cell surface, where it can promote HIV-1 infection.

Streptococcus pyogenes antigen I/II-family polypeptide AspA shows differential ligand-binding properties and mediates biofilm formation

The streptococcal antigen I/II (AgI/II)-family polypeptides are cell wall-anchored adhesins expressed by most indigenous oral streptococci. Proteins sharing 30-40% overall amino acid sequence similarities with AgI/II-family proteins are also expressed by Streptococcus pyogenes. The S. pyogenes M28_Spy1325 polypeptide (designated AspA) displays an AgI/II primary structure, with alanine-rich (A) and proline-rich (P) repeats flanking a V region that is projected distal from the cell. In this study it is shown that AspA from serotype M28 S. pyogenes, when expressed on surrogate host Lactococcus lactis, confers binding to immobilized salivary agglutinin gp-340. This binding was blocked by antibodies to the AspA-VP region. In contrast, the N-terminal region of AspA was deficient in binding fluid-phase gp-340, and L. lactis cells expressing AspA were not agglutinated by gp-340. Deletion of the aspA gene from two different M28 strains of S. pyogenes abrogated their abilities to form biofilms on saliva-coated surfaces. In each mutant strain, biofilm formation was restored by trans complementation of the aspA deletion. In addition, expression of AspA protein on the surface of L. lactis conferred biofilm-forming ability. Taken collectively, the results provide evidence that AspA is a biofilm-associated adhesin that may function in host colonization by S. pyogenes.

Streptococcus mutans biofilm adhesion on composite resin surfaces after different finishing and polishing techniques

This study evaluated Streptococcus mutans biofilm adhesion on the surface of three composite resins (nanofilled, Filtek Z350, 3M ESPE, Salt Lake City, UT, USA; nanohybrid, Vit-1-escence, Ultradent Products, South Jordan, UT, USA; and microhybrid, Esthet X, Dentsply, Milford, DE, USA) following different finishing and polishing techniques. Sixty standardized samples (6 × 3 mm) of each composite were produced and randomly divided into three finishing and polishing treatments (n=20): 1) control group: composite resin surface in contact with Mylar matrix strips with no finishing or polishing performed, 2) Sof-Lex aluminum oxide disc technique (3M ESPE, and 3) carbide bur finishing and Astrobrush polishing technique (Ultradent). Half the samples of each group were incubated in human saliva for 1 hour, and all the samples were subjected to S mutans (ATCC 35688) biofilm development. The mean log of CFU/mL present in the S mutans biofilm was calculated, and data were statistically analyzed by three-way analysis of variance and the Tukey test (p<0.05). Human saliva incubation promoted a significant increase of bacterial adherence on all three of the composites' surfaces, regardless of the polishing treatment performed (p<0.05). Of the three, the nanofilled composite (Filtek Z350) had the lowest bacterial adherence with each of the finishing and polishing techniques despite the presence or absence of human saliva (p<0.05). Mylar matrix strips (control group) promoted the lowest bacterial adhesion on the surface of the microhybrid and nanofilled composites in the absence of human saliva.

MPC-polymer reduces adherence and biofilm formation by oral bacteria

Oral biofilms such as dental plaque cause dental caries and periodontitis, as well as aspiration pneumonia and infectious endocarditis by translocation. Hence, the suppression of oral biofilm formation is an issue of considerable importance. Mechanical removal, disinfectants, inhibition of polysaccharide formation, and artificial sugar have been used for the reduction of oral biofilm. From the viewpoint of the inhibition of bacterial adherence, we investigated whether aqueous biocompatible 2-methacryloyloxyethyl phosphorylcholine (MPC)-polymer can reduce streptococcal colonization and biofilm formation. We examined the effects of MPC-polymer on streptococcal adherence to saliva-coated hydroxyapatite and oral epithelial cells, and the adherence of Fusobacterium nucleatum to streptococcal biofilm. MPC-polymer application markedly inhibited both the adherence and biofilm formation of Streptococcus mutans on saliva-coated hydroxyapatite and streptococcal adherence to oral epithelial cells, and reduced the adherence of F. nucleatum to streptococcal biofilms. A small-scale clinical trial revealed that mouthrinsing with MPC-polymer inhibited the increase of oral bacterial numbers, especially of S. mutans. These findings suggest that MPC-polymer is a potent inhibitor of bacterial adherence and biofilm development, and may be useful to prevent dental-plaque-related diseases. (UMIN Clinical Trial Registry UMIN000003471).

Susceptibility of planktonic cultures of Streptococcus mutans to photodynamic therapy with a light-emitting diode

The objective of this study was to evaluate the effect of photodynamic therapy with erythrosine and rose bengal using a light-emitting diode (LED) on planktonic cultures of S. mutans. Ten S. mutans strains, including nine clinical strains and one reference strain (ATCC 35688), were used. Suspensions containing 10⁶ cells/mL were prepared for each strain and were tested under different experimental conditions: a) LED irradiation in the presence of rose bengal as a photosensitizer (RB+L+); b) LED irradiation in the presence of erythrosine as a photosensitizer (E+L+); c) LED irradiation only (P-L+); d) treatment with rose bengal only (RB+L-); e) treatment with erythrosine only (E+L-); and f) no LED irradiation or photosensitizer treatment, which served as a control group (P-L-). After treatment, the strains were seeded onto BHI agar for determination of the number of colony-forming units (CFU/mL). The results were submitted to analysis of variance and the Tukey test (p ≤ 0.05). The number of CFU/mL was significantly lower in the groups submitted to photodynamic therapy (RB+L+ and E+L+) compared to control (P-L-), with a reduction of 6.86 log₁₀ in the RB+L+ group and of 5.16 log₁₀ in the E+L+ group. Photodynamic therapy with rose bengal and erythrosine exerted an antimicrobial effect on all S. mutans strains studied.

Effect of orthodontic bonding steps on the initial adhesion of mutans streptococci in the presence of saliva

OBJECTIVE

To test the hypothesis that orthodontic bonding has no effect on the initial adhesion of mutans streptococci (MS) in the presence of saliva.

MATERIALS AND METHODS

Hydroxyapatite (HA) and orthodontic adhesive (AD) disks were prepared to a uniform size. HA disks were etched with 37% phosphoric acid (HE, etched group). Some of the HE disks were coated with Transbond XT primer and light cured (HP, primed group). Transbond Plus SEP was applied to a third set of HA disks, dried, and light cured (SEP, self-etching primer group). Adhesion assays were performed using two MS strains in the presence of fluid-phase or surface-adsorbed unstimulated whole saliva (UWS). The MS adhesion patterns were examined by scanning electron microscopy.

RESULTS

MS adhesion was influenced by the bonding steps and the presence of UWS. UWS treatment decreased MS adhesion. However, surface-adsorbed UWS resulted in slightly less inhibition of MS adhesion than fluid-phase UWS. MS adhesion was significantly greater for HE than for the other groups. There were interaction effects between the UWS treatment and surface groups. MS adhesion to HP and AD was significantly diminished in the presence of surface-adsorbed or fluid-phase UWS compared with adhesion to HA, HE, or SEP.

CONCLUSION

The hypothesis is rejected. Our results suggest that MS adhesion is significantly influenced by the bonding procedure used, and the application of conventional primers for the bracket bonding can inhibit MS adhesion to tooth surfaces in the presence of UWS.

A proteomic approach for exploring biofilm in Streptococcus mutans

Biofilm formation by Streptococcus mutans is considered as its principal virulence factor, causing dental caries. Mutants of S. mutans defective in biofilm formation were generated and analyzed to study the collective role of proteins in its formation. Mutants were characterized on the basis of adherence to saliva-coated surface, and biofilm formation. The confocal laser microscopy and scanning electron microscopy images showed that the control biofilms had cluster of cells covered by layer of exo-polysaccharide while the biofilms of mutants were thin and spaced. Two-dimensional protein electrophoresis data analysis identified 57 proteins that are either up (44 proteins) or down (13 proteins) regulated. These data points to the importance of up and down regulated proteins in the formation of biofilm in Streptococcus mutans.

The delta subunit of RNA polymerase, RpoE, is a global modulator of Streptococcus mutans environmental adaptation

The delta subunit of RNA polymerase, RpoE, is widespread in low-G+C Gram-positive bacteria and is thought to play a role in enhancing transcriptional specificity by blocking RNA polymerase binding at weak promoter sites and stimulating RNA synthesis by accelerating core enzyme recycling. Despite the well-studied biochemical properties of RpoE, a role for this protein in vivo has not been defined in depth. In this study, we show that inactivation of rpoE in the human dental caries pathogen Streptococcus mutans causes impaired growth and loss of important virulence traits, including biofilm formation, resistance to antibiotics, and tolerance to environmental stresses. Complementation of the mutant with rpoE expressed in trans restored its phenotype to wild type. The luciferase fusion reporter showed that rpoE was highly transcribed throughout growth and that acid and hydrogen peroxide stresses repressed rpoE expression. Transcriptome profiling of wild-type and ΔrpoE cells in the exponential and early stationary phase of growth, under acid and hydrogen peroxide stress and under both stresses combined, revealed that genes involved in histidine synthesis, malolactic fermentation, biofilm formation, and antibiotic resistance were downregulated in the ΔrpoE mutant under all conditions. Moreover, the loss of RpoE resulted in dramatic changes in transport and metabolism of carbohydrates and amino acids. Interestingly, differential expression, mostly upregulation, of 330 noncoding regions was found. In conclusion, this study demonstrates that RpoE is an important global modulator of gene expression in S. mutans which is required for optimal growth and environmental adaptation.

Role of the Streptococcus mutans irvA gene in GbpC-independent, dextran-dependent aggregation and biofilm formation

Dextran-dependent aggregation (DDAG) of Streptococcus mutans is an in vitro phenomenon that is believed to represent a property of the organism that is beneficial for sucrose-dependent biofilm development. GbpC, a cell surface glucan-binding protein, is responsible for DDAG in S. mutans when cultured under defined stressful conditions. Recent reports have described a putative transcriptional regulator gene, irvA, located just upstream of gbpC, that is normally repressed by the product of an adjacent gene, irvR. When repression of irvA is relieved, there is a resulting increase in the expression of GbpC and decreases in competence and synthesis of the antibiotic mutacin I. This study examined the role of irvA in DDAG and biofilm formation by engineering strains that overexpressed irvA (IrvA+) on an extrachromosomal plasmid. The IrvA+ strain displayed large aggregation particles that did not require stressful growth conditions. A novel finding was that overexpression of irvA in a gbpC mutant background retained a measure of DDAG, albeit very small aggregation particles. Biofilms formed by the IrvA+ strain in the parental background possessed larger-than-normal microcolonies. In a gbpC mutant background, the overexpression of irvA reversed the fragile biofilm phenotype normally associated with loss of GbpC. Real-time PCR and Northern blot analyses found that expression of gbpC did not change significantly in the IrvA+ strain but expression of spaP, encoding the major surface adhesin P1, increased significantly. Inactivation of spaP eliminated the small-particle DDAG. The results suggest that IrvA promotes DDAG not only by GbpC, but also via an increase in P1.

Changes in biochemical and phenotypic properties of Streptococcus mutans during growth with aeration

Oxygen has a potent influence on the expression of genes and the activity of physiological and biochemical pathways in bacteria. We have found that oxygen significantly altered virulence-related phenotypic properties of Streptococcus mutans, the primary etiological agent of human dental caries. Transport of glucose, fructose, or mannose by the sugar:phosphotransferase system was significantly enhanced by growth under aerobic conditions, whereas aeration caused an extended lag phase and slower growth of S. mutans in medium containing glucose, fructose, or mannose as the carbohydrate source. Aeration resulted in a decrease in the glycolytic rate and enhanced the production of intracellular storage polysaccharides. Although aeration decreased the acid tolerance of S. mutans, aerobically grown cells had higher F-ATPase activity. Aeration altered biofilm architecture but did not change the ability of S. mutans to interact with salivary agglutinin. Growth in air resulted in enhanced cell-associated glucosyltransferase (Gtf) activity at the expense of cell-free Gtf activity. These results demonstrate that S. mutans can dramatically alter its pathogenic potential in response to exposure to oxygen, suggesting that the phenotype of the organism may be highly variable in the human oral cavity depending on the maturity of the dental plaque biofilm.

Morphological changes in Streptococcus mutans after chewing gum containing xylitol for twelve months

Few studies have been conducted to evaluate the effect of xylitol on the virulence and morphology of Streptococcus mutans (S. mutans). This study was conducted to evaluate changes in the morphology and virulence of S. mutans in response to long-term consumption of gum containing xylitol. Participants were voluntarily recruited for a women's oral health prevention program, classified into two groups (a control and a xylitol group), and then followed for 1 year. Ten salivary samples were randomly selected from each group. Colony count, field emission scanning electron microscopy, and real-time reverse transcription polymerase chain reaction were used to analyze the characteristics of S. mutans. In the xylitol group, the colony counts of S. mutans decreased steadily over time. In addition, the adherence of the colonies in the xylitol group became weak, and the size of the colonies decreased compared to the control. The secretion of sticky substances from the surface of S. mutans colonies and gtfB gene expression also decreased in the xylitol group. These findings indicate that regular chewing of xylitol gum over a long period may lead to decreased gtfB expression, which can negatively affect the synthesis of extracellular polysaccharides by S. mutans, which could reduce the size and growth of S. mutans colonies and change their morphology as a result.

Identification and characterization of an autolysin gene, atlg, from Streptococcus sobrinus

AtlA is a major cell-lytic enzyme called autolysin in Streptococcus mutans. In this study, we identified the atlg gene-encoding autolysin (Atlg), consisting of 863 residues from Streptococcus sobrinus 6715DP, and confirmed lytic activity of recombinant Atlg by zymography of S. sobrinus cells. An atlA-inactivated mutant was constructed in S. mutans Xc, and the atlg gene product was characterized by plasmid complementation. Microscopic analysis, saliva-induced aggregation assay and autolysis assay of static cultures in air revealed that the atlg gene product partially complemented the role of AtlA. Furthermore, the capability of biofilm formation of the atlA-deficient mutant cultivated in air was restored by plasmid comprising the atlg gene. These findings suggest that Atlg may be involved in cell separation and biofilm formation in S. sobrinus.