Mechanistic, genomic and proteomic study on the effects of BisGMA-derived biodegradation product on cariogenic bacteria

Mechanistic study of the stabilization of dentin-bonded restorative interfaces via collagen reinforcement by multi-acrylamides

Hydrolytically and enzymatically-stable multi-acrylamides have been proposed to increase the long-term durability of dental adhesive interfaces as alternatives to methacrylates. The aim of this study was to investigate the mechanical and biochemical properties of experimental adhesives containing multi-functional acrylamides concerning collagen reinforcement and metalloproteinases (MMP) activity. Multi-functional acrylamides, TMAAEA (Tris[(2-methylaminoacryl) ethylamine) and DEBAAP (N,N-Diethyl-1,3-bis(acrylamido) propane), along with the commercially available DMAM (N,N-dimethylacrylamide) (monofunctional acrylamide) and HEMA (2-Hydroxyethyl methacrylate) (monofunctional methacrylate - control) were tested for stability against enzymatic hydrolysis by cholesterol esterase/pseudocholinesterase (PC/PCE) solutions for up to 30 days. Collagen-derived substrate and gelatin zymography were performed to examine the effect of the compounds on the biological activity of human recombinant and dentin-extracted gelatinases MMP-2 and MMP-9. In situ zymography was carried out by fluorescent collagen degradation combined with confocal microscopy analysis. Hydroxyproline content was measured in collagen derived from dentin extracts though reaction with Ehrlich's reagent p-dimethylaminobenzaldehyde (DMAB), generating a stable chromophore measured at 550 nm. Storage shear modulus of demineralized dentin discs treated with the tested compounds was measured by oscillatory rheometry, in order to investigate potential collagen reinforcement. FT-IR was performed to determine qualitative differences in collagen based on observed changes in amide bands. The results were analyzed by ANOVA/Tukey's test (α = 0.05). Multi-acrylamides survived 30 days of incubation in cholinesterase/pseudo-cholinesterase (PC/PCE) solutions, while HEMA showed approximately 70 % overall degradation. Incubation with multi-acrylamides reduced collagen degradation as evidenced by the reduced hydroxyproline levels and by the 30 % increase inshear storage modulus. Biochemical and zymography assays showed no noticeable inhibition of recombinant and extracted MMPs enzymatic activity. The infra-red spectroscopy results for multi-functional acrylamides treated samples demonstrated shifts of the amide II bonds and marked increase in intensity of the bands 1200 cm-1, which may indicate partial collagen denaturation and some degree of cross-linking of the compounds with collagen, respectively. The multi-acrylamides exhibited not only comparable mechanical properties but also demonstrated significantly enhanced biochemical stability when compared to the widely used methacrylate control. Clinical relevance: These findings highlight the potential of multi-acrylamides to increase the bonding stability to tissues and, ultimately, contribute to the longevity of dental restorations.

Dental composite biodeterioration in the presence of oral Streptococci and extracellular metabolic products

OBJECTIVE

Secondary caries is a primary cause of early restoration failure. While primary dental caries has been extensively researched, our knowledge about the impact of secondary caries on dental restorations is relatively limited. In this study, we examined how different clinically relevant microbially-influenced environments impact the degradation of nano-filled (FIL) and micro-hybrid (AEL) dental composites.

METHODS

Material strength of two commercial dental composites was measured following incubation in aqueous media containing: i) cariogenic (Streptococcus mutans) and non-cariogenic bacteria (Streptococcus sanguinis) grown on sucrose or glucose, ii) abiotic mixtures of artificial saliva and sucrose and glucose fermentation products (volatile fatty acids and ethanol) in proportions known to be produced by these microorganisms, and iii) abiotic mixtures of artificial saliva and esterase, a common oral extracellular enzyme.

RESULTS

Nano-filled FIL composite strength decreased in all three types of incubations, while micro-hybrid AEL composite strength only decreased significantly in biotic incubations. The strength of both composites was statistically significantly decreased in all biotic incubations containing both cariogenic and non-cariogenic bacteria beyond that induced by either abiotic mixtures of fermentation products or esterase alone. Finally, there were no statistically significant differences in composite strength decrease among the tested biotic conditions.

CONCLUSIONS

The results show that conditions created during the growth of both cariogenic and non-cariogenic oral Streptococci substantially reduce commercial composite strength, and this effect warrants further study to identify the mechanism(s).

CLINICAL SIGNIFICANCE

Dental biofilms of oral Streptococci bacteria significantly affect the mechanical strength of dental restorations.

How probiotics, prebiotics, synbiotics, and postbiotics prevent dental caries: an oral microbiota perspective

Dental caries, a highly prevalent oral disease, impacts a significant portion of the global population. Conventional approaches that indiscriminately eradicate microbes disrupt the natural equilibrium of the oral microbiota. In contrast, biointervention strategies aim to restore this balance by introducing beneficial microorganisms or inhibiting cariogenic ones. Over the past three decades, microbial preparations have garnered considerable attention in dental research for the prevention and treatment of dental caries. However, unlike related pathologies in the gastrointestinal, vaginal, and respiratory tracts, dental caries occurs on hard tissues such as tooth enamel and is closely associated with localized acid overproduction facilitated by cariogenic biofilms. Therefore, it is insufficient to rely solely on previous mechanisms to delineate the role of microbial preparations in the oral cavity. A more comprehensive perspective should involve considering the concepts of cariogenic biofilms. This review elucidates the latest research progress, mechanisms of action, challenges, and future research directions regarding probiotics, prebiotics, synbiotics, and postbiotics for the prevention and treatment of dental caries, taking into account the unique pathogenic mechanisms of dental caries. With an enhanced understanding of oral microbiota, personalized microbial therapy will emerge as a critical future research trend.

Biodegradation of Urethane Dimethacrylate-based materials (CAD/CAM resin-ceramic composites) and its effect on the adhesion and proliferation of Streptococcus mutans

OBJECTIVE

To investigate whether urethane dimethacrylate (UDMA) -based dental restorative materials biodegrade in the presence of Streptococcus mutans (S. mutans) and whether the monomers affect the adhesion and proliferation of S. mutans in turn.

METHODS

Cholesterol esterase and pseudocholinesterase-like activities in S. mutans were detected using p-nitrophenyl substrate. Two UDMA-based CAD/CAM resin-ceramic composites, Lava Ultimate (LU) and Vita Enamic (VE), and a light-cured UDMA resin block were co-cultured with S. mutans for 14 days. Their surfaces were characterized by scanning electron microscopy and laser microscopy, and the byproducts of biodegradation were examined by Ultra Performance Liquid Chromatography-Tandem Mass Spectrometry (UPLC-MS/MS). Then, the antimicrobial components (silver nanoparticles with quaternary ammonium salts) were added to the UDMA resin block to detect whether the biodegradation was restrained. Finally, the effect of UDMA on biofilm formation and virulence expression of S. mutans was assessed.

RESULTS

Following a 14-day immersion, the LU and UDMA resin blocks' surface roughness increased. The LU and VE groups had no UDMA or its byproducts discovered, according to the UPLC-MS/MS data, whereas the light-cured UDMA block group had UDMA, urethane methacrylate (UMA), and urethane detected. The addition of antimicrobial agents showed a significant reduction in the release of UDMA. Biofilm staining experiments showed that UDMA promoted the growth of S. mutans biofilm and quantitative real-time polymerase chain reaction results indicated that 50 μg/mL UDMA significantly increase the expression of gtfB, comC, comD, comE, and gbpB genes within the biofilm.

CONCLUSIONS

UDMA in the light-cured resin can be biodegraded to produce UMA and urethane under the influence of S. mutans. The formation of early biofilm can be promoted and the expression of cariogenic genes can be up-regulated by UDMA.

CLINICAL SIGNIFICANCE

This study focuses for the first time on whether UDMA-based materials can undergo biodegradation and verifies from a genetic perspective that UDMA can promote the formation of S. mutans biofilms, providing a reference for the rational use of UDMA-based materials in clinical practice.

The germicidal effect, biosafety and mechanical properties of antibacterial resin composite in cavity filling

In recent years, dental resin materials have become increasingly popular for cavity filling. However, these materials can shrink during polymerization, leading to microleakages that enable bacteria to erode tooth tissue and cause secondary caries. As a result, there is great clinical demand for the development of antibacterial resins. The principle of antibacterial resin includes contact killing and filler-release killing of bacteria. For contact killing, quaternary ammonium salts (QACs) and antibacterial peptides (AMPs) can be added. For filler-release killing, chlorhexidine (CHX) and nanoparticles are used. These antibacterial agents are effective against gram-positive bacteria, gram-negative bacteria, fungi, and more. Among them, QACs has a lasting antibacterial effect, and silver nanoparticles even have a certain ability to kill viruses. Biocompatibility-wise, QACs, AMPs, and CHX have low cytotoxicity to cells when added into the resin. However, nanoparticles with smaller particle sizes have higher cytotoxicity. In terms of mechanical properties, QACs, AMPs, and CHX do not negatively affect the resin. However, the addition of magnesium oxide can have a negative impact. This paper reviews the types and antibacterial principles of commonly used antibacterial resins in recent years, evaluates their antibacterial effect, biological safety, and mechanical properties, and provides references for selecting clinical filling materials.

Biodegradation of Dental Resin-Based Composite—A Potential Factor Affecting the Bonding Effect: A Narrative Review

In recent years, although resin composite has played an important role in the restoration of tooth defects, it still has several disadvantages, including being biodegraded by saliva, bacteria and other enzymes in the oral cavity, which may result in repair failure. This factor is not conducive to the long-term survival of the prosthesis in the mouth. In this article, we review the causes, influencing factors and prevention methods of resin biodegradation. Biodegradation is mainly caused by esterase in saliva and bacteria, which breaks the ester bond in resin and causes the release of monomers. The mechanical properties of the prosthesis can then be affected. Meanwhile, cathepsin and MMPs are activated on the bonding surface, which may decompose the dentin collagen. In addition, neutrophils and residual water on the bonding surface can also aggravate biodegradation. Currently, the primary methods to prevent biodegradation involve adding antibacterial agents to resin, inhibiting the activity of MMPs and enhancing the crosslinking of collagen fibers. All of the above indicates that in the preparation and adhesion of resin materials, attention should be paid to the influence of biodegradation to improve the prosthesis’s service life in the complex environment of the oral cavity.

Autonomous-Strengthening Adhesive Provides Hydrolysis-Resistance and Enhanced Mechanical Properties in Wet Conditions

The low-viscosity adhesive that is used to bond composite restorative materials to the tooth is readily damaged by acids, enzymes, and oral fluids. Bacteria infiltrate the resulting gaps at the composite/tooth interface, demineralize the tooth, and further erode the adhesive. This paper presents the preparation and characterization of a low-crosslink-density hydrophilic adhesive that capitalizes on sol-gel reactions and free-radical polymerization to resist hydrolysis and provide enhanced mechanical properties in wet environments. Polymerization behavior, water sorption, and leachates were investigated. Dynamic mechanical analyses (DMA) were conducted using water-saturated adhesives to mimic load transfer in wet conditions. Data from all tests were analyzed using appropriate statistical tests (α = 0.05). The degree of conversion was comparable for experimental and control adhesives at 88.3 and 84.3%, respectively. HEMA leachate was significantly lower for the experimental (2.9 wt%) compared to control (7.2 wt%). After 3 days of aqueous aging, the storage and rubbery moduli and the glass transition temperature of the experimental adhesive (57.5MPa, 12.8MPa, and 38.7 °C, respectively) were significantly higher than control (7.4MPa, 4.3 MPa, and 25.9 °C, respectively). The results indicated that the autonomic sol-gel reaction continues in the wet environment, leading to intrinsic reinforcement of the polymer network, improved hydrolytic stability, and enhanced mechanical properties.

Development and Assessment of Bioactive Coatings for the Prevention of Recurrent Caries Around Resin Composite Restorations

OBJECTIVE

To develop hydrophilic resin-based surface coatings containing bioactive agents (proanthocyanidins from Vitis vinifera and calcium silicate) and assess their protective role at the dentin and enamel margins of cervical restorations against demineralization under simulated conditions of high and low caries activity.

METHODS

Suboptimal resin composite restorations were placed on cervical cavity preparations on buccal and lingual surfaces of thirty-two molars after a contamination protocol. Groups were divided according to the resin-based coatings (n=8): resin without bioactive (C), resin containing 2% enriched Vitis Vinifera (VVE), and resin coat containing 10% calcium silicate (CaSi). The control group did not receive a resin (NC). To simulate a hydrolyticenzymatic degradation, specimens were subjected to 2-month storage followed by incubation in esterase at 37°C for 8 days. Afterwards, recurrent caries was induced using a pH-proteolytic model on half of the specimens to simulate high caries activity, and the other half remained in simulated body fluid (SBF). Measurements of cross-section microhardness (KHN) and infiltration with rhodamine-B assessed the micropermeability (MP), the extent of demineralization (ED), and the demineralization area (DA). Data were analyzed using analysis of variance (ANOVA) and post-hoc tests (α=0.05).

RESULTS

VVE and CaSi presented higher cross-sectional KHN values for enamel and dentin (p<0.001). The bioactive coatings resulted in lower MP, ED, and DA compared to NC (p<0.005) in enamel and dentin. CaSi coating preserved the enamel from demineralization (p=0.160).

CONCLUSION

The application of bioactive coatings represents a potential strategy to protect the enamel-dentin margins of resin restorations.

Assessing the antimicrobial properties of copper-iodide doped adhesives in an In vitro caries model: A pilot study

Context:

Recurrent caries are the leading cause of composite resin failure.

Aims:

The purpose of this pilot study was to test the efficacy of a novel copper iodide (CuI) containing dental adhesive in an in vitro caries model.

Subjects and Methods:

Streptococcus mutans and Lactobacillus acidophilus were grown individually on the complex medium for 48 h at 37°C. The pH of the mixed medium was 7.0 initially and tested every 24 h. 40 extracted teeth were prepared with standardized cavity preparations and coated with control or experimental CuI adhesives and imaged using a micro-computed tomography (microCT). Four study groups were evaluated: (1) control (2) 0.5 μg/ml CuI (3) 1.0 μg/ml CuI, 4) 5.0 μg/ml CuI. After incubation, the teeth were re-imaged using the microCT. Utilizing AnalyzePro software the three-dimensional data sets were overlaid and demineralization was measured and statistics were run.

Statistics:

Stratified ANOVA models were run to determine if there were differences between the control and experimental adhesive groups. Similarly, pH and bacterial concentrations were evaluated to ensure the viability of polymicrobial specimen.

Results and Conclusions:

Significant differences were found between the control group and the 1.0 and 5.0 CuI adhesive groups. No differences in pH were noted between the groups. Overlaid changes in demineralization were recorded as volume loss. CuI adhesives with 5 mg/ml or higher have the potential to limit tooth demineralization after bacterial penetration of a dental restoration in an in vitro caries model. Further testing is needed.

A universal adhesive containing copper nanoparticles improves the stability of hybrid layer in a cariogenic oral environment: An in situ study

PURPOSE

To evaluate how incorporating copper nanoparticles (CuNp) into a universal adhesive affects the antimicrobial activity (AMA), bond strength (μTBS), nanoleakage (NL), elastic modulus (EM) and nanohardness (NH) of resin-dentin interfaces, at 24 h (24 h) and after in situ cariogenic challenge (CC).

METHODS

CuNp (0% [control] and 0.1 wt%) was added to an adhesive. After enamel removal, the adhesives were applied to dentine surfaces. Each restored tooth was sectioned longitudinally to obtain two hemi-teeth; one of them was evaluated after 24 h, and the other was included in one of the intra-oral palatal devices placed in the mouths of 10 volunteers for 14 days in CC. After that, each hemi-tooth was removed, and any oral biofilm that formed was collected. The AMA was evaluated against Streptococcus mutans. For the 24 h and CC groups, each hemi-tooth was sectioned in the "x" direction to obtain one slice for each EM/NH evaluation. The remains of each hemi-tooth were sectioned in the "x" and "y" directions to obtain resin-dentin beams for μTBS and NL evaluation (24 h and CC). ANOVA and Tukey's test were applied (α = 0.05).

RESULTS

The presence of CuNp significantly improved AMA as well as all of the evaluated properties (24 h; p < 0.05). Although the adhesive properties (μTBS/NL) for all groups decreased after CC (p < 0.05), the adhesive containing CuNp showed higher μTBS and lower NL as compared to the copper-free adhesive (p < 0.05). The incorporation of CuNp maintained NH/EM values after CC (p < 0.05).

CONCLUSIONS

Adding 0.1% CuNp to an adhesive may provide antimicrobial activity and increase its bonding and mechanical properties, even under a cariogenic challenge.

SIGNIFICANCE

This is the first in situ study proving that incorporating CuNp into an adhesive is an achievable alternative to provide antimicrobial properties and improve the integrity of the hybrid layer under in situ cariogenic challenge.

A novel methacrylate derivative polymer that resists bacterial cell-mediated biodegradation

This study tests biodegradation resistance of a custom synthesized novel ethylene glycol ethyl methacrylate (EGEMA) with ester bond linkages that are external to the central polymer backbone when polymerized. Ethylene glycol dimethacrylate (EGDMA) with internal ester bond linkages and EGEMA discs were prepared in a polytetrafluoroethylene (PTFE) mold using 40 μl macromer and photo/co-initiator mixture cured for 40 s at 1000 mW/cm² . The discs were stored in the constant presence of Streptococcus mutans (S. mutans) in Todd Hewitt Yeast + Glucose (THYE+G) media up to 9 weeks (n = 8 for each macromer type) and physical/mechanical properties were assessed. Initial measurements EGEMA versus EGDMA polymer discs showed equivalent degree of conversion (45.69% ± 2.38 vs. 46.79% ± 4.64), diametral tensile stress (DTS; 8.12± 2.92 MPa vs. 6.02 ± 1.48 MPa), and low subsurface optical defects (0.41% ± 0.254% vs. 0.11% ± 0.074%). The initial surface wettability (contact angle) was slightly higher (p ≤ .012) for EGEMA (62.02° ± 3.56) than EGDMA (53.86° ± 5.61°). EGDMA showed higher initial Vicker's hardness than EGEMA (8.03 ± 0.88 HV vs. 5.93 ± 0.69 HV; p ≤ .001). After 9 weeks of S. mutans exposure, EGEMA (ΔDTS-1.30 MPa) showed higher resistance to biodegradation effects with a superior DTS than EGDMA (ΔDTS-6.39 MPa) (p = .0039). Visible and scanning electron microscopy images of EGEMA show less surface cracking and defects than EGDMA. EGDMA had higher loss of material (18.9% vs. 8.5%, p = .0009), relative changes to fracture toughness (92.5% vs. 49.2%, p = .0022) and increased water sorption (6.1% vs. 1.9%, p = .0022) compared to EGEMA discs. The flipped external ester group linkage design is attributed to EGEMA showing higher resistance to bacterial degradation effects than an internal ester group linkage design methacrylate.

Characteristics of biofilm-induced degradation at resin-dentin interfaces using multiple combinations of adhesives and resins

OBJECTIVE

We aimed to evaluate morphological, mechanical and chemical characteristics at resin-dentin interfaces using multiple combinations of adhesives and resins after a short-term biofilm-induced degradation.

METHODS

Cervical cavities were prepared in bovine incisors, treated by Clearfil SE Bond 2 (SE) or FL-Bond II (FL), restored by Clearfil Majesty ES Flow (ES) or Beautifil Flow Plus (BFP) and grouped into SE-ES, SE-BFP, FL-ES and FL-BFP. After biofilm challenge, interfacial gaps and dentin wall lesions were examined by optical coherence tomography (OCT). Gap depth (GD), gap pattern scale (GPS) and dentin wall lesion depth (WLD) were evaluated from confocal laser scanning microscope. Microhardness of dentin lesions was measured with a Vickers microhardness tester. Chemical elements in resins and dentin wall lesions were analyzed by scanning electron microscopy and energy dispersive X-ray spectrometry (SEM/EDS). Morphological structures of interfacial gaps were observed by SEM.

RESULTS

OCT could detect adhesive-dentin-bonded and adhesive-dentin-debonded gaps. SE-containing groups showed significantly lower GPS than FL-containing groups. FL-BFP showed significantly lower WLD than FL-ES. Microhardness of dentin wall lesions was higher than that of outer lesions and they showed significant differences in FL-BFP. SE-BFP showed a lower GPS curve and higher intensities of Ca and P in the upper half of dentin wall lesions than other groups. From SEM, microgaps between filler and matrix, break and loss of matrix, separation of adhesive matrix with hybrid layer occurred at interfacial gaps.

SIGNIFICANCE

The morphological, mechanical and chemical characteristics of resin-dentin interfacial degradation depend on the component and chemistry of restorative materials.

Urethane Dimethacrylate Influences the Cariogenic Properties of Streptococcus Mutans

Concerns regarding unbound monomers in dental composites have increased with the increased usage of these materials. This study assessed the biological effects of urethane dimethacrylate (UDMA), a common monomer component of dental composite resins, on the cariogenic properties of Streptococcus mutans. Changes in the growth rate, biofilm formation, interaction with saliva, surface hydrophobicity, adhesion, glucan synthesis, sugar transport, glycolytic profiles, and oxidative- and acid-stress tolerances of S. mutans were evaluated after growing the cells in the presence and absence of UDMA. The results indicated that UDMA promotes the adhesion of S. mutans to the underlying surfaces and extracellular polysaccharide synthesis, leading to enhanced biofilm formation. Furthermore, UDMA reduced the acid tolerance of S. mutans, but enhanced its tolerance to oxidative stress, thus favoring the early stage of biofilm development. UDMA did not significantly affect the viability or planktonic growth of cells, but diminished the ability of S. mutans to metabolize carbohydrates and thus maintain the level of intracellular polysaccharides, although the tendency for sugar transport increased. Notably, UDMA did not significantly alter the interactions of bacterial cells with saliva. This study suggests that UDMA may potentially contribute to the development of secondary caries around UDMA-containing dental materials by prompting biofilm formation, enhancing oxidative tolerance, and modulating carbon flow.

Detection and analysis of early degradation at resin-dentin interface by optical coherence tomography (OCT) and confocal laser scanning microscope (CLSM)

OBJECTIVE

To evaluate early degradation at resin-dentin interface using non-invasive swept-source optical coherence tomography (SS-OCT) and confocal laser scanning microscope (CLSM).

METHODS

Self-etch adhesives and resin-composites containing bisphenol-glycidyl-dimethacrylate (Bis-GMA), which is one of the most widely used monomers in restorative materials, were investigated in this study. Forty cervical cavities were prepared in bovine incisors and applied by the adhesive with/without Bis-GMA (AdhesiveBG/Adhesive), filled by the resin with/without Bis-GMA (ResinBG/Resin) and then challenged by cariogenic biofilm (37 °C, 24 h). Gap Formation and dentin demineralization around resin-composites were observed by SS-OCT and CLSM.

RESULTS

Three types of resin-dentin interfacial degradation could be detected from SS-OCT. Type I-dentin demineralization around resin without gap, showing feather-shaped dark zones without bright scattered lines at resin-dentin interfaces. Type II-dentin demineralization around resin with adhesive-dentin bonded gaps, showing feather-shaped dark zones with bright scattered lines at resin-dentin interfaces. Type III-dentin demineralization around resin with adhesive-dentin debonded gaps, showing edge-shaped dark zones with bright scattered lines at resin-dentin interface. From CLSM, the groups were compared in gap scale (GS), gap depth (GD), gap width (GW) and dentin wall lesion depth (WLD). Bis-GMA-containing adhesive groups showed significantly lower GS than Bis-GMA-free adhesive groups. Bis-GMA-containing resin groups showed significantly lower WLD than Bis-GMA-free resin groups. However, they did not show significant differences in GD and GW.

CONCLUSION

Three types of early degradation at resin-dentin interface can be noninvasively detected by SS-OCT. Bis-GMA-containing and Bis-GMA-free restorative materials show differences in gap scale and dentin wall lesion depth.

CLINICAL SIGNIFICANCE

SS-OCT can nondestructively detect early resin-dentin interfacial degradation. Gap scale can be used as a parameter to evaluate the risk factor of gaps.

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.

Synthesis of di- and triacrylamides with tertiary amine cores and their evaluation as monomers in dental adhesive interfaces

PURPOSE/AIM

In an attempt to increase the service life of dental adhesive interfaces, more hydrolytically and enzymatically-stable methacrylate alternatives, such as methacrylamides, have been proposed. The aim of this study was to investigate polymerization behavior, as well as mechanical and biological properties of experimental adhesives containing multi-functional acrylamides.

MATERIALS AND METHODS

Multi-functional acrylamides (N,N-Bis[(3-methylaminoacryl)propyl]methylamine - BMAAPMA, Tris[(2-methylaminoacryl)ethyl]amine - TMAAEA, N,N'-bis(acrylamido) 1,4-diazepane - BAADA, N,N-Diethyl-1,3-bis(acrylamido)propane - DEBAAP) or HEMA (2-Hydroxyethyl methacrylate - control) were added at 40 wt% to UDMA. 0.2 wt% DMPA and 0.4 wt% DPI-PF6 were used as initiators. Polymerization kinetics was followed in real-time in near-IR during photoactivation (320-500 nm, at 630 mW/cm²). Water sorption/solubility and flexural strength/modulus were measured according to ISO 4049. ¹H NMR was used to assess monomer degradation kinetics. MTT assay was used to assess cytotoxicity against OD-21 and DPSC cells. Biofilm formation and adhesion were assessed by Luciferase Assay and Impingement technique, respectively. Solvated adhesives (40 vol% ethanol) were used to test interfacial adhesion strength. The results were analyzed by ANOVA/Tukey's test (α = 0.05).

RESULTS

In general, the pure methacrylate mixture had higher rate of polymerization (Rp_max), degree of conversion (DC) at Rp_max, and final DC than the acrylamides. Flexural properties after water storage decreased between 11 and 65%, more markedly for acrylamides. Interfacial bond strength was greater and more stable long-term for the newly synthesized acrylamide formulations (less than 4% reduction at 6 months) compared to the methacrylate experimental control (42% reduction at 6 months). HEMA degraded by almost 90%, while the acrylamides showed no degradation in acidic conditions. Cytotoxicity and biofilm formation, in general, were similar for all groups.

CONCLUSIONS

Despite demonstrating high water sorption, the acrylamide-containing materials had similar mechanical and biological properties and enhanced interfacial bond strength stability compared to the methacrylate control.

Interaction between the Oral Microbiome and Dental Composite Biomaterials: Where We Are and Where We Should Go

Dental composites are routinely placed as part of tooth restoration procedures. The integrity of the restoration is constantly challenged by the metabolic activities of the oral microbiome. This activity directly contributes to a less-than-desirable half-life for the dental composite formulations currently in use. Therefore, many new antimicrobial dental composites are being developed to counteract the microbial challenge. To ensure that these materials will resist microbiome-derived degradation, the model systems used for testing antimicrobial activities should be relevant to the in vivo environment. Here, we summarize the key steps in oral microbial colonization that should be considered in clinically relevant model systems. Oral microbial colonization is a clearly defined developmental process that starts with the formation of the acquired salivary pellicle on the tooth surface, a conditioned film that provides the critical attachment sites for the initial colonizers. Further development includes the integration of additional species and the formation of a diverse, polymicrobial mature biofilm. Biofilm development is discussed in the context of dental composites, and recent research is highlighted regarding the effect of antimicrobial composites on the composition of the oral microbiome. Future challenges are addressed, including the potential of antimicrobial resistance development and how this could be counteracted by detailed studies of microbiome composition and gene expression on dental composites. Ultimately, progress in this area will require interdisciplinary approaches to effectively mitigate the inevitable challenges that arise as new experimental bioactive composites are evaluated for potential clinical efficacy. Success in this area could have the added benefit of inspiring other fields in medically relevant materials research, since microbial colonization of medical implants and devices is a ubiquitous problem in the field.

Effect of bioactive compounds on the regulation of quorum sensing network-associated genes and virulence in Streptococcus mutans-A systematic review

OBJECTIVE

The aim of this study was to systematically review the literature on the effect of bioactive compounds and their regulation of quorum sensing (QS)-related and/or -regulated-virulence genes expression in Streptococcus mutans.

DESIGN

The search strategy was conducted through the electronic databases Pubmed, Scopus, and Web of Science for studies reporting the effects of natural and synthetic bioactive compounds on the regulation of QS-associated and/or -regulated genes of S. mutans.

RESULTS

After full-text reading, 19 studies met the inclusion criteria, in most of them, QS-inhibitors from synthetic origin were evaluated, 16 articles described the effect of the compounds on biofilm formation cultivated in vitro and five studies described these effects on adhesion of biofilm-producing cells. Only 2 studies analyzed the potential target-component of the QS.

CONCLUSIONS

Mostly, the bioactive compounds affected the expression of QS-associated and/or -regulated genes and virulence traits (e.g. adhesion, biofilm formation, acid stress tolerance) of S. mutans. Further studies are necessary to elucidate the target-specific QS-system constituent used by bioactive compounds to achieve QS inhibition as well as validate the use of these compounds in controlling dental caries.

Alternative monomer for BisGMA-free resin composites formulations

OBJECTIVE

Water sorption, high volumetric shrinkage, polymerization stress, and potential estrogenic effects triggered by leached compounds are some of the major concerns related to BisGMA-TEGDMA co-monomer systems used in dental composites. These deficiencies call for the development of alternative organic matrices in order to maximize the clinical lifespan of resin composite dental restorations. This study proposes BisGMA-free systems based on the combination of UDMA and a newly synthesized diurethane dimethacrylate, and evaluates key mechanical and physical properties of the resulting materials.

METHODS

2EMATE-BDI (2-hydroxy-1-ethyl methacrylate) was synthesized by the reaction between 2-hydroxy-1-ethyl methacrylate with a difunctional isocyanate (1.3-bis (1- isocyanato-1-methylethylbenzene) - BDI). The compound was copolymerized with UDMA (urethane dimethacrylate) at 40 and 60wt%. UDMA copolymerizations with 40 and 60wt% TEGDMA (triethylene glycol dimethacrylate) were tested as controls, as well as a formulation based in BisGMA (bisphenol A-glycidyl methacrylate)-TEGDMA 60:40% (BT). The organic matrices were made polymerizable by the addition of DMPA (2.2-dimethoxyphenoxy acetophenone) and DPI-PF6 (diphenyliodonium hexafluorophosphate) at 0.2 and 0.4wt%, respectively. Formulations were tested as composite with the addition of 70wt% inorganic content consisting of barium borosilicate glass (0.7μm) and fumed silica mixed in 95 and 5wt%, respectively. All photocuring procedures were carried out by a mercury arc lamp filtered to 320-500nm at 800mW/cm2. The experimental resin composites were tested for kinetics of polymerization and polymerization stress in real time. Flexural strength, elastic modulus, water sorption, and solubility were assessed according to ISO 4049. Biofilm formation was analyzed after 24h by luciferase assay. Data were statistically analyzed by one-way ANOVA and Tukey's test (α≤0.05).

RESULTS

In general, the addition of 2EMATE-BDI into the formulations decreased the maximum rate of polymerization (RPMAX), the degree of conversion at RPMAX (DC at RPMAX), and the final degree of conversion (final DC). However, these reductions did not compromise mechanical properties, which were comparable to the BT controls, especially after 7-day water incubation. The incorporation of 60wt% 2EMATE-BDI reduced water sorption of the composite. 2EMATE-BDI containing formulations showed reduction in polymerization stress of 30% and 50% in comparison to BT control and TEGDMA copolymerizations, respectively. Biofilm formation was similar among the tested groups.

SIGNIFICANCE

The use of the newly synthesized diurethane dimethacrylate as co-monomer in dental resin composite formulations seems to be a promising option to develop polymers with low-shrinkage and potentially decreased water degradation.

Effect of the photoinitiator system on the polymerization of secondary methacrylamides of systematically varied structure for dental adhesive applications

OBJECTIVE

The aim of this study was to investigate the influence of the photoinitiator system on the polymerization kinetics of methacrylamide-based monomers as alternatives to methacrylates in adhesives dental-based materials.

METHODS

In total, 16 groups were tested. Monofunctional monomers (2-hydroxyethyl methacrylate) - HEMA; (2-hydroxy-1-ethyl methacrylate) -2EMATE, (2-hydroxyethyl methacrylamide) - HEMAM; and (N-(1-hydroxybutan-2-yl) methacrylamide) -2EM; were combined with bifunctional monomers containing the same polymerizing moieties as the monofunctional counterparts (HEMA-BDI; 2EMATE-BDI; HEMAM-BDI; and 2EM-BDI) at 50/50 M ratios. BHT was used as inhibitor (0.1 wt%) and the photoinitiators used were: CQ + EDMAB (0.2/0.8), BAPO (0.2), IVOCERIN (0.2), and DMPA (0.2), in wt%. The polymerization kinetics were monitored using Near-IR spectroscopy (∼6165 cm-1) in real-time while the specimens were photoactivated with a mercury arc lamp (Acticure 2; 320-500 nm, 300 mW/cm2) for 5 min, and maximum rate of polymerization (Rpmax, in %.s-̄1), degree of conversion at Rpmax (DC@Rpmax, in %), and the final degree of conversion (Final DC, in %) were calculated (n = 3). Initial viscosity was measured with an oscillating rheometer (n = 3). Data were analyzed using Two-way ANOVA for the polymerization kinetics and one-way ANOVA for the viscosity. Multiple comparisons were made using the Tukey's test (∝ = 0.05).

RESULTS

There was statistically significant interaction between monomer and photoinitiator (p < 0.001). For the methacrylates groups, the highest Rpmax was observed for HEMA + DMPA and 2EMATE + BAPO. For methacrylamides groups, the highest Rpmax were observed for HEMAM and 2EM, both with DMPA. Final DC was higher for the methacrylate groups, in comparison with methacrylamide groups, independent of the photoinitiators. However, for the methacrylamide groups, the association with BAPO led to the lowest values of DC. In terms of DC@Rpmax, methacrylate-based systems showed significantly higher values than methacrylamide formulations. DMPA and Ivocerin led to higher values than CQ/EDMAB and BAPO in methacrylamide-based compounds. BAPO systems showed de lowest values for both HEMA and HEMAM formulations. For the viscosity (Pa.s), only 2EM had higher values (1.60 ± 0.15) in comparison with all monomers. In conclusion, polymerization kinetics was affected by the photoinitiators for both monomers. Viscosity was significantly increased with the use of secondary methacrylamide.

SIGNIFICANCE

this work demonstrated the feasibility of using newly-synthesized methacrylamide monomers in conjunction with a series of initiator systems already used in commercial materials.

Multifunctional monomer acts as co-initiator and crosslinker to provide autonomous strengthening with enhanced hydrolytic stability in dental adhesives

OBJECTIVE

The purpose of this study was to evaluate a new synthesized multifunctional monomer, aminosilane functionalized methacrylate (ASMA), containing polymerizable methacrylate, tertiary amine, and methoxysilane functionalities in dental adhesive formulations, and to investigate the polymerization kinetics, leachates, thermal and mechanical properties of copolymers.

METHODS

Adhesive contained HEMA/BisGMA (45/55, w/w) was used as a control, and mixtures based on HEMA/BisGMA/ASMA at the mass ratio of 45/(55-x)/x were used as experimental adhesive. Adhesives were characterized with regard to water miscibility, photo-polymerization behavior (Fourier transform infrared spectroscopy, FTIR), leached co-monomers (high performance liquid chromatography, HPLC), thermal properties (modulated differential scanning calorimeter, MDSC), and mechanical properties (dynamic mechanical analyzer, DMA). Stress relaxation times and the corresponding moduli, obtained from stress relaxation tests, are used in a simulated linear loading case.

RESULTS

As compared to the control, ASMA-containing adhesives showed higher water miscibility, lower viscosity, improved monomer-to-polymer conversion, significantly greater T_g and rubbery modulus. HPLC results indicated a substantial reduction of leached HEMA (up to 85wt%) and BisGMA (up to 55wt%) in ethanol. The simulation reveals that the ASMA-containing adhesive becomes substantially stiffer than the control.

SIGNIFICANCE

ASMA monomer plays multiple roles, i.e. it serves as both a co-initiator and crosslinker while also providing autonomous strengthening and enhanced hydrolytic stability in the adhesive formulations. This multifunctional monomer offers significant promise for improving the durability of the adhesive at the composite/tooth interface.

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

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

Esterases affect the physical properties of materials used to seal the endodontic space

Materials used to seal the endodontic space are subjected to enzymatic degradative activities of body fluids and bacteria.

OBJECTIVES

To assess effects of simulated human salivary, blood and bacterial esterases (SHSE) on physical properties of typical restorative material and root canal sealers.

METHODS

Specimens of set methacrylate-based resin composite (BisfilTM2B; RC), calcium-silicate sealer (EndoSequence®; BC) or epoxy-resin sealer (AH-Plus®; ER) were tested after up to 28Days exposure to phosphate buffered saline (PBS) or SHSE, using ANSI/ADA-57:2000 and ISO-6876:2012.

RESULTS

Regardless of media, microhardness increased with time for BC remained unchanged for ER and decreased for RC (p < 0.05). SHSE moderated the increase for BC compared to PBS (28.0 ± 4.8 vs. 38.1 ± 7.9 KHN) at 7Days, and enhanced the decrease for RC at 7Days (55.6 ± 7.1 vs. 66.3 ± 6.5 KHN) and 28Days (52.3 ± 9.2 vs. 62.6 ± 8.5 KHN). Compressive strength was enhanced only for BC by either media. BC expanded with time for both incubation conditions; SHSE moderated the expansion compared to PBS at 7Days (0.026 ± 0.01% vs. 0.049 ± 0.007%). Shrinkage of ER was similar for both incubation media and was lower than that for RC (p < 0.05). Shrinkage of RC was enhanced by SHSE compared to PBS at 7Days (0.5 ± 0.07% vs. 0.38 ± 0.08%). Weight loss was lowest for ER and highest for BC (p < 0.05). It was enhanced by SHSE compared to PBS for BC at 28Days (2.40 ± 0.2 vs. 2.96 ± 0.19 W L%), and for RC at 7Days (0.54 ± 0.09 vs. 0.80 ± 0.1 W L%).

SIGNIFICANCE

Simulated body fluids and bacterial esterases affected the physical properties of test materials, suggesting potential impacts on sealing ability and resistance to bacterial ingress, and tooth strength ultimately affecting their clinical performance.

Rapid Antibiofilm Effect of Ag/ZnO Nanocomposites Assisted by Dental LED Curing Light against Facultative Anaerobic Oral Pathogen Streptococcus mutans

The integration of nanomaterials with clinical therapeutic instruments is a promising approach to improve the effects of nanomaterials. We reported an efficient synergistic antibacterial strategy formed through the combination of Ag/ZnO nanocomposites with a light-emitting diode (LED) curing light, which is a commonly used small instrument in dental clinics. The as-designed integration depicted a significantly enhanced bactericidal effect on facultative anaerobic oral pathogen Streptococcus mutans (S. mutans) both in planktonic and biofilm phases over a very short irradiation time (≤5 min). Further study showed that the combination of LED and Ag/ZnO nanocomposites induced more ·OH and ·O₂^- generation, which is responsible for the enhanced antibacterial activity. Moreover, this combination could destroy S. mutans biofilm by killing the bacteria embedded within biofilm, inhibiting exopolysaccharide production and down-regulating the biofilm-related gene expression. Therefore, it is proposed that this combination could be applied in dental clinics to realize dental caries prevention and dental restoration simultaneously.

Evaluation of biofilm formation on novel copper-catalyzed azide-alkyne cycloaddition (CuAAC)-based resins for dental restoratives

OBJECTIVE

For the past several decades, the resins used in dental restorations have been plagued with numerous problems, including their implication in biofilm formation and secondary caries. The need for alternative resins is critical, and evaluation of biofilm formation on these resins is essential. The aim of this study was to evaluate in vitro biofilm formation on the surface of novel copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC)-based resins and composites.

METHODS

CuAAC-based resins/composites made from varying azide monomers and different copper concentrations were compared with BisGMA-TEGDMA resins/composites that served as the control. Biofilms were formed using a mono-species model containing a luciferase-expressing strain of Streptococcus mutans. Luciferase activity was measured and the number of viable bacteria was enumerated on biofilms associated with each resin and composite.

RESULTS

A significant reduction (p<0.05) in luciferase activity, and the number of viable bacteria recovered from biofilms on CuAAC-based resins and composites was observed in comparison to biofilms associated with the BisGMA-TEGDMA controls.

SIGNIFICANCE

CuAAC-based resins do still allow for the formation of biofilms; however, the statistically significant reduction of growth that was associated with the CuAAC resin may enhance the longevity of restorations that incorporate CuAAC-based materials.

Synthesis and characterization of Ciprofloxacin-containing divinyl oligomers and assessment of their biodegradation in simulated salivary esterase

OBJECTIVE

Two leading causes contributing to dental restoration replacement are the marginal breakdown at the composite/dentin interface and secondary caries mediated by bacteria. The objective of the present study was to synthesize oligomers which incorporated enhanced bio-stability but would also be able to generate antimicrobial function if they underwent degradation.

METHODS

Stability was incorporated into the oligomers by generating structural features that would physically hinder the availability of hydrolytically sensitive groups in the oligomers. As a proof-of concept for the antibacterial feature, antimicrobial function was achieved by covalently incorporating Ciprofloxacin (CF) into the backbone of cross-linking divinyl oligomers (referred to as EDV and HLH-CFPEG). The hydrolytic stability of the oligomers was studied in simulated human salivary esterase and compared to the commercial monomer 2,2-bis[4(2-hydroxy-3-methacryloxypropoxy)-phenyl]propane (BisGMA).

RESULTS

Both drug oligomers were found to be significantly more stable than BisGMA. Upon degradation, both drug oligomers released CF differentially in free form. Polymer synthesis from resin formulations containing 15wt% HLH-CFPEG showed a high degree of vinyl group conversion and gel content, and under hydrolytic conditions showed the release of CF during a 28-day monitoring study period.

SIGNIFICANCE

HLH-CFPEG can be used in dental resin adhesive systems for local delivery of CF to the marginal interface. Minimizing the growth of Streptococcus mutans at the marginal site can improve longevity by reducing esterase activity derived specifically from S. mutans.

Gene expression and protein synthesis of esterase from Streptococcus mutans are affected by biodegradation by-product from methacrylate resin composites and adhesives

An esterase from S. mutans UA159, SMU_118c, was shown to hydrolyze methacrylate resin-based dental monomers.

OBJECTIVE

To investigate the association of SMU_118c to the whole cellular hydrolytic activity of S. mutans toward polymerized resin composites, and to examine how the bacterium adapts its hydrolytic activity in response to environmental stresses triggered by the presence of a resin composites and adhesives biodegradation by-product (BBP).

MATERIALS AND METHODS

Biofilms of S. mutans UA159 parent wild strain, SMU_118c knockout strain (ΔSMU_118c), and SMU_118c complemented strain (ΔSMU_118cC) were incubated with photo-polymerized resin composite. High performance liquid chromatography was used to quantify the amount of a universal 2,2-Bis[4-(2-hydroxy-3-methacryloxypropoxy)phenyl]propane (bisGMA)-derived BBP, bishydroxy-propoxy-phenyl-propane (bisHPPP) in the media. Fluorescence in situ hybridization (FISH) and quantitative proteomic analysis were used to measure SMU_118c gene expression and production of SMU_118c protein, respectively, from biofilms of S. mutans UA159 wild strain that were cultured with bisHPPP.

RESULTS

The levels of bisHPPP released from composite were similar for ΔSMU_118c and media control, and these were significantly lower compared to the parent wild-strain UA159 and complemented strain (ΔSMU_118cC) (p < 0.05). Gene expression of SMU_118c and productions of SMU_118c protein were higher for bisHPPP incubated biofilms (p < 0.05).

SIGNIFICANCE

This study suggests that SMU_118c is a dominant esterase in S. mutans and capable of catalyzing the hydrolysis of the resinous matrix of polymerized composites and adhesives. In turn, the bacterial response to BBP was to increase the expression of the esterase gene and enhance esterase production, potentially accelerating the biodegradation of the restoration, adhesive and restoration-tooth interface, ultimately contributing to premature restoration failure.

STATEMENT OF SIGNIFICANCE

We recently reported (Huang et al., 2018) on the isolation and initial characterization of a specific esterase (SMU_118c) from S. mutans that show degradative activity toward the hydrolysis of dental monomers. The current study further characterize this enzyme and shows that SMU_118c is a dominant degradative esterase activity in the cariogenic bacterium S. mutans and is capable of catalyzing the hydrolysis of the resinous matrix of polymerized composites and adhesives. In turn, the bacterial response to biodegradation by-products from composites and adhesives was to increase the expression of the esterase gene and enhance esterase production, accelerating the biodegradation of the restoration, adhesive and the restoration-tooth interface, potentially contributing to the pathogenesis of recurrent caries around resin composite restorations.

Influence of Dental Prosthesis and Restorative Materials Interface on Oral Biofilms

Oral biofilms attach onto both teeth surfaces and dental material surfaces in oral cavities. In the meantime, oral biofilms are not only the pathogenesis of dental caries and periodontitis, but also secondary caries and peri-implantitis, which would lead to the failure of clinical treatments. The material surfaces exposed to oral conditions can influence pellicle coating, initial bacterial adhesion, and biofilm formation, due to their specific physical and chemical characteristics. To define the effect of physical and chemical characteristics of dental prosthesis and restorative material on oral biofilms, we discuss resin-based composites, glass ionomer cements, amalgams, dental alloys, ceramic, and dental implant material surface properties. In conclusion, each particular chemical composition (organic matrix, inorganic filler, fluoride, and various metallic ions) can enhance or inhibit biofilm formation. Irregular topography and rough surfaces provide favorable interface for bacterial colonization, protecting bacteria against shear forces during their initial reversible binding and biofilm formation. Moreover, the surface free energy, hydrophobicity, and surface-coating techniques, also have a significant influence on oral biofilms. However, controversies still exist in the current research for the different methods and models applied. In addition, more in situ studies are needed to clarify the role and mechanism of each surface parameter on oral biofilm development.

Biostable, antidegradative and antimicrobial restorative systems based on host-biomaterials and microbial interactions

OBJECTIVES

Despite decades of development and their status as the restorative material of choice for dentists, resin composite restoratives and adhesives exhibit a number of shortcomings that limit their long-term survival in the oral cavity. Herein we review past and current work to understand these challenges and approaches to improve dental materials and extend restoration service life.

METHODS

Peer-reviewed work from a number of researchers as well as our own are summarized and analyzed. We also include yet-unpublished work of our own. Challenges to dental materials, methods to assess new materials, and recent material improvements and research directions are presented.

RESULTS

Mechanical stress, host- and bacterial-biodegradation, and secondary caries formation all contribute to restoration failure. In particular, several host- and bacterial-derived enzymes degrade the resin and collagen components of the hybrid layer, expanding the marginal gap and increasing access to bacteria and saliva. Furthermore, the virulence of cariogenic bacteria is up-regulated by resin biodegradation by-products, creating a positive feedback loop that increases biodegradation. These factors work synergistically to degrade the restoration margin, leading to secondary caries and restoration failure. Significant progress has been made to produce hydrolytically stable resins to resist biodegradation, as well as antimicrobial materials to reduce bacterial load around the restoration. Ideally, these two approaches should be combined in a holistic approach to restoration preservation.

SIGNIFICANCE

The oral cavity is a complex environment that poses an array of challenges to long-term material success; materials testing conditions should be comprehensive and closely mimic pathogenic oral conditions.

Responsive antimicrobial dental adhesive based on drug-silica co-assembled particles

Most dental resin composite restorations are replacements for failing restorations. Degradation of the restoration-tooth margins by cariogenic bacteria results in recurrent caries, a leading cause for restoration failure. Incorporating antimicrobial agents in dental adhesives could reduce interfacial bacterial count and reduce recurrent caries rates, inhibit interfacial degradation, and prolong restoration service life, while minimizing systemic exposure. Direct addition of antimicrobial compounds into restorative materials have limited release periods and could affect the integrity of the material. Attempts to incorporate antimicrobial within mesoporous silica nanoparticles showed theoretical promise due to their physical robustness and large available internal volume, yet yielded short-term burst release and limited therapeutic payload. We have developed novel broad-spectrum antimicrobial drug-silica particles co-assembled for long-term release and high payload incorporated into dental adhesives. The release of the drug, octenidine dihydrochloride, is modulated by the oral degradative environment and mathematically modeled to predict effective service life. Steady-state release kills cariogenic bacteria, preventing biofilm formation over the adhesive surface, with no toxicity. This novel material could extend dental restoration service life and may be applied to other long-term medical device-tissue interfaces for responsive drug release upon bacterial infection.

STATEMENT OF SIGNIFICANCE

This study describes a novel dental adhesive that includes a broad-spectrum antimicrobial drug-silica co-assembled particles for long-term antimicrobial effect. The release of the drug, octenidine dihydrochloride, is modulated by the oral degradative environment and mathematically modeled to predict effective release throughout the service life of the restoration. Steady-state drug-release kills caries-forming bacteria, preventing biofilm formation over the adhesive surface, without toxicity. This novel material could extend dental restoration service life and may be applied to other long-term medical device-tissue interfaces for responsive drug release upon bacterial infection. Since recurrent cavities (caries) caused by bacteria are the major reason for dental filling failure, this development represents a significant contribution to the biomaterials field in methodology and material performance.

An Overview of Dental Adhesive Systems and the Dynamic Tooth-Adhesive Interface

From the conception of resin-enamel adhesion to today's contemporary dental adhesive systems, clinicians are no longer afraid of exploring the many advantages brought by adhesive restorative concepts. To maximize the performance of adhesive-based restorative procedures, practitioners must be familiar with the mechanism of adhesion, clinical indications, proper handling, the inherent limitations of the materials and the biological challenges. This review provides an overview of the current status of restorative dental adhesives, their mechanism of adhesion, mechanisms of degradation of dental adhesive interfaces, how to maximize performance, and future trends in adhesive dentistry.

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.

Curcumin as a Promising Antibacterial Agent: Effects on Metabolism and Biofilm Formation in S. mutans

Streptococcus mutans (S. mutans) has been proved to be the main aetiological factor in dental caries. Curcumin, a natural product, has been shown to exhibit therapeutic antibacterial activity, suggesting that curcumin may be of clinical interest. The objective of this study is to evaluate the inhibitory effects of curcumin on metabolism and biofilm formation in S. mutans using a vitro biofilm model in an artificial oral environment. S. mutans biofilms were treated with varying concentrations of curcumin. The biofilm metabolism and biofilm biomass were assessed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and the crystal violet assay. Confocal laser scanning microscopy was used to analyse the composition and extracellular polysaccharide content of S. mutans biofilm after curcumin treatment. The biofilm structure was evaluated using a scanning electron microscope. The gene expression of virulence-related factors was assessed by real-time PCR. The antibiofilm effect of curcumin was compared with that of chlorhexidine. The sessile minimum inhibitory concentration (SMIC50%) of curcumin against S. mutans biofilm was 500 μM. Curcumin reduced the biofilm metabolism from 5 min to 24 h. Curcumin inhibited the quantity of live bacteria and total bacteria in both the short term (5 min) and the long term. Moreover, curcumin decreased the production of extracellular polysaccharide in the short term. The expression of genes related to extracellular polysaccharide synthesis, carbohydrate metabolism, adherence, and the two-component transduction system decreased after curcumin treatment. The chlorhexidine-treated group showed similar results. We speculate that curcumin has the capacity to be developed as an alternative agent with the potential to reduce the pathogenic traits of S. mutans biofilm.

Antibacterial activities of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) against planktonic and biofilm growing Streptococcus mutans

The aim of this study was to evaluate the potential antibacterial activities of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) against planktonic and biofilm modes of Streptococcus mutans (S. mutans). The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were determined. The effects on planktonic growth and biofilm metabolic activity were evaluated by growth curve determination and MTT assay, respectively. Then, colony forming unit (CFU) counting, scanning electron microscopy (SEM) and real-time PCR were performed to further investigate the actions of DHA and EPA on exponential phase-S. mutans. Confocal laser scanning microscopy (CLSM) was used to detect the influences on mature biofilms. The MICs of DHA and EPA against S. mutans were 100 μM and 50 μM, respectively; the MBC of both compounds was 100 μM. In the presence of 12.5 μM-100 μM DHA or EPA, the planktonic growth and biofilm metabolic activity were reduced in varying degrees. For exponential-phase S. mutans, the viable counts, the bacterial membranes and the biofilm-associated gene expression were damaged by 100 μM DHA or EPA treatment. For 1-day-old biofilms, the thickness was decreased and the proportion of membrane-damaged bacteria was increased in the presence of 100 μM DHA or EPA. These results indicated that, DHA and EPA possessed antibacterial activities against planktonic and biofilm growing S. mutans.