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.

Relationships between composite roughness and Streptococcus mutans biofilm depth under shear in vitro.

OBJECTIVE: To investigate the effect of substrate, surface roughness, and hydraulic residence time (HRT) on Streptococcus mutans biofilms growing on dental composites under conditions relevant to the oral cavity. METHODS: Dental composites were prepared with varying amounts of polishing and incubated in a CDC bioreactor with an approximate shear of 0.4 Pa. S. mutans biofilms developed in the bioreactors fed sucrose or glucose and at 10-h or 40-h HRT for one week. Biofilms were characterized by confocal laser microscopy (CLM). Composite surface roughness was characterized by optical profilometry, and pre- and post-incubation composite surface fine structure and elemental composition were determined using scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). RESULTS: Polishing had a significant impact on surface roughness, varying by a factor of 15 between the polished samples and the unpolished control. S. mutans biofilms grew statistically significantly thicker on the unpolished composites. Biofilm thickness was greater at shorter 10-h HRT compared to 40-h HRT. In most cases, biofilm thickness was not statistically significantly greater in sucrose-fed bioreactors than in glucose-fed bioreactors. SEM-EDS analysis did not identify any significant change in elemental composition after aging. CONCLUSIONS: Accurate characterization of oral cavity biofilms must consider shear forces and the use of techniques that minimize alteration of the biofilm structure. Under shear, surface smoothness is the most important factor determining S. mutans biofilm thickness followed by HRT, while sucrose presence did not result in significantly greater biofilm thickness. CLINICAL SIGNIFICANCE: The obvious patterns of S. mutans growth along sub-micron scale grooving created by the polishing process suggested that initial biofilm attachment occurred in the shear-protected grooves. These results suggest that fine polishing may help prevent the initial formation of S. mutans biofilms compared to unpolished/coarse polished composites.

Stoichiometric models of sucrose and glucose fermentation by oral streptococci: Implications for free acid formation and enamel demineralization.

OBJECTIVES: The objective of this study is to develop stoichiometric models of sugar fermentation and cell biosynthesis for model cariogenic Streptococcus mutans and non-cariogenic Streptococcus sanguinis to better understand and predict metabolic product formation. METHODS: Streptococcus mutans (strain UA159) and Streptococcus sanguinis (strain DSS-10) were grown separately in bioreactors fed brain heart infusion broth supplemented with either sucrose or glucose at 37 °C. Cell mass concentration and fermentation products were measured at different hydraulic residence times (HRT) to determine cell growth yield. RESULTS: Sucrose growth yields were 0.080 ± 0.0078 g cell/g and 0.18 ± 0.031 g cell/g for S. sanguinis and S. mutans, respectively. For glucose, this reversed, with S. sanguinis having a yield of 0.10 ± 0.0080 g cell/g and S. mutans 0.053 ± 0.0064 g cell/g. Stoichiometric equations to predict free acid concentrations were developed for each test case. Results demonstrate that S. sanguinis produces more free acid at a given pH than S. mutans due to lesser cell yield and production of more acetic acid. Greater amounts of free acid were produced at the shortest HRT of 2.5 hr compared to longer HRTs for both microorganisms and substrates. SIGNIFICANCE: The finding that the non-cariogenic S. sanguinis produces greater amounts of free acids than S. mutans strongly suggests that bacterial physiology and environmental factors affecting substrate/metabolite mass transfer play a much greater role in tooth or enamel/dentin demineralization than acidogenesis. These findings enhance the understanding of fermentation production by oral streptococci and provide useful data for comparing studies under different environmental conditions.

Logistic Regression Analysis of LC-MS/MS Data of Monomers Eluted from Aged Dental Composites: A Supervised Machine-Learning Approach.

Compound identification by database searching that matches experimental with library mass spectra is commonly used in mass spectrometric (MS) data analysis. Vendor software often outputs scores that represent the quality of each spectral match for the identified compounds. However, software-generated identification results can differ drastically depending on the initial search parameters. Machine learning is applied here to provide a statistical evaluation of software-generated compound identification results from experimental tandem MS data. This task was accomplished using the logistic regression algorithm to assign an identification probability value to each identified compound. Logistic regression is usually used for classification, but here it is used to generate identification probabilities without setting a threshold for classification. Liquid chromatography coupled with quadrupole-time-of-flight tandem MS was used to analyze the organic monomers leached from resin-based dental composites in a simulated oral environment. The collected tandem MS data were processed with vendor software, followed by statistical evaluation of these results using logistic regression. The assigned identification probability to each compound provides more confidence in identification beyond solely by database matching. A total of 21 distinct monomers were identified among all samples, including five intact monomers and chemical degradation products of bisphenol A glycidyl methacrylate (BisGMA), oligomers of bisphenol-A ethoxylate methacrylate (BisEMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA). The logistic regression model can be used to evaluate any database-matched liquid chromatography-tandem MS result by training a new model using analytical standards of compounds present in a chosen database and then generating identification probabilities for candidates from unknown data using the new model.

The impact of long-term aging in artificial saliva media on resin-based dental composite strength.

Although salivary liquid can degrade constituents in resin-based dental composites in short-term incubations, there is a knowledge gap on how longer-term aging impacts their bulk strength. We address this through extended aging studies with resin-based dental composites in different environments. Two commercial composites (FIL and AEL) were aged aseptically at 37°C in air (A, control), artificial saliva (AS), and esterase enzyme amended AS (EAS). Diametral and pushout strength were measured after periods of 120-180 days. At 120 days, the diametral strength of composites aged in air was 69.9 ± 11.0 and 57.7 ± 3.31 MPa in FIL and AEL, respectively. These were significantly greater compared to composites aged in AS (32.1 ± 7.01 and 46.2 ± 9.38 MPa in FIL and AEL, respectively) or EAS (36.7 ± 8.49 and 43.5 ± 5.51 MPa in FIL and AEL, respectively). In contrast, pushout strength for both composites were smaller in A compared to those aged in AS and EAS, results attributed to AS absorption and polymer expansion. No significant change in either diametral or pushout strength occurred after 120 days. There was no significant difference between aging in AS and EAS, suggesting that esterase did not significantly decrease the bulk material strength to a greater extent than AS under the test conditions. Aqueous diffusivities for the composites ranged from 8.4 to 11 × 10-13  m2 /s, with associated porosities ranging from 0.06% to 0.10%. These results indicate that saturation of a typical dental composite occurs over a time frame of 4-5 months, longer than typical aging studies. Together, the results demonstrate the importance of aging time on composite strength.