Biodegradation of Dental Resin Composites by Oral Bacteria

Background: A major reason for dental resin composite restoration replacement is caries at the restoration-tooth margins. Streptococcus mutans is a primary species associated with the formation and progression of dental caries. In addition to attack by acid produced by these bacteria, human salivary and possibly bacterial esterases contribute to the breakdown of the restoration-tooth margins. In order to advance the chemistry of these biomaterials beyond the current formulations, it is necessary to study and understand the effects that bacteria and human salivary esterases have on dental resin composites. Objectives: To measure and compare the hydrolytic-mediated degradation of dental resin-composites by S. mutans UA159 and simulated human saliva esterases (SHSE). Methods: Cholesterol esterase-like (CE) and pseudocholinesterase-like (PCE) activities of S. mutans UA159, and SHSE were measured using p-nitrophenolbutyrate (p-NPB) and butyrylthiocholine (BTC) substrates, respectively. Quantification of biodegradation by-products (BBP) was performed using high performance liquid chromatograph, UV spectroscopy, and mass spectrometry. Bacterial growth was assessed by optical density changes. Results: CE-like activity from S. mutans UA159 over the three cell stages lag, log, and stationary were: 1.62, 2.07, and 0.38 units/mg dry weight of bacteria, respectively (significantly less in the stationary phase, p<0.05). PCE-like activities for the three respective phases were: 31.8, 40.5, and 31.6 nunits/mg dry weight of bacteria. SHSE (267 units/mg of powder for CE activity, 1 unit/mg for PCE) degraded bisphenol-A-diglycidyl dimethacrylate (BisGMA) and triethylene glycol dimethacrylate (TEGDMA) resin composite monomers, releasing the BBP methacrylic acid at a rate of 0.58 µg/hour/ml and 0.27 µg/hour/ml respectively. Carboxylesterase SMU.118c cloned from S. mutans showed an optimum esterase activity at pH5.5 and p-NPB activity of 5 units/mg protein. Conclusion: S. mutans UA159 produce esterases at activities levels potentially capable of degrading dental resin composites. On-going work is using the esterase activities of S. mutans to study the degradation of resin-composites.