Biodegradation By-Products from Dental Composite Resins Modulate the Gene Expression of Oral Pathogen Streptococcus mutans


104th General Meeting of the American Society for Microbiology
May 23-27, 2004, New Orleans, Louisiana
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EMBARGOED UNTIL: Monday, May 24, 9:00 a.m. CDT
(Session 33, Paper D-004)
Peyman Khalichi
Institute of Biomaterials and Biomedical Engineering, University of Toronto
Toronto, ON, Canada
Phone: 416-979-4917 x 4454

New research indicates that composite resin dental restorations, commonly known as white filling materials, can alter activity of various genes in Streptococcus mutans. The results add to the body of evidence pointing to the non-inert nature of composite resins in the biological environment of the mouth. The evidence presented here confirms some of the suspicions that composite resins can impact the biology of oral microbes.

This research was carried out by Peyman Khalichi, a Ph.D. student at the Institute of Biomaterials and Biomedical Engineering (IBBME) of the University of Toronto, under the supervision of Dr. J. Paul Santerre and Dr. Dennis G. Cvitkovitch (IBBME and Dental Research Institute of the University of Toronto). Canadian Natural Sciences and Engineering Research Council, Ontario Graduate Scholarship in Science and Technology, and the Canadian Institute for Health Research strategic training fellowship Cell Signaling in Mucosal Inflammation & Pain funded the work. The results were presented at the 104th annual general meeting of American Society for Microbiology in New Orleans on May 24, 2004.

There are over 500 identified microbial species that reside in the human mouth. Estimates of the total number of undiscovered species run anywhere from 20 to 100 times that amount. The bacteria form communities by first attaching to different surfaces and to each other and then multiplying. The surface associated bacterial communities are called biofilms in general, but they are commonly known as dental plaque in the context of oral cavity. Dental plaque is as divergent as the types of surfaces in the mouth. For instance plaque on the tooth surface is different from that under the margins of gums. Oral health is partially determined by the composition of microbial community in dental plaque. Internal and foreign factors can impact the composition of plaque in a specific location in the mouth. For example certain microbes living on the tooth surface metabolize dietary sugars and carbohydrates and produce acid that facilitates dissolution of enamel, the hard mineralized surface material of teeth. Repeated exposure to such acid attacks results in the onset and progression of tooth cavities. Interestingly, the acid-producing microbes have evolved to tolerate the acidic environments better that other types of microbes. Therefore high sugar consumption can lead to an environmental shift (from neutral to acidic), which in turn can lead to an ecological shift (dominated by acid-producing microbes) in the biofilm covering the surfaces of teeth. Other factors can impact the biofilms favorably. For instance, fluoride, an additive in toothpaste and drinking water, affect the way acid producing microbes metabolize sugars, slowing acid production.

In order to treat tooth cavities, dentists drill away the infected tissues from the cavity sites, which are then filled various "filling materials." Traditionally silver colored mercury amalgams were used for this purpose. However, motivated by aesthetics and fear from toxicity of mercury in amalgams, white restorative materials, known as composite resins in the dental profession, have gradually made their way into the market in the past thirty years. Composite resins are dual phased materials that are composed of organic monomers (reactive organic compounds) and filler particles usually made of glass, quartz, or ceramic particles. The dual phase design of composite resins imparts ease of handling and mechanical stability after implementation. Following cavity preparation, the dentist places the composite in layers, using a light specialized to harden each layer. The light initiates a chain reaction causing the organic monomers to react with each other producing a continuous plastic like organic phase, in a process called polymerization. This step will entrap the filler particles inside the polymeric phase, much like hardening concrete. However as any chemical reaction this process does not proceed to completion and significant portions of the monomers remain unreacted and can leach out. Also, one of the main disadvantages associated with composite resins is the fact that they shrink after the polymerization process creating a gap between the margin of the restoration and the cavity wall, termed marginal gap; while mercury amalgam materials are self-sealing because of volumetric expansion resulting from corrosion. Through the marginal gap, penetration of saliva factors, nutrients, and microbes becomes very feasible. As a result secondary cavities and infections around composite restorations are a fact of life.

Work in Dr. Santerre’s lab has shown that the polymer phase of composite resins can be degraded. It was demonstrated that human saliva contains enzymes, specifically esterases, which have the ability to accelerate the breakdown of the restorations, which is known as biodegradation. The biodegradation by-products are then released into the biological environment of the mouth. One of the more prevalent biodegradation by-products of composite resins identified is a small organic molecule called triethyelene glycol. In an earlier experiment Khalichi had observed that triethylene glycol accelerated the growth of acid-producing microbe S. mutans. Interestingly this effect was most pronounced when the bacterium was grown in an acidic environment where it is more competitive in its natural environment, the dental plaque. This observation spurred the question of whether triethylene glycol can cellular functions through activation/deactivation of genes in this bacterium.

In order to study the activity (expression) of genes that are either activated or deactivated when S. mutans cells are exposed to triethylene glycol, Dr. Cvitkovitch’s research group employed a technique called differential display PCR (ddPCR). This was done by comparing bacterial cells that were exposed to triethylene glycol to those cells that grew in its absence. ddPCR results showed that some of the genes whose expression was altered were involved in promotion of plaque formation by helping bacterial attachment to surfaces, cell surface maintenance, transfer of genetic information among different microbial species, and nutrient metabolism.

These findings are important since some of the biodegradation by-products may influence the formation of biofilms just as we know that acidity, diet, and other factors influence microbial function. Understanding the effects of composite resin biodegradation on the physiological functions of oral bacteria will help in perhaps uncovering mechanisms associated with secondary infections. This knowledge could also be utilized to design novel restorative composite resins that are more biologically stable and host friendly.