Trisha E. O`Hehir, RDH, BS
Just try telling friends or even a taxi driver that you are on your way to a three-day plaque conference and you`ll hear lots of disparaging comments. Last November, I attended the "Dental Plaque Revisited" conference held in London, England. Drs. Hubert Newman and Michael Wilson of the Eastman Dental Institute and University College in London organized the conference. It was dubbed the "slime meeting" by attendees. You`re probably wondering what the interest was in 33 presentations, one every 30 minutes, all on bacterial plaque. To my surprise and delight, the meeting was fascinating!
I`ll give you just a few of the highlights covered in the first scientific presentation of the meeting. After this presentation, I knew I had received my money`s worth from the conference registration! Little did I know that much of this material had been published in the May 21, 1999, issue of Science. The dental community hadn`t noticed it because engineers wrote it! In fact, some of the most exciting biofilm research is taking place at Montana State University in Boseman. You`re right, they don`t have a dental school in Montana. This research is being done in the Center for Biofilm Engineering.
Lots of other fascinating research also is being done in microbiology departments around the world. It seems biofilms are of great interest to the rest of the world, not just to hygienists and dentists. The collaboration among engineers, microbiologists, and dental researchers at this conference provided a broad view of the subject. A 600-page text of the proceedings was presented to each participant - a valuable resource!
Until recently, oral bacteria were evaluated and observed within a test tube. Free-floating or planktonic bacteria actually are quite different from the same bacteria found in biofilms. Different phenotypes develop within the biofilm. As you remember from microbiology, phenotypes are variations in cells due to the influence of the environment. Therefore, different bacterial phenotypes are capable of reacting differently to antibiotics, for instance. Testing the effectiveness of an antibiotic against planktonic bacteria in the lab doesn`t really tell us how effective that antibiotic will be against a different phenotype of that same bacteria found within a biofilm.
Creating and viewing a biofilm in the lab has been a challenge. Engineers are hands-on folks who want to see the structure and development of things. They`re not happy looking at pictures of oral bacteria. To get scanning electron micrographs of plaque bacteria, samples have to be completely dried before viewing. To an engineer, there goes the structure! New microscopes now are available that can focus at different depths within a biofilm (termed "laser confocal microscopy"). You can focus on the outer layer, then refocus on the middle and deeper layers.
This new microscopy technique allows researchers to create an environment similar to the mouth in which bacteria are allowed to form biofilms, providing an on-going view of development. It`s not a stagnant situation, like a petrie dish, but rather a closed, continuous saliva- flow system, with a stage where bacteria are allowed to form a biofilm. This also is where the microscope is focused.
Until now, we have thought bacterial plaque formed a thick mass on tooth surfaces. Confocal microscopy has demonstrated quite a different picture. Plaque bacteria actually form stacks or mushroom-shaped extensions from the tooth surface. A narrow base is attached to the tooth surface and the mushroom-shaped extensions sway in the fluid flowing around them. The mushroom shape is flexible and can readily elongate or change shape in response to sheer forces from saliva, gingival crevicular-fluid flow, and tongue or tissue movements. As more mushrooms of biofilm are formed and pack together, fluid channels form between them, creating both a communication network and a nutrient-supply route. Bacterial-plaque biofilm can be squished and compressed, pushing out the fluid. When the force is removed, new fluid and nutrients are sucked back into the biofilm through the elaborate system of channels.
Each stack or mushroom is a tiny community of bacteria, surrounded by polysaccharide slime. You might think of it as the skyline of a big city, with each building holding thousands of people. Now add the waterways of Venice to your image. Each mushroom of bacterial biofilm is an independent architectural community containing thousands of compatible bacteria. Different mushrooms may contain different combinations of bacteria. As independent communities, they develop and defend themselves independently. This is done through a complex communication system. Signals are needed to know when to form a biofilm from planktonic bacteria, when enough bacteria have accumulated to form the slime to encase them, where to allow waterways, and how to protect themselves. Biofilms release planktonic bacteria from their surface. Small chunks of the biofilm also may break away and form new biofilms nearby.
Several theories of defense have been suggested. Depending on the rate of diffusion, the biofilm may be able to deactivate antibiotics in the outer layers of the biofilm faster than the antibiotic can diffuse to inner layers.
Because of the protective polysaccharide layer, enzymes released by phagocytes are more detrimental to surrounding tissue than to the biofilm.
Another theory holds that bacteria within the biofilm may turn on a mechanism, which allows them to hibernate, more or less. They no longer need food or nutrients and can remain in this state until the destructive force has been eliminated. At that time, they turn on their normal functions and begin to thrive on the dead cells around them, rebuilding the biofilm.
The bacteria may also alter their phenotype to adapt to adverse conditions intended to eliminate them. Some or all of these theories may explain why systemic antibiotic therapy and some local delivery methods seem to control periodontal infection for only a short time.
These are only a few of the highlights of Dr. J. William Costerton`s presentation and chapter in the conference text. We can no longer think of plaque as just "white sticky stuff." It actually is a fascinating, ever-changing ecosystem. Our new understanding of bacterial-plaque biofilms supports what all clinical dental hygienists already know from experience. Old-fashioned mechanical removal of supra- and subgingival bacterial plaque really works! Now, we have a better understanding of why it works so well.
Trisha E. O`Hehir, RDH, BS, is a senior consulting editor of RDH. She also is editor of Perio Reports, a newsletter for dental professionals that addresses periodontics. The Web site for Perio Reports is www.perioreports.com. Her e-mail address is trisha@perioreports. com.