Editorial

An Introduction to Biofilms

John W. Beierle, PhD

Copyright 2001 Journal of the California Dental Association.

The issue of dental waterlines has raised many questions about office safety, effective treatment of biofilms, and the means by which to achieve those solutions. Even further, the issue of office safety has been challenged by the question of whether the waterline biofilm problem exists at a significant level or is merely a tempest in a teapot. In some instances, the dental practitioner feels that the "system" is leaning on him or her personally for singling out on what is, in fact, a universal problem. How common, then, are waterline biofilm problems? Are biofilms established in systems other than dental waterlines?

The Universality of Biofilms

Biofilms are universal. They coat the walls of swimming pools, line the bottom of boats, and live in medical and dental devices and even hospital showerheads. Waterlines in virtually all standard plumbing eventually get a buildup of microbial masses, which we call biofilms. The scientific community is now recognizing the fact that biofilms are a potential problem in many areas of medicine, biology, and public life. In medicine, hip joint replacements are coated with biofilms, as are indwelling catheters, kidney dialysis machines, and numerous other devices.

An intent of this issue of the Journal of the California Dental Association is to bring the worldwide problem into focus in relation to dentistry’s specific niche in the biofilm world. Simply stated, a biofilm is an colonized mass of bacteria attached to some solid or tissue surface. That surface may be dental enamel, a water pipe, or any waterline. The biofilm may even be a slimy mass attached to an instrument panel in a space vessel, which would take the biofilm problem beyond the Earth’s surface. Dentistry also has biofilms below dentino-enamel junctions, in a process we call periodontitis. The coatings on our tongues, such as found in candidiasis, are also biofilm in nature. Bacteria don’t care where they form biofilms; they just want to colonize for survival and protection.

Bacteria have a few key ways to attach to a surface. Pili, short structural appendages connected to the cell wall, can help attach bacteria to a variety of surfaces. Another attachment method is the glycocalyx, a protein-polysaccharide coating that was once called the slime layer. The slime layer is a loose, amorphous mass of carbohydrate-based material secreted by bacteria; it was considered unimportant until the past few decades. This material is critical to the attachment of bacteria to surfaces and has a variety of other functions as well. Once a biofilm forms, a number of amazing things begin to happen. Bacteria start chemically signaling each other by a process known as quorum sensing. The bacteria then send other signals to each other, which, in turn, starts a process known as signaling. Signaling induces bacteria to start behaving not as planktonic free single cells, but rather like multicellular systems acting in concert. Resistance to drugs and chemicals, the initiations of rapid cell division, mucin production, and toxin production are all part of the signaling process that make biofilms extremely well-protected. We in the health professions inherit the problem of dealing with this biomass.

Why do bacteria need to colonize? To survive. There is indeed safety in numbers as any animal flock or school of fish knows. It is estimated that many bacteria live in biofilms, much as we live in cities. Antony van Leeuwenhoek, the first microscopist, initially saw bacteria in dental plaque, making him the first oral microbiologist and biofilm experimenter back in the 1600s. It has taken a long time for humankind to begin to appreciate a bacterium’s "lifestyle."

So What Is a Dental Waterline Biofilm?

A biofilm is a mass of microorganisms coating some solid object, be it organic or inorganic. The microbes secrete a mucinous glue, which allows them to stick to a surface. Other microbes then attach to the adherent ones and, before long a mass of bacteria occur, which appears to be a sticky layer, often called a slime layer. They form in dental waterline units, and originally the bacteria come from the city water supply. We are allowed to receive up to 500 colony forming units per milliliter of water considered drinkable or potable. We are not allowed to have coliforms (enteric microbes such as E. coli) in our water at all. Are there some potentially pathogenic microbes found in safe city (or potable) water? Yes, Pseudomonas lives in normal water supplies, as does Legionella capable of causing Legionnaires disease. Various molds also live in drinking water. These microbes are normal inhabitants of water.

Are there other microbes that may be harmful? Yes, microbes found in the mouth or blood from the oral cavity may be sucked back into the dental waterlines; and those oral microbes may join the natural waterline biofilm colonies already established there. Every time the water in the units is used, sections of the biofilm mass release into the lines and then into patients’ mouths. The harmless water microbes are not the problems. A build-up of human pathogenic bacteria could be a problem, especially to the young and old, the infirmed, or the medically compromised. The medically compromised includes patients undergoing chemotherapy or radiation therapy or who are immunosuppressed. Organ transplant recipients are one example of such a patient. The AIDS or HIV-positive patient also falls into this category. If these people are assaulted by high levels of microbes through waterlines, the portals of entry of ingestion and respiratory route infection, or even blood-borne transfer, are now introduced into the equation. For these reasons, as well as esthetic ones of maintaining a clean practice environment, clean waterlines are important. Other concerns include the patient’s perception of the problem and adverse publicity from the media.

The public view is often that if one dentist has a problem, all dentists have the problem. Adverse publicity through the media can also make life difficult in the realm of public relations. A proactive approach to a problem is often the best approach. Education of the profession on issues is essential to the solution. Education of the public and the media is also crucial.

We must remember that we are not alone with biofilms in dentistry, for the entire world is covered with microbes. Dental biofilms include plaque, calculus, periodontal spaces, the tongue, gingiva, and epithelial linings of the oral cavity. The entire gastrointestinal tract is, in fact, a subway of massive adherent biofilms containing billions of microbes clinging to tissue linings and to each other. The entire elementary tract is indeed a continuous organ-microorganism system.

Dental Operatory Waterlines

The question of the level of contamination in dental waterlines has prompted a variety of studies and propelled development of a series of engineering controls to deal with the problem. Purging of lines between patients and after periods of inactivity ranging from hours to weeks has demonstrated that the build-up of biofilms is rapid and enormously high in colony-forming units. Purging can temporarily reduce this buildup in cell numbers, but the problem persists because of rapid microbial cell division and repopulation.

In instances where one uses an independent water reservoir and a pump system, one can often find high numbers of bacteria still remaining in the waterline. Usually, the solution is easy and dependent on the so-called clean water source. In one instance, it was subsequently learned that the water source was a water cooler’s reservoir. The reservoir itself was contaminated with more than 250,000 cfu/ml, and a biofilm had formed in the interior of the water cooler. Staff members using sipper cups obtained drinking water by placing the sipper mouth to the cooler spigot, thereby inoculating the bottle reservoir. Water bottles of one-gallon size obtained from markets and labeled "drinking water" or "distilled water" are often used for clean water sources. The presence of colony forming bacteria are almost always detected in low numbers, e.g., 2-14/ml. The water in bottles is very clean but not sterile.

If we choose to utilize a bottled water system, we should exercise care in the water source we use and change the bottled water often enough to avoid microbial buildup over longer periods. Sterile water is difficult to keep sterile; but without nutrients, microbes grow slowly. Nutrients may enter the system via connecting tubing and backflow-delivering quantities of saliva and blood as a nutrient source, along with the associated microbes derived from dental unit sources. Frequent changeover of water is essential, as is the cleaning and sterilization of tubing associated with our waterlines. We also have a tendency to forget we can sterilize our lines and water in our own sterilizers, which, in fact, are checked weekly for efficacy. Anecdotal reports trickle in raising the issue of corrosion by water containing added bleach. People have a tendency to do too much of a good thing. If a little bleach is OK, then a whole lot should be better. Bleach is a full disinfectant at 1:100 dilution (0.05 percent) of a 5 percent solution.

Water filtration systems are another means of providing a clean water supply. The cost of filter changes and the potential buildup of biofilms in lines leading to or after the filter are possibilities.

Perhaps the major issue with dental waterlines concerns people with compromised immune systems. Old, young, cancer, HIV-positive, organ transplant recipient, chemotherapy, and radiation therapy patients all have potential problems from a microbial assault. Normal water supplies routinely harbor common water microbes. These microbes are not selectively screened out of the indigent population of microbes in our city water supplies.

A British study has revealed that dental surgical suites in England have the same problems as their American counterparts. Their studies have revealed that oral microbial species such as Moraxella and Flarobacterium often end up in dental waterline biofilms. The British studies also revealed that airline biofilms existed and could contain Streptococci, Candida, Lactobacillus and Legionella. Their ultimate finding was that 95 percent of the British dental waterlines exceeded potable water limits versus 83 percent failure for American Dental Association recommendations (accepted British loads were less than 100 cfu/ml and ADA loads were 200 cfu/ml).

Obviously, dental unit waterlines are a worldwide problem, and equally as obvious is that the poorly understood nature of biofilms will not instantly yield any one-step answers. We must first acknowledge the problem as a worldwide one and not view the issue as a personal affront. It took the microbes a few billion years to get this far, the field of microbiology is about 125 years old. In the past decade, we have made substantial progress.

The Purpose of This Issue

To assist the practicing clinician, we have assembled this issue to highlight some, but not all, of the problem confronting the dental and medical world regarding biofilms. Periodontal disease, an issue to us all, is approached from different, yet similar avenues by Dr. Casey Chen at the University of Southern California and Dr. Wenyuan Shi at the University of California at Los Angeles. Dr. Shi also ties together medical-dental problems of common concern. Elinor Pulcini presents a more technical side of the biofilm problem to illustrate the complexity of microbes in a biomass. She outlines many of the events occurring in the world of biofilms. The understanding of the mechanisms underlying the formation, maintenance, and repair of biofilms will eventually provide solutions to the problem. Ms. Pulcini is a member of the Montana State University Center for Biofilm Engineering in Bozeman, Mont. The center is arguably the leading biofilm research institute in the world. It is hoped that this issue of the CDA Journal will help to build on the base of information available to the dental community and aid in the understanding and solutions to a universal problem.

Contributing Editor

John Beierle, PhD, is an associate professor of basic sciences at the University of Southern California School of Dentistry.

To request a printed copy of this article, please contact/ John W. Beierle, PhD, University of Southern California Department of Basic Sciences, 1321 N. Mission-Livingston Labs, Los Angeles, CA 90033, or at HlthHor@yahoo.com.