Chris Miller, PHD
Dental-unit water appears to be regularly contaminated with microorganisms - a subject we began discussing in last month`s column. This water passes through high-speed handpieces, air/water syringes, and, sometimes, ultrasonic scalers. Certainly no evidence exists for a widespread public health problem as a result of exposure to dental-unit water. The microbial quality of this water, however, needs to be improved.
A recent review of this topic provides more details and references for further reading. These suggestions are listed in the references section of this magazine.
Types of microbes present
More than 20 different scientific studies from around the world have shown that dental-unit water contains microorganisms. They commonly number a few hundred thousand bacteria per teaspoon full.
One United States study in 1993, for example, sampled water from 150 dental units at 54 sites over a three-state area. An average of 49,700 bacteria per milliliter from air/water syringe lines and 72,500 from handpiece lines was found. Since there are about four milliliters in a teaspoon, this equates to a range of 198,800 and 290,000 bacteria per teaspoon of dental-unit water.
Faucet water from those dental office sites usually contained no bacteria. Although some are oral microbes, most of the bacteria present in dental-unit water are common waterborne microbes found in natural water supplies. So, while most of the bacteria in dental-unit waterlines probably come from incoming water, some come from patient`s mouths as a result of entering the lines back through the handpieces or air/water syringes.
More than 25 different genera of bacteria have been detected in dental-unit water, along with occasional fungi and protozoa. Most of these microbes are of little importance in causing disease. Some are, however, opportunistic pathogens (for example, Pseudomonas aeruginosa and Legionella pneumophila) that may have an ability to cause an infection if conditions are right. These conditions include:
- Contamination with a strain of microbes capable of causing an infection.
- Entrance into the body at a proper site for microbe survival.
- Entrance of a sufficient number of the microbe into the body so that some can survive initial body defense mechanisms.
- Travel in the body to a target site where the microbe can survive and multiply.
- Susceptibility of the host to the microbe or depressed body defenses of the host.
So it`s actually fairly hard for a microbe to cause a disease. This is especially true when a weaker type of pathogen (such as an opportunistic pathogen) is involved.
P. aeruginosa is a common inhabitant of soil, natural waters, and almost any domestic water supply or storage tank. The one scientific report strongly implicating dental-unit water as the cause of an infection involved this bacterium. P. aeruginosa from dental-unit water apparently caused an oral infection in two medically compromised dental patients in England.
L. pneumophila is the main cause of Legionnaires` disease (a type of pneumonia) and of a condition called Pontiac fever (symptoms similar to influenza except usually without lower respiratory involvement). In a recent U.S. study, L. pneumophila was found in low numbers in 8 percent of the water samples taken from 28 dental units throughout six states. Other species of legionella (which usually have a lower ability to cause disease) were found in 68 percent.
While there is no direct scientific documentation that L. pneumophila from dental-unit water has caused disease, indirect evidence or anecdotal information does exist. A dentist in California who died from legionellosis had Legionella bacteria in his dental-unit water. The suggested relationship is that he inhaled dental-unit water aerosols containing Legionella and became infected.
Studies also have shown that more dental personnel than non-dental personnel had antibodies to Legionella, suggesting that those in dentistry had a greater chance of exposure to this bacterium.
The presence of biofilm
Once bacteria enter dental-unit waterlines, some attach to the inside walls and begin to multiply, forming a community of microbes called a biofilm. A biofilm is a mass or layer of microbes attached to a surface (dental plaque is a good example of a biofilm).
The trace amounts of nutrients in the flowing dental-unit water are sufficient for supporting growth of the microbes in the biofilm. The flow of the water continually brings fresh nutrients to the site.
It`s hard to imagine how bacteria in the flowing water can attach to the inside walls of the water tubing. Actually, dental-unit water moves through the lines rather slowly. It is not pressurized until it reaches the handpiece or air/water syringe handle, where it is mixed with air.
In addition, water flowing through any line flows the slowest near the walls of the line. This slow flow coupled with frequent periods of stagnation (overnight and on the weekends) provides the right conditions for biofilm development inside the lines.
Studies with newly installed dental units have shown that bacteria begin to attach to the inside walls of dental-unit water tubing within two weeks. They form easily recognized biofilm in three to six months.
So a type of biofilm ("plaque") commonly forms on the inside walls of dental-unit waterlines in the control box and in the hoses, and this microbial community continuously sheds microbes into the flowing water.
Information on methods to control dental-unit waterline contamination will be discussed next month.
Chris Miller is director of Infection Control Research and Services and professor of oral biology at Indiana University.