Formation and decontamination of biofilms in dental unit waterlines

Background: Biofilms are a natural occurrence in aquatic environments, including community drinking water systems. The interior of small-diameter tubings in dental unit waterlines (DUWL) are also sites of biofilm formation. In the lumen of the tubings, the flow is minimal, and the water becomes stagnant when the units are not in use. Molecules precipitate from the water onto the interior wall and promote the adherence of planktonic microorganisms from the water. Once they become sessile, the microorganisms change their phenotype. After adherence, there is a so-called surface-associated lag time, and the organisms then enter a growth phase and produce exopolysaccharides that coat the organisms in a slime layer. Within the biofilm, the microorganisms can signal one another, transfer nutrients, and exchange genetic material. The insoluble exopolysaccharides shield the microorganisms from displacement and from penetration by predator organisms, antibiotics, and disinfectants. The external surface layer of microorganisms is faster growing and may detach as "swarmer" cells. Detachment of microorganisms from dental unit biofilm flushed into the oral cavity could theoretically infect the patient. Splatter and aerosols from dental procedures may possibly infect health care personnel. Methods: This study compared three DUWL cleaners (an alkaline peroxide product, a freshly mixed chlorine dioxide product, and a buffer-stabilized chlorine dioxide product) in 16 dental units with self-contained water systems, 6 months after installation in a periodontal teaching clinic. One unit treated by flushing and drying served as a control. Units were sampled daily for 10 days with heterotrophic plate count (HPC) sampler plates. The plates were incubated for 7 days at room temperature, and colonies were counted at 10.5x magnification. Samples of internal water tubing before and after the use of waterline cleaners were processed and examined by scanning electron microscopy. Results: The estimated mean HPC was derived from original and replicate independent counts of two investigators of undiluted and diluted samples, reported as colony forming units (CFU)/ml. Shock treatments with the alkaline peroxide product (n = 5) reduced the HPC from baseline, but in the ratio of daily counts to control, there was a large variance and a trend to return of high counts as days passed. The mean daily HPC was significantly better than the control for only 3 of the 9 days of treatment and exceeded the goal of 200 on 3 days. Freshly mixed chlorine dioxide (n = 4) and the buffer-stabilized chlorine dioxide (n = 5) both reduced HPC to near 0 on all days. Their ratios of daily estimated means to that of the control were significantly (P <0.001) better at all times. In comparing treatments, the freshly mixed chlorine dioxide was better (P <0.001) than the alkaline peroxide on 8 of 9 days. The buffered chlorine dioxide treatment was better than the alkaline peroxide at all times. The two chlorine dioxide treatments each had so many HPC counts of 0 that a meaningful statistical difference between them was not calculated. Scanning electron microscopy of plastic waterline tubing samples taken before and after treatments showed reductions in biofilm coverage, but the differences were not statistically significant. Conclusions: Chlorine dioxide waterline cleaners are effective in decontaminating DUWL biofilm. Chlorine dioxide has advantages over other chlorine products. Controlling DUWL biofilm may have beneficial effects on nosocomial infections.