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Dental Unit Waterlines: Questions and Answers 

What is biofilm?

"Biofilm" is a community of bacterial cells and other microbes that adhere to surfaces and form a protective slime layer. Found in virtually all places where moisture meets a suitable solid surface, biofilm can contain many types of bacteria as well as fungi, algae, protozoa, and nematodes. The polysaccharide slime produced by many microbial inhabitants protects the cells from physical and chemical challenges, while water channels within the biofilm carry nutrients to the cells inside the film. Individual organisms, or even portions of the biofilm near the surface, break off into flowing water. In dental waterlines, resulting in contamination of coolant and irrigating solutions.

Dental unit waterlines (DUWLs) provide particularly suitable conditions for biofilm formation. The tubing is an attractive substrate for bacterial growth and the narrow diameter (1/8- to 1/16-inch) provides a high internal surface area for biofilm growth in comparison to a given volume of water. Low water pressure, low flow rates, and frequent periods of stagnation also encourage bacteria present in non-sterile source water (e.g., tap water, bottled drinking water), to form a biofilm layer on tubing walls. Once biofilm is formed, output water often exhibits high levels of bacterial contamination even when the source water is sterile or has very low bacterial counts.

Water heaters and prefilters in dental units may further exacerbate bacterial proliferation and colonization of dental unit waterlines. Heating water to near body temperature may enhance the number of microorganisms adapted for growth within a warm-blooded human host. Although they are intended to remove particles from municipal water as it enters the dental unit, prefilters have pores that are too large to trap bacteria. They not only slow the flow of water but also may provide additional surface area for microbial colonization.

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What are the health implications of waterline biofilm?

The presence of opportunistic human pathogens in DUWLs, such as Pseudomonas aeruginosa, non-tuberculous mycobacteria (NTM), and Legionella species have provided cause for concern. Two cases of postoperative Pseudomonas infections in immunocompromised patients were the direct consequence of exposure to contaminated procedural water. Biofilms can be important replication sites for NTM and Legionella species as they can survive and replicate in free-living amoebae and protozoa found in biofilms. NTM are innately tolerant to disinfectants and are typically resistant to disinfectant residuals present in potable water. NTM have been found in the effluent immediately after DUWL treatment. Mycobacterium abscessus, isolated from DUWLs were found to be the source in separate outbreaks of pediatric post-operative infections in Georgia and California. Fatal cases of Legionnaires’ pneumonia associated with contaminated dental unit water have been reported in elderly patients in Italy and Sweden. Both altered nasal flora and serological evidence of exposure to Legionella bacteria have been reported in dental healthcare workers. A recent post-hoc review of screening for serologic markers of Legionella exposure in dentists conducted as part of the American Dental Association dentist health screening program, however, found that dentists appeared to be no more likely to exhibit evidence of exposure than the general population.

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How do we know if our waterlines are contaminated? Should we check our water quality?

Bacterial biofilm is present in virtually all dental unit waterlines unless procedures specifically designed to eliminate, trap, or kill biofilm are performed. Bacterial counts in untreated dental greater than 100,000 colony forming units per milliliter have been reported.

The American Dental Association (ADA) Council on Scientific Affairs' 1999 report to the profession on dental unit waterlines has stood the test of time and is still current in 2021, evaluating water quality before a treatment protocol is implemented is controversial. Because the scientific literature suggests that all untreated units are highly contaminated, pre-testing to confirm contamination is of little value. However, testing water quality after initiation of a treatment regimen ascertains whether a waterline product or protocol achieves the desired outcome. Monitoring water quality according to an established schedule can help identify problems in performance and compliance and provides documentation of water quality.

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We've just implemented a waterline treatment device or protocol. How and when should we monitor?

Monitoring procedural water quality and inspection of dental procedural water systems provide an important margin of safety for dental providers and patients by confirming that dental equipment and/or aftermarket water treatment products are performing according to manufacturer specifications. Regular monitoring and inspection can also identify problems with water quality management including but not limited to:

• Staff non-compliance with directions for use
• Dental unit or device design variables such as dead legs that compromise water quality management.
• Units with excessive biofilm growth that may be refractory to treatment.
• Incompatibility of aftermarket products or devices with specific dental units or treatment devices.
• Contaminated source water.

While recent reports of outbreaks of NTM and a report of a fatal Legionellosis death in dental settings have raised concerns about current monitoring recommendations, OSAP concurs with current CDC guidelines that do not recommend routine environmental testing for potential pathogens such as Legionella species, Pseudomonas aeruginosa, NTM or other waterborne pathogen in healthcare settings. Testing for specific pathogens in procedural water should only be performed to investigate the source of infection(s) caused by a water-associated opportunistic pathogen as directed by local or state health authorities. A negative test for a difficult-to-culture potential pathogen such as Legionella or other pathogens at a given point in time may give false reassurance of the safety of dental treatment water.

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What is "acceptable quality" for treatment water?

According to the 2003 Guidelines for Infection Control in Dentistry and the 2016 CDC Summary of Infection Prevention Practices in Dental Settings:

DHCP should be trained regarding water quality, biofilm formation, water treatment methods, and appropriate maintenance protocols for water delivery systems. Water treatment and monitoring products require strict adherence to maintenance protocols, and noncompliance with treatment regimens has been associated with persistence of microbial contamination in treated systems (345). Clinical monitoring of water quality can ensure that procedures are correctly performed and that devices are working in accordance with the manufacturer's previously validated protocol.

Dentists should consult with the manufacturer of their dental unit or water delivery system to determine the best method for maintaining acceptable water quality (i.e., <500 CFU/mL heterotrophic bacteria) and the recommended frequency of monitoring. Monitoring of dental water quality can be performed by using commercial self-contained test kits or commercial water-testing laboratories. Because methods used to treat dental water systems target the entire biofilm, no rationale exists for routine testing for such specific organisms as Legionella or Pseudomonas, except when investigating a suspected waterborne disease outbreak (244).

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Does flushing lower microbial counts in dental unit water?

Mechanical flushing alone does little to control contamination in waterlines because it does not eliminate or control the biofilm. Although it can temporarily reduce the number of microbes in the water delivered to patients by clearing away many of the free-floating organisms in the waterline, biofilm bacteria continually break free and re-contaminate dental unit water during clinical treatment. Flushing for several minutes between patients, however, may be valuable in removing contaminants that can enter the water system during patient treatment.

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What are the advantages of self-contained water systems?

Self-contained water systems, also referred to as independent water systems or reservoirs, isolate the dental unit from office plumbing and publicly owned water supplies to permit the use of water of known microbiological quality (e.g., sterile water, bottled drinking water, or water that has been boiled to kill micro-organisms). They also allow the introduction of chemical agents to control biofilm growth.

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What about chemical agents? Should we disinfect our waterlines?

Waterline treatment products can be delivered either intermittently at high concentrations to remove biofilm and kill microorganisms and at lower levels continuously to maintain acceptable water quality. Periodic disinfection involves purging the waterlines, adding the treatment product to the water reservoir, filling the lines for the recommended time, and flushing to remove the agent. Continuous chemical treatment refers to waterline treatment tablets or liquids added to the reservoir or gradually added to water by metering devices or slow-release resin cartridges that may also provide filtration and water conditioning. Some products may require both intermittent and continuous line treatments to maintain water quality and can be technique sensitive.

Devices are also available that filter or condition water and add germicidal agents to incoming water from clinic plumbing. Always follow the manufacturer’s instructions for use for both dental equipment and waterline treatment products to achieve the best results.

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Are filters effective in controlling water quality?

Filters can trap bacteria present in water used for dental treatment, but without control of biofilm in waterlines cannot produce water that meets CDC recommendations for dental water quality. Filtration products currently marketed for use in dentistry also release low levels of germicide (e.g., iodine or silver) to inhibit biofilm and inactivate suspended organisms.

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Should we invest in a sterile water delivery system?

The “Guidelines for Infection Control in Dental Health-Care Settings –2003” states that only sterile irrigating solutions (e.g., water, saline) should be used for surgical procedures including periodontal surgery, tooth extraction (other than exfoliating primary teeth), and other procedures which enter previously sterile spaces or contacts the bloodstream by way of incision, rotary instrumentation, or manual manipulation. Sterile irrigation and cooling systems or manual sterile water or saline delivery methods are recommended also for endodontic access if a disinfection solution is not used as irrigation upon pulpal exposure in primary or permanent teeth.

Sterile water cannot be delivered through dental waterlines in any standard dental unit. Solutions must be delivered using sterile disposable or autoclavable tubing and reservoirs or with an irrigating syringe. If using air-driven handpieces connected to a dental unit, the water must be turned off and the site irrigated using sterile syringe or a device that uses sterile tubing to bypass the dental unit water system. Using a dedicated oral surgery handpiece or scaler with integrated sterile water delivery capability is a good investment for practices that routinely provide surgical procedures.

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What are the benefits of source water treatments ("water purifiers")?

No dental unit can provide water that is cleaner than the water that enters it. However, unless source water treatment systems also address the colonized tubing (i.e., the biofilm) within the dental unit, they will provide little improvement in water quality.

Currently, available water purifiers treat source water with ultraviolet germicidal irradiation (UVGI), filtration, or both to remove or inactivate planktonic microorganisms. One currently marketed device uses UVGI and a silver electrode to produce ozone and silver ions that are said to discourage the growth of waterline biofilm. A simple method to provide bacteria-free source water is to use a device like a tabletop electric teakettle to boil tap water and allow it to cool to room temperature before adding to separate water reservoirs.

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In summary...

Careful compliance with treatment protocols appears to be a critical factor for long-term success. As with other clinical infection control practices, successful control of biofilm in dental unit waterlines depends on technique factors, effective personnel training, and an established standard operating procedure. Failure to establish consistent and rational office procedures could result in damaged equipment as well as harm to patients and dental healthcare workers.

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