Are Your Waterlines Safe?

Over the past three decades, the concept of biofilms has revolutionized the way we view the microbial world. By studying biofilms, scientists have been able to explain how bacteria establish themselves, replicate, and develop the complex architecture and communications systems that allow them to survive in almost every environment where moisture and a solid surface exist.^1-4 Understanding biofilms has enhanced our understanding of oral disease processes and has provided improved methods to diagnose and treat plaque mediated oral diseases.^5,6 The recognition that biofilms can colonize the waterlines in dental units and contaminate water used for dental treatment with high levels of bacteria has stimulated changes in the design of dental equipment and has led to new guidelines for clinical aseptic procedures.⁷

A BRIEF HISTORY OF BIOFILM

Bacteria suspended in a liquid-like water or saliva are attracted to nonpolar solid surfaces (dental waterlines or teeth) where they readily attach using specialized structures including fimbriae, flagella,¹ and cell surface molecules called adhesins.⁵ Once attached, their genetic code directs these early colonizers to undergo transformation from the free-swimming state to a mode adapted to life on a solid surface.^1,5 Individual organisms soon begin extruding specialized extracellular polymeric substance (EPS) made up of a variety of soluble and insoluble polysaccharide molecules.^1,5 Early colonizers are soon joined by other bacteria recruited to join the microbial community from the planktonic state. The final result of this process is a microbial community known as a biofilm that can include structures of surprising complexity.⁸ The biofilm architecture is highly heterogeneous, consisting of microcolonies protected by the EPS matrix and separated by voids between colonies that act as water channels.¹ The biofilm provides a protected environment that is tightly adherent to the colonized surface and protects its microscopic inhabitants from desiccation,¹ predation by protozoa,¹ and antimicrobial agents.^1,56 Superbly adapted to the oral environment, plaque bacteria provide a persistent challenge to the most determined efforts by patients and dental health care professionals to eradicate them.

In medicine, biofilms play an important role in the pathogenesis of other conditions including bacterial endocarditis, pneumonia, and infections related to implants and indwelling catheters.⁸ Pharmaceutical, food processing, and manufacturing industries must also contend with biofilm formation that can cause spoilage and clogging (biofouling) of lines that carry liquids—including not only water but crude oil and petroleum products as well.⁹

CURRENT RECOMMENDATIONS

Water delivery systems in dental units are often heavily contaminated with bacteria and other microorganisms.^10-16 The long narrow-bore waterlines used in dental units and other devices provide an environment that favors biofilm formation. The geometry of these systems, with large surface areas for biofilm growth, relative to small volumes of fluid and low flow rates can result in bacterial loads in water from handpieces and air water syringes that may exceed 100,000 colony forming units per milliliter (CFU/mL).¹⁷

The American Public Health Association and the American Water Works Association recommend that drinking and recreational water should contain no more than 500 CFU/mL of noncoliform heterotrophic water bacteria.¹⁸ An American Dental Association (ADA) panel on dental unit waterlines that convened in 1995 recommended that manufacturers of dental equipment provide methods to provide water for nonsurgical dental treatment that contain less than 200 CFU/mL of bacteria.¹⁹ Current recommendations from the Centers for Disease Control and Prevention (CDC) recommend that water used for nonsurgical treatment in dentistry meet drinking water standards for bacterial contamination and contain no more than 500 CFU/mL of heterotrophic water bacteria.²⁰

WATERLINE BIOFILMS

Most bacteria growing in dental water lines are heterotrophic water bacteria that survive public utility disinfection processes and are delivered to the dental office in municipal water.²¹ Although most of these microorganisms appear to have limited capacity to cause disease, some bacteria with recognized pathogenic potential such as Pseudomonas aeruginosa,²² Legionella species,²³ and aquatic nontuberculous Mycobacteria^24,25 have been found in water used for dental treatment. Despite this fact, only one report describing two confirmed Pseudomonas aeruginosa infections in immune compromised patients linked to contaminated dental treatment water has been published in the scientific literature. The same study, however, found that 78 other asymptomatic patients treated during the same time period were colonized with the strain of Pseudomonas recovered from the dental units.¹³ Though less commonly recovered than environmental bacteria, oral microorganisms can be retracted into dental waterlines and later discharged during patient treatment.²⁶

Infections are not the only way biofilms can affect human health. The Gram-negative bacteria found in dental water systems may produce substances with potent physiological effects. One such substance is a structural lipopolysaccharide molecule that forms part of the cell wall of these bacteria. Also known as endotoxin, these molecules can cause increased heart rate, fever, respiratory distress, and shock in susceptible individuals. This complex of symptoms, known as a pyrogenic reaction, may occur in patients who are undergoing kidney dialysis when numbers of bacteria in water used for dialysis exceed recommended levels.²⁷ Endotoxin can also cause respiratory distress in patients with asthma. Respiratory complications can occur in workers exposed to aerosols from contaminated water sources in various workplaces including metalworking²⁸ and in an indoor swimming pool facilities where decorative waterfalls aerosolized water contaminated with Gram-negative bacteria.²⁹

Water from untreated dental units can contain levels of endotoxin greater than 2,500 endotoxin units per milliliter (EU/mL). In comparison, the permissible level of endotoxin in USP water for irrigation is only 0.25 EU/mL. The potential for high levels of endotoxin to affect wound healing in periodontal surgery³⁰ is a concern and a recent study suggests that contaminated dental aerosols may also be linked to asthma in dentists.³¹ Because dental hygiene procedures using ultrasonic scalers may produce greater quantities of aerosols than other dental procedures,³² the potential for exposure to endotoxin may be concomitantly greater. No studies to date have investigated the risk of long-term exposure to aerosolized bacteria or endotoxin among dental hygienists or other dental health care professionals.

Despite the paucity of documented cases of illness, the use of water for dental treatment that contains high numbers of potentially pathogenic bacteria is inconsistent with the principles of aseptic practice that emphasize reducing the number of microorganisms present in the clinical environment.

PRODUCTS/PROCEDURES

Since the ADA waterline panel challenged the dental industry to develop methods to improve the quality of water used in dental practice in 1996,¹⁹ the number of products available in the dental marketplace for control or elimination of biofilms has grown. Currently available devices and products include:

• Independent reservoir systems that can be used to introduce chemical agents to control biofilms and to provide water of known microbiological quality.
• Chemical germicides or cleaners (available in liquid powder or tablet forms) that are formulated to: inactivate and remove biofilms or to prevent attachment of new biofilm in new or cleaned systems.
• Slow release resins or metering devices that release low concentrations of agents designed to prevent attachment of biofilm.
• Antimicrobial reservoirs and tubing that inhibit attachment of organisms.
• Bottled water and water conditioning systems that treat water by filtration, sedimentation, and/or ultraviolet irradiation to remove bacteria and other contaminants entering the water system (these devices generally have no direct effect on biofilms).

Virtually all dental unit manufacturers provide independent reservoir systems and some are now offering factory-installed water treatment devices as well. Chemical agents commercially available for use in dental waterlines include: oxidizing agents such as alkaline peroxide;^33,34 chlorine dioxide;³⁵ elemental iodine; chlorhexidine gluconate; and alcohol,³⁴ silver, and various proprietary compounds.³⁶

The initial purchase cost and operating expense of these products varies greatly, with a premium placed on passive or automated devices that reduce user intervention. Waterline treatment products that are technique sensitive or require greater user intervention to control or prevent biofilm formation may be more prone to clinical failure.³⁷

The quality of water entering the system is also important in controlling biofilm formation. Treating a dental water system to control biofilm growth has little benefit if the water introduced into the system has high levels of bacterial contamination. Conversely, sterile water passing through a water delivery system colonized by biofilms will quickly become contaminated.

SONIC AND ULTRASONIC SCALERS

Ultrasonic scalers are used extensively by dental health care professionals to remove plaque and calculus on teeth in both surgical and nonsurgical settings. Like dental units, ultrasonic scalers use narrow bore plastic tubing to provide water to cool and irrigate teeth and root surfaces. Relatively few articles specifically addressing water contamination in ultrasonic scalers have been published^15,38 and only two in recent years.^39,40 As with dental units, these devices should provide an accommodating environment for bacterial biofilm if left untreated. Aerosols and spatter produced by sonic and ultrasonic scalers may therefore contain a mixture of both plaque and environmental bacteria.³²

The 2003 CDC guidelines recommend that only sterile solutions should be used for oral surgical procedures that “involve the incision, excision, or reflection of tissue that exposes the normally sterile areas of the oral cavity.”²⁰ Under this definition, the use of sonic or ultrasonic scalers during open flap periodontal surgery is considered an oral surgical procedure requiring the use of sterile irrigating solutions. To be considered sterile, the irrigating solutions must be delivered to the operative site by sterile tubing. The tubing may be single-use disposable or sterilizable. For nonsurgical procedures such as supra- or subgingival scaling without a surgical flap, the water or other irrigating solution should contain fewer than 500 CFU/mL.²⁰

Few ultrasonic scalers currently approved for sale in the United States are capable of providing sterile water irrigation. In addition, design characteristics of some ultrasonic scalers may complicate efforts to control biofilms and not all manufacturers provide explicit instructions for decontamination of water lines connected to scalers. If the water supply for the scaler comes from a dental unit that is being treated to control biofilm, the same treatment method used in the dental unit can be used on the scaler. In order to be effective, however, the treatment agent must be present for the recommended contact time in waterlines connected to or within the device. In some older dental units with separate reservoirs that have been retrofitted, the ports used to provide water to the ultrasonic scalers may remain connected to municipal water.

Some ultrasonic units use self-contained reservoirs designed to deliver specialized irrigants during scaling procedures. While irrigants such as chlorhexidine gluconate have well-documented antimicrobial properties, intrinsic bacterial resistance has been reported.41 If water is used instead, biofilms are likely to form in the waterlines. To control biofilm formation, periodic treatment with a product designed to control biofilm formation in dental equipment may be required. The manufacturers of dental units, ultrasonic scalers, and other devices that provide water to patients should be able to recommend waterline treatment products and methods that are compatible with their products.

SUMMARY

Biofilms colonize dental waterlines in all types of dental equipment and can contaminate water used for dental treatment. While the risks of exposure to contaminated dental water among patients and dental health care professionals are uncertain, current recommendations from the CDC recommend that water used for nonsurgical dental treatment meet current standards for drinking water. To meet these standards, some form of treatment to remove or prevent formation of biofilm is necessary. Manufacturers of dental units that use water should provide directions for use that will enable compliance with current guidelines for dental water quality. Additional research regarding biofilm contamination in dental waterlines is needed.

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From Dimensions of Dental Hygiene. June 2007;5(6): 28-30.