Microbiology

 

To flush or not to flush ?

 

A.J. Smith

BDS, Ph.D, FDS RCS (Edin.), MRCPath.

Lecturer/ Hon Sp Registrar Microbiology,

Infection Research Group, Glasgow Dental Hospital and School, 378 Sauchiehall Street, Glasgow, UK.


Title: To flush or not to flush ?

Abstract

The problem of microbial contamination of dental unit water lines continues to gather momentum. Many studies have established that the water emerging from dental units contains a wide range of micro-organisms at levels far higher than that found in drinking water. This paper highlights some of the problems assoicated with microbial contaminated water supplies and dicusses the risks to patients and dental staff from some of these organisms.

 

Clinical relevance: Patients and dental staff are continually exposed to high levels of microbial contamination in dental unit water supplies. This article discusses the risks involved and current recommendations and products designed to reduce these risks.


Introduction

The saga of microbial contamination in dental unit water lines continues to hot up. Judging by last years tabloid headlines and TV/ radio coverage the issue will continue to generate more ammunition to throw at the beleaguered general practitioner. So if the media won’t go away - what about the bugs ? Do they really matter ? Can we eliminate them and if so how? This article will discuss the background to microbial contaminated water lines and how we control (or not) the problem.

The problem

Bacteria proliferate on the walls of the tubes that supply water to dental handpieces to form a biofilm (Figure 1). These bacteria enter the water supply and may consitute an infection hazard to the operator and patient.

What is a biofilm?

Well we used to think of biofilms as pretty lifeless gelatinous lumps of goo (usually stuck to teeth and usually called plaque). However, researchers from many different branches of microbiology have been shedding light on a phenomenon that far from being lifeless is apparently dynamic and thriving with bacteria in various metabolic stages communicating via intricate channels that supply nutrients (Figure 2). The goo is still there of course but now called glycocalyx and is vital for soaking up disinfectants and antibiotics. Even if the disinfectants do get through the bacteria may be in such a dormant state that it has little effect. Some bacteria, such as Legionella species can even hide within other micro-organisms such as protozoa! Bio-films are important because they are the source of microbial contamination in dental unit water lines and monitoring the quality of water gives an indication of the level of biofilm build up in dental units.

Most microbiologists would agree that the answer to the problem lies either in preventing biofilm formation or removing biofilm as and when it forms. Unfortunately, biofilms are incredibly difficult to completely remove. Bacteria are shed from the surface of a biofilm and enter into the surrounding solution to form the planktonic phase (Figure 2), it is these bacteria that we can detect growing in the water emerging from dental lines. The numbers of bacteria in the water are not just one or two  or even ten or twenty times above what are usually found in drinking water, usually they are 10,000 to 100,000 times more concentrated than in drinking water1.

What micro-organisms are found in dental unit water systems and do they pose a risk to health?

Many of the micro-organisms found in dental unit water systems (DUWS) are “environmental” organisms that are found in many aquatic environments. These types of organisms do not usually pose a threat to healthy individuals but may behave as opportunistic pathogens in individuals with impaired host defences, for example, patients with end stage AIDS. Cases have been reported of oral wound infections caused by environmental organisms probably acquired from dental unit water in two immunocompromised patients2. However, another study examining a group of cystic fibrosis patients, who are particularly prone to lung infections, did not find an increase in infections in these patients undergoing dental treatment3. One of the main concerns of water microbiologists is proliferation of the Legionella bacteria. Legionella infection manifests in two very different forms: Pontiac fever and Legionnaires disease. Pontiac fever is an acute self limiting flu-like illness without pneumonia. In Legionnaires disease a broad spectrum of illness may be apparent ranging from a mild cough and slight fever to pneumonia with multi-organ failure. There are over 40 species of Legionella , but most disease is caused by L. pneumophila. Interestingly, the Legionella bacteria appear to occupy a unique niche within the biofilm by surviving as intracellular parasites of protozoa (organisms similar to amoeba). This intracellular existence may partly explain the difficulties in eradicating Legionella species from some water systems. Legionella bacteria have been found in dental unit water supplies and there is an obvious concern that this will cause Legionella infections in patients or dental staff. Some studies have indicated elevated levels of antibody to Legionella bacteria in dental staff when compared to controls4, however, no clinical cases of legionellosis were noted. There is one reported case in the literature of a Californian dentist dying from legionaires disease, subsequent investiagtions found a similar strain of Legionella in the dental unit water supply in the dentists practice5.

Some recently published work has also found a wide range of organisms that are more commonly found in the oral cavity, this implies that there has been suck back of the patients oral fluids into the water system, this has obvious infection control connotations1.

If ordinary tap water goes into my dental unit why does it come out full of bacteria?

There are two main reasons for this;

1. The materials used in the internal construction of dental unit water lines commonly contain polyurethane tubing which encourages attachment and proliferation of bacteria.

2. The large number of narrow bore plastic tubes creates a high surface area to volume ratio, i.e., there is a large surface area of biofilm formed compared to the volume of water it contains (Figure 3).

Paradoxically, this phenomenon also helps protect micro-organisms from the effects of disinfectants as the large surface area of bio-film acts like a sponge to soak up the active ingredients of many disinfectants. This explains the poor efficacy of many disinfectants and the fact that many solutions have to be repeatedly applied to control bacterial proliferation.

Should we be concerned about microbial contamination in DUWS ?

Among immunocompromised individuals there is undoubtedly a potential for infection via this route. For most healthy patients, however unpalatable it appears to inhale and drink a microbial soup there is little epidemiological evidence that this constitutes a significant risk of infection, though collecting the appropriate data is difficult. However, for surgical procedures where an irrigant is required a sterile solution must always be used.

For dental staff, again there is little evidence that inhaling millions of microbes 7-8 hours a day, 5 days a week over several years constitutes a significant health hazard, although no long term studies have been performed to assess the risks.

As clinical microbiologists we constantly strive to be pragmatic in our risk assessments for infection control within the community and hospital environments. Often as in the case of the water supply there are legislative guidelines based on sound microbiological methods to aid in the risk assessment process and safeguard public health. Water-borne pathogens such as Legionella species and coliforms, for example E. coli, should always be absent from drinking water supplies. It is usually recommended that the number of colony forming units (cfu) in drinking water is less than 10 cfu/ml at 37°C (or less than 100 cfu/ml at 22°C). Why should water from dental units (a medical device) be any different ?

 

What are the current recommendations?

The most commonly used procedure of flushing the handpiece with water prior to use may lower bacterial counts but high levels of microbial contamination can still persist6. Nevertheless, the British Dental Association (BDA)7 and Centers for Disease Control (CDC)8 recommend that all water lines should be allowed to run and discharge water for several minutes at the beginning of each day and for a shorter interval between patient appointments.

Who’s responsibility is it to ensure drinking quality water emerges from dental units?

Dental chairs and units are considered to be medical devices and must therefore meet the relevant Essential Requirements of the Medical Devices Directive (93/42/EEC). In the UK the Directive was implemented by the Medical Devices regulations SI 1994 No 3017. The Medical Devices Regulations require that device manufacturers supply the necessary information to allow the safe use of their devices. This information includes cleaning, disinfection or sterilisation instructions as appropriate. It is the responsibility of the dental unit manufacturers to provide dentists with the information to provide drinking quality water from dental units.

What’s available on the market ?

All agents used to treat microbial contamination of DUWS have to overcome the remarkable tenacity of biofilm material but must also be non-corrosive to the materials within a dental unit and have a high margin of safety for use in a health care environment. A list of some of the products marketed for reducing microbial contamination in DUWS are shown in Table 1.

There have been several attempts to address this problem including autoclaving of handpieces, handpiece replacement between patients, flushing of the unit prior to use, "anti-contamination" devices to prevent retrograde aspiration of oral secretions into the water supply line, connection to a separate water supply (for example, connection to bottles of distilled water), chemical disinfection of waterlines, ultra-violet radiation disinfection and the use of in-line water filters. These have been developed and implemented in many dental practices. The research to date suggests that which ever method is used, microbial recolonisation of the waterline is inevitable and repeat disinfecting treatments are necessary.

Should I be testing my dental unit for Legionella ?

No - there is no indication for the routine monitoring of DUWS for Legionella bacteria. Treatment of DUWS should target removal of the entire biofilm or better still reduce its formation in the first place. High microbial counts within the DUWS is indicative of high levels of bio-film formation within the dental unit.

 

What should I do ?

Ultimate responsibility lies with the dental profession in insisting on drinking water quality from dental units. Under the medical devices directive equipment manufacturers are required to provide users with information on appropriate methods of decontamination, this includes biofilms within the DUWS. Practitioners should contact the manufacturers of their dental unit for suitable advice. In the interim it would be prudent to flush some water through dental units that have been left standing for several hours. If the handpieces being used do not contain anti-retraction devices a flush between patients would be useful to remove any aspirated micro-organisms from previous patients. For surgical procedures it is important to use only sterile irrigants.

Conclusions

There is little epidemiological evidence that microbial contamination of DUWS constitutes a significant risk of infection to either patients or their dentists. Although this may be due to difficulties in collecting the appropriate data. Among immunocompromised individuals, however, there is undoubtedly a potential for infection via this route. Therefore, every effort should be made to ensure that water of potable quality emerges from dental unit handpieces.

How do we achieve drinking water standards from dental unit water lines ? The problem in dentistry is exacerbated by the complex engineering of the unit. The use of biocides to solve the problem is ill fated, as demonstrated by the plethora of methods claimed to solve the problem, all short lived. The answer (from a scientific and microbiological viewpoint) lies in redesigning the system to eliminate stagnation of water and reduce the formation of biofilms.

References

1. Walker JT, Bradshaw DJ, Bennett AM, Fulford MR, Martin MV, Marsh PD. Microbial biofilm formation and contamination of dental-unit water systems in general dental practice. Appl Environ Micro 2000; 8: 3363-3367.

2. Martin MV. The significance of bacterial contamination of dental unit water systems. Br Dent J 1987; 163: 152-154.

3. Jensen ET, Giwercman B, Ojeniyi B et al. Epidemiology of Pseudomonas aeruginosa in cystic fibrosis and the possible role of contamination by dental equipment. J Hosp Infect 1997; 36: 117-122.

4. Fotos PG, Westfall HN, Synder IS, Miller RW, Mutchler BM.. Prevalence of Legionella specific IgG and IgM antibody in a dental clinic population. J Dent Res 1985; 64: 1328-1385.

5. Atlas RM, Williams JF, Huntington MK. Legionella contamination of dental unit waters. Appl Environ Microbiol 1995; 61: 1208-1213.

6. Smith AJ, Blacker S, MacKenzie D, Bagg J, Hood J.  Microbiological quality and efficacy of two decontamination regimens on dental unit water supplies. Journal of Hospital Infection 40; Suppl A; P.9.5.1 1998.

7. British Dental Association. Infection Control in Dentistry: Advice Sheet A12; 2000.

8. Centers for Disease Control. Recommended infection control practices for dentistry. MMWR 1993; 42: 1-12.