The association between dental unit waterlines (DUWL) and bacterial biofilms is well documented. A recent field study has illustrated how practices can deal with the presence of biofilm and how by the effective management of microbial load in DUWL, practices can more effectively adhere to their compliance obligations.
Dental unit water lines are an integral part of dental treatment systems and previous studies have evaluated the microbiological quality of water obtained from DUWL systemsindicating that high levels of bacteria can be present in the water delivered from the unit to the patient’s mouth.
High bacteria levels can be a result of several factors including poor feed water quality or the presence of established biofilm, that could support pathogenic species, constituting a source of potential infection to both patients and dental staff (via aerosol inhalation, ingestion or surgical site infection).
In aqueous environments, biofilms form when individual planktonic (free floating) bacteria adhere to a surface such as the wall of a tube. In DUWL tubing the water flow rate is low and the tube bore is small (typically 2-5mm), providing ideal conditions for the formation of biofilm.
There are three generally accepted methods of managing microbial contamination in DUWL.
- Water treatment producing an oxidising biocide in situ. Water is treated by electrolysis and addition of reagents to produce a biocide such as hypochlorous acid, which is fed directly or indirectly into the DUWL system. The disadvantage of this approach is the requirement for capital equipment.
- The addition of a biocide to the water fed to the DUWL. A reagent is added to the water in the bottle used to feed the treatment unit. This remains constantly in the water that is fed through the system to the patient. The disadvantages of this approach are that biocide solution contacts the mucous membranes of patients (irritation/allergy/ingestion potential), adhesion of materials to tooth surfaces may also be adversely affected by the presence of biocide formulation in the irrigation waterand internal surfaces and mechanisms of hand pieces are in constant contact with the biocide, with the potential for corrosion or wear effects over time.
- Regular off-line treatment with a biocide to remove and control biofilm. A biocide is added to the system when it is not in use. This may be left overnight or over a weekend and is flushed out with water before the system is used. During use no biocide is present in the water. The advantage of this type of system is that biocide does not contact the patient, tooth surfaces during treatment or the internal surfaces of hand pieces.
Before assessing the presence of biofilm in output water, it is necessary to measure the microbiological contamination of water stored in practices ready for use in DUWL systems. Current UK guidelines recommend the use of distilled or reverse osmosis (RO) water in bottle fed DUWL systems.
The initial field study involved evaluation of stored water for use in DUWL at four UK practices. The results of monitoring over a three month period showed that only seven samples (20 per cent) had bacterial counts below the 200cfu/ml limit. Of these, six (85.7 per cent) were from a mains supply. The two sites using distilled water showed the highest counts, in the range 10,000 to 1,000,000cfu/ml. The third site, using mains water demonstrated the largest difference in TVC results; 3-1,000,000cfu/ml. The fourth site, using bottled purified water showed results between 30-10,000cfu/ml. These results indicate that in most cases (80 per cent of 35 samples); the water being introduced into a DUWL system does not meet the required standard for microbial contamination specified for output water. A unit fed with such water cannot be in compliance with guidelines as microbial contamination can only increase as water passes through the unit.
An evaluation of contamination of output water was then carried out in four dental units. High levels of bacterial contamination were measured in all the units with only seven of 66 samples (11 per cent) found to be compliant. When the test was repeated using water with a measured TVC of zero as the feed water, we found there was limited reduction in the output counts obtained in some cases, but this was not sufficient to bring the units into compliance with the guidance. After thorough flushing, 14 of 66 samples were compliant (21 per cent) and after repeated flushing, this figure rose to just 15 of 66 samples, see table 1.
Following these tests a disinfectant designed for offline treatment was introduced to determine its potential impact on contamination levels. The disinfectant was allowed to reside in the system overnight before being flushed out with water. The unit was then re-sampled before being returned to normal use. Evaluation in four units showed that the disinfection process was able to reduce bacterial counts to acceptable levels when used weekly, as demonstrated in the table.
Following these controlled tests, a user evaluation was instigated using a total of 19 units at five UK practices. Users were trained to apply the disinfectant and given stocks of material together with a user record sheet to provide information on usage patterns. The same monitoring system was used to obtain microbiological results throughout the evaluation period.
This research confirms that if no preventive measures are undertaken,biofilm will readily form in DUWLs and in addition to the water line system itself, any containers used for the storage of water and the bottles provided as reservoirs within the systems should be considered as part of the DUWL system and be treated accordingly. Water supply quality and storage conditions must also be effectively managed in order to ensure that adequate output water quality can be achieved and the potential for formation of biofilm minimised.
We conclude from this study that despite the guidance within HTM 01-05, the flushing of DUWL between patients has no significant effect on biofilm or on planktonic levels of bacteria in effluent water. It may, however, be useful in reducing the effects of retraction from handpieces.
When used on a regular weekly basis, the disinfectant intervention tested in this study was effective in bringing about a sustained reduction in TVC values to below the levels set out in HTM 01-05.
References available on request.