With the heightened emphasis on decontamination and cross infection control within dentistry over the past few years and the legislative demands for compliance with HTM 01-05, dental practices are increasingly aware of their obligations in regard to patient health and safety. One such long standing issue, certainly not exclusive to dentistry, is the problem of biofilm.
Biofilm, which can cause water quality to fall below the minimum standards required by HTM 01-05, is prevalent in dental unit water lines (DUWLs) because in the majority of today's most commonly used treatment centres, water is delivered via a tubing mechanism, providing an ideal environment for the development of bacterial biofilms. It is the biofilm itself that gives rise to problems in two key areas: the physical restriction of flow in narrow lumens; and in significant numbers of planktonic bacteria in the output water.
The term biofilm refers to a collection of microorganisms adhered to a surface and surrounded by a protective and adherent slime (known as the extracellular matrix) secreted by the bacteria. Biofilms are particularly prevalent where water with only low concentrations of solids and low levels of nutrients are found. As well as DUWLs, examples can be found in streams and rivers, cooling towers and piped water systems.
Biofilms form when a few individual bacteria in the free floating (planktonic) state in water adhere to a solid surface such as the wall of a pipe or tube. The initial attraction to the surface is weak, but subsequent bacteria can adhere directly to those already attached and behaviour of the aggregated organisms changes and starts to produce the extra-cellular matrix. This in turn increases adhesion and enables more planktonic bacteria to adhere easily to the film.
Biofilms are found extensively in nature because they provide favourable conditions for bacterial survival. Susceptibility of biofilms to external agents such as detergents and disinfectants is greatly reduced compared with that of individual planktonic organisms. Another feature of the biofilm form is the reduction in nutrient requirements for each bacterium.
Significance
The following reasons clearly show why dental unit water lines provide the ideal substrate for biofilm growth.
- They are non-toxic. The materials used for tubing are selected for their non-toxic properties and flexibility to suit the mechanical operation of dental units. Low toxicity to humans also means low toxicity to bacteria.
- Water has a low flow rate. The small bore tubes used in dental units provide water at a typical flow rate of 30ml/min. The behaviour of water flowing through a tube means the linear flow rate decreases from the centre of the tube to its wall. Low linear flow rates favour bacterial adhesion.
- Water flows intermittently. Water only flows through the tube when instruments are in use, causing minimal disruption of the growing biofilm.
- Regular replacement of supernatant liquid. When instruments are used or the system is flushed, fresh liquid is brought into contact with the film, bringing with it nutrients and new recruits to join the film. The liquid moving downstream towards the distal end of the system carries bacteria released by the maturing film along with excreted matter.
- A surgery provides ideal temperature range. Dental surgeries are normally maintained at room temperature for the comfort of patients and staff. This is good news for the film-forming bacteria as they can thrive in this temperature range.
The presence of biofilm is bad for the dental practice on a number of practical levels, not least because the quality of output water is covered by HTM 01-05 guidance and the presence of biofilm makes compliance with this standard very difficult. The existence of biofilm can also lead to a reduction in water flow and even tube blockage leading to unit downtime and repair costs. Biofilm can potentially harbour many pathogens which may originate from the input water or by retraction from handpieces. High bacterial loads in output water have the potential to cause infection in patients or dental professionals either by direct contact or through the inhalation of aerosolised mist. It is the practice putting water into a patient's mouth that does not meet the standards required for drinking water that is morally (and possibly legally) indefensible.
References available on request.