why and how biofilms are formed, what risks are connected with the biofilms and microorganisms in the water lines used for dental treatment procedures, and what can be done to minimize the hazard.
Biofilm Formation
Biofilms are complex and heterogeneous formations on most surfaces in nature and in man-made water systems where there is a slow streaming water flow [5]. All water systems contain minute amounts of organic materials and microorganisms unless the water is distilled and sterilized. Biofilms are a natural habitat for microorganisms to live and grow because they offer them many advantages in comparison to the free-living planktonic stage. The adherence to a surface and a biofilm makes it possible to build complex communities that can withstand flushing, dehydration, and antimicrobial measures. More importantly, due to nutrient limitation, most microbial cells are slow growing or even dormant. Also, the production of extracellular polysaccharides that build up the biofilm, constituting over 90% of a biofilm’s dry weight, forms a barrier that protects the microorganisms in deeper layers of the biofilm from antimicrobials. Other advantages are cooperation for nutrients (food web), which is important for the microorganisms living in the biofilms of DUWLs, where the level of nutrients is extremely low. In established biofilms there is a constant detaching of microbial cells, which can be measured in the output water. More than 100,000 colony-forming units (CFU)/mL have been reported, which illustrates that the water lines in DUWLs constitute an ideal environment for biofilm formation and bacterial growth [3, 6–8].
There are several reasons why biofilms are easily formed in DUWLs. The main reason is the low water flow, estimated to be 0.5 mL/s, and the total volume of water in a dental unit is around 100 mL. In addition, the units are standing still most of the day, which makes it possible for the microorganisms in the biofilm to reorganize, cooperate, and multiply, increasing the biofilm’s size and thickness. During weekends and vacations there could be several days without use. The comparatively higher temperature (room temperature) than the incoming community water makes it possible for the attachment and growth of more thermophile and pathogenic bacteria. The incoming water contains minute amounts of organics (proteins, polysaccharides) as well as dead and viable microorganisms that easily attach to the inner surface of the lines. In the periphery of the lines, immediately adjacent to the surface and liquid interface, the flow velocity is negligible compared to that of the center where the flow is the highest. The surface roughness and hydrophobicity increase the attachment, which mainly occurs between hydrophobic areas of organic molecules and bacteria on the one hand, and the surface of the water lines on the other, today usually made of hydrophobic non-polar plastic material (polyvinyl or polyurethane). Once the first layer (a pellicle) has become established more microorganisms attach to this layer. Established bacteria will multiply and the growth rate depends on available nutrients. Heterotrophic (requiring organic material for growth) environmental bacteria, mostly Gram-negative aerobic or facultative but low fastidious microorganisms, will grow in the biofilm inside the DUWLs. The established biofilm structure comprises microbial cells and extracellular polymeric substances (EPS). The biofilms are highly complex, with microcolonies of bacterial cells, EPS matrix, cell communication, exchange of genes, quorum sensing, predation, and competition representing an ecological community that differs from one unit to another. Each unit has its own “inner life.”
During periods of no flow, the microbial growth and biofilm formation can be substantial and can even cause total stagnation of the flow. The problem is obvious when the flow is turned on and bigger masses are loosened, potentially causing the lines and valves to become clogged [4]. Filters have been attached to dental units but have the drawback of reducing the flow rate significantly [9]. The recommendation to flush the system for 3 min in the morning to reduce the microbial level in the output water only has a marginal and temporary effect [1]. A number of commercially available treatment products for DUWLs have been marketed that efficiently reduce the number of bacteria in the output water or reduce/eliminate the biofilms in the water lines [4, 7].
Aerosols
The combination of biofilm formation and the use of the contaminated water are of high concern during patient treatment in health care and the dental environment [10]. The presence of a biofilm constantly delivers a number of detached microorganisms in a planktonic stage to the output water used for treatment procedures, such as spraying, cooling, and ultrasonication, which all form aerosols. Aerosols are easily inhaled by patients as well as the dentist and nurse. The personnel can protect themselves with a mask that should cover both the mouth and nose to fulfil its purpose. However, the patients cannot be protected and may be in a more vulnerable situation since the aerosols are formed in and around their mouths and they are exposed to a higher degree to the aerosols by inhalation. It should be pointed out that the contaminated water from DUWLs is not hazardous to drink, since gastrointestinal pathogens such as coliforms are extremely uncommon in DUWLs [2–4]. However, high numbers of microorganisms should be regarded as poor quality and in the worst cases the water can even be discolored and smell and taste bad, and unacceptable for used for irrigation in the mouth. With a high number of microorganisms in DUWLs, the potential risk is entirely connected to the risk of inhalation and development of respiratory tract infections. This is especially important in immune and medically compromised patients, the elderly, and other individuals more susceptible to developing respiratory tract infections, who can even develop infection from normally innocent microorganisms or their products (e.g., endotoxins) if they are exposed to a high number of microorganisms. The risk of complications increases when respiratory pathogens are present (see below).
The risk with aerosols is well known from other similar systems, such as air conditioners, air moisteners, water towers, and showers. In fact, the first well-recognized incidence with respiratory tract infection occurred in 1976 with the outbreak of pneumonia caused by Legionella among veterans who had assembled in Philadelphia (PA, USA) and were exposed to an air condition system containing L. pneumophilia [11]. Since then, a number of similar outbreaks have been reported in hotels, old people’s home, and hospitals [11]. Although only a few case reports are documented in dentistry [12], the theoretical risk of gaining respiratory tract infection from DUWLs is considered to be high. However, the actual frequency of respiratory tract infections caused by aerosols from dental unit water is unknown due to difficulties in tracing the origin of the bacteria.
Microorganisms in DUWLs
The microbiota of the water and biofilms in DUWLs can be very complex [2, 4]. If the biofilms are not regularly removed they can build up complex microbial communities composed of heterotrophic environmental Gram-negative bacteria in the water. Opportunistic pathogens such as Legionella and Pseudomonas species have gained the most attention, but other heterotrophic species such as mycobacteria, staphylococci (S. aureus, S. epidermidis), and a number of Gram-negative rods and fungi may also be present [2, 4,