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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

      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.”

      Aerosols

      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,

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