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7]. Potential pathogens grow rapidly (within 2 days) on ordinary solid bacteriological media in 36 °C. Samples could be incubated at room temperature (22 °C) but grow slower [8]. A longer incubation time (1 week) will increase the number of CFU due to the slow-growing nature of most heterotrophic bacteria for which the water lines are their natural habitat [4]. High numbers of slow-growing bacteria therefore indicate poor water quality, the presence of a biofilm, and a rich inner life of the DUWL.

      Specific Pathogens in DUWLs

      Although numerous microorganisms have been found in the water of DUWLs, some opportunistic pathogens are of special concern for patients and personnel, for example aerobic Gram-negative bacilli such as Pseudomonas spp. and Legionella spp. [4, 13].

      Methods to Reduce Bacterial Counts in DUWLs

      Reasons for Failure

      While most DUWLs with functioning water-cleaning systems show few or no bacteria, others still harbor bacteria to an unacceptable level. A 4-year follow-up study [28] showed that the installation of water-cleaning systems (added systems or inbuilt systems) leads to a significant improvement in output water quality. For 1,200 DUWLs at 140 clinics followed between 2013 and 2016, a 95% acceptance of the water quality was attained. The Alpro/Bilpron system was used in the majority of DUWLs (81.6%), but also Oxygenal (2.0%), Unit-Clean (5.2%), Sterilox (0.6%), as well as inbuilt systems (4.8%) were included in 2016. This study did not show any significant differences between the systems. In the remaining 5% of DUWLs that did not reach an acceptable level, the failures could be divided into problems related to the unit or to the clinic. Even if the unit is equipped with a water-cleaning system there is no guarantee of successful results. The most common reason for failure is neglect to run the water-cleaning system regularly. If the bacterial level in the output water is not acceptable, additional cleaning steps have to be taken. It is of outmost importance that the personnel are sufficiently informed and instructed and that necessary precautions are taken for servicing each unit.

      Clinic-related

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