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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].

Pseudomonas spp. are regularly present in water, soil, and moist environments, and should be considered an indicator of poor water quality. Pseudomonas should not be present in drinking water and therefore not in the water of DUWLs. Pseudomonas spp. typically cause opportunistic infections, and respiratory tract infections in particular. Patients with respiratory diseases such as COPD and cystic fibrosis are a particular risk for Pseudomonas infections [14–17], with P. aeruginosa being the most common species.

Legionella infections are typically spread through aerosols from water lines containing various Legionella spp. [3, 18, 19]. L. pneumophila is responsible for most Legionella infections and can be serologically classified, which makes it possible to trace the origin [11]. Some outbreaks can be endemic, when many compromised patients can be exposed at the same time, for example in hotels, hospitals, and public baths. An aggravating circumstance of Legionella in water lines is their tendency to hide intracellularly in protozoa/amoebae residing in the biofilms [20, 21]. This makes the action of some disinfectants, especially chlorine, less efficient [19].