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Jurnal Sains dan Teknologi Lingkungan 58 Ursula Ubst drinking water such as Aquabacterium commune and Caulobacter crescentus could survive radiation intensities of 100 J/m2, exceptionally up to 400 J/m2 (German technical standard). Hygienically relevant bacteria such Pseudomonas aeruginosa and Enterococcus faecium, however, could survive even stronger intensities up to 600 J/m2. Repair of DNA was not induced and did not start until 2 hrs after disinfection so that immediatelly after disinfection (the regular point for sampling and analysing) no repair or survival indicator could be detected. Repair started as recently as during transport through the pipe system. Enterococcus faecium represented an extreme example by starting repair after 27 hrs, being viable again at the point of drinking water use by the customer. Thus, bacterial stress response proved to be an important factor for hygiene during drinking water treatment and distribution which is mostly neglected up to now. Table 1. Occurence of hygienically relevant bacteria in source water, during treatment and distribution Species Surface Raw Water Activated Ultraviolet Distribution Water Carbon Radiation Net Filtration Saprophytic atypical positive positive positive positive positive Mycobacteria Pathogenic atypical Mycobacteria negative negative negative positive positive Enterococci (E. faecium/E. faecalis) negative negative negative negative negative Legionella spp. positive positive positive positive positive Legionella pneumophila negative negative negative negative negative Pseudomonas aeruginosa positive negative negative positive positive 5. Water distribution As drinking water is not sterile but has still low bacterial numbers, microbes also colonize the distribution system and may form biofilms. These biofilms cannot be detected by the regular controls using water sampling and cultivation techniques. Thus, we used devices specific for biofilm sampling (Kalmbach et al., 1997; Schwartz et al., 2003a, Obst et al., 2007) consisting of hollow stainless steel cylinders. Stainless steel bolts holding platelets of different materials (plastics, metals) for biofilm settlement were screwed into place (fig. 2). The samplers were installed at