[Bacterial regrowth in drinking water. IV. Bacterial flora in fresh and stagnant water in drinking water purification and in the drinking water distribution system].

Six dead end water pipes were installed inside a Zurich drinking water plant and five others over a distance of 12 km along the distribution system and the water was left stagnating in there for 2 weeks. A total of 1508 bacteria from fresh and stagnating water were isolated and identified. Of these, 241 bacterial isolates from the distribution system were examined using the nutrient-tolerance test, i.e. testing the ability to grow in tap water and in media with low and very high nutrient content. In the fresh water of the treatment plant specific bacterial populations were obtained, these occurring particularly after the filters. According to different chlorine dosage and chlorine demand, they were finally washed into the distribution system in varying amounts and compositions. It was shown that in the fresh water of the distribution system the genera of Pseudomonas, Azotobacter and Actinobacteria were each present at a level of approximately 30%. After two weeks stagnation non-fluorescing pseudomonads were dominating in the treatment plant as well as in the fresh water of the distribution system. All isolated Actinobacteria and Azotobacter and almost half of the Pseudomonads proved to be oligotrophic oligocarbotolerants or oligocarbophilic organisms in the nutrient-tolerance test. The other half of the Pseudomonads plus the Flexibacter species were mesotrophic oligocarbotolerants, since they could grow in tap water and in culture media with very high nutrient content. Attention is drawn to the unrecognized danger of recontamination of mesotrophic bacteria growing rapidly in stagnating drinking water, which is used as rinsing water for cleaning food processing equipment.