Diversity and structure of Hyphomicrobium populations in a sewage treatment plant and its adjacent receiving lake

Budding methylotrophic bacteria resembling Hyphomicrobium spp. were counted for 12 months in a German sewage treatment plant by most-probable-number (MPN) methods. Influent samples contained up to 2 x 10(4) cells ml(-1), activated sludge consistently contained 1 x 10(5) to 5 x 10(5) cells ml(-1), and the effluent contained 1 x 10(3) to 4 x 10(3) cells ml(-1), The receiving lake had only 2 to 12 cells ml(-1). Six morphological groups with different growth requirements could be observed among 1,199 pure cultures that had been isolated from MPN dilutions, With dot blot DNA hybridizations, 671 isolates were assigned to 30 hybridization groups (HGs) and 84 could not be classified. Only HG 22 hybridized with a known species, Hyphomicrobium facilis IFAM B-522. Fourteen HGs (HGs 8 to 20 and HG 22) were specific for the lake; most others occurred only in the treatment plant. HGs 1, 3, and 26 were found in the activated sludge tank throughout the year, and HGs 27 and 28 were found for most of the year. In summary, it was demonstrated that bacteria with nearly identical and specific morphologies and nutritional types showed a high level of genetic diversity, although they were isolated under the same conditions and from the same treatment plant or its receiving lake, A directional exchange of these genetically different populations was possible but less significant, as was shown by the establishment of distinct populations in specific stations.