Phylogenetic relationships of filamentous sulfur bacteria (Thiothrix spp. and Eikelboom type 021N bacteria) isolated from wastewater-treatment plantsand description of Thiothrix eikelboomii sp nov., Thiothrix unzii sp nov.,Thiothrix fructosivorans sp nov and Thiothrix defluvii sp nov.

The relationship of mixotrophic and autotrophic Thiothrix species to morpho logically similar chemoorganotrophic bacteria (e.g. Leucothrix species, Eik elboom type 021N bacteria) has been a matter of debate for some years. Thes e bacteria have alternatively been grouped together on the basis of shared morphological features or separated on the basis of their nutrition. Many o f these bacteria are difficult to maintain in axenic culture and, until rec ently, few isolates were available to allow comprehensive phenotypic and ge notypic characterization. Several isolates of Thiothrix spp. and Eikelboom type 021N strains were characterized by comparative 16S rRNA sequence analy sis. This revealed that the Thiothrix spp. and Eikelboom type 021N isolates formed a monophyletic group. Furthermore, isolates of Eikelboom type 021N bacteria isolated independently from different continents were phylogenetic ally closely related. The 16S rRNA sequence-based phylogeny was congruent w ith the morphological similarities between Thiothrix and Eikelboom type 021 N. However, one isolate examined in this study (Ben47) shared many morpholo gical features with the Thiothrix spp. and Eikelboom type 021N isolates, bu t was not closely related to them phylogenetically. Consequently, morpholog y alone cannot be used to assign bacteria to the Thiothrix/type 021N group. Comparative 16S rRNA sequence analysis supports monophyly of the Thiothrix /type 021N group, and phenotypic differences between the Thiothrix spp. and Eikelboom type 021N bacteria are currently poorly defined. For example, bo th groups include heterotrophic organisms that deposit intracellular elemen tal sulfur. It is therefore proposed that the Eikelboom type 021N bacteria should be accommodated within the genus Thiothrix as a new species, Thiothr ix eikelboomii sp. nov., and three further new Thiothrix species are descri bed: Thiothrix unzii sp. nov., Thiothrix fructosivorans sp. nov. and Thioth rix defluvii sp. nov.