THAUERA-SELENATIS GEN-NOV, SP-NOV, A MEMBER OF THE BETA-SUBCLASS OF PROTEOBACTERIA WITH A NOVEL TYPE OF ANAEROBIC RESPIRATION

A recently isolated, selenate-respiring microorganism (strain AX(T) [T = type strain]) was classified by using a polyphasic approach in which both genotypic and phenotypic characteristics were determined. Strain AX(T) is a motile, gram-negative, rod-shaped organism with a single polar flagellum. On the basis of phenotypic characteristics, this organism can be classified as a Pseudomonas sp. However, a comparison of the 16S rRNA sequence of strain AX(T) with the sequences of other organisms indicated that strain AX(T) is most similar to members of the beta subclass (level of similarity, 86.8%) rather than to members of the gamma subclass (level of similarity, 80.2%) of the Proteobacteria. The presence of the specific polyamine 2-hydroxyputrescine and the presence of a ubiquinone with eight isoprenoid units in the side chain (ubiquinone Q-8) excluded strain AX(T) from the authentic genus Pseudomonas and allowed placement in the beta subclass of the Proteobacteria. Within the beta subclass, strain AX(T) is related to Iodobacter fluvatile. The phylogenetic distance (level of similarity, less than 90%), as well as a lack of common phenotypic characteristics between these organisms, prevents classification of strain AX(T) as a member of the genus Iodobacter. In addition, strain AX(T) possesses a unique mechanism for anaerobic respiration, which allows it to utilize selenate as an electron acceptor without interference by nitrate. Therefore, we propose that strain AX(T) should be the first member of a new genus and species, Thauera selenatis.