A Gram-negative and tryptophanase-positive thermophile, whose growth is dependent on co-culture with an associating Bacillus strain, had been reported and tentatively named Symbiobacterium thermophilum strain T-T. Axenic culture of strain T-T was recently established by dialysing cultures with the supporting bacterial strains or adding their culture broth. Phylogenetic analysis of strain T-T, based on the 16S rDNA sequence, was conducted for the validation of S. thermophilum, the sequence of strain T-T was located at the outermost position in the high-G+C Gram-positive group distinctly isolated from any other branches hitherto known. Ten sequences identical to that of strain T-T, and one sequence closely related to it, were identified for the first time from soil and compost samples. The outer membrane of strain T-T had a three-layered structure, outside the cytoplasmic membrane, which is similar to the S-layer in the cells of members of the Bacillaceae. Chemical analysis of the cells revealed that menaquinone-6 is a major component of the quinone system. According to these results, along with several previous observations (i.e. a G+C DNA content of 65 mol% and the identification of iso-C-15:0 and iso-C-17:0 acids as major cellular fatty acids), the new taxon Symbiobacterium thermophilum gen, nov., sp. nov. is proposed. The type strain is S. thermophilum strain T-T (= IAM 14863(T)).
