Taxonomic relationships of Thiobacillus halophilus, T. aquaesulis, and other species of Thiobacillus, as determined using 16S rDNA sequencing.

Total base sequences of the 16S rRNA genes of Thiobacillus halophilus and Thiobacillus aquaesulis show that these bacteria fall into the gamma- and beta-subdivisions, respectively of the Proteobacteria. The closest relative of T. halophilus is Thiobacillus hydrothermalis (with 98.7% similarity), and the closest relative of T. aquaesulis is Thiobacillus thioparus (93.2% similarity). Physiological properties and mol% G+C content of their DNA serve to confirm that these four organisms are all distinct species. It is reiterated that the species currently assigned to the genus Thiobacillus are clearly so diverse that they need reclassification into several genera. The type species, T. thioparus, is unequivocally placed in the beta-subdivision of the Proteobacteria, thus requiring that the use of the genus name Thiobacillus be restricted to the chemolithoautotrophic species falling into that group. T. aquaesulis and T. thioparus may thus be regarded as true species of Thiobacillus. The relatively large number of obligately chemolithoautotrophic Thiobacillus species falling in the gamma-subdivision of the Proteobacteria need further study in order to assess the case for reclassification into one or more new or different genera.