Molecular phylogeny and taxonomy of colorless, filamentous sulfur bacteria of the genus Thiothrix

摘要

Comprehensive investigation combining molecular genetic techniques and comparative studies of morphological and physiological properties made it possible to resolve the disputed issue of the taxonomic status of the groups "T. nivea" and "Eikelboom type 021N" of the genus Thiothrix. The phylogenetic trees constructed on the basis of 16S rRNA and gyrB gene sequences demonstrated that members of the genus Thiothrix formed a cluster within the order Thiotrichales. According to the "ribosomal" tree, the cluster of the genus Thiothrix was divided into two main groups, I and II, corresponding to the groups "T. nivea" and "Eikelboom type 021N". The levels of similarity between the 16S rRNA gene sequences of Thiothrix species reached 88. 9-100%. On the contrary, in the "gyrase" tree, these species were not divided into "T. nivea" and "Eikelboom type 021N" groups. The levels of similarity between the amino acid sequences of the gyrB gene fragments of Thiothrix species varied from 74. 5 to 99. 2%. Importantly, members of the groups "T. nivea" and "Eikelboom type 021N" formed very similar 16S rRNA secondary structures in the variable region V3, where a 30-nucleotide deletion characteristic of all Thiothrix species was detected. Phenotypic analysis of the studied bacteria revealed some morphological and physiological properties shared by the groups "T. nivea" and "Eikelboom type 021N". The data obtained indicate that members of the groups "T. nivea" and "Eikelboom type 021N" are phenotypically and genetically heterogeneous species within the single monophyletic genus Thiothrix.