Phylogenetic structure of the genera Flexibacter, Flexithrix, and Microscilla deduced from 16S rRNA sequence analysis

The 16S rDNA sequences of 40 strains of 17 species in the genus Flexibacter, 5 strains of 4 species in the genus Microscilia, and 1 strain of Flexithrix dorotheae, including all type strains of approved and validated species in these genera, were determined to reveal their phylogenetic relationships. The 16S rRNA sequence analysis demonstrated the extreme heterogeneity of the genera Flexibacter and Microscilla. The strains examined diverged into 24 distinct lines of descent (1 group included both flexibacteria and flexithrix, and 1 group included both flexibacteria and microscilla) that were remote from each other at the genus level or higher. Flexibacter strains were scattered across the cytophaga-flavobacteria-bacteroides phylum and divided into 20 phylogenetic groups, and the genus Microscilia was separated into 5 groups. Flexibacter flexilis, the type species of the genus Flexibacter, and Microscilla marina, the type species of the genus Microscilla, were isolated from other organisms in their respective genera. This means that each genus should be restricted to only the type species. Flexithrix dorotheae, the type species of the genus Flexithrix, clustered with Flexibacter aggregans. The heterogeneity was found not only within genera but also within species. Flexibacter aggregans, Flexibacter aurantiacus, Flexibacter flexilis, Flexibacter roseolus, Flexibacter tractuosus, and Microscilla sericea each contained phylogenetically distant strains. The taxonomic concept of the genera Flexibacter, Flexithrix, and Microscilla should be reorganized in accordance with the natural relationships revealed in this study.