Taxonomy of the Genus Microcyclus Orskov 1928: Reintroduction and Emendation of the Genus Spirosoma Migula 1894 and Proposal of a New Genus, Flectobacillus
The type strains of the three currently recognized species of the genus Microcyclus Ørskov 1928 (M. aquaticus Ørskov, strain ATCC 25396; M. major Gromov, strain BKM 859; and M. flavus Raj, strain ATCC 23276), together with strains apparently belonging to each species, were examined to determine their taxonomic relationships. There were many differences between the type strains in their morphological and physiological characteristics as well as in the guanine-plus-cytosine (G+C) contents of their deoxyribonucleic acids (DNAs). Therefore, we propose to split the genus Microcyclus into three genera, each containing a single species. M. aquaticus, the type species of Microcyclus, remains as the only species in the genus. It is characterized by the formation of cellular rings which are produced when the ends of the curved cells overlap. It is nonpigmented and is respiratory in its metabolism; the G+C content of its DNA ranges from 66.3 to 68.4 mol%. M. flavus ATCC 22376 and three similar strains were found to possess characters in agreement with those given in the original description of Spirosoma linguale, and they were placed in that species. The name M. flavus Raj 1970 thus becomes a later subjective synonym of Spirosoma linguale (Eisenberg 1891) Migula 1894. This species is characterized by the formation of wavy and coiled filaments, yellow pigmentation on Microcyclus-Spirosoma (MS) agar, and respiratory metabolism. The G+C content of its DNA ranges from 51.0 to 52.9 mol%. Strain DSM 74 is designated as the neotype strain of S. linguale. M. Major BKM 859 and one similar strain were placed in a new genus, Flectobacillus. F. major (Gromov) comb, nov., the type species, is composed of rods which are straight to curved, the degree of curvature varying among individual cells within a culture. The cells may form long, sinuous filaments and rings. Colonies on MS agar are pale pink to rose-colored. Metabolism is respiratory, and the G+C content of its DNA ranges from 39.5 to 40.3 mol%. The type strain of F. major is strain BKM 859.