Taxonomic characterization of Sphaerotilus microaerophilus sp. nov., a sheath-forming microaerophilic bacterium of activated sludge origin

A microaerophilic Gram-stain-negative bacilliform bacterial strain, FB-5 ^T, was isolated from activated sludge in Yokohama, Japan, that exhibited filamentous growth and formed a microtube (sheath). Cells were motile using a single polar flagellum. The optimum growth temperature and pH were 30 °C and 7.5, respectively. Strain FB-5 ^T was catalase-negative. Peptides and amino acids were utilized as energy and carbon sources. Sugars and organic acids were not utilized. Vitamin B₁₂ enhanced the growth of strain FB-5 ^T. Sulfur-dependent lithotrophic growth was possible. Major respiratory quinone was UQ-8. Major fatty acids were C_16:1ω7 and C_16:0. The genomic DNA G + C content was 69.16%. Phylogenetic analysis of the 16S rRNA gene suggested that strain FB-5 ^T belongs to the genus Sphaerotilus. The close relatives were S. natans subsup. sulfidivorans and S. natans subsup. natans with 98.0% and 97.8% similarity based on the 16S rRNA gene analysis, respectively. The genome size (6.06 Mbp) was larger than that (4.39-5.07 Mbp) of the Sphaerotilus strains. The AAI values against the related strains ranged from 71.0 to 72.5%. The range of ANI values was 81.7 - 82.5%. In addition to these distinguishable features of the genome, the core genome and dDDH analyses suggested that this strain is a novel member of the genus Sphaerotilus. Based on its physiological properties and genomic features, strain FB-5 ^T is considered as a novel species of the genus Sphaerotilus, for which the name S. microaerophilus sp. nov. is proposed. The type strain is FB-5 ^T (= JCM 35424 ^T = KACC 23146 ^T).