Taxogenomics of the Genus Cyclobacterium: Cyclobacterium xiamenense and Cyclobacterium halophilum as Synonyms and Description of Cyclobacterium plantarum sp. nov.

The genus Cyclobacterium belongs to the phylum Bacteroidetes and includes eight species. Our study, based on the genomic parameters in silico DNA–DNA hybridization (GGDC), average nucleotide identity (OrthoANI), and average amino acid identity (AAI), confirmed that all current species of Cyclobacterium belong to this genus and constitute a coherent phylogenomic group, but with species forming two separate branches. In addition, the genome-based analyses revealed that Cyclobacterium xiamenense and Cyclobacterium halophilum are members of the same species. Besides, we carried out a taxonomic characterization of the new strain GBPx2(T), isolated from the halophytic plant Salicornia sp. Analysis of its 16S rRNA gene sequence showed the highest sequence similarity (97.5%) to Cyclobacterium lianum HY9(T). Percentages of GGDC and OrthoANI between strain GBPx2(T) and species of the genus Cyclobacterium were lower than the threshold value for species delineation. The DNA G+C content was 43.0 mol%. The polar lipids included phosphatidylethanolamine as well as one unidentified phospholipid and four unidentified lipids, and its major cellular fatty acids were iso-C(15:0) and summed feature 3 (C(16:1)ω7c and/or iso-C(15:0) 2-OH). The only quinone present was menaquinone 7. Based on a combination of phenotypic, chemotaxonomic, and phylogenomic features, the GBPx2(T) strain represents a novel species of the genus Cyclobacterium, for which the name Cyclobacterium plantarum sp. nov. is proposed. The type strain of Cyclobacterium plantarum is GBPx2(T) (= IBRC-M 10634(T) = LMG 28551(T)).