Discovery of genomically distinct Arcanobacterium isolates from companion animals: a candidate for novel species status

The genus Arcanobacterium has expanded rapidly, with an increasing number of species described from domestic and wildlife hosts, underscoring the need for clearer delineation of species boundaries. In this study, 12 isolates initially presumed to belong to Arcanobacterium canis were examined using comparative phenotypic and genomic approaches to clarify their taxonomic position. The isolates showed partial phenotypic similarity to the type strain A. canis DSM 25104ᵀ, including consistent haemolysis, but differed in several enzymatic reactions and carbohydrate utilization patterns. Matrix-assisted laser desorption/ionization time-of-flight MS identified all isolates at the genus level, with only one displaying spectral similarity to A. canis . Phylogenetic analysis of five gene targets revealed that although the isolates shared sequence similarity with the type strain A. canis DSM 25104ᵀ, they consistently formed a separate clade. Core-genome phylogeny likewise placed the isolates on an independent branch. Pangenome analysis identified 2,178 gene clusters (58.8% core genes) and demonstrated that the cytolysin-like gene present in the type strain A. canis DSM 25104ᵀ is completely absent from all 12 isolates. Digital DNA–DNA hybridization and average nucleotide identity values fell below accepted species-level thresholds, indicating that the isolates represent a genomically distinct lineage within Arcanobacterium . Because chemotaxonomic analyses required for formal species description were not performed, the taxonomic status of this lineage remains unresolved. Nevertheless, the present findings refine species boundaries within Arcanobacterium and provide a robust foundation for future taxonomic and comparative genomic studies.