Abstract

The Chilean salmon farming industry faces major challenges from bacterial diseases, notably tenacibaculosis, which is primarily caused by Tenacibaculum maritimum and T. dicentrarchi. However, T. finnmarkense was recently identified as an emerging pathogen. This bacterium was previously described only in Norway, where two genomovars (finnmarkense and ulcerans) were reported. The present study focuses on the phenotypic and genetic characterisation of isolates recovered from salmonids in Chile to elucidate intraspecific diversity, identify the dominant genomovar, and evaluate pathogenic potential. Thirty-three isolates were analysed through phylogenetic analyses based on the 16S rRNA, fusA, and atpA genes, along with phenotypic characterisation, genotyping by RAPD and ERIC-PCR, and comparative genome analysis. All isolates were confirmed as T. finnmarkense and grouped within the genomovar ulcerans, which was further supported by whole-genome phylogenetic analysis of two representative isolates. Phenotypic characterisation was consistent with the general description of T. finnmarkense from previous studies, although some metabolic variations were observed compared to the homogeneity reported in Norwegian isolates. Genotypic analyses also revealed genetic variability among the presently studied isolates. Pathogenicity was assessed in Atlantic salmon (Salmo salar) through immersion challenge tests under controlled conditions, using isolates obtained from Coho salmon (Oncorhynchus kisutch; SC-T20) and Atlantic salmon (PB-L5). Koch's postulates were confirmed only with the SC-T20 isolate, which caused significant mortality (57.14%) in experimental infections. This study highlights the predominance of genomovar ulcerans in Chilean salmon farming and its varying pathogenic potential. The findings underscore the need to monitor the impacts of tenacibaculosis and to accurately identify the specific etiological agent responsible for skin lesions, particularly in cases of coinfection.