Comparative genomics of seven Tenacibaculum maritimum isolates from farmed Atlantic salmon (Salmo salar), Atlantic halibut (Hippoglossus hippoglossus), and lumpfish (Cyclopterus lumpus) in Canada and Norway reveals conserved features

Tenacibaculum maritimum is a major bacterial pathogen of marine fish responsible for ulcerative disease outbreaks and high mortality in numerous fish species and represents an important challenge in aquaculture worldwide. In this study, we characterized seven newly sequenced T. maritimum isolates recovered from farmed Atlantic salmon (Salmo salar), Atlantic halibut (Hippoglossus hippoglossus), and lumpfish (Cyclopterus lumpus) in British Columbia (Canada) and Norway, including the first complete genome of T. maritimum isolated from Atlantic salmon in Norway. Comparative genomic analysis revealed the presence of a complete Type IX secretion system (T9SS) and the canonical gliding-associated machinery. We also identified a largely conserved repertoire of candidate T9SS substrates and several major host-interaction proteins. Phylogenetic analysis placed the isolates in several well-supported clades comprising strains from different geographic origins and host species. Characterization of the O-antigen gene clusters identified three different types (1-0, 2-1, and 4-0), whose distribution was not congruent with the phylogenetic relationships among isolates. Colony development of the four Norwegian isolates on agar was strongly influenced by incubation temperature, with rapid growth at 20-24°C, slower growth at 12-16°C, and only limited growth at 8°C. This study expands the available genomic resources for T. maritimum and highlights the importance of integrating genomic and environmental surveillance to improve tenacibaculosis management in aquaculture systems.