Antagonistic mechanisms of probioticAliivibriosp. strain Vl2 againstMoritella viscosa: Evidence from co-cultivation and transcriptomic analysis

Winter ulcers are a significant challenge to the Norwegian aquaculture industry and the primary causative agent is Moritella viscosa . Control measures are limited due to the lack of effective vaccines and limited use of antibiotics, reflecting the global effort to combat antibiotic resistance. It was recently demonstrated that probiotic Aliivibrio spp. colonize the skin and ulcers of Atlantic salmon and their association with a reduced prevalence of winter ulcers. These observations suggest that M. viscosa and Aliivibrio spp. may interact within ulcers in vivo . In this study, we have investigated how the probiotic Aliivibrio sp. strain Vl2 modulates M. viscosa in vitro , using both co-cultures and CHSE cell cultures. We found that the probiotic strain exhibits antagonistic effects against M. viscosa , reducing its growth and pathogenicity toward salmonid cells. Transcriptome analysis of Aliivibrio Vl2 revealed potential mechanisms impeding the growth of the competing pathogen. Together, our findings demonstrate how the probiotic bacterium can inhibit M. viscosa in vitro and suggests potential mechanisms that may explain the reduced prevalence of winter ulcers observed in the field.