Antagonistic Mechanisms of Probiotic Aliivibrio sp. Strain Vl2 Against Moritella viscosa: Evidence from Co-cultivation and Targeted Transcriptomic Analysis.

Winter ulcers, primarily caused by Moritella viscosa, represent a significant challenge for the Norwegian aquaculture industry. Effective control measures are hampered by the lack of effective vaccines and limited use of antibiotics, driven by the global effort to combat antibiotic resistance. Recent studies have shown that probiotic Aliivibrio spp. colonize the skin and ulcers of Atlantic salmon and are linked to 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 investigated how the probiotic Aliivibrio sp. strain Vl2 (hereafter Aliivibrio Vl2) modulates M. viscosa in vitro, using both co-cultures and cultures within the salmonid cell line CHSE-214. We found that this probiotic strain antagonizes M. viscosa, reducing its growth and its pathogenicity toward the salmonid cells. Targeted transcriptome analysis of Aliivibrio Vl2 during co-culture revealed potential antagonistic mechanisms that impede the growth of this competing pathogen. Together, our findings demonstrate that this probiotic bacterium inhibits M. viscosa growth in vitro. Furthermore, the identified transcriptomic changes suggest potential mechanisms underlying the reduced prevalence of winter ulcers observed in field studies after probiotic administration.