Research

Disease in farmed Atlantic salmon occurs in all its life stages. Salmon are particularly vulnerable to infectious diseases at transition from the freshwater stage to the saltwater stage. Our aim in these studies reported was to investigate the possibility that waterborne delivery of a probiotic comprised of naturally occurring marine bacterial species would reduce the mortality and improve the health and growth of farmed Atlantic salmon.

Cleaner wrasse eat sea lice (Lepeophtheirus salmonis) directly from the skin of the Atlantic salmon (Salmo salar). To reduce harvest from the wild wrasse populations and to increase the quality and availability of cleaner fish, lumpfish (Cyclopterus lumpus) have been increasingly developed as a service industry to the farming of Atlantic salmon in the last few years. Acquired resistance against the de-lousing chemicals is occuring at an increasing speed particularly in mid- and western-Norway has made the farming of lumpfish more important in the control of the sea lice infestations in farmed Atlantic salmon.

Ulcerative conditions present a major challenge in Norwegian salmon farming. Probiotic Aliivibrio species have previously been demonstrated to provide health benefits in both Atlantic salmon and lumpfish. This study aimed to investigate whether these bacteria could colonize Atlantic salmon following bath administration, determine the tissue tropism, and assess the duration of colonization. We examined the host microbiota using culture-based methods, qPCR and immunohistochemistry techniques specifically designed to target the applied Aliivibrio strains. Our findings reveal that the probiotic bacteria can successfully colonize Atlantic salmon and persist for at least nine months post-administration. We identified the administered strains in the skin and underlying tissue with all three methods. The probiotics were also identified in the distal intestine and the visceral organs. Additionally, we isolated the probiotic Aliivibrio species from mixed cultures in ulcerated areas. While viable bacteria were recoverable from recently euthanized fish, tissue decay promoted bacterial recovery of the administered species across all experiments. Given prior evidence on ulcer reduction associated with these probiotics, competitive exclusion appears to be a plausible mechanism of action, though further investigation is warranted.

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.

Salmon lice (Lepeophtheirus salmonis) constitute a major challenge during the production of farmed Atlantic salmon in Norway. Preventive measures are considered to have a higher impact on sustainable control than lice treatment. Therefore, the studies presented here aimed to document the preventive effects of probiotic Aliivibrio spp. on lice infestation in experimental challenges. A reduction in salmon lice attachment success (58–65%) was observed in two separate aquarium trials, where Atlantic salmon were exposed to different compositions of Aliivibrio species 91 and 155 days prior to lice challenge. In a third trial, no difference in attachment was observed in groups exposed to probiotics 58 days prior to lice challenge compared to controls. However, a relative reduction in lice counts was seen on movable stages later in the trial. High levels of probiotic bacteria had no impact on lice viability in an in vitro bioassay on the preadult life stage; thus, the mechanism behind the preventative effect remains unknown. In conclusion, probiotic Aliivibrio bacteria can likely be used as a preventive tool to reduce salmon louse infestations in the salmon industry. The mechanism is still unknown, and this novel a