Antagonistic Effect of Indigenous Skin Bacteria of Brook Charr Salvelinus fontinalis Against Flavobacterium columnare and F. psychrophilum

Aquaculture industry has grown for the last decade and massive production in fish farms exposes fish to stressful conditions, which induce infections by opportunistic pathogens. Probiotics appear to be the more promising way to prevent opportunistic infections in aquaculture. The current strategy to develop probiotic for a given host species is to test a probiotic agent already proven to be efficient for another host species. However, when transferred into a different interacting environment, the probiotic agent is likely to lose its probiotic properties, and possibly becoming harmful for its secondary host. Therefore, to ensure the harmless of a probiotic for our host species, we sampled it directly from the host endogenous bacterial community. We assumed first, that some mutualist bacteria living in the skin mucus of Salvelinus fontinalis were able to compete against pathogens under normal condition and second, that the stress itself induces changes in the bacterial community, and third, that this change allows opportunistic pathogen to infest the host. In order to identify which bacteria were able to compete against pathogens, we screened the bacterial community with a culture based method and tested in vitro the potential competitive effects against the pathogens. Our results from both agar diffusion assay and broth co-culture assay showed unambiguously that the mechanism involved is a competitive exclusion. We found eight bacterial strains, collected from unstressed fish, which exerted strong exclusive competition against both F.psychrophilum and F.columnare, suggesting they are all promising probiotic candidates. All those candidates were mixed together equally and use in an in vivo experiment. Probiotics candidate were added in the surrounding water of four different families. Those families responded differently to the infection in control tanks. One was strongly resistant (~5% of death), two other were medium sensitive (~15% of death) and the last was strongly sensitive (~25% of death). After 3 weeks of treatment with probiotics, an important diminution of death was observed with respectively 54%, 57%, 74% and 86% less death. Those results confirm that our candidates had a strongly antagonistic effect against F.columnare and confirm that the interaction between probiotics and fishes have genetics basis.