Endogenous Probiotics As a Sustainable Strategy Against Opportunistic Pathogens of Salmonids

Furunculosis is a major lethal opportunistic infection affecting salmonids worldwide.  The brook charr (Salvelinus fontinalis), primary salmonid species in Quebec aquaculture, is heavily affected by this disease.  The etiological agent, Aeromonas salmonicida, is naturally present in the resident microbial community of salmonids.  Under inadequate conditions, however, the pathogen undergoes a shift in virulence as a response to the destabilisation of the host microbial flora.  Currently available methods to treat furunculosis have been pushed to their limits, as they either favour antibiotic resistance or induce harmful adverse effects to the farmed fish.  Endogenous probiotics are a promising alternative, since they are innocuous to the fish itself, to the environment and to human consumers.  In this study, we use in vitro antagonism screening assays and next-generation sequencing to unravel efficient probiotic candidates from the brook charr against A. salmonicida.  Our experimental data highlights: we show that among forty-three isolates from mucus, gills and intestines, eight candidates exhibit efficient in vitro antagonism against A. salmonicida.  Four mechanisms of action are postulated: diffusion of antimicrobial compounds, competitive exclusion, virulence factor inhibition and modulation of the host immune response.  Further investigation of these mechanisms will shed new light on new management approaches for opportunistic diseases affecting fish.