Stimulated Growth and Innate Immunity in Brook Charr (Salvelinus fontinalis) Treated with a General Probiotic (Bactocell(®)) and Two Endogenous Probiotics That Inhibit Aeromonas salmonicida In Vitro

Aeromonas salmonicida subsp. salmonicida is a Gram-negative bacterium causing furunculosis, an opportunistic infection of farmed salmonid fish. Current treatment methods against furunculosis rely heavily on antibiotherapy. However, strains of this opportunistic fish pathogen were found to possess genes that confer resistance to major antibiotics including those used to cure furunculosis. Therefore, dispensing bacterial symbionts as probiotics to susceptible hosts appears to be a promising alternative. Here, we present the genomic characterization and in vivo safety assessment of two brook charr (Salvelinus fontinalis) bacterial symbionts that inhibited A. salmonicida subsp. salmonicida growth in vitro (Pseudomonas fluorescens ML11A and Aeromonas sobria TM18) as well as a commercialized probiotic, Pediococcus acidilactici MA18/5M (Bactocell(®)). The genomic sequences of ML11A and TM18 obtained by whole-genome shotgun sequencing lack key virulence factor genes found in related pathogenic strains. Their genomic sequences are also devoid of genes involved in the inactivation (or target modification of) several key antimicrobial compounds used in salmonid aquaculture. Finally, when administered daily to live brook charr fingerlings, ML11A, TM18 and Bactocell(®) helped improve several physiological condition metrics such as mean body weight, Fulton’s condition factor and blood plasma lysozyme activity (an indicator for innate immune activity).