Intensive aquaculture and globalization of trade contribute to the emergence and spread of infections in fish farms. In a context of restriction of antibiotics use, probiotics could represent an interesting bioalternative to control infections and improve the zootechnical performances while minimizing the environmental impact. We investigated the probiotic potential of sporulated bacteria extracted from the marine environment on seabass, Dicentrarchus labrax, one of the major marine fish species produced in Mediterranean.
Bacillus strains were screened in vitro and fully sequenced to identify their potential probiotic properties and assess their safety. The efficacy of probiotics was evaluated by dietary supplementation of juveniles seabass (average initial weight : 5g) with the candidates of interest for several months at 21°C. During the supplementation, their ability to persist in the intestine, their effects on growth (length and size), immune system (blood formulation, phagocytosis activity) were evaluated. After two to five months of supplementation, some fish were experimentally infected with Vibrio harveyi, Vibrio anguillarum or nervous necrosis virus (NNV) to evaluate their disease resistance.
Genome analyses confirmed the absence of antibiotic resistance and pathogenicity genes and revealed interesting capacities of production of amino acids, vitamins and digestive enzymes. Strains selected showed antibacterial properties against various genus of pathogenic bacteria isolated from farms, associated with the identification of gene clusters encoding secondary metabolites with potential antimicrobial activities.
The survival of probiotics in the intestinal tract of supplemented fish was confirmed with the detection of concentrations up to 1.105 CFU per intestine at different time. Probiotic supplementation increased survival rates after infection with Vibrio harveyi and Vibrio anguillarum. However, survival rates are not impacted for NNV infection. Furthermore, fish supplemented with probiotics showed significantly higher leukocytes counts, with an increased proportion of phagocyte (expression of immunity genes ongoing).
The In vitro and bioinformatics analyses performed confirm the probiotic potential and the safety of the marine strains of Bacillus selected. These probiotics have beneficial effects on components of the immune system and improve resistance to bacterial pathogens of juveniles sea bass.