The rapid development of aquaculture, its intensification, and the occurrence of fish health problems on farms push to develop alternative methods to antibiotics and chemotherapy for controlling fish diseases. Probiotics may provide a potential alternative method to protect fish from opportunistic and pathologic bacteria and promote a balanced environment.

In this work, we have assessed the in vitro probiotic properties of twenty one bacteria from aquatic and fish origin, for their application in aquaculture. Selection was based on their antimicrobial activity (Bacteriocin) against fish pathogens and their in vitro safety assessment. This includes the evaluation of their haemolytic, proteolytic and mucinolytic activities, bile salt deconjugation ability and antibiotic susceptibility.

Twelve of the twenty one bacteria isolated from several showed strong antibacterial activity against several pathogenic species such as Lactococcus garvieae, Vibrio anguillarum, Vibrio harveyi, Aeromonas hydrophila and Aeromonas salmonicida, and were taxonomically identified by partial 16S rDNA gene sequencing. The cell-free culture supernatants from cultures of these twelve strains were treated with proteinase K (10 mg/ml; 37ºC, 1h) and submitted to heat treatment (100ºC, 10 min), which showed that eleven strains exert extracellular antimicrobial activity against fish pathogens due to the production and secretion of thermo-stable antimicrobial peptides (i.e., Bacteriocins). The tested strains showed a great heterogeneity respect to their safety and antibiotic susceptibility.

Given their antimicrobial (Bacteriocin) activity against fish pathogens and their safety, some of the tested strains may be considered as potential fish probiotics, and their effectiveness will be further tested in vivo.