<em>In vitro</em> Assessment of the probiotic characteristics of Bacillus strains from gut of gilt-head bream, <em>Sparus aurata</em>

Abstract

Background and aim: 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. In this regard, the application of antibiotics as therapeutic and/or prophylactic strategies during fish production may lead to serious health and environment problems such as bacterial drug resistance development and dissemination, and food safety concerns. Probiotics may provide a potential alternative method to protect fish from opportunistic and pathologic bacteria and promote a balanced environment. Methods: Evaluating the in vitro probiotic properties of nine bacteria of aquatic or gut of gilt-head bream for their application in aquaculture. Selection was was assayed by subtractive screening with several criteria including, their antimicrobial activity (Bacteriocin) against fish pathogens, tolerant to low pH and bile salts, their secretion of enzymes and finally their in vitro safety assessment Results: Two of the nine bacteria isolated from several biotopes showed strong antibacterial activity against several pathogenic species such as Lactococcus garvieae, Vibrio anguillarum, Vibrio harveyi, Aeromonas hydrophila and Aeromonas salmonicida. These endospores-forming strains were withstand gastrointestinal conditions, produced a notable amylase, protease and lipase activities, showed high percentages of hydrophobicity, auto-aggregation and co-aggregation with fish pathogens and a strong adhesion to several fishes mucus and presented also a strong inhibition of pathogens adhesion. The tested strains showed a great heterogeneity respect to their safety and antibiotic susceptibility and were taxonomically identified by partial 16S rDNA gene sequencing. Conclusion: Given their antimicrobial 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.