Selection of putative probiotics based on antigen-antibody cross-reaction with Photobacterium damselae subsp. piscicida and Vibrio harveyi for use in Senegalese sole (Solea senegalensis)

Probiotics can be used to reduce disease outbreaks in aquaculture. Some of them are characterised by their antagonistic activity against pathogens or the stimulation of the fish immune response, including the production of specific antibodies. If a probiotic has common antigens with a determined pathogen, it could produce antibodies with a cross-reaction to that pathogen. Thus, a probiotic with these characteristics could be used in a similar way to a live vaccine. The aim of this study was to select bacteria with antigenic similarity and antagonistic activity against the pathogens Photobacterium damselae subsp. piscicida and Vibrio harveyi, and to determine their ability to stimulate the production of antibodies in sole (Solea senegalensis, Kaup) with crossreaction against these pathogens. Dot blot was used to detect strains with cross-reaction using sera immunized against P. damselae subsp. piscicida Lg41.01 and V. harveyi Lg16.00. A total of 138 strains were selected from 718 strains, based on the intensity of the dot blot reaction. A second selection was performed to detect their ability to inhibit the pathogens growth. Five strains inhibited the growth of P. damselae subsp. piscicida, four strains inhibited the growth of V. harveyi, while two strains inhibited both pathogens. A Western blot confirmed the cross-reactions of the selected strains with the pathogens. Selected strains were subsequently inoculated into sole specimens by intraperitoneal injection. Four strains produced antibodies with cross-reaction against the pathogens. None mortality was observed in the inoculated fish. Further research demonstrated the storage capability of the selected strains in saline solution and feed, their growth at low pH, and identified their enzymatic characteristics. In conclusion, the selected strains showed antimicrobial activity and capacity to activate a specific immune response against fish pathogens.