Molecular characterization and bivalent vaccine development of Aeromonas hydrophila and Aeromonas veronii in Nile tilapia (Oreochromis niloticus)

In the present study, nine strains of Aeromonas spp. were isolated from diseased Nile tilapia in Thailand and identified based on 16S rRNA and gyrB gene sequences and multilocus sequence typing (MLST) analysis. Phenotypic characterization was performed for all the isolates. For the antimicrobial susceptibility test, all isolates were resistant to amoxicillin, whereas most of the isolates were susceptible to enrofloxacin. Pathogenicity analysis revealed that most A. hydrophila strains were highly virulent to fish, corresponding with the hemolytic activity test demonstrating that clear hemolysis occurred on sheep blood agar. To develop a vaccine for fish against A. veronii and A. hydrophila infection, a bivalent vaccine based on the formalin-killed cells (FKC) of A. veronii A5 and A. hydrophila A9, which showed higher virulence to fish, was prepared. This FKC vaccine was administered to Nile tilapia by intraperitoneal injection. The booster vaccination was conducted at 4 weeks after the first vaccination. The specific antibody titer was evaluated during rearing since the first vaccination, and then at 8 weeks after the first vaccination, the fish were challenged with A. veronii A5 and A. hydrophila A9 via intraperitoneal injection. There is not significance different in relative percentage survival of the vaccinated fish against A. veronii (63.49 +/- 19.51%) and A. hydrophila (93.33 +/- 6.67%). These results suggest that this candidate bivalent vaccine is effective against both A. veronii and A. hydrophila infection in Nile tilapia and could be an "motile Aeromonas septicemia (MAS)" control measure. However, to obtain the promising vaccine efficacy, the optimization of vaccine formulation might be achieved and be validated vaccine efficacy in field trial.