Epr3 is a conserved immunogenic protein among Aeromonas species and able to induce antibody response in Nile tilapia

The present study retrospectively identified sixteen putative aeromonad isolates associated with previous disease outbreaks in farmed tilapia, unspecified fish, and striped catfish. Based on a combination of biochemical characterization and homology identity of 16S rDNA, the majority of the isolates were identified as Aeromonas caviae (n = 10) while the remaining isolates were Aeromonas veronii (n = 4), Aeromonas jandaei (n = 1) and Plesiomonas shigelloides (n = 1). Potential antigenic gene of epr3 from A. caviae isolates was identified and recombinant proteins were expressed in E. coli system for investigation of immunogenicity in vivo. The result indicated the beta-sheet domain (comprising amino acid residue 23-171) expressed in a soluble form while the alpha-helix domain (residues 172-346) of Epr3 protein expressed in an insoluble form. Interestingly, all 15 identified Aeromonas strains naturally produced Epr3 evidenced by Western blot analysis detected with Epr3-specific polyclonal antibodies. Serum collected from the surviving fish after exposure to A. caviae AH15 was able to react with two domains of the recombinant Epr3. Two immunogenic domains of Epr3 had the capability of enhancing fish humoral immunity but a soluble form (beta-sheet) induced a higher antibody response compared to an insoluble form (alpha-helix). A combination of two forms provoked the strongest antibody response as indicated by dot blot assay. Furthermore, Western blot results showed that sera from fish immunized with Epr3 beta-sheet domain, alpha-helix domain, and pool of both domains could also react with specific protein bands from bacterial lysates of A. caviae, A. veronii and A. jandaei. The result suggests that the common immunogenic protein Epr3 is a promising candidate antigen for development of a cross-protective vaccine against different isolates of Aeromonas spp. Statement of relevance: The authors strongly believe that our manuscript would provide significant knowledge to fish aquaculture especially to that of the tilapia (Oreochromis spp.) farming and vaccine development for Aeromonas spp. infection. (C) 2016 Elsevier B.V. All rights reserved.