Immunoproteomic identification of OmpA with potential stimulation of serum-specific antibody in Nile tilapia (Oreochromis niloticus) and its ability to protect against Edwardsiella ictaluri infection

Edwardsiella ictaluri is a Gram-negative bacterium that has been reported to be disease causing in several economic fish including Nile tilapia. Here, immunoproteomic identification found that outer membrane protein A (OmpA) could react with serum from fish naturally infected with E. ictaluri. The expression and production of recombinant OmpA protein could be successfully conducted in Escherichia coli strain BL21(DE3) with the molecular mass of 38 kDa after inclusion bodies were successfully solubilized and refolded. Immunization of fish with the recombinant OmpA protein showed that the recombinant proteins could induce the production of specific antibodies in serum of immunized fish (p < 0.05) at 1-week postimmunization (7 dpi) and reached the highest antibody titre at 3 weeks postimmunization (21 dpi). The serum-specific antibody from fish immunized with the recombinant OmpA exhibited bacterial agglutination property. Furthermore, the partial protection of Nile tilapia was observed in OmpA immunized fish with the relative per cent survival (RPS) of 39.81%. In summary, this study explored the immunoproteomic method as a starting point to investigate OmpA of E. ictaluri and might be considered as a candidate protein for recombinant vaccine development in Nile tilapia against this bacterial infection.