Generation and use of Edwardsiella ictaluri ghosts as a vaccine against enteric septicemia of catfish (ESC)

• Edwardsiella ictaluri ghost (EIG) was first generated by PhiX174 lysis gene E.
• The Edwardsiella ictaluri ghost could elicit significant immune responses and protection in channel catfish.
• Edwardsiella ictaluri ghost may be developed as a vaccine candidate against ESC in aquaculture.

The enteric septicemia of channel catfish, caused by Edwardsiella ictaluri, is one of the most severe diseases in channel catfish worldwide. Currently, a safe and highly effective vaccine is urgently needed for the control of this disease. In this study, E. ictaluri ghosts (EIGs) were first generated by PhiX174 lysis gene E, and the hematological indices and relative percent of survival (RPS) were examined in channel catfish (Ictalurus punctatus) after immunization. Generation of ghosts in the transformant of E. ictaluri carrying the plasmid pBV-lysisE was enhanced by increasing the incubation temperature up to 42 °C. Lysis of E. ictaluri occurred 2 h after induction and the process was complete in 6 h. The efficiency of ghost induction in non-lyophilized E. ictaluri was 99.997%. Additionally, no bacterial growth was detected by culture on BHI plates after lyophilization of the ghosts. The immune response of channel catfish immunized with the E. ictaluri ghosts indicated that the number of erythrocytes and leucocytes reached a maximum value of 3.53 × 106 and 5.25 × 105 cell/ml, at day 4 post-immunization, respectively, which were both significantly higher than in the control group inoculated with PBS only (P < 0.01). The percentage of monocyte and neutrophils reached 7.33 ± 1.52% and 26.3 ± 2.08% at day 4 post-immunization, respectively. The phagocytic percentage (PP) and phagocytic index (PI) reached maximum values of 49.33 ± 1.52% and 5.67 ± 1.15, respectively, which were significantly higher than in control group (P < 0.01). The percentage of lymphocytes and the serum antibody titers also increased significantly and peaked at 61.33 ± 0.58% and a titer of 1:682.67 at day 21 post-immunization, respectively. The challenge evaluation showed that the RPS of immunized group was 89.3%. These results demonstrated that EIGs could induce significant immunological responses in channel catfish that resulted in high protection and suggest that EIGs may be a potential vaccine against ESC in channel catfish.Statement of relevanceIn the paper, we illustrated the generation method of Edwardsiella ictaluri ghosts and the immunological responses and protection in channel catfish after being immunized with E. ictaluri ghosts. The result of this paper indicated that E. ictaluri ghosts would be an effective approach for preventing ESC in channel catfish in the future.