Protective effects of OmpA exposure via Bacillus subtilis spore bathing on Japanese eel (Anguilla japonica) against Edwardsiella anguillarum infection

Edwardsiella anguillarum is a major pathogen responsible for causing hemorrhagic septicemia in Japanese eels (Anguilla japonica). Currently, no commercial vaccines are available against E. anguillarum infection. In our previous study, the outer membrane protein A (OmpA) was identified as a protective antigen of E. anguillarum. In this study, we constructed a spore-displayed OmpA strain of Bacillus subtilis (WB800N-cotC-OmpA) and successfully displayed OmpA on the B. subtilis spore. Japanese eels were then immunized by bath immersion in suspensions of WB800N spores (WC group), WB800N-cotC-OmpA spores (WA group), inactivated E. anguillarum (EA group), or PBS (Con group). We measured the phagocytic activity of leukocytes, plasma bactericidal activity, and plasma-specific antibody titers. At 28 days post-immunization, all groups were challenged with E. anguillarum, and cumulative mortality rates were recorded. Compared to the Con group, the EA and WA groups exhibited significantly increased plasma bactericidal activity, and the EA group showed significantly elevated anti-E. anguillarum antibody titers at 28 dpi. However, antibody titers in the WA group were not significantly different from those in the WC group. Notably, the mortality rates in the EA and WA groups were significantly lower than those in the Con group, with 52.6% and 57.9% relative percent survival (RPS), respectively. In conclusion, we successfully displayed OmpA on the B. subtilis spore, and bath immunization of Japanese eels with the spore-displayed OmpA was an effective strategy against E. anguillarum infection. Overall, this study provides a novel approach for developing fishery vaccines.