Comparative protection of two antigens (whole-cell and outer membrane vesicle) of Yersinia ruckeri in rainbow trout (Oncorhynchus mykiss)

Yersinia ruckeri (Y. ruckeri)-associated disease remains a problem in fish farming, despite widespread vaccine use. Y. ruckeri is still one of the main causes of economic loss in the aquaculture industry. In this study, we developed outer membrane vesicles (OMV) as a new antigen and compared its efficacy with the classical antigen (whole-cell bacteria). Rainbow trouts were immunized with Y. ruckeri-derived OMVs and formalin-inactivated whole-cell bacteria by immersion route. The fish were then subjected to an intraperitoneal challenge with live bacteria during 2-10 weeks post-immunization. Subsequently, relative percentage of survival (RPS), antibody titer, lysozyme serum activity, lymphocyte percent and kidney pathological signs were evaluated. The OMV morphology was confirmed by electron microscopy. OMV and whole-cell bacteria immunized fish had higher RPS relative to the control group, but whole-cell bacteria resulted in higher RPS than the OMV group. Moreover, in the whole-cell bacteria group antibody titer was higher than the control and OMV groups. Lymphocyte percentage and lysozyme level in the whole-cell bacteria group was increased compared to OMV and control groups. Y. ruckeri's pathological damage was observed in the control group, while only eosinophilic secretions were identified for both immunized groups. These findings proved the protection of Y. ruckeri-derived OMV against Y. ruckeri, but whole-cell bacteria in the immersion method had a better effect on disease protection.