Characterization of Streptococcus iniae ghost vaccine and its immunization in Nile tilapia (Oreochromis niloticus)

Streptococcus iniae (S. iniae) is a Gram‐positive bacterium associated with disease in several commercial species and other animals including humans. This zoonotic pathogen has led to high worldwide morbidity and mortality and huge economic losses in aquaculture. Here a S. iniae ghost (SIG) vaccine was successfully generated with a chemical method, and its immunogenicity and protective efficacy were examined in this study. The results showed that the titres of immunoglobulin M (IgM) in serum were significantly higher in group A (SIG‐immunized) than group B (formalin‐inactivated S. iniae, FSI‐immunized), which was significantly higher than the control group (PBS‐treated). Also, the phagocytic activity was significantly higher in group A than group B, which was significantly higher than the control group. Moreover, AKP activity and lysozyme activity of serum in group A were significantly higher than group B, which was significantly higher than the control group. Also, IL‐1β, IFN‐γ and HSP70 in the head kidney of fish immunized with SIG and FSI showed significantly higher than the control group throughout the whole experimental period. In addition, IL‐1β and IFN‐γ in the spleen of fish immunized with SIG showed significantly higher than that of in the FSI‐immunized fish. Most important, the survival rate and relative per cent survival of group A were higher than group B, and both of them were all significantly more protected against the S. iniae challenge compared to the PBS‐treated group. These results suggest that immunization with SIG induces immune responses and provides protection against a virulent S. iniae challenge.