An effective live attenuated vaccine against Streptococcus agalactiae infection in farmed Nile tilapia (Oreochromis niloticus)

Streptococcus agalactiae is an important pathogen associated with various aquatic animals, especially tilapia. Streptococcosis has greatly limited the healthy development of tilapia aquaculture in recent times. The development of novel effective vaccines is important for the prevention and control of streptococcosis in fish. We previously constructed a non-encapsulated S. agalactiae strain Delta cps by the in-frame deletion method. Here, we evaluated whether this mutant Delta cps is safe for tilapia and suitable for protection against streptococcosis. We observed that the Delta cps strain was non-pathogenic to tilapia, and there was no reversion of virulence when it was passaged in tilapia. Moreover, the Delta cps strain survived for at least 11 d in the main immune organs of tilapia. The tilapia vaccinated via intraperitoneal (IP) injection with Delta cps strain induced a high antibody titer, and the IgM antibody levels were significantly higher in the vaccinated group than in the control group. The vaccination protected tilapia against the S. agalactiae challenge with a relative percent survival of 90.47%. In addition, tilapia immunized with the /cps strain showed significantly higher expression level of IFN-gamma, IL-1 beta, MyD88, IgM, and MHC-I alpha in the head kidney than those in the control during the entire observation period. The expression of MHC-143 was inhibited during 1-7 d of immunization. These results revealed that the Delta cps strain is able to induce humoral and cell-mediated immune response in tilapia. Therefore, the strain Delta cps has a broad application prospect as a target for attenuation in vaccine development.