Preparation and immunogenicity of Lactococcus lactis vaccine expressing Sip-Pgk fusion protein of Streptococcus agalactiae isolated from tilapia

In order to develop a more effective Oreochromis sp. oral vaccine of Streptococcus agalactiae, we constructed a recombinant plasmid pNZ8148-sip-pgk which expressed Sip-Pgk fusion protein of S. agalactiae by using homologous recombination, and Lactococcus lactis NZ9000 pNZ8148-sip-pgk was obtained by electrotransformation of L. lactis NZ9000. The Sip-Pgk fusion protein was induced by nisin and tested by Western blot so that a Sip-Pgk fusion protein Lactobacillus oral vaccine was prepared. The tilapia was vaccinated orally by gavage with recombinant L. lactis NZ9000 pNZ8148-sip-pgk for twice or three times. ELISA was used to detect the level of serum antibody after immunization, and the relative immune protection rate was obtained by intraperitoneal injection of S. agalactis on the 18^th day after immunization. The results show that the inducible protein size of the constructed recombinant Lactobacillus was 92 kD, which was consistent with the size of the target protein. Compared with the second immunization, the third immunization with the fusion protein Lactobacillus vaccine could significantly improve the serum antibody level of tilapia and the immune protection against S. agalactiae. The serum level of Lactobacillus vaccine immunized with three times Sip-Pgk fusion protein was significantly higher than that of single protein group and PBS group, with the highest relative immune protection rate of 45.56%.