Immunogenicity and cross-protective potential of a sip-derived recombinant vaccine in hybrid red tilapia (Oreochromis sp.)

The recombinant vaccine was prepared from the Streptococcus iniae surface immunogenic gene (Sip) to determine its potential as a vaccine candidate and its ability to cross-protect against Streptococcus iniae and Streptococcus agalactiae. The vaccine was administered intraperitoneally (i.p.). Group 1 was vaccinated with the formalin-killed recombinant vaccine (Rvac), while groups 2 (Sivac), 3 (Savac) and 4 (Vvac) were immunised with the formalin-killed whole cells of S. iniae, S. agalactiae and E. coli (vector only), respectively. Group 5 was an unvaccinated control group (Cx). A booster dose was administered on day 14 following the first vaccination. Serum, mucus and gut lavage fluids were collected and evaluated using the enzyme-linked immunosorbent assay (ELISA). At challenge week 4, all groups were challenged intraperitoneally with an inoculum containing 1.43 × 109 CFU mL-1 live S. agalactiae and 1.32 × 109 CFU mL-1 live S. iniae, respectively. The Rvac group showed a strong immune response in tilapia; however, high antibody levels did not necessarily confer adequate protection, particularly when cross-infected with S. agalactiae. Moderate protection (60%) and excellent protection (100%) were observed in the homologous challenge with S. iniae. Therefore, further improvement may be required to provide outstanding protection, particularly against S. agalactiae.

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