Potential of naturally attenuated Streptococcus agalactiae as a live vaccine in Nile tilapia (Oreochromis niloticus)

Abstract

Streptococcosis, caused by Streptococcus agalactiae, among other Streptococcus species, seriously harms the global tilapia aquaculture industry. Immunization is a preferred method to control this disease. A naturally attenuated S. agalactiae strain TFJ0901 was previously isolated from Nile tilapia (Oreochromis niloticus). In this study, the vaccine potential of TFJ0901 in Nile tilapia via intraperitoneal (IP) injection immunization was analyzed by the safety, immune efficacy and immunological response assessments. The virulence test through IP challenge showed that TFJ0901 had extremely low virulence with an LD50 of 8.8 × 108 CFU/fish. The results of serial passage tests in vivo demonstrated that TFJ0901 had stable virulence with no potential of reversion. At 28 days post-vaccination, the relative percent survival (RPS) of fish vaccinated with TFJ0901 at 1.0 × 106 and 1.0 × 107 CFU/fish were 71.8% and 97.4%, respectively, which were significantly higher than those fish vaccinated with 1.0 × 102–1.0 × 105 CFU/fish. Additionally, the duration of immune protection was determined for 1.0 × 106 and 1.0 × 107 CFU/fish with single or booster vaccinations. At 28, 56 and 84 days post-initial vaccination (DPIV), fish were IP challenged to determine the duration of protection. The booster vaccination at 1.0 × 107 CFU/fish showed an RPS of 100% at each time point. The RPS for the single vaccination at 1.0 × 107 CFU/fish were 92.8%, 83.0% and 72.5% at 28, 56 and 84 DPIV, respectively, which were similar with those for the booster vaccination at 1.0 × 106 CFU/fish. The RPS for the single vaccination at 1.0 × 106 CFU/fish were the lowest at 65.7%, 60.3% and 31.9% for 28, 56 and 84 DPIV, respectively. Vaccinated fish developed specific serum antibodies; however, the antibody levels of the single vaccination were higher than those of the booster vaccination at 1.0 × 107 CFU/fish. The expression levels of IgM corresponded to the antibody levels, while the expression levels of humoral immune-related genes (MHC IIβ and CD4) also increased in the vaccinated fish. This study indicates that TFJ0901 can be developed as a safe live vaccine to protect Nile tilapia from S. agalactiae infection for at least 84 days.