Abstract
Streptococcus agalactiae is an important pathogen in fish, causing great losses of intensive tilapia farming. To develop a potential live attenuated vaccine, a re-attenuated S.agalactiae (named TFJ-ery) was developed from a natural low-virulence S.agalactiae strain TFJ0901 through selection of resistance to erythromycin. The biological characteristics, virulence, stability and the immunization protective efficacy to tilapia of TFJ-ery were determined. The results indicated that TFJ-ery grew at a slower rate than TFJ0901. The capsule thickness of TFJ-ery was significantly less (p<0.05) than TFJ0901. When Nile tilapia were intraperitoneally (IP) injected with TFJ-ery, the mortality of fish was decreased than that injected with TFJ0901. The RPS of fish immunized with TFJ-ery at a dose of 5.0×107 CFU was 95.00%, 93.02% and 100.00% at 4, 8 and 16weeks post-vaccination, respectively. ELISA results showed that the vaccinated fish produced significantly higher (p<0.05) antibody titres compared to those of control at 2 or 4weeks post-vaccination. Taken together, our results suggest that erythromycin could be used to attenuate S.agalactiae, and TFJ-ery is a potent attenuated vaccine candidate to protect tilapia against S.agalactiae infections.
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