Efficacy of pcDNA-Alp1 DNA vaccine against Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus)

Streptococcosis caused by Streptococcus agalactiae and S. iniae is one of the most important bacterial diseases in Nile tilapia. To prevent Streptococcus infection, various forms of vaccine including inactivated bacterial vaccines and DNA vaccines have been developed. In this study, a plasmid containing the alpha- like protein1 (alp1) gene was constructed. Samples of 120 Nile tilapia (Oreochromis niloticus) from a provincial fish farm in Thailand were divided into four equal groups: 1) the control, injected with phosphate-buffered saline (PBS), 2) injected with pcDNA3.1 (+), 3) injected with pcDNA-Alp1 and 4) injected with inactivated Streptococcus vaccine. At four weeks after vaccination, the fish were challenged with 10(8) CFU/mL S. agalactiae using intraperitoneal injection. The relative percent survival (RPS) rates for pcDNA3.1 (+), pcDNA-Alp1 and inactivated S. agalactiae vaccine were 26.6%, 51.4% and 46.6%, respectively. Notably, the efficacy of pcDNA-Alp1 was equivalent to the inactivated S. agalactiae vaccine. The results demonstrated that pcDNA-Alp1 vaccine could provide protection to tilapia against S. agalactiae infection.