Protection of Nile tilapia (Oreochromis niloticus L.) against Streptococcus agalactiae following immunization with recombinant FbsA and α-enolase

Streptococcus agalactiae, a Gram-positive bacterium, can infect a variety of fish species reared in marine and freshwater aquaculture facilities, which can result in serious economic losses. We identified, expressed, and purified, two putative cell surface proteins - fibrinogen-binding proteins A (FbsA) and α-enolase - from S. agalactiae THN strain, and determined their roles as subunit vaccines to protect tilapia against S. agalactiae infections. We also investigated the effects of both proteins on the respiratory burst activity of blood leukocytes and on serum lysozyme activity. Specific antibody titers were carried out on a weekly basis. Results indicated that both putative proteins shared similar structure characteristics with those from other bacterium species. Using the Escherichia coli BL21 system, we expressed and purified recombinant FbsA (rFbsA) and α-enolase (rEnolase). Following immunization of tilapia we found that both recombinant proteins can confer protection against S. agalactiae, with the relative percentage survival (RPS) of 40.63 ± 17.21% and 62.50 ± 18.75%, respectively. Inactivated S. agalactiae and adjuvant control groups produced RPS of 93.75 ± 5.41% and 16.67 ± 17.21%, respectively. The respiratory burst activity and lysozyme activity were significantly enhanced in groups rFbsA and rEnolase when compared with that of the PBS control. The serum-specific antibody titer against rFbsA or rEnolase was also increased and we found that rFbsA induced a higher antibody titer, post immunization. These results indicated that, although both FbsA and α-enolase can regulate the host's innate immune responses and specific antibody responses, α-enolase can also function as a candidate vaccine antigen, and can provide a high level of protective immunity against S. agalactiae. We also noted that different proteins were associated with different levels of protection.