Streptococcus agalactiae is a Gram-positive bacterium and a severe aquaculture pathogen that can infect a
wide range of warmwater fish species. The outer-surface proteins in bacterial pathogens play an
important role in pathogenesis. We evaluated the immunogenicity of two of the identified surface
proteins namely phosphoglycerate kinase (PGK) and ornithine carbamoyl-transferase (OCT). PGK and
OCT were over-expressed and purified from Escherichia coli and used as the subunit vaccines in tilapia.
Tilapia immunized with the S. agalactiae modified bacteria vaccine (whole cell preparations with recombinant
PGK and OCT proteins) individually were tested for the efficacy. OCT and PGK combined with
WC had a higher survival rate. A high-level protection and significant specific antibody responses against
S. agalactiae challenge was observed upon the vaccinated tilapia with the purified PGK protein and
S. agalactiae whole cells. The specific antibody titer against S. agalactiae antigen suggested that increased
antibody titers were correlated with post-challenge survival rate. Il-1b expression profile was higher in
PGK þ WC-treated group. Tnf-a expression in the PGK þ WC group was significantly increased. Taken
together, our results suggested the combinations of recombinant protein and whole cell may elicit immune
responses that reach greater protection than that of individual S. agalactiae components.