Superiority of PLGA microparticle-encapsulated formalin-killed cell vaccine in protecting olive flounder against Streptococcus parauberis

Olive flounder (Paralichthys olivaceus) is a valuable maricultured fish in Asian countries, especially Korea, Japan, and China. Streptococcosis caused by Streptococcus parauberis has resulted in significant economic loss of olive flounder aquaculture in these countries. Since the conventional formalin-killed cell vaccine (FKC vaccine) is reportedly effective only for short period, the main objective of this study was to develop new vaccine of protecting olive flounders against streptococcosis for longer period. To assess the protective effect of poly(D,L-lacfide-co-glycolic acid) (PLGA) microparticle-encapsulated formalin-killed cell vaccine (PLGA vaccine), it was compared to the FKC vaccine. A bacterial agglutination test was performed using the serum of olive flounders after intraperitoneal administration of PLGA vaccine, FKC vaccine, or sterile phosphate-buffered saline (PBS). The PLGA vaccinated group showed higher agglutination titers than the FKC vaccinated group. Additionally, the percentage survival of the olive flounders was monitored after an S. parauberis challenge 10 weeks post-vaccination. In the first bioassay (challenge with 4.0 x 10(4) CFU/fish), percentage survival 15 days post-challenge (dpc) were 100% for the PLGA vaccine, 80% for the FKC vaccine, and 65% for PBS. In the second bioassay (4.0 x 10(5) CFU/fish), percentage survival 15 dpc were 100% for the PLGA vaccine, 55% for the FKC vaccine, and 0% for PBS. The results of this study showed that PLGA vaccination of olive flounder led to greater protection against streptococcosis, compared with conventional FKC vaccination.