Application of alginate microparticles incorporating formalin-inactivated Streptococcus parauberis for oral vaccination in olive flounder

Streptococcus parauberis is an etiological agent that causes streptococcosis in fish, exerting a significant impact on aquaculture production. The available injectable vaccine against S . parauberis is commonly used in olive flounder species ( Paralichthys olivaceus ) in Korea. However, an oral alternative that overcomes the disadvantages of the injectable vaccine has long been desired in the aquaculture industry. Alginate-based microparticles have been widely used as delivery systems for oral administration in aquaculture. In the present study, we produced alginate microparticles encapsulating formalin-inactivated S . parauberis via ionic gelation, which remained stable under acidic conditions (pH 2 and 4) and at low temperatures (4 °C and 25 °C). Mixing the alginate–Gelzan complex with a commercial feed for ease of application induced a specific antibody response and exerted significant protective effects (relative survival rate: 50.0%) in small olive flounder fingerlings. Less pronounced protective effects were also observed in large fingerlings (relative survival rate: 37.5%), suggesting that protection against S . parauberis may be dependent on the size of the fish. Thus, our results indicate that alginate microparticles encapsulating an S . parauberis antigen can be used for initial vaccination in olive flounder at the fingerling stage.