Effects of Extracellular Products of Edwardsiella tarda on the Innate Immunity in Olive Flounder Paralichthys olivaceus

Edwardsiella tarda causes a serious economic loss in olive flounder Paralichthys olivaceus farming industry. Extracellular products (ECP) of the pathogen contain various bio-molecules, some of which are responsible for the virulence. ECP of E. tarda virulent strain, KE-1, was used to investigate the effects on the nature of phagocytes isolated from olive flounder. High doses of ECP down-regulated phagocytosis and bactericidal activity on engulfed bacteria in phagocytes and chemiluminescence (CL) activity of macrophages. However, production of nitric oxide of macrophages soared under high concentrations of ECP. In contrast, low doses of ECP induced increases in bacterial invasion activity and CL activity. These results suggest that appropriate amounts of ECP could effectively stimulate cellular immunity, and that E. tarda ECP is a good candidate for vaccine adjuvant.