Effect of Bacterial Infection on the Expression of Stress Proteins and Antioxidative Enzymes in Japanese Flounder

Stress induced by environmental stimuli, such as pollutants and infection, in fish is thought to influence their fitness. Bacterium Edwardsiella tarda is a virulent intracellular pathogen of commercial fish species. Edwardsiellosis caused by E. tarda is one of the most severe diseases in Japanese flounder Paralichthys olivaceus. However, little is known about the effect of bacterial (such as E. tarda) infectious-induced stress on the expression of stress proteins and antioxidative enzymes in fish. In the present study, we examined the expression of heat shock protein (HSP) 70 and superoxide dismutases (SODs) in the hepatopancreas of Japanese flounder in response to E. tarda infection. HSP70 expression was rapidly increased and was significantly higher in fish 48 h after being infected with E. tarda, compared to that in control fish. The expression level of Cu, Zn–SOD in infected fish increased from 24 to 48 h and peaked at 48 h post-infection. On the other hand, the expression level of Mn–SOD in infected fish gradually increased from 12 h post-infection and remained high between 24 and 48 h post-infections. The changes in expression levels of HSP70 and SODs are suggestive of reactive oxygen species-induced oxidative stress in Japanese flounder. The results also suggest that both HSP70 and SODs play roles in mediating immune response upon E. tarda infection in Japanese flounder. We hypothesize that these proteins are important factors that protect cells against oxidative stress.