Evaluating Type III Secretion System Genes (escE, esaE and eseJ) of Edwardsiella piscicida for Virulence in Japanese Flounder (Paralichthys olivaceus).

Abstract

Edwardsiella piscicida, an etiological agent of edwardsiellosis in finfish, causes severe damage to the aquaculture industry in the world. The virulence of E. piscicida is known to be associated with the type III secretion system (T3SS) which is a protein transport nanomachine that injects effector proteins directly into host cells to be resistant to phagocyte killing. The functions and importance of the T3SS genes for the virulence of E. piscicida have been identified using murine macrophages or model fish. In this study, we evaluated the contribution of the T3SS genes consisting of orf13 (escE), orf19 (esaE) and orf29/30 (eseJ) for the virulence in a natural host using Japanese flounder (Paralichthys olivaceus). Intracellular replication ability in macrophage-like cells isolated from Japanese flounder was significantly decreased in all knockout mutants compared to the wild-type strain. In the challenge test using Japanese flounder, the virulence of Δorf13 and Δorf19 mutants was significantly decreased, but not for Δorf29 and Δorf30 mutants. Thus, escE and esaE genes are considered to highly affect the virulence in Japanese flounder. However, the eseJ gene may not significantly contribute to the virulence, although it could regulate intracellular survival in phagocytes of Japanese flounder.