Edwardsiella tarda invasion of fish cell lines and the activation of divergent cell death pathways

Edwardsiella tarda is an important gram-negative intracellular pathogen of fish. However, the invasive features of E. tarda to fish cells and the pathogenesis of host cell death have not been thoroughly investigated. In this study, two fish cell models were used to investigate the interactions between E. tarda and its cellular hosts. E. tarda invaded and replicated in both cell lines. Epithelioma papulosum cyprini (EPC) cells were more sensitive to E. tarda infection than the flounder gill cell line FG-9307, with higher levels of intracellular bacteria in the former. The invasion and intracellular replication of E. tarda in FG-9307 cells were studied at the ultrastructural level, and infected cells with large amounts of replicated bacteria and destroyed organelles were observed. Apoptosis was observed in EPC cells upon infection, characterized by the occurrence of apoptotic bodies, DNA ladder, increased Annexin V binding and the activation of caspase-3, whereas E. tarda infected FG-9307 cells were negative for all of those features. E. tarda infection in FG-9307 cells failed to protect the staurosporine-induced apoptosis. Moreover, both intrinsic and extrinsic pathways were activated in EPC cells upon E. tarda infection. The present study revealed that E. tarda interacts with fish cells in different manners, and divergent pathways were activated in these cellular hosts to mediate cell death. These results provided new information on the interactions between E. tarda and fish cells.