A critical role of the T3SS effector EseJ in intracellular trafficking and replication of Edwardsiella piscicida in non-phagocytic cells

Edwardsiella piscicida (E. piscicida) is an intracellular pathogen within a broad spectrum of hosts. Essential to E. piscicida virulence is its ability to survive and replicate inside host cells, yet the underlying mechanisms and the nature of the replicative compartment remain unclear. Here, we characterized its intracellular lifestyle in non-phagocytic cells and showed that intracellular replication of E. piscicida in non-phagocytic cells is dependent on its type III secretion system. Following internalization, E. piscicida is contained in vacuoles that transiently mature into early endosomes, but subsequently bypasses the classical endosome pathway and fusion with lysosomes which depends on its T3SS. Following a rapid escape from the degradative pathway, E. piscicida was found to create a specialized replication-permissive niche characterized by endoplasmic reticulum (ER) markers. We also found that a T3SS effector EseJ is responsible for intracellular replication of E. piscicida by preventing endosome/lysosome fusion. Furthermore, in vivo experiments confirmed that EseJ is necessary for bacterial colonization of E. piscicida in both mice and zebrafish. Thus, this work elucidates the strategies used by E. piscicida to survive and proliferate within host non-phagocytic cells.\n\nAuthor summaryE. piscicida is a facultative intracellular bacterium associated with septicemia and fatal infections in many animals, including fish and humans. However, little is known about its intracellular life, which is important for successful invasion of the host. The present study is the first comprehensive characterization of E. piscicidas intracellular life-style in host cells. Upon internalization, E. piscicida is transiently contained in Rab5-positive vacuoles, but the pathogen prevents further endosome maturation and fusion with lysosomes by utilizing an T3SS effector EseJ. In addition, the bacterium creates an specialized replication niche for rapid growth via an interaction with the ER. Our study provides new insights into the strategies used by E. piscicida to successfully establishes an intracellular lifestyle that contributes to its survival and dissemination during infection.