Mutations of the lon protease and tryptophan operon gene trpA in Edwardsiella tarda regulate colonization and pathogenesis in olive flounder fish.

Edwardsiella tarda is a severe aquaculture and zoonosis pathogen that infects a wide range of fish, especially the Oliver flounder fish. Investigation of its pathogenetic mechanisms is important for developing an effective protective agent, such as a vaccine or antibiotics. In this study, we characterized the involvement of two target genes, lon protease and tryptophan operon gene trpA, in the virulence and pathogenesis pathways of E. tarda. Deletion of the trpA gene induced tryptophan auxotrophy. This ensured clearance from the host and made the microbe more vulnerable to acidic and oxidant stress, which are the main antimicrobial implements of host innate immunity. Lon deletion caused impaired motility and intracellular invasion by downregulating the overall flagella assembly pathway genes, as revealed by a transcriptome analysis. Those biological features were in line with the in vivo challenge results. The groups challenged with the mutant strain showed significantly higher survival rates and delayed mortality onsets than the group challenged with the wild-type strain, which showed rapid and extensive mortality. Most importantly, the cytokine profiles of the fish after challenge contribute to understanding the mechanism of this enhanced survival. Instead of an uncontrolled inflammatory state, the mutant strain administration elicited distinct immune responses. The Δlon strain induced a robust IFN-γ-mediated response, whereas the ΔtrpA strain was characterized by the downregulation of pro-inflammatory cytokines; notably, both mutants contributed to restored IL-10 regulation. By identifying the target genes and their involvement in virulence pathways, this study provides insights for the development of effective vaccine candidates against edwardsiellosis in aquaculture and its surroundings.