LAUSR

Abstract Edwardsiella piscicida is a leading pathogen that threats seawater and fresh aquaculture, in which the emergence and prevalence of bacterial resistance to antibiotics is a tricky problem. Currently, the pathogenesis of E. piscicida remains unclear, some new pathogenic factors need to be identified. Serine permease and dehydratase exist ubiquitously in microorganisms and participate in pathogenicity, but their functions in E. piscicida remain unknown. In this study, a serine permease YhaO and serine dehydratase YhaM were characterized in E. piscicida. yhaOEp and yhaMEp are expressed in the form of a bicistronic operon. YhaOEp possesses multiple transmembrane structures and YhaMEp is predicted to be located in the cytoplasm. To explore the biological roles of yhaOEp and yhaMEp, markerless in-frame mutant strains, TX01ΔyhaO and TX01ΔyhaM, were constructed, respectively. TX01, TX01ΔyhaO, and TX01ΔyhaM exhibited similar growth under normal and stress conditions, which indicates yhaOEp and yhaMEp are irrelevant to adversity. However, TX01ΔyhaO and TX01ΔyhaM displayed markedly reduced resistance against antibiotics, compared to TX01. Further studies indicated that the yhaOEp mutation damaged E. piscicida's ability to maintain membrane integrity and biofilm formation, decreased its adhesion and invasion to non-phagocytic FG cells, weakened the survival and proliferation in macrophage cells, and decreased bacterial dissemination in fish tissues. Complementary strain TX01ΔyhaOC restored these phenotypes of TX01ΔyhaO. The inactivation of yhaMEp has no effect on bacterial pathogenicity to FG cells and macrophage cells, but in vivo experiment showed that TX01ΔyhaM exhibited attenuated capability of bacterial dissemination in fish tissues, compared to TX01. Taken together, our findings demonstrate for the first time that YhaO-YhaM is requisite for full pathogenicity of E. piscicida to host cells and host tissues, and provide new insights into the pathogenesis of E. piscicida.