Identification of protective immunogens from extracellular secretome of Edwardsiella tarda

Edwardsiella tarda is an opportunistic pathogen that causes a great loss in aquaculture. Identification of immune protective immunogens is a key step for development of subunit vaccines and control of the infectious diseases caused by the bacterium. This study aims to identify the protective antigens from extracellular secretory proteome of E. tarda. Out of 38 extracellular secretory proteins predicted by PSORTb, 20 genes were randomly cloned and their recombinant proteins were expressed in Escherichia coli BL21 and purified by either affinity chromatography or inclusion body washing. The purified recombinant proteins were used for investigation of immune protection in zebrafish model using active immunization approach. Half of them had significant immune protection compared with the control. Out of them, four, EseC, ETAE_2088, FlgD and ETAE_2130, showed approximately 60% relative percent survivals as a result of the highly protective antigens identified. Except for FlgD, the other three were first reported here. Moreover, the present study identified EseC and ETAE_2088 in bacterial extracellular fraction. These results indicate that secretory proteome is an interesting pool used for identification of immune protective antigens, and the four highly protective antigens identified provide useful candidates for development of subunit vaccines.