Edwardsiella tarda Eta1, anIn Vivo-Induced Antigen That Is Involved in Host Infection
ABSTRACTEdwardsiella tarda, a Gram-negative bacterium, is a severe fish pathogen that can also infect humans. In this study, we identified, viain vivo-induced antigen technology, anE. tardaantigen, Eta1, and analyzed its function in a Japanese flounder (Paralichthys olivaceus) model. Eta1 is composed of 226 residues and shares homology with putative bacterial adhesins. Quantitative real-time reverse transcriptase (RT)-PCR analysis indicated that when culturedin vitro,eta1expression was growth phase dependent and reached maximum at mid-logarithmic phase. During infection of flounder lymphocytes,eta1expression was drastically increased at the early stage of infection. Compared to the wild type, theeta1-defective mutant, TXeta1, was unaffected in growth but exhibited attenuated overall virulence, reduced tissue dissemination and colonization capacity, and impaired ability to invade flounder lymphocytes and to block the immune response of host cells. The lost virulence of TXeta1 was restored when a functionaleta1gene was reintroduced into the strain. Western blot and immunodetection analyses showed that Eta1 is localized to the outer membrane and exposed on the surface ofE. tardaand that recombinant Eta1 (rEta1) was able to interact with flounder lymphocytes. Consistent with these observations, antibody blocking of Eta1 inhibitedE. tardainfection at the cellular level. Furthermore, when used as a subunit vaccine, rEta1 induced strong protective immunity in flounder against lethalE. tardachallenge. Taken together, these results indicate that Eta1 is anin vivo-induced antigen that mediates pathogen-host interaction and, as a result, is required for optimal bacterial infection.