Edwardsiella tarda Sip1: A serum-induced zinc metalloprotease that is essential to serum resistance and host infection

Edwardsiella tarda is a severe bacterial pathogen to a wide arrange of farmed fish. One salient virulent feature of E. tarda is a remarkable ability to survive in host serum. In this study, in order to identify E. tarda proteins involved in serum resistance, we conducted proteomic analysis to examine the extracellular protein profiles of TX01, a pathogenic E. tarda isolate, in response to serum treatment. Five differentially expressed proteins were identified, one of which was a putative zinc protease (named Sip1). Western blot confirmed extracellular production of Sip1 by E. tarda. Sequence analysis revealed that Sip1 possesses a conserved zinc metalloprotease motif and shares low homology with the putative zinc proteases/aureolysin of several bacterial species. Purified recombinant Sip1 (rSip1) exhibited zinc-dependent proteolytic activity that reached maximum at 40 degrees C and pH 8. Compared to the wild type, the sip1 knockout mutant, TX Delta sip1, was dramatically reduced in the ability to cause mortality in the host (Japanese flounder) and to survive in host serum. These lost virulence capacities of TX Delta sip1 were restored by complementation with the sip1 gene. Further study showed that rSipl enhanced the serum resistance of TX01 and TX Delta sip1, whereas antibody blocking of the Sip1 produced naturally by TX01 impaired serum resistance. Vaccination study showed that rSipl as a subunit vaccine was able to induce effective protection in flounder against E. tarda challenge. Taken together, these results indicate that Sip1 is a novel zinc metalloprotease that is essential to serum resistance and host infection. (C) 2015 Elsevier B.V. All rights reserved.