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

- Proteomic analysis identified Sip1 as an E. tarda protein induced by host serum.
- rSip1 was a zinc-dependent protease with maximum activity at 40 °C and pH 8.
- sip1 knockout was significantly impaired in host lethality and serum resistance.
- rSip1 enhanced serum survival, whereas rSip1 antibody reduced serum survival.
- rSip1 as a subunit vaccine induced effective protection against E. tarda challenge.

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 °C and pH 8. Compared to the wild type, the sip1 knockout mutant, TXΔsip1, 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 TXΔsip1 were restored by complementation with the sip1 gene. Further study showed that rSip1 enhanced the serum resistance of TX01 and TXΔsip1, whereas antibody blocking of the Sip1 produced naturally by TX01 impaired serum resistance. Vaccination study showed that rSip1 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.