Pathogenic Mechanism Of Edwardsiella Tarda And Streptococcus Iniae In Marine Fish

Edwardsiella tarda and Streptococcus iniae are severe pathogens to a wide range of economically important fish species. In our study, we research the pathogenic mechanism of E. tarda and S. iniae. In the study of E. tarda, we analyzed the functions of lysozyme inhibitor(MliCEt) and characterized MliCEt in comparison with the other lysozyme inhibitor(IvyEt). We found that recombinant MliCEt(rMliC) inhibited lysozyme-induced lysis of a Gram-positive bacterium, and reduced serum-facilitated lysozyme killing of E. tarda. When rMliC introduced into turbot, rMliC promoted bacterial dissemination in fish tissues. The C33 S mutation had no influence on the activity of rMliC, while the W79 A mutation slightly but significantly enhanced the activity of rMliC. Compared to the wild-type strain, singleknockout strains of either TXΔmliC or TXΔivy were severely attenuated for the ability of tissue invasion, host lethality, serum survival, and intracellular replication. TXΔmliC was restored by complementation with an introduced mliCEt gene. Compared to the TXΔmliC or TXΔivy singleknockout strains, the TXΔmliCivy double-knockout strain was significantly impaired in most of the virulence features. These results indicate that the conserved cysteine is functionally dispensable to MliC, MliCEt most likely works in a concerted and parallel manner with IvyEt. Except MliC, we researched the anti-serum mechanism of E. tarda. We found that, when incubated with flounder serum, E. tarda exhibited a high survival rate(87.6%). And E. tarda-incubated serum possessed strong hemolytic activity and bactericidal activity. Compared to the serum incubated with a complement-sensitive laboratory Escherichia coli strain, E. tarda-incubated serum exhibited much less chemotactic activity; in contrast to the serum incubated with live E. tarda, the serum incubated with heat-inactivated E. tarda exhibited no apparent hemolytic capacity. Taken together, these results indicate that E. tarda circumvents serum attack by preventing, to a large extent, complement activation via the alternative pathway, and that heatlabile surface structures likely play an essential role in the complement evasion of E. tarda.In the study of S. iniae, we identified a putative bacteriocin, Sil, and examined its biological activity. We found that purified recombinant Sil(rSil) exhibited a dose-dependent inhibitory effect on the growth of Bacillus subtilis but had no impact on the growths of other 23 different strains. rSil bound to the surface of B. subtilis but induced no killing of the target cells. Cellular study revealed that rSil interacted with turbot(Scophthalmus maximus) head kidney monocytes and inhibited the innate immune response of the cells, which led to enhanced cellular infection of S. iniae. Antibody blocking of the extracellular Sil produced by S. iniae significantly attenuated the infectivity of S. iniae. Study showed that administration of turbot with rSil prior to S. iniae infection significantly increased bacterial dissemination and colonization in fish tissues. These results indicate that rSil is a novel virulenceassociated bacteriostatic and an immunoregulator that promotes S. iniae infection by impairing the immune defense of host.