Ferric hydroxamate uptake system contributes to Edwardsiella ictaluri virulence.

Abstract

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia in fish, particularly in channel catfish. Ferric iron is an essential micronutrient for bacterial survival, and some bacterial pathogens use secreted hydroxamate-type siderophores to chelate iron in host tissues. Siderophore-iron complexes are taken up by these bacteria via the ferric hydroxamate uptake (Fhu) system. In E.&#xa0;ictaluri, the Fhu system consists of fhuC, fhuD, fhuB, and fhuA genes. However, the importance of the Fhu system in E.&#xa0;ictaluri virulence has not been investigated completely. Here, we present construction of E.&#xa0;ictaluri fhuD and fhuB mutants (Ei&#x394;fhuD and Ei&#x394;fhuB) by in-frame gene deletion and evaluation of the mutants' virulence and immunogenicity in channel catfish fingerlings and fry. Immersion challenges showed that Ei&#x394;fhuD was not significantly attenuated (p&#xa0;<&#xa0;0.05) in catfish fingerlings, whereas Ei&#x394;fhuB was significantly attenuated (p&#xa0;<&#xa0;0.01). Catfish fingerlings immunized with Ei&#x394;fhuD and Ei&#x394;fhuB showed 100% and 97.62% survival, respectively. Fry immersion challenges indicated Ei&#x394;fhuB was also significantly attenuated (p&#xa0;<&#xa0;0.05) in two-week old fry compared to the wild-type (48.96% vs. 82.14% mortalities). The survival rate in the fry vaccinated with Ei&#x394;fhuB was significantly higher (p&#xa0;<&#xa0;0.05) than that of non-vaccinated fry (96.77% vs. 21.42% survival). Our data indicates that the fhuB gene, but not the fhuD gene, contributes to E.&#xa0;ictaluri virulence.