Construction and evaluation of an Edwardsiella ictaluri fhuC mutant.

Edwardsiella ictaluri is a Gram-negative facultative intracellular pathogen causing enteric septicemia in channel catfish. Iron is an essential micronutrient needed for bacterial virulence, and to acquire iron, many Gram-negative bacteria secrete ferric iron chelating siderophores. The ferric hydroxamate uptake (Fhu) system consists of four genes (fhuC, fhuD, fhuB, and fhuA), and is involved in the uptake of hydroxamate type siderophores across bacterial membranes. However, the Fhu system and its importance in E. ictaluri virulence have been uninvestigated. Here, we present construction and evaluation of an E. ictaluri ΔfhuC mutant. The E. ictaluri fhuC gene was deleted in-frame by allelic exchange, and the mutant's growth in media and virulence in catfish were determined. Our results indicated that deletion of the E. ictaluri fhuC gene did not affect the growth of E. ictaluri largely in both iron-replete and iron-depleted media. Addition of ferric iron sources into the iron-depleted medium improved the growth of both E. ictaluri ΔfhuC and wild type (WT). Catfish mortalities indicated that E. ictaluri ΔfhuC mutant was attenuated 2.05-fold compared with the parent strain. The catfish immunized with the E. ictaluri ΔfhuC mutant showed a high relative percent survival rate (97.50%) after re-challenge with the WT E. ictaluri strain. Taken together, our data indicates that the fhuC gene contributes to E. ictaluri virulence.