WcaJ, the initiating enzyme for colanic acid synthesis, is required for lipopolysaccharide production, biofilm formation and virulence in Edwardsiella tarda

Edwardsiella tarda, a Gram-negative bacterium, is the causative agent of edwardsiellosis, which is a major problem for the aquaculture industry worldwide. Undecaprenyl-phosphate glycosyltransferase, WcaJ, as the initiating enzyme for colanic acid synthesis, is hypothesized to be involved in the pathogenesis of infection. In this study, a wcaJ in-frame deletion mutant (ΔwcaJ) of a virulent isolate of E. tarda EIB202 was constructed through double crossover allelic exchange by means of the suicide vector pRE118. A complementary strain (ΔwcaJ (pACYC184K-wcaJ)) was obtained by electrotransformation of a low-copy plasmid pACYC184K carrying the intact wcaJ into the ΔwcaJ mutant. Several virulence-associated phenotypes of the wild-type, the wcaJ mutant and complementary strains were tested. It was found that the deletion of wcaJ reduced biofilm formation, lipopolysaccharide (LPS) production, adherence and internalization to Epithelioma papulosum cyprini (EPC) cells and pathogenicity to zebrafish embryos. All the phenotypes displayed by the deletion mutant were restored in the complementary strain. These results revealed that WcaJ is required for pathogenicity in E. tarda.