A mutation in rcsB, a gene encoding the core component of the Rcs cascade, enhances the virulence of Edwardsiella tarda

Edwardsiella tarda, a Gram-negative bacterium of the family Enterobacteriaceae, is the causative agent of the systemic disease edwardsiellosis, which is a major problem in aquaculture industry worldwide. Many virulence-related genes in E. tarda have been investigated, but the Rcs phosphorelay, a two-component pathway, which regulates several cell-surface-associated structures related to invasion and survival in host cells, has not yet been thoroughly studied. In the present study, an rcsB in-frame deletion mutant ArcsB was constructed through double-crossover allelic exchange. To complement the rcsB mutation, the Delta rcsB (pACYC184K-rcsB) mutant was constructed by transformation of a low-copy plasmid carrying the intact rcsB into the Delta rcsB mutant of E. tartda. Several virulence-associated characters of the mutants and wild-type strain were tested. Compared with wild-type strain EIB202, biofilm formation decreased significantly in Delta rcsB, while Delta rcsB (pACYC184K-rcsB) recovered the phenotype to some extent. In addition, the capacity for autoagglutination, the percentage of adherence and internalization to Epithelioma papulosum cyprini cells and lethality toward zebrafish embryos significantly increased in Delta rcsB. All these phenomena displayed by mutant Delta rcsB showed a certain degree of recovery, though incomplete, in strain Delta rcsB (pACYC184K-rcsB). Present results indicate that rcsB is involved in regulating the gene expression of virulence factors in E. tarda, as shown in other members of Enterobacteriaceae. (C) 2014 Institut Pasteur. Published by Elsevier Masson SAS. All rights reserved.