The role of the outer membrane protein gene <i>ybjX</i> in the pathogenicity of avian pathogenic <i>Escherichia coli</i>

<p>Avian pathogenic <i>Escherichia coli</i> (APEC) are important pathogens that cause serious economic losses to the poultry industry worldwide. The PhoP/Q system is essential for APEC pathogenicity, and its mutation leads to a decrease in <i>ybjX</i> gene expression, but whether <i>ybjX</i> regulated by PhoP/Q plays a role in pathogenicity remains unclear. The role of <i>ybjX</i> in APEC pathogenicity was investigated using a <i>ybjX</i> mutant strain constructed from APEC strain AE17 by the Red recombination method. The mutant strain displayed significantly reduced adherence to and invasion of chicken embryo fibroblast cell line (DF-1) cells. Moreover, N-phenyl-1-naphthylamine (NPN) assays revealed an increase in outer membrane permeabilization for the <i>ybjX</i> mutant, suggesting <i>ybjX</i> affected susceptibility to avian beta-defensin through this mechanism. The <i>ybjX</i> mutation increased susceptibility to avian beta-defensin 9 (AvBD9), and <i>in vivo</i> LD₅₀ assays revealed reduced bacterial virulence in a chicken model. Taken together, these results indicated that mutation of <i>ybjX</i> may attenuate APEC pathogenicity, not only by affecting bacterial adhesion and invasion, but also via reducing resistance to avian beta-defensin by increasing outer membrane permeabilization of bacteria.