Abstract: Escherichia coli type III secretion system 2 (ETT2) is involved in the pathogenesis of avian pathogenic Escherichia coli (APEC). This study investigated the biological characteristics and pathogenic effects of ETT2 structural gene epaPQR on APEC in order to provide the basis for further understanding of the pathogenic effects of ETT2. The epaPQR mutant strain and complemented strain were constructed based on CRISPR-Cas9 gene editing technology. The effects of epaPQR gene on the biological characteristics of APEC were analyzed by detection of growth curve, motility and biofilm formation ability, etc. The effect of epaPQR gene on the pathogenicity of APEC was analyzed by serum bactericidal and tissue bacterial load tests. The results showed that the deletion and complemented strains of epaPQR gene of ETT2 were successfully constructed. Deletion of epaPQR gene did not significantly influence the growth and biofilm formation ability (P>0.05). After deletion of epaPQR gene, the motility was significantly reduced (P<0.05), and the number of flagella observed by transmission electron microscopy was significantly reduced. RT-qPCR showed that the transcription levels of flagellar T3SS structural genes and flagellar output protein genes were significantly down-regulated (P<0.05). After the deletion of epaPQR gene, its antiserum bactericidal ability was significantly increased (P<0.05), and the colonization ability of AE81ΔepaPQR strain in different organs of chicks was significantly decreased. The results indicated that epaPQR was involved in the regulation of APEC flagellar formation, and affected antiserum bactericidal ability of APEC, as well as the ability to colonize tissues and organs in vivo, suggested that epaPQR played an important role in pathogenesis of APEC. This study provides a reference for the further understanding of the function of ETT2 and the pathogenesis of APEC.

Key words: avian pathogenic Escherichia coli, ETT2, epaPQR gene, biological characteristics, pathogenicity