Effects of the ecpA gene on the biological characteristics and pathogenicity of avian pathogenic Escherichia coli strain FJLY68.

Abstract

Avian pathogenic Escherichia coli (APEC) is a major cause of colibacillosis in poultry, yet the role of the ecpA gene, which encodes the major structural subunit of the Escherichia coli common pilus (ECP), remains incompletely defined in APEC pathogenesis. To investigate the role of ecpA in the biological characteristics and pathogenicity of Avian Pathogenic Escherichia coli (APEC) strain FJLY68, an ecpA deletion mutant (ΔecpA) and its corresponding complemented strain (CΔecpA) were constructed using the CRISPR/Cas9 system and verified by PCR and Sanger sequencing. Phenotypic analyses revealed that the ΔecpA mutation significantly impaired bacterial motility, biofilm formation, adherence to chicken embryonic fibroblast (DF-1) cells, and fimbriae assembly. Transcriptomic analysis identified 1720 differentially expressed genes in the ΔecpA mutant, significantly enriched in pathways associated with flagellar assembly, chemotaxis, and metabolism, consistent with the observed phenotypic changes. Although in vitro growth was unaffected, the ΔecpA mutant exhibited markedly attenuated virulence in a chick infection model, as indicated by an increased LD50, attenuated clinical signs and pathological lesions, and reduced bacterial colonisation in tissues. Full genetic complementation restored all observed defects to wild-type levels. This study identifies ecpA as a critical determinant of APEC pathogenesis, directly linking its function to bacterial motility, biofilm formation, adhesion, and in vivo virulence, and provides a theoretical basis for developing novel control strategies targeting this virulence factor.