Extensively drug‐resistant (XDR) avian pathogenic Escherichia coli (APEC) poses a global threat to poultry and public health. In this study, we isolated an APEC strain, SDLYRA415, from broilers in Shandong, China, during an outbreak with ~20% mortality. To investigate the genomic characteristics, resistance mechanisms, and pathogenicity of SDLYRA415, drug susceptibility was tested by disk diffusion and minimum inhibitory concentration (MIC); whole‐genome sequencing analyzed genomic features and resistance genes; conjugation assays evaluated resistance transfer; animal experiments (BALB/c mice, chicks) assessed pathogenicity. Drug susceptibility test results showed that SDLYRA415 was resistant to 32 antibiotics, showed intermediate resistance to polymyxin B and amikacin, and was only susceptible to tigecycline. Whole‐genome sequencing showed SDLYRA415 had a 5.51 Mb genome, including one chromosome and four plasmids. Phylotyping, multilocus sequence typing (MLST), and serotyping classified SDLYRA415 as phylogroup A, ST48, and O6: H16. Plasmids 1, 2, and 4 belonged to IncHI2/IncHI2A/IncN, IncFIB(AP001918)/IncFIC(FII), and p0111 groups, respectively, while plasmid 3 was a novel type. A total of 78 antibiotic resistance genes (ARGs) were detected, including bla CTX-M−55 , MCR-1 , bla CTX-M−65 , NDM-5 , etc. Conjugation assays showed that SDLYRA415 transferred polymyxin B resistance to EC600 at 6.5 × 10 −7 efficiency, and EC600 recipient strains levels of resistance showed significant increases. Pathogenicity tests showed that SDLYRA415 caused 60% mortality in both BALB/c mice and chicks, demonstrating its potential to cause severe disease in mammalian models and underscoring its zoonotic threat. This study offers crucial insights into drug resistance and pathogenic patterns of XDR‐APEC, aiding in the development of targeted strategies to prevent poultry colibacillosis and mitigate the public health risks of drug‐resistant strains entering the human food chain.
Ni et al. (Thu,) studied this question.