This study examines phenotypic antimicrobial resistance (AMR) and its genetic background in Escherichia coli isolated from poultry flocks in Türkiye, with a particular focus on extended spectrum β-lactamase (ESBL)-producing strains. A total of 918 E. coli isolates obtained from ceacal samples of chickens (n = 745) and turkeys (n = 173) were subjected to antimicrobial susceptibility testing using the agar disk diffusion method. Overall, high resistance rates were observed to tylosin (99.6%), ampicillin (90.8%), and oxytetracycline (84.0%), while resistance to third-generation cephalosporins (cefotaxime/ceftazidime) was detected in 11.4% of the isolates. Notably, AMR profiles varied significantly between poultry companies, indicating heterogeneous antimicrobial selection pressures within the production sector. ESBL-producing E. coli isolates exhibited high levels of multidrug-resistance, particularly to sulfamethoxazole (91.4%) and chloramphenicol (90.5%). Whole-genome sequencing (WGS) of ESBL-producing E. coli isolates (n = 87) identified several ESBL-encoding genes, with blaCTX-M-55 being the most prevalent (51.7%). Plasmid analysis demonstrated frequent associations of blaCTX-M-15 with IncFIB replicon, while blaCMY-2 was mainly linked to IncHI2A and IncI1-I plasmid types. In silico typing identified 44 distinct serotypes and 35 sequence types (STs), with O23:H4 and ST1011 being the most detected, highlighting the broad population structure of poultry associated E. coli. Virulence-associated genes were widely distributed among ESBL-producing isolates and predominantly related to adhesion, iron acquisition, stress response, and secretion systems. To the best of our knowledge, this study provides the first comprehensive WGS-based analysis of AMR in commensal E. coli from poultry in Türkiye, revealing significant public health concerns and the need for enhanced monitoring strategies.
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