Antibiotic resistance patterns of Escherichia coli isolates in Passeriformes and non-Passeriformes: insights from migratory and resident avifauna

Antimicrobial resistance (AMR) represents a major global One Health challenge, with increasing recognition of the role of wildlife in the environmental circulation of resistant bacteria. Wild birds, particularly those inhabiting urbanized migratory bottlenecks, may act as sentinels reflecting anthropogenic selective pressures. However, the relative contribution of migratory behaviour versus local environmental exposure to AMR carriage in avifauna remains insufficiently characterized. A total of 339 fecal samples were collected from 44 avian species during migration periods in Istanbul, Turkey. E. coli isolates were identified using standard bacteriological methods and an automated identification system. Antimicrobial susceptibility testing was conducted using 16 agents representing six antimicrobial classes. Selected antimicrobial resistance genes were screened by PCR, and resistance patterns were compared according to migratory status. Escherichia coli was recovered from 46 of the 339 samples (13.5%). Phenotypic resistance to at least one antimicrobial agent was observed in 65.2% of isolates. A high proportion of isolates exhibited increased tigecycline minimum inhibitory concentrations exceeding epidemiological cut-off values, indicating non-wild-type phenotypes. No resistance to colistin or meropenem was detected, and none of the isolates were classified as ESBL-producing or multidrug-resistant. Although no statistically significant differences were detected between migratory and resident bird groups with respect to overall AMR prevalence, 83.3% of the detected resistance genes were identified in isolates from resident or mixed-status birds. The findings suggest that local environmental exposure in urban areas may play a more significant role than long-distance migratory movements in the acquisition of AMR markers in wild birds. Simultaneously, the identification of non-wild-type tigecycline phenotypes highlights their consideration in comprehensive One Health monitoring initiatives for wildlife.