Reassessing the role of wild birds in the spread of antibiotic resistance: the white stork as a model species in studying populations from Central European river valley.

It is commonly accepted that the emergence of multidrug-resistant bacteria in the hospital environment, in food production, and in the wild is the consequence of the overuse of antibiotics. The presence of antibiotic-resistant bacterial strains was reported in both wild and farmed animals. It was suggested previously that wild birds may be carriers of antibiotic-resistant bacteria acquired from agriculture and/or industrial/urban habitats. Here, we used the white stork (Ciconia ciconia) as a model species to assess the presence and origin of antibiotic-resistant bacteria in a breeding population located in river valley landscapes of western Poland. Cloacal swabs from 50 nestlings across 19 nests were collected during the 2019 breeding season, along with 49 environmental samples (soil, water, and food items) from known foraging sites. A total of 147 bacterial isolates representing 49 species were obtained. The majority originated from cloacal samples (58.5%), followed by water (16.3%), food (15.0%), and soil (10.2%). Resistance testing against 19 antibiotics revealed significantly higher resistance in food-derived isolates, while no significant differences were found among soil, water, and cloacal isolates. Correlation analyses based on resistance profiles showed no significant relationship between cloacal isolates and those from soil (rs = 0.11; P = 0.647) or food (rs = 0.25; P = 0.289), but a strong correlation with water isolates (rs = 0.68; P = 0.001). These results might suggest that wild birds' role in transmitting antibiotic-resistant microorganisms might be more limited than predicted previously. Therefore, the role of wild birds, especially the white stork, in the transmission of antibiotic-resistant bacteria appears unclear and requires further consideration and re-examination.IMPORTANCEDue to the overuse of drugs during animal breeding and the use of manures from breeding farms, antibiotic residues are present in the natural environment, especially in soil and water reservoirs. This causes a selection pressure on bacteria, creating a reservoir of antibiotic-resistant bacteria. It was suggested previously that wild birds may be carriers of antibiotic-resistant bacteria acquired from agriculture and/or industrial/urban habitats. Here, using a model of the white stork as a synanthropic bird, we asked whether this species is colonized by antibiotic-resistant bacteria, and if there is any significant correlation between bacteria present in samples of cloacal swabs and environmental samples from their habitats. We found no significant correlations between antibiotic-resistance patterns in bacteria isolated from white stork cloaca and those identified in bacteria isolated from soil and stork's food. Thus, roles of wild birds in transmitting antibiotic-resistant microorganisms might be constrained and less significant than predicted previously.