# From ecology to evolution: plasmid- and colicin-mediated persistence of antibiotic-resistant Escherichia coli in gulls

## TL;DR

This study shows how antibiotic-resistant E. coli can persist in gulls for months, spreading through competitive interactions and plasmid-encoded traits.

## Contribution

The study presents the longest in vivo experiment tracking AMR persistence in wild birds, revealing ecological and evolutionary mechanisms.

## Key findings

- Resistant E. coli was sustained in gulls for three months, the longest documented carriage in wild birds.
- ST11138 outcompeted other strains, aided by plasmids encoding colicins and immunity genes.
- Long-term in vivo tracking revealed how plasmid-encoded traits and bacteriocin interactions maintain AMR in wildlife.

## Abstract

Antimicrobial resistance (AMR) in wildlife is an emerging concern within the One Health concept. Gulls, due to their synanthropic behavior and long-distance migration, are recognized as vectors and secondary reservoirs of resistant bacteria. These birds can facilitate the environmental spread of resistant strains across ecosystem boundaries. Understanding their role in shaping microbial communities is essential for assessing the broader ecological impact. This study investigates the persistence and competitive dynamics of cephalosporin-resistant Escherichia coli in Caspian gulls (Larus cachinnans) captured at their breeding colony at a water reservoir and subsequently monitored in captivity for three months, representing the longest in vivo experiment of its kind conducted on wild birds. We observed sustained colonization and long-term shedding of resistant E. coli throughout the entire study, marking the longest documented carriage of resistant bacteria in wild birds to date. Notably, rapid dissemination of various E. coli sequence types (STs) with CTX-M-1 was observed, with ST11138 rapidly outcompeting other strains, including the initially dominant ST11893. Genomic analyses revealed that ST11138 harboured F24:A-:B1 and IncI1/ST3/CTX-M-1 plasmids encoding colicins and corresponding immunity genes, likely conferring a competitive advantage. Our findings underscore the role of bacteriocin-mediated interactions in shaping microbial communities and highlight the importance of plasmid-encoded traits in the persistence of resistant strains in wildlife. Importantly, our findings underscore the ecological novelty of longitudinal in vivo tracking of AMR persistence in natural hosts and highlight the need to consider ecological and microbiome-level interactions when assessing the environmental dimension of AMR under the One Health concept.

Antimicrobial resistance (AMR) in wildlife is an emerging concern within the One Health framework, with gulls recognised as important vectors and secondary reservoirs of resistant bacteria. Due to their synanthropic behavior and long-distance migration, these birds can facilitate the spread of resistant strains across ecosystems. However, the role of wildlife in resistance dynamics remains underexplored, especially in long-term, natural settings. Our study is unique in its scope and duration, representing the longest in vivo experiment of its kind conducted on wild birds. By capturing these processes in live hosts under naturalistic conditions and across an extended period, our study provides rare and ecologically grounded insights into how AMR is maintained outside clinical or laboratory settings. Our findings show sustained colonisation and long-term shedding of resistant E. coli, with strain ST11138 outcompeting others. Genomic analyses reveal plasmid-encoded traits, highlighting the ecological and evolutionary mechanisms underlying resistance maintenance in wildlife.