# Conjugation frequency of ESBL- and pAmpC- E. coli in broiler chickens in vivo and in vitro

## TL;DR

This study examines how antibiotic resistance genes spread between E. coli bacteria in chickens, both in the gut and in lab conditions.

## Contribution

The study confirms conjugation of mcr-1 between pAmpC- and ESBL- E. coli in vivo and compares it with in vitro conditions.

## Key findings

- Transconjugants were detected in 35 out of 138 broiler chickens after 49 days.
- In vitro conjugation frequencies varied minimally across different conditions.
- In vivo conjugation suggests the gut environment significantly influences HGT.

## Abstract

Plasmid-mediated conjugation is a major form of horizontal gene transfer (HGT), facilitating dissemination of antimicrobial resistance (AMR) and the emergence of multi-drug-resistant (MDR) strains. In poultry, Escherichia coli producing extended-spectrum β-lactamases (ESBL) and plasmid-mediated AmpC β-lactamase (pAmpC) enzymes are common and contribute to antibiotic resistance. Additionally, plasmid-mediated colistin resistance gene mcr-1 in poultry requires attention, as it is a last-resort antibiotic in human medicine. Although plasmid-mediated conjugation is known to play a role in spreading antimicrobial resistance, its specific impact on resistance transmission within the broiler microbiota is still not well understood. We assessed conjugation dynamics of the mcr-1 gene from a pAmpC- E. coli to an ESBL- E. coli observed in an in vivo broiler chicken trial and compared them to conjugation frequencies under different in vitro conditions (LB broth, intestinal chicken cells in DMEM/F-12 medium, and DMEM/F-12 medium alone), with two initial bacterial loads.

From in vivo trial, among 138 broiler chickens sampled after a 49-day fattening period, transconjugants were detected in the cecal content of 35 broilers. The median conjugation frequency observed was of -5.02 log10 transconjugants/donor. Median conjugation frequencies across all in vitro conditions varied by less than one log unit (between − 6.8 and − 6.0 log10 transconjugants/donor), and no significant differences in conjugation efficiency were observed between initial bacterial concentrations.

We confirmed bacterial conjugation between pAmpC-producing E. coli carrying the mcr-1 gene and ESBL-producing E. coli both in vitro and in vivo. The similar conjugation efficiencies observed across different in vitro methods suggest that experimental conditions have minimal influence under controlled settings. In contrast, the in vivo results underscore the significance of the host’s physiological environment in HGT. The presence of transconjugants after a 49-day fattening period indicates that intestinal bacteria function as reservoirs for resistance plasmids and could facilitate their spread throughout the broiler production chain. However, limitations like the possibility of plasmid transfer to other bacteria, unknown persistence of the plasmid in the gut, and potential modulations of transfer efficiency under antibiotic selection must be considered when interpreting the results.

The online version contains supplementary material available at 10.1186/s12866-026-04822-1.