Post-transfer instability, not initial transfer, limits dissemination of IncI1-blaCTX-M-1 plasmids between chicken and human Escherichia coli.

Plasmids are key vectors in the dissemination of antibiotic resistance genes. Within Escherichia coli, IncI1 plasmids carrying blaCTX-M-1 significantly contribute to extended-spectrum beta-lactamase (ESBL) resistance. To investigate their zoonotic dissemination potential, we compared the conjugative transfer rates, post-transfer stability, and short-term evolution of two IncI1-blaCTX-M-1 plasmids transferred from their original chicken E. coli host to a panel of chicken and human E. coli. In vitro conjugation assays revealed that transfer rates were not affected by recipient host origin or temperature and were primarily governed by recipient genotype. However, post-transfer plasmid loss rates were host-dependent, with four-fold higher loss rates in human than chicken-derived transconjugants. The total number of mutations accumulated post-acquisition was determined by the identity of the specific plasmid rather than the host origin, with each plasmid following a distinct evolutionary trajectory involving convergent parallel deletions. Specifically, one trajectory involved the loss of genes associated with plasmid stability, observed more frequently in human than in chicken strains, while the other involved the loss of part of the conjugation machinery. Our findings show that, although these plasmids transfer easily between bacteria from chicken to human hosts, they are less stable in their new host environment, which may restrict their overall spread. A comprehensive One Health risk assessment must consider these critical downstream dynamics of plasmid stability and host-specific evolution.