The IncI1 plasmid carrying the bla _CTX-M-1 gene persists in in vitro culture of a Escherichia coli strain from broilers

Background: Commensal bacteria are a reservoir for antimicrobial-resistance genes. In the Netherlands, bacteria producing Extended Spectrum Beta-Lactamases (ESBL) are found on chicken-meat and in the gut of broilers at a high prevalence and the predominant ESBL-gene is the bla _CTX-M-1 located on IncI1 plasmids. We aim to determine the fitness costs of this plasmid for the bacterium.We investigated the conjugation dynamics of IncI1 plasmids carrying the bla _CTX-M-1 gene in a batch culture and its impact on the population dynamics of three E. coli populations: donors, recipients and transconjugants. The intrinsic growth rate (ψ), maximum density (K) and lag-phase (λ) of the populations were estimated as well as the conjugation coefficient. Loss of the plasmid by transconjugants was either assumed constant or depended on the effective growth rate of the transconjugants.Parameters were estimated from experiments with pure culture of donors, recipients and transconjugants and with mixed culture of donors and recipients with a duration of 24 or 48 hours. Extrapolation of the results was compared to a 3-months experiment in which a mixed culture of recipient and transconjugant was regularly diluted in new medium. Results: No differences in estimated growth parameters (ψ, K or λ) were found between donor, recipient and transconjugant, and plasmid loss was not observed. The conjugation coefficient of transconjugants was 10⁴ times larger than that of the donor. In the 3-months experiment, the proportion of transconjugants did not decrease, indicating no or very small fitness costs. Conclusions: In vitro the IncI1 plasmid carrying the bla _CTX-M-1 gene imposes no or negligible fitness costs on its E. coli host, and persists without antimicrobial usage.