Competition between Escherichia coli populations with and without plasmids carrying a gene encoding extendedspectrum beta-lactamase in the broiler chicken gut

Extended-spectrum-beta-lactamase (ESBL)/AmpC-producing Escherichia coli strains are widely found in E. coli isolates from broiler feces, largely due to the presence of the bla_CTX-M-1 gene on IncI1 plasmids. Plasmid carriage is theorized to cause fitness loss and thus should decrease under conditions of reduced antibiotic use. However, in vitro studies showed plasmid carriage to increase in the absence of antimicrobials, due to plasmid conjugation. We investigated whether this translates to increased levels of plasmid in the gastrointestinal tracts of chickens, where conjugation rates may be different and subtle differences in growth rates may have a larger impact on colonization. Eight groups of five chickens were orally inoculated at 4 days of age with a 0.5-ml volume containing 10⁶ CFU/ml E. coli cells, of which 0%, 0.1%, 10%, or 100% carried the IncI1 plasmid with the gene bla_CTX-M-1. At 13 time points during 41 days, fecal samples were taken from each chicken. E. coli strains with and without plasmids were quantified. Trends in E. coli subpopulations were analyzed using generalized linear mixed models, and population dynamics were studied by fitting to a mechanistic model. Trends in E. coli subpopulations were different between groups rather than between individual chickens, suggesting substantial levels of E. coli exchange between chickens in a group. The IncI1 plasmid carrying bla_CTX-M-1 was transferred with conjugation coefficients at levels higher than those observed in vitro. Across groups, the plasmids disappeared or were established independently of the initial fraction of plasmid-carrying E. coli, but no major increase occurred as observed in vitro. Differences in growth rates were observed, but competitive exclusion of plasmid-carrying variants was counteracted by conjugation.