Competition between Escherichia coli with and without Extended-Spectrum-Beta-Lactamase-carrying plasmids in the broiler chicken gut

ESBL/AmpC-producing Escherichia coli are widely found in E. coli isolates from broiler faeces, much of which is due to the blaCTX-M-1 gene on IncI1 plasmids. Plasmid carriage is theorized to cause fitness loss, thus should decrease when reducing antibiotic use. However, in vitro studies showed plasmid carriage to increase in the absence of antimicrobials, due to plasmid conjugation. We investigate if this translates to plasmid increase 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 four days of age with 0.5 ml containing 106 CFU/ml E. coli of which 0%, 0.1%, 10% or 100% carried the IncI1 plasmid with the gene blaCTX-M-1 At thirteen timepoints during 41 days, faecal samples were taken from each chicken. E. coli with and without plasmids were quantified. Trends in E. coli subpopulations were analysed 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 individual chickens, suggesting substantial E. coli exchange between chickens in a group. The IncI1 plasmid carrying blaCTX-M-1 was transferred with conjugation coefficients larger than observed in vitro Across groups, the plasmids disappeared or 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.Importance Bacteria that produce extended spectrum beta-lactamases are resistant to an important class of antimicrobials in human and veterinary medicine. Reduction in antibiotic use is expected to decrease the prevalence of resistance. However, resistance genes often lie on plasmids which can be copied and transferred to other bacteria by conjugation, so in vitro resistance was observed to increase in absence of antimicrobials. We sought to determine whether this also occurs in the chicken gut and if competitive exclusion by similar E. coli variants without the resistance occurred. We studied the excretion of E. coli carrying IncI1 plasmids with the blaCTX-M-1 resistance gene in small groups of broiler chickens, after inoculating with E. coli suspensions containing different fractions of plasmid-carrying cells. Our results show little variation between chickens within groups, but large differences between groups independent of the ratio of variants with and without the plasmid, with persistence or extinction of the plasmid. However, there was no major plasmid increase as observed in vitro We conclude that in vivo studies with sufficient independent replications are important for intervention studies on plasmid-mediated antimicrobial resistance.