Dissemination of IncI plasmid encodingbla_CTX-M-1is not hampered by its fitness cost in the pig’s gut

Abstract

Multiresistance plasmids belonging to the IncI incompatibility group have become one of the most pervasive plasmid types in extended-spectrum beta-lactamase producingEscherichia coliof animal origin. The extent of the burden imposed on the bacterial cell by these plasmids seems to contribute to the emergence of "epidemic" plasmids. However,in vivodata in the natural environment of the strain are scarce. Here, we investigated the cost of ablaCTX-M-1-IncI1 epidemic plasmid in a commensalE. colianimal strain, UB12-RC, before and after oral inoculation of fifteen 6- to 8-week-old specific pathogen-free pigs. Growth rate in rich medium was determined on (i) UB12-RC and derivatives, with or without plasmid,in vivoand/orin vitroevolved, and (ii) strains that acquired the plasmid in the gut during the experiment. AlthoughblaCTX-M-1-IncI1 plasmid imposed no measurable burden on the recipient strain after conjugation and during the longitudinal carriage in the pig's gut, we observed a significant difference in the bacterial growth rate between IncI1 plasmid-carrying and plasmid-free isolates collected duringin vivocarriage. Only a few mutations on the chromosome of the UB12-RC derivatives were detected by whole-genome sequencing. RNA-Seq analysis of a selected set of these strains showed that transcriptional responses to theblaCTX-M-1-IncI1 acquisition were limited, affecting metabolism, stress response, and motility functions. Our data suggest that the effect of IncI plasmid on host cells is limited, fitness cost being insufficient to act as a barrier to IncI plasmid spread among natural population ofE. coliin the gut niche.