Fecal carriage of ESBL-producing Escherichia coli in veal calves results from two non-exclusive strategies: bacterial clone and plasmid spreads

Despite a high prevalence of extended spectrum ß-lactamase (ESBL)-producing Escherichia coli in veal farms, little is known about plasmid and clone dynamics at play in intestinal carriage of ESBL-encoding genes by calves. We isolated 173 ESBL-producing E. coli strains from 514 rectal swabs collected over six months from 45 calves in three French commercial farms. We performed genomic characterization of selected ESBL-producing E. coli isolates (n= 43) and plasmids (n= 35) to explore the diffusion of ESBL-encoding genes in calves. Calves were colonized successively by different clones. The persistence of a clone in a farm was significantly associated with the number of calves colonized (rs= 0.85, p= 4 x 10-9). Despite a high diversity of ESBL-producing E. coli clones and plasmids, two main scenarios were at play in the spread of blaCTX-M genes among calves: the first one consisted in an efficient colonization of a few bacterial clones, and the second one in successful plasmid spread in various bacterial clones. The scenario (clone versus plasmid spread) was not exclusive and depended on the farm. Our findings demonstrated that the fitness of its molecular supports is a key factor for persistence and diffusion of ESBL-encoding genes in veal farms.