Selection of ESBL-producing Escherichia coli in the gut of calves experimentally fed with milk containing antibiotic residues.

In the bovine sector, the spread of Enterobacterales producing extended-spectrum and AmpC β-lactamases (ESBL/AmpC) mostly concerns veal calves, and the use of waste milk containing antibiotic residues has been recurrently incriminated. In this study, calves were experimentally fed with milk containing either 2,000 μg/L or 20,000 μg/L of the critically important antibiotic cefquinome. The total counts of enterobacterales and ESBL-producing E. coli were monitored using non-selective and selective media. Our data highlighted the important combination of two main factors (cefquinome exposure and initial ESBL colonization level) in the ESBL selection and amplification process in the gut of calves. Results also proved the dose-independent effect of cefquinome administration on the selection and amplification of ESBL-producing E. coli. Finally, the blaCTX-M-1/IncI1 ST3 plasmid was systematically recovered after cefquinome exposure, highlighting its epidemic success. Altogether, this work is one of the rare experimental studies providing quantitative information on the impact of waste milk containing antimicrobials on the ESBL load in calves' microbiota, and the first one using cefquinome. These data emphasise the need for global guidelines on the use of waste milk on dairy farms in order to decrease the antimicrobial resistance burden in this sector.