Ceftiofur formulation differentially affects the resistance of fecal bacteria and the microbiome of steers

Antimicrobial drug concentrations in the gastrointestinal tract likely drive antimicrobial resistance in enteric bacteria. Therefore, the first objective of this study was to determine the concentration of ceftiofur and its metabolites in the gastrointestinal tract of steers that are treated parenterally with either ceftiofur crystalline-free acid (CCFA) or ceftiofur hydrochloride (CHCL). The second objective was to determine the effect of these drugs on the minimum inhibitory concentration (MIC) of fecal E. coli and to evaluate shifts in the microbiome after drug administration.