Therapeutic Use of Ceftiofur Crystalline Free Acid (Excede) and its Impact on Numbers and Susceptibility of Generic E. coli in Feedlot Cattle

Findings: Exposure to ceftiofur crystalline free acid was associated with a reduction in E. coli within the gastrointestinal tract. The consequence of which was that ceftiofur-resistant variants were more likely to be recovered. Repeated exposure to ceftiofur crystalline free acid was associated with greater numbers of ceftiofur-resistance E. coli. Ceftiofur resistance is typically mediated by the blaCMY-2 gene in North America and is often carried on a conjugatable plasmid. In the present study, the expansion of resistance to ceftiofur was also associated with expansion of resistance to ampicillin, chloramphenicol, streptomycin, sulfamethoxazle, and tetracycline. The ACSSuT pattern has been frequently associated with an integron complex and in this study, appears to be linked to determinants conferring resistance to ceftiofur. The number of E. coli and the likelihood of recovering ceftiofur- (and ACSSuT-) resistant E. coli approached pre-exposure levels by ~2 weeks after exposure. More research will be performed to evaluate the molecular characteristics underpinning the phenomena observed herein. In particular, we will determine if the resistance determinants are housed on a conjugatable plasmid, if the genes are similar to those reported elsewhere, and if the expansion of resistance was associated with clonal expansion or the selection of a mobile plasmid in unrelated E. coli.