Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/21/2013
Publication Date: 4/1/2013
Publication URL: http://handle.nal.usda.gov/10113/56423
Citation: Schmidt, J.W., Griffin, D., Kuehn, L.A., Harhay, D.M. 2013. Influence of therapeutic ceftiofur treatments of feedlot cattle on fecal and hide prevalences of commensal Escherichia coli resistant to expanded-spectrum cephalosporins, and molecular characterization of resistant isolates. Applied and Environmental Microbiology. 79(7):2273-2283.
Interpretive Summary: Some classes of antibiotics are critically important to human medicine and are prescribed for the treatment of serious E. coli and Salmonella infections. Concerns have been raised that therapeutic treatment of feedlot cattle with antibiotics in the same classes as those used for humans increases the prevalence of resistant E. coli. A feedlot cattle herd was sampled over a 10 month period that included fecal samples before, during and after antibiotic treatment for disease. A baseline, low level of antibiotic resistant E. coli was detected in cattle upon arrival at the feedlot, resistance temporarily increased after antibiotic treatment, and returned to baseline levels after several weeks. Genetic analysis of 312 resistant E. coli isolates obtained from this study demonstrated that the baseline level of resistant E. coli in this cattle herd was more likely due to the persistence of a few feedlot-adapted resistant E. coli strains rather than the transfer of the genes conferring resistance between E. coli strains. These results indicate that antibiotic treatment of disease in cattle feedlots does not increase the prevalence of antibiotic-resistant E. coli in those cattle when they are harvested.
Technical Abstract: In the United States the bla**CMY-2** gene contained within incompatibility type A/C (IncA/C) plasmids is frequently identified in extended-spectrum cephalosporin-resistant Escherichia coli (ESCr) from both human and cattle sources. Concerns have been raised that therapeutic use of ceftiofur in cattle may increase the prevalence of ESCr E. coli. We report that herd ESCr E. coli fecal and hide prevalences throughout the residency of cattle at a feedlot, including during the period of greatest ceftiofur use at the feedlot, were either not significantly different (P > or = 0.05) or significantly less (P < 0.05) than the respective prevalences at arrival. Longitudinal sampling of cattle treated with ceftiofur demonstrated that once the transient increase of ESCr E. coli shedding that follows ceftiofur injection abated, ceftiofur-injected cattle were no more likely than untreated members of the same herd to shed ESCr E. coli. Pulsed-field gel electrophoresis (PFGE) genotyping, antibiotic resistance phenotyping, screening for presence of the bla**CMY-2** gene, and plasmid replicon typing were performed on 312 ESCr E. coli isolates obtained during six sampling periods spanning the 10-month residence of cattle at the feedlot. The identification of only 26 unique PFGE genotypes, 12 of which were isolated during multiple sampling periods, suggests that clonal expansion of feedlot-adapted bla**CMY-2** E.coli strains contributed more to the persistence of bla**CMY-2** than horizontal transfer of IncA/C plasmids between E. coli strains at this feedlot. We conclude that therapeutic use of ceftiofur at this cattle feedlot did not significantly increase the herd prevalence of ESCr E.coli.