Effects of Ceftiofur and Chlortetracycline on the Resistome of Feedlot Cattles

Margaret D. WeinrothH. Morgan ScottBo NorbyGuy H. LoneraganNoelle R. NoyesPablo RoviraEnrique DosterXiang YangDale R. WoernerPaul S. MorleyKeith E. Belk

abstract

Treatment of food producing animals with antimicrobial drugs (AMD) is controversial because of concerns regarding promotion of antimicrobial resistance (AMR). To investigate this concern, resistance genes in metagenomic bovine fecal samples during a clinical trial were analyzed to assess the impacts of treatment on beef feedlot cattle resistomes. Four groups of cattle were exposed, using a 2×2 factorial design, to different regimens of antimicrobial treatment. Injections of ceftiofur crystalline free acid (a third generation cephalosporin) were used to treat all cattle in treated pens vs. only a single animal, and chlortetracycline was included in the feed of all cattle in a pen vs. no treatment. On days 0 and 26, respectively, pre- and post-trial fecal samples were collected and resistance genes were characterized using shotgun metagenomics. Treatment with ceftiofur was not associated with changes to β-lactam resistance genes. However, cattle fed chlortetracycline had a significant increase in relative abundance of tetracycline resistance genes. There was also an increase of an AMR class not administered during the study—a possible indicator of co-selection of resistance genes. Samples analyzed in this study had previously been evaluated by culture characterization (E. coli and Salmonella) and qPCR of metagenomic fecal DNA, which allowed comparison of results with this study. In the majority of samples, genes that were selectively enriched through culture and qPCR were not identified through shotgun metagenomic sequencing in this study, suggesting that changes previously documented did not reflect changes affecting the majority of bacterial genetic elements found in the predominate fecal resistome.

Link to “Applied and Environmental Microbiology” Paper
Citation

Margaret D. WeinrothH. Morgan ScottBo NorbyGuy H. LoneraganNoelle R. NoyesPablo RoviraEnrique DosterXiang YangDale R. WoernerPaul S. MorleyKeith E. Belk. Applied and Environmental Microbiology, 04 May 2018. doi: 10.1128/AEM.00610-18