Comparative metagenomics reveals limited differences in antimicrobial resistance gene abundance across conventional and natural livestock production systems.

The livestock industry has been a source of concern in terms of antimicrobial resistance (AMR) development and spread, especially from a One Health perspective. Raising livestock without antimicrobials, so called natural (NAT) production, is an increasingly popular practice. This study used metagenomics to compare this practice to conventional (CONV) antimicrobial use (AMU) on the microbiome and resistome in the feces of beef cattle and swine and the cecal contents of broiler chickens. In cattle, Bacteroidetes, Euryarchaeota, and Spirochaetes were more abundant (q < 0.01) in CONV than NAT systems, with no differences (q > 0.05) in bacterial profiles in either swine or chickens. Classes of antimicrobial resistant genes (ARG) were not impacted regardless of AMU in any of the livestock species. However, many tetracycline resistance genes were more abundant in CONV as compared to NAT swine (q < 0.05), but this difference was not observed in cattle or chickens. This study confirmed that elimination of AMU does not necessarily result in an immediate decline in the abundance or diversity of ARGs within a single livestock production cycle.