Antimicrobial resistance genes as environmental pollutants

The transformation of food animal production to an industrial scale has been accompanied by industrial scale environmental releases of microbes, including pathogens; biological products, such as allergens; and chemical pollutants, such as pesticides and greenhouse gases. Because of the continued nontherapeutic use of antimicrobial drugs as feed additives for food animals, releases from industrial food animal production operations contain antimicrobial drugs, antimicrobial-resistant bacteria, and genetic resistance determinants. We and others have measured these constituents in air, soils, and surface waters near industrial food animal production operations and along the consumer pathway as animals are transported to slaughter and food products are processed for sale. In terms of framing research and evaluating pathways of exposure and human health risks, we can consider these releases of antimicrobials, the drug-resistant bacteria, and genes for resistance collectively as pollutants. Moreover, these releases have cumulative impacts through expanding the microbial resistome, which is defined as the collective resources of resistance genes available to microbes in a community (microbiome) linked by efficient mechanisms of horizontal gene transfer. Traditional environmental chemical pollutants may impact the resistome as well. At the molecular level, antimicrobial resistance is often transferred among bacteria in “genetic exchange communities” through plasmid-mediated cassettes of multiple resistance genes. These cassettes can include genes encoding resistance to metals such as mercury. In this example, mercury exposure can drive plasmid dissemination of antimicrobial resistance. Host responses to this combination of microbiological and chemical factors further complicate the ecology of host, animal, and environmental microbiomes. For this reason, it is necessary to take a systems approach bridging infectious disease biology, landscape ecology and traditional environmental health in order conduct research and inform policies that maintain healthy communities and prevent disease.