Antibiotic Resistance in Soil: The Anthropogenic, the Native, and the Unknown.

Community acquired antibiotic resistance is one of the greatest medical challenges of the 21^st century due to propagation of antibiotic resistant bacterial pathogens that harbor an array of antibiotic resistant genes (ARGs). The past decade has produced a myriad of studies indicating that anthropogenic activities such as animal husbandry, manure amendment and wastewater treatment expand environmental ARG reservoirs; creating “hotspots” that potentially contribute to community acquired antibiotic resistance. Soil contains extremely high levels of antibiotic resistant bacteria, which harbor a diverse array of ARGs. It has been previously hypothesized that many ARGs associated with human pathogens originated in antibiotic-producing soil bacteria, and that novel soil-associated ARGs can potentially reach clinically relevant bacteria via horizontal gene transfer. This presentation presents data from two comprehensive studies in attempt to provide a holistic overview of how external and intrinsic factors influence antibiotic resistance in soil. The first study assessed the impact of treated-wastewater irrigation, which potentially results in proliferation of AR in irrigated soil microcosms due to release of residual antibiotic compounds, ARB, and ARGs; while the second directly evaluated the impact of selective pressure (i.e. antibiotic concentration) on antibiotic resistance in soil bacteria. Combined, these data suggest that the soil microbiome is extremely resilient to anthropogenic impact, but that native soil bacteria represent a highly diverse reservoir of ARGs, which can potentially be transmitted to clinically-relevant bacteria. It also provides additional support for the theory that the origin of many clinically-relevant ARGs is in soil bacteria.