Quantifying Dissemination of Antibiotic Resistance Genes in Air from a Dairy Farm and Swine Farm

Farms are a suspected source of dissemination of antibiotic resistance genes (ARGs) to the atmosphere, but their contribution remains poorly quantified. This study investigated the concentrations, emission rates, and particle size distributions of ARGs in air around a dairy farm and swine farm, as well as in farm wastewater and soil as potential sources, during a yearlong sampling campaign. Analysis targeted genes corresponding to a cross-section of antibiotic classes used in human and veterinary medicine, along with 16S rRNA and <i>intI1</i> as indicators of total bacterial load and anthropogenic sources of ARGs, respectively. Two approaches were demonstrated for estimating emissions to account for the physical configurations of the farms. A custom sampler that collected size-resolved aerosol particles at a flow rate of 2.25 L/min only when the wind originated from the direction of interest was used to collect aerosol particles near potential sources. At the dairy and swine farms, <i>bla</i> _{<i>CTX‑M1</i>} concentrations varied significantly by sampling location, averaging 10² gene copies per cubic meter (gc m^-3) across seasons and peaking at 10⁴ gc m^-3 during the summer sampling period. At the swine farm, maximum concentrations reached 10⁵ gc m^-3 for <i>intI1</i>, <i>ermF</i>, and <i>qnrA</i> near the buildings' exhaust fans. Emission rates reached ∼ 10⁵ gc s^-1 for some ARGs, including <i>bla</i> _{<i>CTX‑M1</i>} , and 10⁶ gc s^-1 for <i>intI1</i>. ARGs were predominantly associated with coarse particles (>5 μm) near emission sources and were also present in fine (<1 μm) and accumulation (1-5 μm) mode particles near the source and at downwind locations, indicating potential for inhalation exposure and long-range transport.