Airborne Dissemination of Antibiotic Resistance Genes Near Farms and Effectiveness of Ionization Against Airborne Bacteria in a Classroom

Abstract

The Covid-19 pandemic heightened attention to airborne microorganisms and their widespread impacts. This dissertation examines two facets about airborne microorganisms: (1) dissemination of antimicrobial resistance in the environment and (2) ionization for disinfection of indoor air. Antimicrobial resistance (AMR) poses a significant threat to public health, exacerbated by the dissemination of antibiotic resistance genes (ARGs) in the atmosphere to an extent that is not yet well understood. Chapters 2 and 3 of this dissertation characterize ARGs in the atmosphere through a literature review and experimental observations at two agricultural sites, respectively. A critical review of 52 studies revealed that ARGs are present in aerosols in urban, rural, hospital, industrial, wastewater treatment plants, composting and landfill sites, and indoor environments. Commonly studied genes include sul1, intI1, beta-lactam ARGs, and tetracycline ARGs, with abundances varying by season and setting. Temporal trends varied based on the type of environment and human activity. Characterization methods included qPCR, ddPCR, and metagenomic analysis; standardized methodologies are needed to unify findings about the dissemination of ARGs in the atmosphere. To address knowledge gaps identified in the literature review, we designed an experimental study at a dairy farm and swine farm, where beta-lactam was the dominant antibiotic used. We quantified ARG concentrations, size distributions, and emission rates in the air and related these to ARGs found in nearby water and soil samples over four seasons. Concentrations of most ARGs were higher during warmer months but varied more by sampling location or exhaust fan usage than time of year. At both farms, blaCTX-M1 concentrations peaked at 104 gene copies per cubic meter (gc m-3), while the exhaust from a building at the swine farm contained genes like intI1, ermF, and qnrA at concentrations up to 105 gc m-3. ARGs were found in aerosol particles of all sizes, and the fraction in coarse particles (>5 m) was enhanced near emission sources. The presence of ARGs in fine (<1 m) and accumulation mode (1-5 m) particles indicates potential for long-range transport. Emission rates reached ~105 gc s-1 for some ARGs, including blaCTX-M1, and 106 gc s-1 for intI1. Inhalation exposure to blaCTX-M1 was comparable to ingestion exposure from soil at the dairy farm. In chapter 4, the effectiveness of an in-duct, bipolar ionization system for reducing airborne pathogens was evaluated in a real-world setting: an in-use lecture hall at a university. There were no significant differences in positive, in-room ion concentrations between periods with the ionizer turned on and turned off; however, negative, in-room ion concentrations were significantly lower when the ionizer was on with constant fan speed. To account for day-to-day variability in total bacteria concentrations, related to occupancy and other factors, we examined the ratio of bacterial colony forming units to 16S rRNA gene copies (CFU gc–1). There were no significant differences in this ratio whether the ionizer was on or off, suggesting limited real-world effectiveness of the treatment technology. Factors such as occupancy and the heating, ventilation, and air conditioning (HVAC) system emerged as the primary drivers of bacterial load in the air. This study highlights the need for further research to validate the potential of ionization to reduce levels of airborne pathogens.