Browsing by Author "Shawver, Sarah Elizabeth"

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    Antimicrobial resistance in soil: long-term effects on microbial communities, interactions with soil properties, and transport of antimicrobial elements
    Shawver, Sarah Elizabeth (Virginia Tech, 2022-06-08)
    Since penicillin was discovered in 1928, antibiotic usage in human and veterinary medicine and prevalence of antibiotic resistant bacteria (ARB), has been increasing. While antibiotics and antibiotic resistance genes (ARGs) naturally occur in soils, increasing abundances of ARGs correlate with increased antibiotic usage in agricultural settings. When livestock are treated with antibiotics, the antibiotic compounds, ARB, and ARGs can enter soil via manure excreted onto pastures or applied to other fields as fertilizer, thereby spreading antimicrobial resistance (AMR) in the environment. In addition to human health implications, increased AMR has negative impacts on ecosystem services such as carbon and nitrogen cycling. While many studies have researched antibiotic persistence in agricultural systems and their impacts on soil microbial communities, there are still significant knowledge gaps around the long-term effects of antibiotic exposure in soils, how those impacts differ among soils, and how elements of AMR may differentially transport through soil. To address these knowledge gaps, our objectives were to 1) examine the impact of multi-year repeated additions of manure from cattle administered antibiotics on soil microbial communities, 2) determine the interactive effects of soil moisture and type on soil microbial communities exposed to antibiotics and manure, and 3) differentiate between vertical transport of AMR in the form of viable ARB or ARGs in extracellular plasmids. Our results demonstrate that soil bacterial community structures were consistently altered by 3-year additions of manure from cattle administered antibiotics compared to soil amended with antibiotic-free manure. Furthermore, ARG abundances were higher in soils with manure additions compared to soil without manure, although this was true regardless of whether the cattle were administered antibiotics, suggesting that manure and antibiotic impacts on soil microbial communities can persist over multi-year of repeated manure applications. Additionally, in microcosms, effects of manure from cattle administered antibiotics on ARG abundances, microbial community structures, respiration, and nitrogen pools in soil were seen across multiple soil types and moisture contents, suggesting environmental conditions can alter how manure and antibiotics impact microbial community structure and nutrient cycling. Finally, ARB flowed readily through saturated soil, but were also detectable in the top 5 cm of soil columns. However, ARGs on extracellular plasmids did not flow through soil columns and were not detected in soil, indicating that extracellular DNA does not persist or transport through the soil to any meaningful degree. Overall, these results indicate a nuanced approach is required to mitigate the environmental spread of AMR. Soil management strategies for addressing the AMR crisis should consider the broader context of manure management, as high ARG abundances can come from application of manure from antibiotic-free cattle, and soil microbial communities in individual environments may have varied responses to manure antibiotic exposure. Furthermore, the transport of AMR through soil is complex and dynamic, as elements of AMR may transport differently through soil and require separate consideration in modeling and management. Future AMR management practices that consider diverse factors that affect persistence and spread of AMR in the environment can help protect livestock productivity and maintain the efficacy of antibiotics to protect human and animal health.