Prevalence and Transmission of Antibiotic Resistance Genes in the Soil Environment

The increasing prevalence of antibiotic resistance genes (ARGs) in environmental settings and their potential acquisition by human and animal pathogens have become a global public health concern in the 21st century. The use of antibiotics in livestock industry and the enrichment of ARGs in animal manure and arable soils have engendered the concern that recycling of manure onto agricultural land could disseminate antimicrobial resistance to crops/vegetables, which might represent a potential route for migration of environmental ARGs from farm to fork. However, the pathways for transmission of ARGs from soil to plant remain unclear. Growing evidence points to the pivotal role of the environmental factors in influencing the prevalence of ARGs in the natural environment, while our understanding of the ecological and evolutionary environmental factors that contribute to the development and dissemination of antibiotic resistance in natural environment is lacking. It is imperative to decipher the diversity, prevalence, and environmental determinants of antibiotic resistance before we could design management approaches to control the transmission of environmental resistomes. A soil microcosm incubation experiment was conducted to compare the effects of poultry, cattle and swine manures spiked with or without the antibiotic tylosin on the temporal changes of soil ARGs. The high-throughput quantitative PCR detected a total of 185 unique ARGs (out of 295 targeted genes) among all the soil samples. The diversity and abundance of ARGs significantly increased following manure application. The level of antibiotic resistance gradually decreased over time in all the manured soils but was still significantly higher in the soils treated with swine and poultry manures than in the untreated soils after 130 days’ incubation. Tylosin-amended soils consistently showed higher abundances of ARGs than soils treated with manure only, suggesting a strong selection pressure of antibiotic-spiked