Recovery of Antibiotic Resistance Genes From Agricultural Runoff

The reduced capacity of antibiotics to treat infections is one of the greatest health concerns that society faces. There is substantial evidence that links this reduced capacity with the widespread use of antibiotics in livestock production. Livestock can act as reservoirs of antibiotic resistance genes (ARGs) and antibiotic resistant bacteria, which can pass resistance on in the livestock's manure. It is important to understand the fate of antibiotic resistance genes and resistant bacteria in the environment after land-application of manure-based amendments. The goal of this field-scale study was to identify the effects of soil amendments (inorganic fertilizer, compost, or raw manure) and crop cover (lettuce or radish) on sediment transfer, fecal indicator bacteria (FIB), and release of ARGs in runoff over six storm events. Two FIB (Escherichia coli and enterococci) and two ARGs (sulI and ermB) were quantified in runoff from each of the constructed plots throughout the growing season. FIB and ARGs were recovered from all plots, including control plots indicating a background level within the soil. Additionally, only the effects of variability among individual storms had an impact on the concentration of FIB in runoff. Vegetative cover and storm variability affected sediment release. A trend of higher sul1 and ermB in runoff from compost and raw manure-amended plots for at least 2 months after planting crops was observed. Only one of these ARGs (ermB) is associated with the class of drugs given to the dairy cows used for the manure and compost, indicating inherent carriage of some ARGs independent of the type of antibiotic administered, and such genes can persist in the environment. These results suggest that there is a risk of ARGs being carried into areas downgradient from agricultural plots that have been amended with compost or manure.