Assessing uptake of antimicrobials by Zea mays L. and prevalence of antimicrobial resistance genes in manure-fertilized soil

Manure-borne antimicrobials and antimicrobial resistance genes (ARGs) are of environmental concern due to their potential to be transferred into the food-web via plant-uptake. In this study, Zea mays L. seeds were grown in three different soil conditions: soil without dairy manure, dairy manure-amended soil, and antimicrobial spiked dairy manure-amended soil, to investigate the potential uptake of antimicrobials and ARGs present in manure. The antimicrobial spiked manure consisted of dairy manure fortified with 1 mg/Kg of each individual antimicrobial compounds belonging to the sulfonamide and tetracycline classes. Samples of the Zea mays L. plants were harvested over the course of three weeks to determine potential uptake of antimicrobials from soil to plant shoots, and to compare prevalence of ARGs in manure amended soils and plant tissue. Antimicrobial analysis was performed using liquid chromatography with tandem mass spectrometry (LC-MS/MS), and ARGs (sul1, tetO, and OXA-1) were analyzed using quantitative polymerase chain reaction (qPCR). The study found that both tetracycline and sulfamerazine antimicrobials bioaccumulated in the Zea mays L., reaching concentrations of nearly 3000 ng/g and 1260 ng/g, respectively. Tetracycline residues predominated in the soil, while sulfonamides had mainly bioaccumulated in Zea mays L. tissue. The greatest average uptake factor within the Zea mays L. tissue was 8 for tetracyclines and 110 for sulfonamides indicating larger bioaccumulation of sulfonamides. Additionally, three ARGs (sul1, tetO, and OXA-1) were detected in the soil, only after manure application. However, ARGs were not detected in any of the plant samples.