Application of biosolids drives the diversity of antibiotic resistance genes in soil and lettuce at harvest

High-throughput quantitative polymerase chain reaction (PCR) of antibiotic resistance genes (ARGs) was used to profile the composition and diversity of ARGs in biosolid-amended soil and in lettuce roots and shoots. Biosolid application significantly increased the ARGs in soil and influenced the soil antibiotics resistome mainly through exogenous introduction. Four dominant ARGs occurred in sterilized and water-washed leaves, namely catB8, php, vanB and sir, which accumulated to >50% of total abundance. Nine subclasses of ARGs were determined in both sterilized and water-washed leaves, and they were shared ARGs among soil, roots and shoots. This suggests that the ARGs in ready-to-eat lettuce were mainly located in endophytes and the internal route may contribute more to the ARGs in routinely consumed raw lettuce. Moreover, a previously unsuspected shift in multridrug ARGs to root and shoot compartments due to biosolid application was discovered. These results reveal that biosolid application drives the evolution and dissemination of ARGs in the soil-plant system.