Does organically produced lettuce harbor higher abundance of antibiotic resistance genes than conventionally produced?

The demand for organic food products, especially for organic vegetables has been growing rapidly in the last few decades. However, the risk of introducing more antibiotic resistant genes (ARGs) and antibiotic resistant bacteria (ARB) to the vegetables by organic production procedures has long been overlooked. In our study, by using high-throughput quantitative PCR and 16sRNA Illumina sequencing technology, we investigated the abundance and diversity of ARGs and the microbial communities in conventionally (CPL) and organically produced lettuce (OPL). A total of 134 ARGs were detected in the phyllosphere and leaf endophyte of the samples. Absolute copy numbers of ARGs in phyllosphere were 8-fold higher in the OPL than in CPL. We also observed a significant difference in the microbial communities between OPL and CPL, and a lower diversity of both phyllosphere and leaf endophytic bacteria in OPL than in CPL. The Mantel test and variation partitioning analysis (VPA) suggested that the profile of ARGs is strongly affected by bacterial community compositions. Network analysis between ARGs and bacterial taxa indicated that eight bacterial families were implicated to be the potential hosts of ARGs. These results provide insights into the impacts of organic farming on the profiles of bacterial and ARG compositions in vegetables.