Rare earth oxide nanoparticles promote soil microbial antibiotic resistance by selectively enriching antibiotic resistance genes

The booming application of manufactured nanoparticles raises concerns about their unintentional environmental consequences, one of which is whether nanoparticles will co-selectively enrich soil antibiotic resistance genes (ARGs) which may be transferred to human pathogens through the food chain. We examined soil microbial resistance to tetracycline in three agricultural soils after pre-exposure to 0-100 mg kg(-1) nanoparticulate La2O3, Nd2O3, and Gd2O3 for up to 60 days. Rare earth oxide nanoparticle pre-exposure promoted soil microbial antibiotic resistance, as reflected by the decreased effects of tetracycline on soil microbial biomass and activity in nanoparticle-amended soils. Quantification of ARGs by high-capacity quantitative polymerase chain reaction revealed that the enhanced antibiotic resistance was attributed to the increased relative abundance and diversity of total ARGs which targeted all major classes of antibiotics. Among the 168 observed ARGs, 40 ARGs (e.g., 8 for La2O3, 21 for Nd2O3, and 23 for Gd2O3) were significantly enriched in the presence of nanoparticles (e.g., tetracycline and multidrug resistance genes, P < 0.05), indicating the selective pressure imposed by rare earth oxide nanoparticles in promoting the proliferation of antibiotic resistance genes in soil microbial communities. There was a significantly positive correlation between the relative abundance of mobile genetic elements and ARGs (P < 0.05), suggesting that horizontal gene transfer may have aided the spread and proliferation of ARGs in nanoparticle-amended soils. Taken together, these findings demonstrate that nanoparticulate La2O3, Nd2O3, and Gd2O3 can aggravate soil microbial antibiotic resistance by enriching ARGs through co-selection and horizontal gene transfer, and thus forewarn of the environmental and health risks of rare earth oxide nanoparticles in soil ecosystems.