Effects on resistant bacteria, resistance genes and taxonomic composition after long-term application of sewage sludge on farmland

The widespread practice of applying sewage sludge to arable land makes use of nutrients indispensable for crops and diminishes the need for inorganic fertilizer, while it at the same time provides a route for human exposure to contaminants in the sludge. A more recent concern is that such practice could promote selection and dissemination of antibiotic resistant bacteria or resistance genes. Understanding the risks of sludge amendment in relation to antibiotic resistance development is important for sustainable agriculture, waste treatment and infectious disease management. To assess such risks, we have taken advantages of a research project initiated in 1981 where land used for growing different crops have been amended with sludge every four years in a randomized plot design. We sampled raw sludge, semi-digested sludge and fully treated and long-term stored sludge together with soils from the experimental plots before and two weeks after the most recent amendment in 2017. LC-MS/MS was used to determine the concentrations of selected antimicrobials while microbial effects were evaluated using both culture-independent metagenome sequencing and conventional culturing.