The effect of sulfadiazine in manure on accumulation of sulfonamide resistance genes in freshly consumable plants

Abstract Background Antibiotic resistance genes will spread via soil fertilized with animal manure to food products. Especially plants whose harvested products can be consumed freshly are of concern. The aim of this study was to assess the impact of sulfadiazine (SDZ)-manured soil on the occurrence of sulfonamide (SA) resistance genes in freshly consumable plants. Methods Sulfadiazine-containing manure was administered via soil to lettuce and leek plants. At harvest, the rhizosphere soil, roots and leaves were investigated on the presence of SDZ-resistant bacteria and sul1 and sul2 genes via qPCR. Further, the impact of SDZ in manure on bacterial community and antibiotic resistance gene composition via amplicon sequencing and shotgun metagenomics was investigated in rhizosphere soils. Results Amendment of SDZ to manure resulted in an increase in sul2 genes in manure. However, abundances of sul genes in rhizosphere soils was strongly determined by plant growth and not by soil treatments with SDZ-manure, which was also the case for the bacterial community composition. Effects of SDZ at low or undetectable levels in leek rhizosphere soil became evident by bacterial association network and resistome analyses, and also in roots and leaves by SDZ-selective bacterial cultivation. Conclusions Antibiotic residues present in animal manure can lead to an increase in antibiotic resistances in food products. Plants play an important role in selection of antibiotic resistance genes present in manured soil. Transmission of antibiotic resistances via manure to the soil–plant ecosystem must be placed into the context of soils as vast reservoirs of ARGs.