Introduction: Wastewater containing pathogens, antibiotics, and antibiotic resistance determinants raises concerns about the risk of antibiotic resistance genes (ARGs) and mobile genetic elements (MGEs) spreading to the environment, being mobilized from the resistome and transferred to potentially pathogenic bacteria.

Goals: This study investigates the impact of the change from untreated to treated wastewater for irrigation on the abundance and diversity of ARGs, MGEs, and the associated soil microbiome. The objective is to improve our understanding of the impact of plasmids and pollutants present in wastewater on the soil microbiome and the dissemination of ARGs.

Materials & Methods: In a soil incubation experiment, three different soil types were subjected to irrigation with treated or untreated wastewater, either spiked or not with antibiotics and biocides. Samples were collected at both four days and four weeks post-irrigation. DNA-based analysis was conducted by qPCR and amplicon sequencing of 16S rRNA gene.

Results: Soils irrigated with spiked wastewater containing antibiotics showed a higher relative abundance of sulfonamide resistance (sul1) gene, class 1 integron-integrase (intI1) gene, and plasmids belonging to the IncP-1 and pSK1 group. In soils irrigated with unspiked wastewater, trimethoprim and erythromycin resistance genes as well as Staphylococcus plasmids of the pI258 family were detected. Principal component analysis revealed a distinct separation in the distribution of ARGs and MGEs depending on the spiking level, sampling date and soil type. Conversely, the analysis of the soil microbial community by NMDS showed that only the soil type and sampling date had an impact on the composition of the soil microbial community.

Conclusions: Antibiotics and disinfectants applied to irrigation water increased the relative abundance of several ARGs in soils irrigated with wastewater. Antibiotics and disinfectants may have enhanced the occurrence of ARGs and the spread of MGEs, but they had little impact on the soil microbial composition.