Changes in Antibiotic Resistance Gene Levels in Soil after Irrigation with Treated Wastewater: A Comparison between Heterogeneous Photocatalysis and Chlorination

Abstract

Wastewater (WW) reuse is expected to be increasingly indispensable in future water management to mitigate water scarcity. However, this increases the risk of antibiotic resistance (AR) dissemination via irrigation. Herein, a conventional (chlorination) and an advanced oxidation process (heterogeneous photocatalysis (HPC)) were used to disinfect urban WW to the same target of <i>Escherichia coli</i> <10 CFU/100 mL and used to irrigate lettuce plants (<i>Lactuca sativa</i>) set up in four groups, each receiving one of four water types, secondary WW (positive control), fresh water (negative control), chlorinated WW, and HPC WW. Four genes were monitored in water and soil, 16S rRNA as an indicator of total bacterial load, <i>intI1</i> as a gene commonly associated with anthropogenic activity and AR, and two AR genes <i>bla</i>_OXA-10 and <i>qnrS</i>. Irrigation with secondary WW resulted in higher dry soil levels of <i>intI1</i> (from 1.4 × 10⁴ copies/g before irrigation to 3.3 × 10⁵ copies/g after). HPC-treated wastewater showed higher copy numbers of <i>intI1</i> in the irrigated soil than chlorination, but the opposite was true for <i>bla</i>_OXA-10. The results indicate that the current treatment is insufficient to prevent dissemination of AR markers and that HPC does not offer a clear advantage over chlorination.