Transport of Antibiotics in Wastewater Irrigated Soil Columns and Their Impact on the Occurrence of Resistance Genes.

Wastewater irrigation potentially enhances the spread of antibiotic resistance genes, since it contains resistant bacteria and antibiotics in sub-inhibitory concentrations. We investigated the transport of two antibiotics (sulfamethoxazole, SMX, and ciprofloxacin, CIP) and their effect on the abundance of resistance genes in a column experiment. Intact soil monoliths collected from either a field irrigated with untreated wastewater (ww) or from a rain-fed field, were cultivated with alfalfa (Medicago sativa). The monoliths were irrigated once or twice with ww spiked with 200 mg SMX and CIP per liter, and with the dye brilliant blue to visualize the water infiltration path. Also a monolith irrigated with 10 mM CaCl₂ was set up as control. After the irrigations we sampled the stained soil separately from the unstained soil at different depths. The accumulation of both antibiotics in the different soil compartments and their effect on the abundance of the resistance genes sul1, sul2, qnrB and qnrS was measured. SMX was even more mobile than the dye tracer, and diffused through the entire soil monolith, while CIP was readily sorbed and accumulated exclusively in the stained areas. The resistance genes qnrB and qnrS were below the detection limit in all experimental treatments. While the sul1 gene was part of the soil native resistome, as we measured 2x10⁵ gene copies/g soil in the control rain-fed soil, the sul2 gene was introduced with the ww, as this gene was below the detection limit in the control rain-fed soil and became detectable after the irrigation with ww (2.6x10⁶ gene copies/g soil). Irrigation with ww spiked with antibiotics increased the presence of resistance genes, particularly in the rain-fed soil where the increase of sul1 due to the ww amounted to two orders of magnitude, and to one additional order of magnitude after SMX application.