Reclaimed wastewater (WW) is often used for irrigation. After changing irrigation with untreated to treated WW it is supposed, that there could be short-term increase in antimicrobial resistant bacteria (ARB) following release of adsorbed pollutants. We hypothesize heterotrophic ARB abundance increase after changing irrigation (untreated to treated) in soils.

Here, an incubation experiment was performed with three soil types, Phaeozem, Vertisol and Leptosol with historically extended irrigation with untreated WW. Soils were incubated with untreated-unspiked, treated-unspiked, untreated-spiked and treated-spiked WW. Soil samples before incubation with WW were used as controls. A spot-assay technique was established to determine ARB abundance on MH (37°C/24h/potential pathogens) and R2A (25°C/48h/environmental bacteria) both without and with CIP, TRI/SUL, ERY/CLI and BAC-C12. Samples were taken 4 weeks after irrigation.

WW treatment and spiking effected the abundance of ARBs (on MH, R2A) in irrigation water. Especially Pseudomonas and E. coli were present in treated and spiked WW. Independent of the soil type the irrigation shift from untreated to treated WW showed no significant effects on the relative abundance of ARBs (on MH, R2A). However, the relative abundance of potential pathogens (MH) was significantly higher (p<0.05) in the presence of TRI/SUL and ERY/CLI upon irrigation (all WW types) compared to the control for phaeozem. Presence of BAC-C12 caused significantly higher relative abundance of potential pathogens after irrigation (exception: untreated-unspiked) for phaeozem. Similarly, potential pathogen relative abundance was significantly higher after irrigation (all WW types) in presence of TRI/SUL (exception, treated-unspiked vs control) and BAC-C12 for leptosol. No interactions were observed between soil-types, treatments and spiking considering abundance of ARB (on MH, R2A).

In phaeozem and leptosol irrespective of the WW types, the fraction of TRI/SUL and BAC-C12 resistant potential pathogens increased compared to non-irrigated soil. Resistance screening will provide insights into the WW treatment based modification of the ARBs resistome.