Effect of irrigation with urban wastewater and sustainable soil management on the presence and persistence of potential HPMOs in olive trees

Under suitable conditions, low-quality, urban wastewater is an additional water resource for irrigation in water-scarce environments but its use in agriculture requires a careful monitoring of a range of hygiene parameters, including HPMOs. Culture-based and DNA-based microbiological analyses on soil, xylem sap, leaves and fruits were carried out in an olive (Olea europaea L.) grove located in Southern Italy (Basilicata region). The experimental grove has been managed in two plots for 18 years. The first plot (IRR), non-tilled, was drip irrigated daily with reclaimed wastewater (293 mm yr‒1). The second plot (N-IRR) was unirrigated (i.e. rainfed) and subject to conventional soil and plant management. Escherichia coli concentration in the wastewater varied considerably, being frequently above the stringent Italian mandatory limit of 10 CFU 100 mL‒1 and also the WHO limit of 1000 MPN 100 mL‒1. A detailed metagenomic analysis revealed slight increases in other potential HPMOs belonging to the Enterobacteriaceae, Pseumonadaceae and Clostridiaceae families, occasionally observed in IRR soil and plant compartments. In the IRR plot, no significant HPMO bacterial contamination was recorded in the surface and pulp of the fruits harvested directly from the canopy or sampled from the ground. The results confirmed that fertigation urban wastewater did not cause significant increases or persistence of bacterial HPMOs in the soil and plants of the IRR plot and that, among the ecological niches where HPMOs live, xylem sap could be a reservoir of bacteria, mainly deriving from the soil but partially also from the canopy.