Evaluation of possible persistence of potential human pathogenic bacteria in olive orchards irrigated with treated urban wastewater

Under suitable conditions, low-quality, treated urban wastewater (TWW) 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 human pathogenic bacteria (HPB). DNA-based microbiological analyses on soil, xylem sap, and leaves surface (phyllosphere) 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 experimental plot (WWtr) was drip irrigated daily with TWW (2800 m3 ha−1 year−1 ), while the control plot (RFtr) was rainfed. The results of the 16S-rRNA-based metagenomic analysis demonstrated that the phyllosphere had the lowest number of potential HPB (6), compared to soil (22) and xylem (26) compartments. Gammaproteobacteria, including potential HPB, like Pseudomonas and Acinetobacter spp., were significantly higher in WWtr soil and xylem sap, compared to RFtr. A similar trend was observed for Burkholderia spp. (Betaproteobacteria) and Mycobacterium spp. (Actinobacteria). The Firmicutes Enterococcus, Staphylococcus and Streptococcus spp. were more abundant in WWtr xylem sap. The pathogenic Clostridium perfringens was found higher on WWtr leaves (relative abundance 7.17 in WWtr and 1.33 in RFtr) and Enterococcus faecalis in WWtr xylem sap (93.22 in WWtr and 7.08 in RFtr). On the basis of the results obtained, the irrigation with TWW can be considered a realistic and safe agronomic practice in Mediterranean orchards, and an opportunity for farmers and consumers.