Transmission of Escherichia coli from irrigation water to preharvest leafy greens

AIM: The aim of this study was to examine the fate of Escherichia coli strain 0611, used as model for a water-borne bacterial contaminant, in leafy green production systems. METHODS AND RESULTS: Irrigation water from two different sources were inoculated with the strain and applied via overhead irrigation to endive and spinach plants grown under open-field conditions. Inoculum levels were 10³ CFU ml⁻¹, representing occasional water contamination, and a 1,000-fold higher density to enable detailed quantification across plant compartments. Three experiments assessed bacterial persistence after single or double irrigation events at varying intervals, and under continuous irrigation. Colony-forming units were measured on leaves and in rhizosphere soils. Under continuous irrigation with the low inoculum dose, E. coli 0611 was only sporadically detected on leaves. At the higher dose, the interval between irrigation and harvest had a greater impact on bacterial presence than water source. Decline patterns on leaves and in endive rhizosphere soil followed double-exponential models. Rhizosphere soil represents an additional risk factor for transmission of human pathogens to fresh produce. Although bacterial populations declined rapidly on leaves, they did not completely disappear from rhizosphere soil. CONCLUSIONS: A preharvest interval of at least four days after the final irrigation substantially reduces the risk of pathogen transfer to fresh produce. The experimental data support evidence-based recommendations for producers to reduce the risk of microbial contamination of fresh produce, thereby contributing to safer production of ready-to-eat vegetables.