Evaluation of the Uptake of Microbial Surrogates by Leafy Greens Using Subsurface Irrigation

Abstract

Foodborne outbreaks have been traced to contaminated irrigation water. The use of contaminated water to irrigate crops by surface, sprinkler, or conventional drip irrigation may pose a public health hazard due to the potential presence of pathogens. It is immediately apparent that crops that come in direct contact with contaminated irrigation water can become contaminated. Thus, this study assessed a subsurface irrigation system quantitatively with romaine lettuce in terms of crop contamination when microbial-contaminated water was used for irrigation. A cropping system for bottom watering pots at the greenhouse and a subsurface drip irrigation (SDI) system commonly used for lettuce production in Arizona was modeled with the well-known software, HYDRUS 2D/3D, to ensure that the soil surface does not become wet and consequently contaminated. Experiments were conducted during the full growth cycle of romaine lettuce to fulfill the study purposes. Escherichia coli ATCC 25922 and bacteriophage MS2 were used as surrogate microorganisms for this study. No microbial contamination of produce occurred in romaine lettuce irrigated with contaminated irrigation water utilizing bottom watering pots. Maintenance of a dry soil surface appeared to avoid crop contamination in subsurface irrigated lettuces. Subsurface irrigation systems with proper management can be excellent strategies to not only conserve water resources but to reduce or eliminate contamination of leafy green vegetables when harmful microbes are present in the irrigation water. In the second aspect of this study, microbial contamination of lettuce plants irrigated by surface irrigation was investigated to aid in the development of irrigation water quality standards for enteric bacteria. Surface irrigation was evaluated with the use of Escherichia coli O157:H7 transfer data to the plants. The Beta-Poisson model was used to estimate the microbial risk of annual pathogen ingestion. The risk of infection was found to be variable depending on the E. coli concentration in water, and the pathogen transfer to the crop days post-exposure. The worst-case scenario, in which produce could be harvested and consumed the day after the last irrigation event, and maximum exposure is assumed, indicated that concentrations of 104 CFU (colony forming units) /ml in 30-day-old lettuce plants would result in an annual risk of 1:10,000 if the crop would be consumed. Similarly, concentrations of 103 CFU/ml, and 104 CFU/ml in 12-day-old lettuce would result in an annual risk of 1:10,000 if the crop would be ingested.