Evaluation of subsurface irrigation on microorganism contamination of romaine lettuce

¹ University of Arizona, Department of Biosystems Engineering. 1177 E. 4th Street, Shantz Building, Room 403. Tucson, AZ, USA 85721.
² University of Arizona, Department of Environmental Science. 1177 E. Lowell Street, Animal and Comparative Biomedical Sciences Building, Room 211. Tucson, AZ, USA 85721.

Abstract

The use of contaminated water to irrigate crops by surface, sprinkler, or conventional drip irrigation represents a significant public health concern due to the presence of multiple microbial pathogens associated with gastrointestinal disease. In this study, Escherichia coli and MS2 bacteriophage were used as microbial surrogates to evaluate the contamination of romaine lettuce (Lactuca sativa L. var. longifolia) using bottom watering pots simulating a subsurface drip irrigation (SDI) system in a greenhouse. The primary goal was to determine whether the exposure of lettuce plants to E. coli and MS2 would result in detectable levels of these microorganisms associated with the edible portions of plants. Plants were grown in bottom watering pots and were irrigated the last twelve days before harvesting with contaminated water containing E. coli and MS2 phage at 10⁹ CFU/ml and 10¹¹ PFU/ml, respectively. Harvested plants were processed to determine if E. coli or MS2 was associated with the plant surfaces or within the plant tissues. None of the samples was positive for either E. coli or MS2 bacteriophage, suggesting that subsurface irrigation systems effectively reduce the risk of produce-contamination with bacterial and viral pathogens.