Interactions between the foodborne pathogen Escherichia coli O157:H7 and plant tissue

Fresh produce has emerged as a vehicle of foodborne illness over the past decade. Escherichia coli O157:H7, a pathogen previously associated with animal products has repeatedly been linked to the consumption of fresh fruits and vegetables, most notably, leaf lettuce. The interactions between E. coli O157:H7 and pre-harvest plants are poorly understood, impeding the introduction of effective control and sanitation methods. Experiments were conducted to determine whether uptake of E. coli O157:H7 by the root systems of growing lettuce plants was perhaps a reason for the ineffectiveness of surface sanitation. Results indicated that the pathogen, when introduced in irrigation water or manure, could be transported upward from the roots into the edible portions of growing plants, putatively by the plant vasculature. Further studies were conducted to investigate the effect of irrigation of plants using contaminated water by either spray or surface methods. Spray irrigation using contaminated water resulted in a greater number of plants testing positive for E. coli O157:H7 at time of harvest. Regardless of irrigation method, surface sanitation using chlorine was not effective in eliminating the pathogen. E. coli O157:H7 was demonstrated to persist for long periods of time following application by spray irrigation. Fluorescent microspheres were used to investigate whether the internalization of E. coli O157:H7 into lettuce plants was mediated by the microbe. Imaging demonstrated the presence of microspheres within the edible tissue of the lettuce plants. This indicates that the associations between E. coli O157:H7 and pre-harvest plants may be passive events. The attachment of E. coli O157:H7 to lettuce leaf tissue has not been characterized. Fluorescent microspheres, live and glutaraldehyde-killed cells of E. coli O157:H7 attached to lettuce at similar levels, regardless of exposure time. The attachment could not be attributed to hydrophobicity. Results suggest that the attachment of E. coli O157:H7 to lettuce plant tissue is not mediated by specific bacterial processes or cell surface moieties.