Survival of Escherichia coli O157:H7 in soil and on lettuce after soil fumigation.

Long-term survival of Escherichia coli O157:H7 in soil and in the rhizosphere of many crops after fumigation is relatively unknown. One of the critical concerns with food safety is the transfer of pathogens from contaminated soil to the edible portion of the plants. Multiplex fluorogenic polymerase chain reaction was used in conjunction with plate counts to quantify the survival of E. coli O157:H7 in soil after fumigation with methyl bromide and methyl iodide in growth chamber and microcosm laboratory experiments. Plants were grown at 20 degrees C in growth chambers during the first experiment and soils were irrigated with water contaminated with E. coli O157:H7. For the second experiment, soil microcosms were used in the laboratory without plants and were inoculated with E. coli O157:H7 and spiked with the two fumigants. Primers and probes were designed to amplify and quantify the Shiga-like toxin 1 (stx1) and 2 (stx2) genes and the intimin (eae) gene of E. coli O157:H7. Both fumigants were effective in reducing pathogen concentrations in soil, and when fumigated soils were compared with nonfumigated soils, pathogen concentrations were significantly higher in the nonfumigated soils throughout the study. This resulted in a longer survival of the pathogen on the leaf surface especially in sandy soil than observed in fumigated soils. Therefore, application of fumigant may play some roles in reducing the transfer of E. coli O157:H7 from soil to leaf. Regression models showed that survival of the pathogen in the growth chamber study followed a linear model while that of the microcosm followed a curvilinear model, suggesting long-term survival of the pathogen in soil. Both experiments showed that E. coli O157:H7 can survive in the environment for a long period of time, even under harsh conditions, and the pathogen can survive in soil for more than 90 days. This provides a very significant pathway for pathogen recontamination in the environment.