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item Ibekwe, Abasiofiok - Mark
item Grieve, Catherine

Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/24/2006
Publication Date: 6/22/2007
Citation: Ibekwe, A.M., Grieve, C.M., Yang, C. 2007. Survival of Escherichia Coli O157:H7 in soil and on lettuce after fumigation. Canadian Journal of Microbiology. Vol. 53:623-635

Interpretive Summary: Contamination of food and water by microorganisms from animal manures has become a major public health concern. Non-point source contamination may result from pastured animals, wild animals or from manure spread on fields as fertilizer or waste. Risks assessed in this study include the ability of potential pathogens, namely E. coli O157:H7 to survive in different types of soils for a long time. We also assessed whether this pathogen can survive on plant roots and leaves as they survived in soils. We found that E. coli O157:H7 can survive in soil for over 90 days. We also found that during the 90 day period, majority of the pathogen will die, but few will remain and regrow in the soil even if the soil is left dried. Our study suggests that E. coli O157:H7 could persist in most soil for many months and replicate when conditions are favorable. This can provide a pathway for the pathogens to infect farm workers or contaminate produce grown after the initial contamination events.

Technical Abstract: Long term persistence of Escherichia coli (E. coli) O157:H7 in soil and in the rhizosphere of many crops is relatively unknown. Many groups have voiced concerns about the safety of land application of manure and the potential for food and water contamination by E. coli O157:H7 from agricultural runoffs. Multiplex fluorogenic PCR was used in conjunction with plate counts to quantify E. coli O157:H7 in soil after fumigation with methyl bromide (MeBr) and methyl iodide (MeI). E. coli O157:H7 strain 72 with green fluorescent protein (pGFP) was used for two experiments. Plants were grown at 20°C in growth chambers during the first experiment. For the second experiment, soil microcosms were used in the laboratory without plants and spiked with 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. A direct correlation was determined between the fluorescence threshold cycle (CT) and the starting quantity of E. coli O157:H7 DNA. The detection limit for the real time PCR assay was 2400 CFU/ g E. coli O157:H7. Both fumigants were effective in reducing pathogen concentrations in soil in the short run, but there were no significant long term effects of fumigants on the persistence of pathogens even when fumigants were applied at five times the normal application rate. Persistence of pathogen was consistently higher in sandy soil than clay soil used in this study. Regression models showed that persistence of pathogen in the growth chamber study followed a linear model while that of the microcosm followed a curvilinear model suggesting regrowth due to survival of a sub-population within the general population. Both experiments showed that E. coli O157H:7 can persist in the environment for a long period of time, and, even under harsh conditions, the pathogen can persist and may regrow. This provides a very significant pathway for pathogen recontamination in the environment.