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We hypothesize that vertebrate populations (especially cattle and wild birds) function as a key source of E. coli O157:H7 (EcO157) contamination of watersheds where lettuce and other leafy vegetables are grown; that climate, landscape attributes, and irrigation practices correlate with increased risks of EcO157 and commensal E. coli contamination; and in-field contamination of lettuce plants with EcO157 relates to combinations of production practices and environmental risk factors in the Salinas Valley. <P>The major objectives of this proposal are to (1) quantify environmental loading by vertebrate sources, (2) characterize predisposing conditions for hydrological transport of EcO157 and E. coli to lettuce fields, (3) determine if concentrations of non-O157 E. coli predict an increased risk of contamination with EcO157 in water, (4) identify the in-field mechanism(s) of contamination of lettuce, (5) create a molecular subtyping database of EcO157 strains to characterize the genetic relatedness of environmental and outbreak-associated isolates, and (6) develop and disseminate educational materials for growers of fresh produce and the livestock community about microbial water quality, potential impacts on down-stream stakeholders, and effective BMPs for improving water quality.
Approach: Our overall study design combines epidemiological and microbiological methods followed by outreach and education activities to disseminate prevention and control information. We will conduct an in-depth longitudinal study that identifies the key biotic and abiotic processes that sufficiently load, then hydrologically link and disseminate, primary environmental reservoirs of EcO157 within and between lettuce fields, resulting in bacterial contamination of this raw agricultural commodity. For each node (vertebrate sources, water, soil, lettuce) of the system, we will collect a detailed set of covariates that will be used to identify critical control points, points of environmental amplification, and management practices that either elevate or decrease the risk of in-field contamination and dissemination of EcO157 on lettuce. Strains will be analyzed by Multi-Locus Variable number tandem repeat Analysis (MLVA) and Pulsed Field Gel Electrophorsis (PFGE) to characterize isolates by genetic differences, determine molecular epidemiologic linkages among samples isolated or obtained from public health collaborators, and thus, pinpoint the mechanisms that link and disseminate vertebrate sources of EcO157 within and between fields of lettuce. Hierarchical regression models will identify significant covariates for our outcomes of interest.