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We hypothesize that key biotic and abiotic processes link primary environmental reservoirs of E. coli O157 with fields of Leafy Vegatables (LV) located in the largest U.S. producing region for LVs. The specific hypotheses and research objectives of the proposed research are that vertebrate populations (e.g., cattle and wild pigs) located in the interior of the Central California Coast (CCAC) function as the key source of EcO157 contamination of LV either through direct fecal deposition in LV fields or indirectly via fecal contamination of adjoining watersheds draining into and alongside the fields used for LV production. <P>We propose to quantify environmental loading by vertebrate sources (especially cattle and wild pigs) that may function as key sources of EcO157 contamination of LV directly through fecal deposits, or indirectly, via fecal contamination of watersheds, wells or soil in contact with LV row crop fields; to create a molecular subtyping database of EcO157 strains in the CCAC to characterize the genetic relatedness of environmental and outbreak-associated isolates; to determine if increased commensal E. coli concentration and presence of shigatoxin-producing E. coli strains are associated with fecal contamination and an increased risk of EcO157 contamination in LV production areas; and to develop and disseminate educational materials for growers of fresh produce regarding specific strategies to prevent pre-harvest microbial contamination; to educate the livestock community about microbial water quality, potential impacts on down-stream stakeholders, wildlife management strategies, 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 LV 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 LV. 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 LV. Hierarchical regression models will identify significant covariates for our outcomes of interest. We will also screen for non-O157 enterohemorrhagic E. coli, to determine if relevant serovars are also present in these environments.