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The occurrence of Escherichia coli in agricultural watersheds, either in waterways or in food products, is detrimental to the sustainability and security of U.S. agriculture and food systems. To reduce the abundance of E. coli, it is imperative to determine its fate in the natural environments. The goal of this project is to understand how changes in soil parameters related to agricultural practices affect the survival of E. coli cells in soil.
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Two specific research objectives are proposed. The first objective studies the survival spectrum of soil E. coli strains by examining their spatial concentration dynamics and genetic diversities in the Opaeula watershed. It is hypothesized that different E. coli strains colonize different soil environments according to their particular stress resistances. The second objective investigates how soil parameters, particularly those that are subject to alteration by agricultural operations, affect the survival of natural E. coli strains. The working hypothesis is that soil E. coli cells, by attaching to the soil particle surfaces, exhibit enhanced resistance to environmental stresses, and the agriculture-related soil condition changes exacerbate this phenomenon. The expected outcomes of the project can improve the best management practices of agricultural operations for the reduction of E. coli in agricultural watersheds.
NON-TECHNICAL SUMMARY: The over presence of Escherichia coli in agricultural watersheds contributes water quality deterioration, contaminates fresh produces, and introduces uncertainty and anxiety in food security. Due to the enormous genetic diversity of E. coli, to understand its survival in agricultural watersheds requires investigations beyond typical laboratory type strains. In this study, we aim to examine the abundance and genetic diversity of soil E. coli in a tropical Hawaiian watershed that features different microclimates and distinct land uses within short geographic distances. In addition, we will examine the soil E. coli isolates for their survival on soil particles under environmental stresses typical of agricultural lands. Knowledge obtained from this study will help us more accurately assess the impacts of different agricultural practices on water biological quality and food safety, and may also enable us to devise prevention and remediation strategies against E. coli contaminations.
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APPROACH: The research approach combines field sampling and laboratory experimentation. First of all, the abundance and genetic diversity of soil E. coli in the Opaeula watershed will be determined (Objective 1). Soil samples along the Opaeula stream will be collected at multiple sampling transects within reaches of rainforests, urbanized regions, and agricultural fields. The abundance of E. coli throughout the watershed will be analyzed with respect to water and soil characteristics (such as pH, salinity/water potential, TOC, etc.), to the different microclimates, and to the ecological-economic zoning of the watershed. Representative E. coli isolates will also be obtained from each sampling site; genetic compositions of the isolates will be determined by using genomic DNA fingerprinting and by assaying for survival-related genes (such as rpoS and fimH). The establishment of correlations between the genetic compositions of environmental isolates and the environmental parameters will be attempted using statistic methods such as cluster analysis. The second objective aims to understand the survival of soil E. coli strains on particle surfaces under prevailing environmental conditions in agricultural soils. The surface-associated E. coli cells, including lab type strains and the soil isolates, will be developed by growing two-phase culture containing quartz sand and LB broth. Survival of the surface-associated E. coli cells will be determined under common agricultural soil conditions, including soil acidification, organic accumulation, reduced desiccation, and improved nitrogen/phosphorous availability.