Assessing the risk of in-field microbial contamination of leafy greens using inoculated soil

Scope and method of study. The objective of the study was to investigate the survival of non-pathogenic Escherichia coli in soils using different inoculation methods and to determine whether the microorganism would be transmitted to edible parts of the plants when planted in contaminated soils. Different field studies were conducted starting fall 2007 through early spring 2009 at Oklahoma State University Vegetable Research Station in Bixby, Oklahoma. Treatments included soils applied with previously inoculated spent mushroom compost, wood shavings, and direct inoculation using water-bacteria mixture sprayed to soils with tilling and without tilling. Two different variety of leafy greens were planted in treated soils include Top White Globe turnip greens and spinach variety Padre. Soil samples were collected in weekly and monthly basis for all experiments and analyzed for E. coli populations using direct plating method. Ground leaves of turnip greens and spinach were also analyzed for presence of E. coli with and without surface sterilization.Findings and conclusions. Results from different experiment suggested that media used for inoculation, season variability and inoculation methods to land influence survival rate of E. coli in soils. E. coli survived better in winter season than in spring due to temperature differences and moisture in soils. E. coli was able to survive in soils for at least 161 days in soils amended with inoculated wood shavings, whereas spraying of water bacteria mixture to soils had the lowest survival of E. coli. Flooding over green turnip crops caused transmission of E. coli from contaminated soils to leaves although the site of attachment was not determined. E. coli was not recovered from any spinach leaf samples. However, the maximum amount of sample assayed was very small. Therefore, future studies should include assay of larger amount of spinach sample.