Summary of Awards to Date

Investigation of E.coli survival on contaminated crop residue.

Date

Jan. 1, 2011 - Dec. 31, 2012

Award Number

2011-104

Amount Awarded

$118,000.00

Investigator

Steven Koike
University of California Cooperative Extension, Monterey County

Co-Investigator(s)

Michael Cahn, Ph.D.

Resources
Summary

Our research team has conducted several in-field studies in the Salinas Valley and found that if leafy green crops were exposed to E. coli either prior to emergence from soil or early in their growth cycle, the plants at harvest were rarely contaminated and E. coli was rarely recovered from soil or water. However, we found that if a mature spinach crop was contaminated with E. coli and then disked into the field, we could recover E. coli from soil for a much longer period of time (over 85 days). We propose to investigate the dynamics of E. coli survival on lettuce residue incorporated into soil. We will establish experimental plots in commercial field settings and inoculate plants prior to disking. As treatments, we will employ commercial ground preparation practices (multiple diskings, addition of irrigation water) and evaluate their effects on survival and decline of E. coli in soil. Such experiments should provide the grower with practical information on the duration of bacterial survival in soil and the role of post-harvest field practices in reducing bacterial carry over. Because the research will be conducted in the Salinas Valley, our results should reflect real world dynamics of the production environment in coastal California. 

Technical Abstract

Fresh market leafy green vegetables are periodically contaminated with foodborne human pathogens such as E. coli. As extension researchers stationed in the Salinas Valley, our approach has been to conduct field studies under commercial situations in order to study the ecology of E. coli and how this bacterium might survive in the field. Our in-field studies in the Salinas Valley indicate that if lettuce or spinach crops were exposed to E. coli either prior to emergence from soil or early in their growth cycle, the plants at harvest were rarely contaminated and E. coli was rarely recovered from soil or water. However, if a mature spinach crop was contaminated with E. coli and then disked into the field, we could recover E. coli from soil for over 85 days. (Note that for these plots, spinach was inoculated with E. coli, disked into the soil, and left undisturbed for the 85+ days.)

Because of the extended soil survival as observed in this previous field trial, we propose to investigate further the dynamics of E. coli survival on crop residue incorporated into soil. We will establish experimental plots in commercial field settings and inoculate lettuce with generic, rifampicin-resistant E. coli strains prior to disking and incorporating the crop residue. Treatments will consist of farm practices, such as disking the soil one or more times and irrigating plots, used in preparing the soil for leafy green production. We will evaluate the effects of these practices on survival and decline of E. coli in the soil by using direct plating and enrichment recovery methods. Such experiments should provide the grower with practical information on the duration of bacterial survival in soil and the role of post-harvest field practices in reducing bacterial carry over. Information generated from such experiments could guide the grower if faced with positive test results that would prevent harvest and use of the crop.

Because the research will be conducted in the Salinas Valley, our results should reflect real world dynamics of the production environment in coastal California. In addition, our research team is very familiar with field production aspects of leafy vegetables in coastal California. Our experience as field researchers and extension educators provides us with strong industry connections, and our project will therefore have a prominent extension component as we work directly with growers and other industry personnel.