Examination of the survival and internalization of E.coli on spinach under field production environments.

Examination of the survival and internalization of E.coli on spinach under field production environments.

Jan. 1, 2009 - Dec. 31, 2009

2008-6

$142,790.00

Steven Koike
University of California Cooperative Extension, Monterey County

Michael Cahn, Ph.D., Trevor Suslow, Ph.D.

Final Report

For many years, fresh market leafy green vegetables periodically have become contaminated with foodborne human pathogens. Despite this on-going problem, little is known about where such pathogens come from, how they reach leafy vegetables, how they persist in farming environments, the role of post-harvest contamination, and other aspects. While many laboratory, growth chamber, and greenhouse studies provide some information on how E. coli and other pathogens might interact with leafy vegetables such as lettuce and spinach, it is notable that few in-field research projects have been used to address such issues. Our proposal has the advantage of being a practical field study conducted in the Salinas Valley in a commercially farmed crop of spinach. By inoculating replicated spinach plots with either a generic or a non-toxigenic O157:H7 strain of E. coli, we will monitor the survival of this organism under coastal California, field production conditions. With this simulation of a contamination event in spinach, we should gain a better understanding of the ecology and dynamics of E. coli in an agricultural setting. A second aspect of our proposal will examine the phenomenon of 'internalization,' which holds that a plant can absorb a human pathogen (via root uptake, for example) and transport the pathogen to leaves that will be later consumed. Researchers that feel internalization takes place have mostly dealt with leafy vegetables grown in pots under artificial conditions. In our proposal we will test commercially grown spinach to see if internalization can take place in a field setting.

Technical Abstract

Fresh market leafy green vegetables periodically have become contaminated with foodborne human pathogens for many years; hence this is not a new problem or concern. Despite this on-going food safety issue, there is little documented information about where such pathogens come from, how they reach leafy vegetables, how they persist in farming environments, the role of post-harvest contamination, the role of wild animals, and other aspects of the epidemiology of this system. A number of laboratory, growth chamber, and greenhouse studies provide some information on how E. coli and other pathogens might interact with leafy vegetables such as lettuce and spinach. However, it is notable that few in-field research projects have been used to address such issues and advance our knowledge of foodborne pathogens in plant agriculture settings. In our proposal, we will simulate a field contamination event and introduce both generic and attenuated, non-toxigenic O157:H7 strains of E. coli to a commercially planted and grown spinach crop in coastal California. The experiment will be replicated and randomized. We will monitor the survival of these strains under field production conditions and will also be able to compare persistence between generic and attenuated strains as surrogate isolates for future field research. Part 1 of this study involves inoculating bed top soil (inoculum level: 107 CFU/ml) after seeding but prior to plant emergence. In addition, we will inoculate mature spinach plants prior to crop disking and soil incorporation. With a different set of plants for part 2, we will inoculate plants with various contamination levels [126 MPN/100ml (log 2.1); 235 MPN/100ml (log 2.37); 576 MPN/100ml (log 2.76); 5760 MPN/100ml (log 3.76); and 57600 MPN/100ml (log 4.76)]. We will monitor soil, plant, and water runoff for E. coli presence. With these simulations of contamination events in spinach, we should gain a better understanding of the ecology and dynamics of E. coli in an agricultural setting. A second aspect of our proposal will examine the possible internalization of E. coli from spinach root uptake and to spinach leaves. While some internalization studies demonstrate possible pathogen uptake by plants grown in pots and under greenhouse or growth chamber environments, we will conduct internalization studies in a commercial field in coastal California. By using buried irrigation drip tape, we will deliver E. coli strains (both generic or attenuated,non-toxigenic O157:H7 strains) to the roots of spinach plants and will in this way be able to prevent surface contamination to foliage. We will be able, therefore, to test for plant uptake and internalization under field production conditions. Our project is further strengthened because our research team is well acquainted with field production aspects of leafy vegetables, including spinach. Our role as field researchers and educators provides us with strong industry support, and our project will therefore have a prominent extension component as we work directly with growers and other industry personnel.