Survival of E. coli on soil amendments and irrigation water in leafy green field environments.

Fresh market leafy green vegetables periodically are subject to contamination from foodborne human pathogens such as E. coli. Field aspects of such contamination are not well understood, and there is a lack of information on where and how E. coli comes in contact with leafy greens in the field, how E. coli survives there, and how production factors influence this pathogen. It is notable that few in-field projects have been conducted to address such issues as they pertain to commercial environments for leafy greens in California. Our plan is to continue to develop field-generated information on the survival of E. coli under actual production environments for coastal California leafy greens. We will validate our initial findings regarding survival of generic and non-toxigenic O157:H7 strains of E. coli when introduced to irrigation water, soil, and plants of a spinach field. With this simulation of a contamination event in spinach, we document survival of E. coli under field conditions; such information will be useful in further improving metrics and regulatory measures. We will evaluate survival of generic and non-toxigenic O157:H7 strains when introduced as contaminants in fertilizer inputs (compost, amendments, teas or other extracts) that are subject to soil cultivation and other practices.

Fresh market leafy green vegetables are periodically contaminated with foodborne human pathogens such as E. coli. However, there is little documented information about the ecology and survival of E. coli in production settings. 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, though, that few in-field research projects have been used to address such issues. In our proposal, we will further validate and confirm how both generic and attenuated, non-toxigenic O157:H7 strains of E. coli survive when introduced to soil, water, and plants in a spinach production field. In a replicated field trial we will inoculate soil with both E. coli strains, grow a spinach crop in this field, and then monitor survival by sampling and testing soil, water runoff, and spinach plants. In another experiment, we will inoculate mature spinach plants, incorporate such plants into the field by disking, and test soil for E. coli survival. This information should contribute to our understanding of how E. coli actually functions in a commercial production setting. We will examine how generic and non-toxigenic O157:H7 strains of E. coli survive when introduced into field settings via production inputs. We will inoculate fertilizer and amendment materials such as compost, organic fertilizers, and compost teas. We will incorporate materials into the soil and then test soil for survival of the inoculated strains following soil cultivation. We will also grow a lettuce crop in these plots and evaluate plants for any indications of contamination. A strength of our research team is our familiarity with and expertise in field production aspects of leafy vegetable in coastal California. Our roles as field researchers and educators provide 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.

Objective 1: To further document survival of E. coli strains when introduced into a spinach production system 

Objective 2: To monitor survival of E. coli strains when introduced into a lettuce field in the form of composts, organic amendments, and extracts such as teas

We investigated how generic and attenuated, non-toxigenic O157:H7 strains of E. coli survive when introduced to soil, water, and spinach plants in a commercial production setting. Our overall results are consistent with previous experiments conducted under commercial Salinas Valley agricultural environments. Both generic E. coli and attenuated E. coli O157:H7, when applied to soil, survived for relatively short periods of time. In addition, both inoculum types failed to move significantly into irrigation water runoff or move in the soil. Bacterial inoculum was not recovered from spinach plants that were grown in inoculated plots. However, when mature spinach plants were inoculated with either E. coli strain and disked back into the soil, both types of bacteria were recovered from soil and crop residues for an extended period of time (over 100 days). This was an unexpected out come and additional studies would be appropriate to examine factors that could enhance decline of such inoculum. also examined how generic and non-toxigenic O157:H7 strains of E. coli survive when introduced into field settings via production inputs. We inoculated compost, a liquid supplement, and a pelleted supplement as standard amendment materials, then introduced the contaminated materials into the soil and tested soil for survival of the inoculated strains. Plants from the romaine crops grown in these plots were evaluated for any indications of contamination. Contaminated compost did not result in persistent survival in soil and did not result in contaminated romaine. For liquid and solid supplements, which were inoculated with generic E. coli and non-toxigenic Salmonella, the lettuce had low levels of recoverable bacteria until day 35, at which time recovery was negative. We also collected additional evidence that when E. coli is introduced to the roots of spinach plants, the plants do not absorb these bacteria and that “internalization” does not occur under field conditions. Spinach and lettuce are both high-value leafy vegetable crops that are extensively grown in California. The coastal spinach and lettuce producing area is the most important and productive region for these commodities. Because spinach and lettuce have been subject to E. coli contamination, it is critical to develop practical information on how E. coli may behave in these cropping systems under coastal California conditions. Field-generated research information developed in commercial coastal California conditions contributes significantly to our understanding of E. coli ecology and assists the industry in further understanding the dynamics of this foodborne pathogen. Field studies conducted under coastal California conditions are not widely available, and this research reduces such information gaps.