10/16/2006

E. coli in Vegetables: Ohio State Study Looks at ‘Under the Surface’ Contamination, Role of Plant Diseases

Writer:

Mauricio Espinoza
espinoza.15@osu.edu
(330) 202-3550

Martha Filipic
filipic.3@cfaes.osu.edu
(614) 292-9833

Source:

Jeff LeJeune, Food Science and Technology
lejeune.3@osu.edu
(330) 263-3739

Ken Lee, Food Science and Technology
lee.133@osu.edu
(614) 292-7797

Sally Miller, Plant Pathology
miller.769@osu.edu
(330) 263-3678

WOOSTER, Ohio -- Escherichia coli has reared its ugly and deadly bacterial head once more, sickening almost 200 people and killing three during the past month in a national outbreak linked to a source that was rare only a few years ago: fresh vegetables.

But as cases of foodborne-pathogen contamination in produce are on the increase -- just last week, lettuce grown in California was recalled after growers found E. coli in irrigation water -- scientists’ understanding of how such contamination takes place and the factors that contribute to it is lagging behind, said Jeff LeJeune, a microbiologist with Ohio State University’s Ohio Agricultural Research and Development Center (OARDC).

He’s hoping he can change that.

LeJeune leads a new research project aimed at determining the processes that impact growth and survival of E. coli O157 (the strain involved in the spinach outbreak) on and in vegetables -- practical knowledge that could lead to new ways to reduce bacterial contamination in the veggies people are so encouraged to eat every day.

Working with LeJeune are OARDC colleagues Ken Lee, a food safety expert with the Department of Food Science and Technology, and Sally Miller, a vegetable pathologist with the Department of Plant Pathology and OSU Extension state specialist. Other key players in the study are Dan Aruscavage, a graduate student in the Department of Food Science and Technology working with both LeJeune and Lee, and Melanie Lewis Ivey, an OARDC research associate and graduate student working with Miller and LeJeune.

Other collaborators in the project -- funded at more than $600,000 by the U.S. Department of Agriculture’s Cooperative State Research, Education, and Extension Service (USDA-CSREES) -- are the College of Wooster and Michigan State University.

“By far the most foodborne illnesses occur from improper handling of foods at home,” said LeJeune, who is also a veterinary specialist with OSU Extension. “But contamination can happen anywhere in the farm-to-fork continuum. In the recent E. coli outbreak involving spinach, given the widespread distribution of illnesses, contamination most likely occurred at the time of processing or in the field. If it happened in the field, there are a number of potential sources of contamination, including soil, fertilizer, water or wildlife. This pre-harvest vegetable contamination is what we are closely examining in our study.”

Regardless of the source of contamination in the field, LeJeune explained, E. coli finds a way to survive and reproduce on the surface of vegetables -- and even worse, inside the plant tissue, where it cannot be washed off or killed by disinfectants. LeJeune and colleagues propose that the interaction between E. coli O157 and plant pathogens results in increased E. coli uptake, proliferation, exchange of antibiotic resistance genes, and protection from post-harvest disinfection.

In other words, if vegetables are under siege by plant diseases and become tainted with E. coli, the nasty foodborne bacteria will have a better chance of surviving and multiplying in our next fresh salad -- and it will be harder, if not impossible, to get rid of it.

“Did you ever notice that lettuce turns brown at its cut edges first?” LeJeune asked. “That’s because pathogens and other food-spoilage bacteria tend to survive and grow better at sites of plant-tissue injury. Pathogens such as E. coli can be recovered from fruits and vegetables that researchers have tried to disinfect using extreme laboratory procedures, far beyond anything that would be used in a home of commercial setting. This suggests that the pathogens are deeper in the plant tissues where they are protected from disinfecting solutions.”

Lee, who is also director of Ohio State’s Center for Food Safety, said graduate student Aruscavage already has preliminary findings indicating that this is true.

"In nature, very few plants are perfect," Lee said. "They are injured -- from wind, rain, animals, humans. And when a plant is injured, it reacts by exuding sugars. And guess what loves sugars? E. coli. So far, Dan's work has shown that traditional methods of washing produce are not going to remove E. coli that has found a home in that wound."

That's how foodborne bacteria actually benefit from plant pathogens that may be present in vegetables, said Miller, an internationally recognized vegetable-disease expert.

“Plant pathogens come in contact with vegetables and break down tissue, which releases sugars and other compounds that can be used by human pathogens as a food source,” Miller said. “Human pathogens can also get into the tissue thanks to the injuries caused by plant pathogens, where they find a safe environment to survive.”

Research about the interaction between human pathogens and plants is limited, so this study is expected to fill critical gaps in understanding how foodborne illnesses work and what can be done to prevent outbreaks and save lives in the future. Specifically, LeJeune and colleagues are trying to prove the following hypotheses:

• In the presence of plant-pathogenic bacteria that secrete plant tissue-degrading enzymes, E. coli O157 will proliferate to greater numbers and is more likely to be systematically disseminated to edible plant tissues than in the absence of plant pathogens.
• Pre-harvest contamination of edible plant surfaces with bacterial foodborne pathogens reduces the chance of success of post-harvest disinfection procedures, as E. coli O157 becomes protected in the plant tissue.
• Plant pathogens serve as a reservoir of antibiotic resistance genes present in human pathogens, and these genes are transferred from plant pathogens to E. coli O157 in plant lesions.
• Seeds produced by plants contaminated with foodborne pathogens will be contaminated internally with bacterial pathogens and produce contaminated products.

In addition to these research objectives, the project includes an outreach component aimed at delivering educational programs that vegetable growers need to improve food safety. Miller said the team will conduct a survey to find out exactly what farmers know about foodborne-pathogen contamination and come up with targeted programs in areas where the knowledge gap is wider.

"This is a step in the right direction, but we don't have a quick and easy solution," Lee said. "We do know this: The safety procedures that are now in place are not enough." It's a credit to the produce industry's management practices that the nation doesn't see more cases of E. coli poisoning, he said, "but it's not a fail-safe system. ... Maybe with this research, we will be able to build one more hurdle against E. coli in the field."

OARDC and OSU Extension are part of Ohio State’s College of Food, Agricultural, and Environmental Sciences.

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