Sep 17, 2008 (CIDRAP News) – Nine research teams that shared $2 million from Fresh Express, a California produce company, recently presented their findings on Escherichia coli O157:H7, revealing some clues on the pathogen's behavior on leafy greens that could lead to safer produce.
Researchers presented their studies in Monterey, Calif., at a Sep 11 conference hosted by Fresh Express and attended by about 300 experts from government, academia, and the produce industry, according to a Sep 11 report from the Associated Press.
In January 2007, Fresh Express, which produces bagged salads and other produce items, announced it would provide $2 million to fund research on how to keep E coli O157:H7 out of produce. The move came in the aftermath of several high-profile E coli outbreaks that were linked to leafy greens (though none of the company's products have been implicated in a foodborne illness outbreak).
No passage through plants
Craig Hedberg, PhD, a foodborne disease expert and associate professor of environmental health sciences at the University of Minnesota School of Public Health in Minneapolis, said one of the research themes was exploring E coli's ability to infect the interior tissues of plants through roots and leaves. Hedberg was a member of the Fresh Express advisory panel for the initiative.
Based on the bulk of the findings, including those from the University of Georgia's Center for Food Safety, led by Michael Doyle, E coli O157:H7 does not appear to enter through roots or leaves and disseminate through the internal tissues of the plant, Hedberg said. "Several studies lay to rest that concern."
One of the more surprising findings was that the pathogen appears to have unique biochemical mechanisms for interacting with plants, Hedberg said. An investigation by University of Arizona researchers, led by Jorge Girón, revealed that E coli O157:H7 can open stomata (tiny pores) on spinach leaves using a secretion system that is similar to those it uses to colonize cattle and human hosts. Girón and his colleagues suggested that E coli's apparent ability to hide out in stomata may explain how the pathogen can evade produce cleaning processes.
Hedberg said, "It's fascinating biology, and now we understand more about how things happen in nature. The dynamic nature of the interaction between these enteric bacteria and plants has not previously been recognized."
Another notable finding was evidence suggesting that flies and other insects can spread E coli O157:H7 to leafy greens, he said. In May 2007 a research team from the University of Oklahoma, led by Jacqueline Fletcher, collected insects on farms in California's Salinas Valley. They found several "filth fly species" that tested positive for E coli O157:H7, along with evidence of fly fecal and regurgitation spots on leaf surfaces. The group reported that the findings suggest a possible contamination route between cattle pastures and vegetable growing areas.
However, when the investigators returned in 2008 to collect insects again from the same sites, they found that the fly populations were significantly lower, and they weren't able to culture E coli O157:H7 from the insects. They suggested that the presence of the pathogen may be transient in some settings.
The insect findings raise new questions and warrant further study, Hedberg said.
In some of the other research studies, investigators reported that:
- Leafy greens can be sanitized with ozone treatment during vacuum cooling (a group from Ohio State University led by Ahmed Yousef)
- Composting doesn't always inactivate E coli O157:H7 and other pathogens, and weather may play a role in survivability (researchers from Clemson University, headed by Xiuping Jiang)
- Naturally occurring microorganisms on fresh lettuce and spinach may have an antagonistic effect on E coli O157:H7 growth (investigators from the University of Georgia, led by Mark Harrison)
- Shredding, cleaning, and other processing methods provide multidirectional transfer points for E coli O157:H7 in iceberg lettuce and baby spinach, and a new predictive model can help guide risk assessment and safety efforts (a group from Michigan State University, headed by Elliot Ryser).
Expert panel set objectives
Fresh Express has said it funded the research and shared the results to benefit the produce industry and consumers. The company's scientific advisory panel, consisting of unpaid volunteers that have been meeting since May 2006 to identify research gaps and administer the initiative, is chaired by Michael T. Osterholm, PhD, MPH. He directs the University of Minnesota Center for Infectious Disease Research and Policy, which publishes the CIDRAP Web site.
Osterholm told CIDRAP News that the Fresh Express research funding sparked the accomplishment of a great deal of applied research in a short time. "What we did in 16 months would have taken traditional academic or government-funded researchers 3 or 4 years," he said.
The key to the initiative's success was having an expert review group establish research objectives ahead of time, Osterholm said. The review panel also helped determine who among the 65 grant applicants received funding.
Fresh Express and its science advisory panel hope to publish all nine of the studies in a single issue of a journal, he said. "And we're looking at what we can do to keep this research model moving."
Officials from Fresh Express said in a statement to CIDRAP News that they were thankful for the advisory panel's guidance and extremely pleased with the results of the research initiative. Company President Tanjos E. Viviani and Executive Vice-President Jim Lugg said they were happy with "the remarkable [conference] turn-out from growers and harvesters, other key manufacturers, regulators, customers and food safety experts." They added, "We are also very excited about the [research] results. We intend to carefully evaluate them and determine how or when we might be able to act on those that are most relevant for us."
A future for 'fast-track' research?
Jim Prevor, a produce industry expert who hosts a blog called Perishable Pundit, attended the Monterey conference and praised the "fast track" research initiative. However, he warned produce industry officials not to dismiss the benefits of traditional peer-reviewed academic research.
"Although this type of quick turnaround can provide important clues for further research and provide the trade and regulators with some notion of how research is progressing, we think demanding instantaneous revolutions in horticultural and processing practices is too much," Prevor wrote.
He said the Fresh Express initiative showed that a rapid, applied research model can work, but he questioned how similar projects in the future might be feasible. He wrote that the Center for Produce Safety, a University of California, Davis, research institute funded through produce industry support, might be a good site to continue studies based on the Fresh Express model. However, he added that fundraising, staffing, and balancing research priorities would pose big challenges.
"For today, however, the industry owes a big hat tip to Fresh Express. We know more and have a clearer path to food safety than we did last week. That is a formidable accomplishment," Prevor wrote.
Apr 13, 2007, CIDRAP News story "Produce firm names winners of E coli research grants"
Jan 19, 2007, CIDRAP News story "California produce firm to fund E coli research"
Perishable Pundit blog site