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Salad lovers, take note: Lettuce is more vulnerable to E. coli contamination than other leafy greens, researchers report. The physical composition of green leaf and romaine lettuce makes it a happy home for E. coli bacteria, particularly at room temperature, according to a report published recently in the journal Food Microbiology.
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Other leafy greens -- spinach, kale and collards -- are more resistant to E. coli, a bacteria with toxic strains that can cause severe stomach cramps, bloody diarrhea and vomiting.
"At room temperature or higher, E. coli grows very fast on lettuce," said lead researcher Mengyi Dong, a postdoctoral research associate in food safety at Duke University in Durham, N.C. "But if lettuce is refrigerated at 39 degrees F, we see a sharp decline in the E. coli population."
The new study was prompted by reports of food-borne illness caused by lettuce, said Dong, who conducted the research as a doctoral student at the University of Illinois, Urbana-Champaign.
"We are seeing a lot of outbreaks on lettuce, but not so much on kale and other brassica vegetables," Dong noted in an UI news release. "We wanted to learn more about the susceptibility of different leafy greens."
For the study, Dong and her colleagues infected whole leaves from five different leafy greens -- romaine lettuce, green leaf lettuce, spinach, kale, and collards -- with E. coli. They then observed what happened after storage at 39, 68 and 98.6 degrees F.
They found that a green's susceptibility to E. coli was determined in part by leaf surface properties like roughness and natural wax coating.
On kale and collards, E. coli grows slower in warmer temperatures, but can survive longer under refrigeration, researchers found. Even so, kale and collards are overall less susceptible to E. coli contamination.
Further, kale and collards are usually cooked before eaten, while lettuce is consumed raw, Dong noted. Cooking kills or inactivates E. coli.
Researchers also infected cut leaves with E. coli, to compare the difference in bacterial growth between damaged and whole leaves.
"Whole leaves and freshly cut leaves present different situations. When the leaf is cut, it releases vegetable juice, which contains nutrients that stimulate bacterial growth," Dong said.
However, the researchers found that the juice from spinach, kale and collards actually had antimicrobial properties that protect against E. coli.
In fact, when they isolated kale and collards juice and applied the liquid to lettuce leaves they found the juice can be used as a natural antimicrobial agent.
People can protect themselves by rinsing lettuce, Dong said, although she warned that E. coli bacteria tend to attach themselves tightly to lettuce leaves.
Consumers also should store their lettuce in the fridge and pay attention to any food safety recalls involving leafy greens, researchers said.
"We can't completely avoid pathogens in food. Vegetables are grown in soil, not in a sterile environment, and they will be exposed to bacteria," said researcher Pratik Banerjee, an associate professor of food safety with the University of Illinois Urbana-Champaign.
"It's a complex problem to solve, but we can embrace best practices in the food industry and food supply chain," Banerjee said. "There's a lot of interest from the research community and federal agencies to address these issues, and the USDA imposes high standards for food production, so overall the U.S. food supply is quite safe."
More information
The Mayo Clinic has more about E. coli.
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