Bagged lettuce may carry—and enhance—salmonella bacteria

November 28, 2016 Providence Health Team

If you, like many grocery shoppers, buy pre-cut lettuce or other leafy vegetables packaged in bags or plastic containers, you should eat it promptly.

If you don’t, researchers say, you are courting an increased risk of salmonella poisoning.

A study out of the University of Leicester in England has found that juices from cut or damaged leaves stimulate the presence of salmonella in packaged greens. Further, the spread of salmonella inside a bag can make the bacteria more potent, increasing the likelihood it will infect the person who eats it.

Researchers say they recognize that salad leaves are important to a healthy diet, but wanted to examine why they are often associated with food poisoning. They chose this particular study, they said, because little is known about what happens to salmonella bacteria after it is sealed in a plastic bag or container.

Salmonella is a leading cause of food-borne illness, hospitalizing 19,000 Americans a year and killing 380. Its symptoms, which occur 12 to 72 hours after infection, usually include abdominal cramps, fever and diarrhea.

Surprising findings

“We found that even microliters of the juices (less than 1/200th of a teaspoon) which leach from the cut-ends of the leaves enabled salmonella to grow in water, even when it was refrigerated,” said Primrose Freestone of the University of Leicester’s Department of Infection, Immunity and Inflammation.

“These juices also helped the salmonella to attach itself to the salad leaves so strongly that vigorous washing could not remove the bacteria and even enabled the pathogen to attach to the salad bag container,” she said. "This strongly emphasizes the need for salad leaf growers to maintain high food safety standards as even a few salmonella cells in a salad bag at the time of purchase could be become many thousands by the time a bag of salad leaves reaches its use by date, even if kept refrigerated.”

Further reading

If you become infected with salmonella and visit a health care provider, your treatment will depend on such factors as your age and overall health, and how sick you are. You can find a Providence provider here.

The Providence Health Library can tell you more about salmonella infections, including who is most at risk.

The U.S. Centers for Disease Control and Prevention offers resources related to salmonella, from diagnosis and treatment to an atlas of infections since 1962.

The Clemson University Cooperative Extension Service offers a series of tips on safe handling of lettuce and leafy green salads.

The study, “Salad leaf juices enhance Salmonella growth, fresh produce colonization and virulence,” was published in the journal Applied and Environmental Microbiology. The University of Leicester published a reader-friendly story about the study.

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