Lettuce Salmonella: How the Bug Invades Leaves
One of the confounding mysteries about lettuce Salmonella outbreaks and E. coli spinach outbreaks is how the microbes harbor inside the plants, too deep to be washed off the surface by consumers who are diligent about protecting themselves against potential food poisoning.
Now scientists in Israel have evidence of how the dangerous contamination occurs. In an experiment published this month in the journal Applied and Environmental Microbiology, researchers from the Microbial Food Safety Research Unit at the Volcani Center in Beth Dagan, Israel, witnessed rod-shaped Salmonella bacteria clustering at stomata openings on the leaves of iceberg lettuce. In fact, the journal’s cover photo this month shows exactly that.
The stomata are tiny pores plants use to obtain and release gases during photosynthesis, where light energy is captured and turned into sugars. The experiment found that exposure to light makes the internal invasion more likely. The Los Angeles Times reported that the scientists even created movies in which the pathogens are seen “moving toward and vanishing within the substomatal cavity when the experiments were performed in light, but not in dark.”
The microbiologists say their finding “has important implications for both pre- and post-harvest handling of lettuce and probably other leafy vegetables.”
Any advance in this area is welcomed by food safety advocates around the globe, especially since outbreaks of E. coli O157:H7 and Salmonella involving leafy greens seem to be chronic occurrences, at least in the U.S. This summer, for instance, public health officials confirmed an outbreak of Salmonella Typhimurium that included at least 124 confirmed cases around the country, but mostly in the West.
An Oregon health official told The Oregonian newspaper that the outbreak peaked in Oregon in August but is now over. No recalls were associated with the outbreak because investigators couldn’t identifiy the source of the outbreak with enough certainty, but many victims reported eating shredded lettuce and fast food. At least two victims were seriously affected, requiring hospitalization.
Lettuce Salmonella Outbreak Under Investigation
State and federal health investigators are trying to determine the source of a possible lettuce Salmonella outbreak that sickened at least 124 people across the country before the outbreak subsided late last month.
William Keene, senior epidemiologist at Public Health Division in Oregon, told reporter Lynne Terry of The Oregonian newspaper that no recalls were associated with the outbreak. Health investigators’ interviews of victims suggested that shredded lettuce was a leading suspect in causing the outbreak, but by now the product is likely consumed. In addition, many victims said they had eaten at fast food restaurants, but Keene said officials are not warning people to avoid lettuce or fast food restaurants.
According to the newspaper, the first cases began in mid-July and the outbreak peaked and subsided by the end of August. Illnesses matching the outbreak strain of Salmonella Typhimurium were found across the country, but there was a clustering of cases in the West. In Oregon, two people were hospitalized, including one who was severely injured. No deaths were reported.
Outbreaks of Salmonella and E. coli O157:H7 caused by contaminated lettuce, spinach, parsley and other leafy green vegetables continue to be a problem in the United States despite hygiene and clean-water standards set by the U.S. Food and Drug Administration and the industry itself. Both pathogens are carried in animal feces, which can travel into crop fields via irrigation or flooding. Manure itself, if it comes in contact with plants, can contaminate the crops and the problem can be exacerbated if contaminated lettuce or spinach is bagged in ready-to-eat packages.
That was the case with the Dole baby spinach E. coli outbreak in 2006.
Salmonella prevention strategies for consumers include washing fresh produce and being careful not to cross-contaminate kitchen surfaces with unwashed fruits and vegetables. But as the Centers for Disease Control and Prevention (CDC) noted in a study of the baby spinach outbreak, washing leafy greens sometimes does not affect odds of becoming infected.
Possible reasons for this include that pathogens could get internalized into the plant structure by entering the roots. The CDC concluded that another possibility is that bacteria are more likely to adhere to cut surfaces of leafy greens (e.g. shredded lettuce?).
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