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.
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