Light helps bacteria get into lettuce (and presumably spinach, and other leafy produce)

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If scientists want to cut down on nasty contamination of produce, it helps to understand how bacteria end up inside the leaves of the plants, too deep to be washed off the surface. That’s what happened in the E. coli O157:H7 outbreaks in 2006 traced to bags of spinach, to name just one example, and salmonella does the same trick too.

In an interesting experiment just published in the journal Applied and Environmental Microbiology, researchers reported that exposure to light makes this internal invasion more likely.

The Israeli microbiologists bought fresh iceberg lettuce from grocery stores or fields and cut the inner leaves into little pieces. These they placed in little tubes to which they added salmonella. After incubation, they looked at the leaves under a microscope to see where the bacteria had ended up.

The findings: Salmonella penetrates the lettuce leaf’s deeper surfaces by entering little pores called stomata. These are the pores plants use to obtain and release gases during photosynthesis--the vital process by which light energy is captured and turned into sugars. There are rather nasty pictures in the article of rod-like salmonella clustered all around these stomata, seemingly going down into them and thus into the inner parts of the leaf.

Stomata tend to be open when it’s light and closed when dark. The scientists repeated the experiment with lettuce disks incubated in the dark--and this time, didn’t see the salmonella gaining entry through stomata.

The scientists even created movies in which the bacteria could be seen ‘moving toward and vanishing within the substomatal cavity when the experiments were performed in the light but not in the dark.’ It could be the bugs are attracted by sugar being generated in the leaf.

The scientists say their finding ‘has important implications for both pre- and post-harvest handling of lettuce and probably other leafy vegetables.’ Maybe, for example, a way could be found to inhibit the movement of the bacteria.

-- Rosie Mestel