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TEL-AVIV – A study conducted by Israeli researchers suggests that exposure to light, and possibly photosynthesis, may help disease-causing bacteria to invade fresh produce, making them impervious to washing.
According to background information in a report published in journal Applied and Environmental Microbiology, past studies have already shown that salmonella enterica attaches to the surface of fresh produce, and finds its way below the surface of the skin through pores called stomata, where it can hide from and resist washing and food sanitizers.
In the new study, researchers from the Agricultural Research Organization at the Volcani Center in Israel and Tel-Aviv University examined the role that light and photosynthesis might play on the ability of salmonella bacteria to infiltrate lettuce leaves via stomata.
They exposed sterile iceberg lettuce leaves to bacteria either in the light, in the dark, or in the dark after 30 minutes of exposure to light.
Incubation in the light or pre-exposure to light resulted in aggregation of bacteria around open stomata and invasion into the inner leaf tissue.
Incubation in the dark, on the other hand, resulted in a scattered attachment pattern and very little internalization.
According to the researchers, the increased propensity for internalization in the light may be due to several factors.
First, they say, in the absence of light plants enter a period of dormancy, where stomata are closed and no photosynthesis takes place. In the light, the stomata are open.
Additional findings also suggest that the bacteria are attracted to the open stomata by the nutrients produced during photosynthesis, which are not present in the dark.
“The elucidation of the mechanism by which Salmonella invades intact leaves has important implications for both pre- and postharvest handling of lettuce and probably other leafy vegetables. The capacity to inhibit internalization should limit bacterial colonization to the phylloplane and consequently might enhance the effectiveness of surface sanitizers,” say the researchers.