Light Helps Microbes Invade Leafy Greens

Results of a study conducted by researchers from the Volcani Center and the Tel Aviv University in Israel showed that exposure to light, and possibly photosynthesis itself, could be helping bacteria such as the gastroenteritis-causing Salmonella enterica, to be internalized in lettuce leaves, making them impervious to washing. The findings could prove useful in limiting bacteria's access to leafy greens.

The researchers observed that incubation in the light or preexposure to light of iceberg lettuce leaves exposed to bacteria resulted in aggregation of bacteria around open stomata and invasion into the inner leaf tissue. In contrast, incubation in the dark resulted in a scattered attachment pattern and very little internalization.

Bacteria penetrate the lettuce leaf's deeper surfaces by entering little pores called stomata, the scientists believe. These pores, which plants use to obtain and release gas during photosynthesis, are open in the light. The researchers also believe that the bacteria might be attracted to the stomata by compounds produced during photosynthesis that are not produced in the dark. Indeed, the scientists found that mutations affecting Salmonella motility or chemotaxis, the phenomenon in which bacteria direct  their movement according to a chemical in their environment, inhibited bacterial internalization.

The paper published by the journal Applied and Environmental Microbiology is available at http://dx.doi.org/10.1128/AEM.01084-09