What happens to lettuce leaves and other leafy greens that become contaminated with E. coli O157:H7 and then suffer insect or mechanical damage?

Can the bacteria then become internalized, in other words, get inside the greens and become impossible to wash off?

That’s what a dozen University of Georgia researchers tried to find out.  The results of their study have been published in the current issue of the Journal of Food Protection.

“Environmental pests may serve as reservoirs and vectors of zoonotic pathogens to leafy greens,” the study abstract explained, “however, it is unknown whether insect pests feeding on plant tissues could redistribute these pathogens on the surface of leaves to internal sites.”

After using various means to apply E. coli to the tops and undersides of lettuce and spinach leaves, through mist, droplets and by inoculation, the researchers from UG’s Department of Crop and Soil Sciences then exposed the greens to cabbage loopers, aphids, whiteflies, and thrips.  They later found that:

-No internalized E. coli O157:H7 was detected in individual lettuce leaves 48 hours after being inoculated with a lower amount of pathogens per leaf.

-In contrast, pathogen internalization was observed 48 hours after leaves were spray inoculated with a much higher number of E. coli O157:H7 bacteria.

The researchers concluded that while the toxic E. coli O157:H7 could get into leafy greens via insect damage,  it would take such a high concentration of the bacteria for this to happen that “contamination in the field … is unlikely.”

On the “rare occasion” that a field of greens might become contaminated with a high concentration of E. coli, the researchers said internalization of the pathogen “may be minimized” by the plants’ natural defenses to insects or physical injury, which can inhibit  bacterial growth.

The UG team, located at Tifton, GA, included Marilyn C. Erickson, Jean Liao, Alison S. Payton, David G. Riley, Cathy C. Webb, Lindsey E. Davey, Sophia Kimbrel, Li Ma, Guodong Zhang, Ian Flitcroft, Michael P. Doyle, and Larry S. Beuchat.

The complete results can be found in the Nov. 10, 2010 Journal of Food Protection.