Fruit & Vegetable Magazine

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Leaf age may contribute to lettuce contamination


April 28, 2008
By Marg Land


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Apr. 28, 2008 – A new study shows
that harmful pathogens are more commonly found on younger inner leaves
than on older outer leaves of romaine lettuce.

Apr. 28, 2008 – A new study shows that harmful pathogens are more commonly found on younger inner leaves than on older outer leaves of romaine lettuce.

Researchers from the Produce Safety and Microbiology Research Unit , Albany, California and the University of California, Berkley reported their findings in the April 2008 issue of the journal Applied and Environmental Microbiology .

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Lettuce (Lactuca sativa) is the fresh produce item most commonly implicated in epidemics of food-borne illness, while Escherichia coli O157:H7 and Salmonella enterica are the most frequently attributed bacterial agents. Although previous studies have focused on E. coli O157:H7 colonization on cut or shredded lettuce leaves, little is known of its ability to colonize whole lettuce leaves in both pre- and post-harvest environments.

In the study, researchers investigated the growth of E. coli O157:H7 and S. enterica on romaine lettuce leaves both pre- and post-harvest. The increased population size of E. coli O157:H7 on young lettuce plants ranged from 16- to 100-fold in the presence of warm temperatures and free water on the leaves.  The increase in population size also varied significantly with leaf age, however the colonization was consistently 10-fold higher on the young (inner) leaves than on the middle leaves. Growth rates of S. enterica were found to be similarly leaf age dependent. Both bacterial pathogens also displayed higher population rates on younger leaves than on middle leaves harvested from mature lettuce heads.

The results indicate that leaf age and nitrogen content contribute to shaping the bacterial communities of preharvest and postharvest lettuce and that young lettuce leaves may be associated with a greater risk of contamination with E. coli O157:H7.

M.T. Brandl, R. Amundson.  2008.  Leaf age as a risk factor in contamination of lettuce with Escherichia coli O157:H7 and Salmonella enterica. Applied and Environmental Microbiology, 74. 8: 2298-2306.