Fruit & Vegetable Magazine

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Scientists keeping salad mixes safe to eat


June 15, 2011
By USDA-ARS

Topics

safesaladsJune 15, 2011 – Packaged
salad mixes offer convenience, selection, and quality, and perhaps best of all,
they free us from the chore of washing and chopping, slicing, or shredding
salad greens.

June 15, 2011 – Packaged
salad mixes offer convenience, selection, and quality, and perhaps best of all,
they free us from the chore of washing and chopping, slicing, or shredding
salad greens.

But outbreaks of
foodborne illness have, from time to time, been associated with bagged salad
greens. The outbreaks have led the fresh-cut produce industry to voluntarily
adopt stringent quality-control standards.

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safesalads 
ARS research
microbiologists Maria Brandl and Craig Parker have identified genes in Escherichia
coli O157:H7
that help this foodborne pathogen colonize fresh-cut lettuce.
 

U.S. Department of Agriculture
(USDA)
food safety researchers are pitching in to help keep salad mixes safe to
eat. Innovative studies led by Agricultural
Research Service (ARS)
microbiologist Maria T.
Brandl are providing new information about the impressive array of
genes that a major foodborne pathogen, Escherichia coli O157:H7, calls into
action when attempting to colonize leaves of fresh-cut lettuce.

Mechanical cutting of
lettuce leaves into large pieces or shredding of leaves into narrow strips,
like those in taco filling, breaks lettuce cells, explains Brandl. The broken
cells exude carbohydrates, which the microbe can use as a source of energy. But
injured cells can also leak natural compounds such as antimicrobials that are
problematic for the pathogen.

A study with romaine
lettuce that Brandl and her co-investigators published in Applied and Environmental Microbiology in 2010
showed that E. coli, when exposed in lab tests to the contents of broken
lettuce leaf cells, can adapt quickly. Using an approach known as
microarray-based whole genome transcriptional profiling, the researchers
determined that the pathogen uses its genetic arsenal to protect itself against
not only the antimicrobial compounds, but also against oxidative stress,
osmotic stress, damage to its DNA and other threats to its ability to survive
and multiply.

The investigation – the
first to provide extensive details about the biology of E. coli O157:H7 in
fresh-cut lettuce – has paved the way for follow-up experiments that Brandl and
coworkers hope will lead to new technologies to overcome the pathogen's defenses.

Brandl works at the ARS Western
Regional Research Center
in Albany, Calif. She conducted this
research in collaboration with research associate Jennifer L.
Kyle and microbiologist Craig T.
Parker at Albany, and with Danielle
Goudeau of the University of
Pittsburgh
in Pennsylvania