ARS researchers have found that younger romaine
leaves may be nutritionally richer for E. coli bacteria, a finding that
could lead to new ways to protect romaine and other leafy greens from harmful
organisms.
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ARS research assistant Danielle Goudeau inoculates
a romaine lettuce leaf with E. coli O157:H7 to study the pathogen's
biology on salad greens.
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Protecting Romaine Lettuce From Pathogens
By Marcia Wood
June 27, 2008 Knowing the preferences of foodborne
pathogens such as Escherichia coli O157:H7 is essential to a successful
counterattack on these microbes. That's why Agricultural Research Service (ARS) microbiologist
Maria
T. Brandl and University of
California-Berkeley colleague Ronald G. Amundson are scrutinizing the
little-understood ability of E. coli O157:H7 and Salmonella enterica
to contaminate romaine lettuce.
Brandl is with the ARS
Produce
Safety and Microbiology Research Unit, part of the agency's
Western
Regional Research Center in Albany, Calif.
In experiments, the scientists exposed romaine lettuce leaves to E. coli
O157:H7 and found that, after 24 hours, populations of the microbe were 10
times higher on young leaves than on middle ones.
One explanation: The young leaves are a richer nutritional "hunting
ground" for E. coli. They exude about three times more nitrogen and
about 1.5 times more carbon than do the middle leaves, Brandl and Amundson
reported.
Scientists have known for decades that plants exude compounds--from leaves
and roots--that bacteria and fungi can use as food. But the romaine lettuce
study, published earlier this year in Applied
and Environmental Microbiology, is the first to document the different
exudate levels in romaine lettuce leaves of the two age classes. It's also the
first to show that E. coli can do more than simply bind to the leaves;
it also can multiply.
Adding nitrogen to the middle leaves boosted E. coli growth and
further pointed to a key role of nitrogen in helping this pathogen. For that
reason, a strategy that decreases nitrogen fertilizer use in romaine lettuce
fields may be worth investigating, Brandl noted.
According to
James
A. Lindsay, ARS national program leader for food safety research,
commodity-specific food safety guidelines for producing and harvesting leafy
greens such as lettuce have been developed. That was done through industry,
government and academic collaboration, in an effort to support Good
Agricultural Practices, or GAPs.
ARS is a scientific research agency of the U.S. Department of Agriculture.