July 3, 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,
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
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
Source : USDA/ Agricultural Research Service