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Solutions for Small
Farms
In the United States, the
delicious, sprouted seeds of red clover, broccoli, wheat, radish,
soybean, mung bean, alfalfa, and other vegetables or grains "are
produced primarily by small operations," says Charkowski. Many are
family-run. These 'sprouters,' as they are nicknamed in the industry,
may not be able to afford expensive techniques to kill foodborne
pathogens.
"Also," notes
Charkowski, "many sprouters run organic operations." That
means they simply won't use irradiation or another option—bathing
sprouts in a chlorine-containing chemical called calcium hypochlorite.
This compound—the same white powder that's used to disinfect swimming
pools—is recommended by the Food and Drug Administration for
sanitizing sprouts.
The FDA currently advises
all consumers to cook sprouts before eating them. And it recommends that
the young, the elderly, and those with compromised immune systems should
not eat sprouts.
Salmonella
and E. coli can flourish in the warm, moist indoor environment in
which seeds are forced to sprout. Typically, unsprouted seeds are placed
on trays or in rotating drums. There, they are kept warm and are
periodically moistened with a fine mist of clean water.
There are many
opportunities for the seeds to become contaminated with the microbes
before they're even purchased by sprouters. Salmonella or E.
coli could be harbored in bird droppings, in manure applied to
fields as fertilizer, in contaminated water that's used to irrigate
fields, or perhaps in dirt left over in improperly cleaned seed-sorting
machinery. The pathogens might also live in droppings of rodents that
eat seeds stored in bags, bins, or silos.
Microbes' Preferred
Attack Sites
Charkowski's studies with
sprouted seeds of radish, alfalfa, broccoli, and mung bean may yield
tactics that sprouters can use to ensure their products are free of Salmonella
and E. coli. Experiments in her laboratory at the ARS Western
Regional Research Center have revealed that these pathogens prefer to
attack the roots and—secondarily—the seedcoats. Her studies, which
required tracking the microbes' progress in thousands of sprouts, are
among the first few to reveal the pathogens' priorities.
Related tests may
indicate what sprout-produced compounds—such as amino
acids—inadvertently nurture the attacking microbes. In addition, the
experiments should show whether harmless bacteria could be applied to
the sprouts to deprive the pathogens of the vital compounds.
"We might be able to
undermine the harmful pathogens," says Charkowski, "if we can
use beneficial bacteria to outcompete them in the race for essential
compounds."
Salmonella
Genes Scrutinized
In other approaches,
Charkowski is probing the genetic makeup of Salmonella. Her
intent: To discover which genes Salmonella
"expresses"—that is, activates—when it contaminates
sprouts.
For one group of
experiments, she's producing genetically engineered lab strains of Salmonella
by randomly knocking out genes. Then, she's determining whether removing
those genes will reduce the microbe's ability to infect sprouts.
In a second set of
genetic engineering investigations, Charkowski and postdoctoral fellow
Jeri D. Barak will track the genes that Salmonella expresses as
it colonizes the sprouts. They will do that with Salmonella that
contain a gene borrowed from a jellyfish. The jellyfish gene causes Salmonella
to fluoresce a bright green whenever a Salmonella gene turns on.
In a competition among
ARS labs nationwide, Charkowski won special ARS funding that allows
Barak to join in this genetic research effort.
Less Effective Salmonella
Found
"So far,"
Charkowski reports, "one of our most interesting strains of
genetically modified Salmonella is only about one-tenth as
effective in colonizing sprouts as conventional Salmonella."
But which of Salmonella's 4,000 to 5,000 genes are missing or
disabled in that strain? Help in identifying those genes—and their
makeup, or sequence—may come from biotechnologists who are part of an
international venture to sequence Salmonella genes.
In the meantime,
Charkowski's quest to find out which Salmonella genes are crucial
to successful attacks on sprouts is likely unique. "Once we know
which Salmonella genes are critical in an invasion," she
says, "we may be able to develop a strategy to activate and amplify
the natural protective response by the sprouts."
"The genes that Salmonella
activates when it invades sprouts," says Charkowski, "are
likely the same as those it uses when it colonizes other fresh produce
and perhaps meats and poultry. That means the food safety strategies
developed from our genetic studies may help protect these other foods
from Salmonella, as well."—By Marcia Wood,
Agricultural Research Service Information Staff.
This research is part
of Food Safety, an ARS National Program (#108)
Amy O. Charkowski is
in the USDA-ARS Food Safety and Health Research Unit, Western Regional
Research Center, 800 Buchanan St., Albany, CA 94710; phone (510)
559-5647, fax (510) 559-5948.
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