Searching for E. coli Fingerprints
To find where E. coli O157:H7 contamination is most
likely to occur in a packing plant, MARC scientists employ a method that public
health officials use to track the source of bacteria. The procedure, called
pulsed-field gel electrophoresis, produces a DNA fingerprint that is unique to
a particular strain.
"By using this method we've found that at least 68 percent of
the E. coli O157:H7 on meat traces back to a live animal of the same
group of cattle," says microbiologist Genevieve A. Gallagher.
At five points in the processing plants the researchers sought
evidence of E. coli contamination, just as plant employees do routinely
in a troubleshooting program that complies with standards set by USDA's Food
Safety and Inspection Service (FSIS). Such programs are known as Hazard
Analysis and Critical Control Point procedures.
The Meats Research Unit helps to fulfill FSIS' information
needs, such as developing protocols for microbiological testing or researching
steam vacuuming of beef carcassesa practice approved by FSIS and now used
in most meat packing plants.
"Presently, we are researching the question, 'Have some strains
of E. coli O157:H7 become well adapted to acids in the gastrointestinal
tract of cattle in the feedlot?' " says microbiologist Elaine D. Berry. Here's
why the answer is important: In many processing plants, an organic acid-wash
solution is used for cleaning carcasses. Acid-resistant strains could render
the wash solution ineffective.
A Special Glow
To better assess potential protocols for decontaminating meat
and to gain basic insights into how E. coli O157:H7 attaches to carcass
surfaces, the researchers have genetically engineered the microbe so that it
glows. A very sensitive video camera known as an intensified charged-coupled
device system, equipped with a 60-millimeter lens, enables the scientists to
see for the first time contamination patterns on large areas of carcass surface
"By studying the attachment as it happens, we don't have to
first sample the surface and culture the bacteria to find where they are
located," says microbiologist Gregory R. Siragusa, who is now at the ARS
Richard B. Russell Research Center's Microbiological Safety Research Unit,
The Leader So Far
The scientists hope to find ways to prevent E. coli
O157:H7 from binding to the carcass surface because attempts to wash off
bound bacteria may be futile. Then, during further processing, contamination
The scientists recently developed a multistep treatment before
meat grinding that holds promise against E. coli O157:H7 proliferation.
The most effective treatment so far is a multistep process: A high-pressure
water wash, followed by a hot water spray, then a hot air treatment, and
finally a spray of 2-percent food-grade lactic acid. While the treatment almost
imperceptibly darkened the color of refrigerated ground beef patties, it
reduced microbe numbers in beef trim by 99.9 percent.
Hunting Harmful Genes
With a tenacity akin to E. coli O157:H7's own penchant
for posing health concerns, the scientists are striving to determine which
genes make the bacteria harmful. They're studying the DNA piece by piece in a
procedure called subtractive hybridization. They take DNA from the E.
coli, remove parts known to be harmless, and then isolate and identify the
remaining pieces. The scientists then research the cellular processes encoded
by the unique pieces.
For producers, the scientists are working toward breeding farm
animals that resist infection with pathogenic E. coli strains. Using new
techniques, Laegreid and his colleagues have identified cattle genes that are
expressed in response to a molecule called a lipopolysaccharide, which is
produced by E. coli O157:H7.
"Knowing which genes respond to foodborne pathogens is a first
step toward developing interventions," says Laegreid. Next, scientists will
pinpoint genetic markers that one day may be used to select farm animals that
won't harbor the harmful bacteria or help them propagate.By
Ben Hardin, Agricultural
Research Service Information Staff.
This research is part of Food Safety, an ARS National Program
(#108) described on the World Wide Web at
Koohmaraie and William W.
Laegreid are at the USDA-ARS Roman L. Hruska
U.S. Meat Animal Research Center, P.O.
Box 166, State Spur 18D, Clay Center, NE 68933; phone (402) 762-4100, fax (402)