foodborne bacteria in their tracks
In the past, health officials had two recurrent nightmares associated
with food poisoning.
The first was that they would move too slowly in the event of an
outbreak: reluctant to name a particular food product or company,
they would wait until hundreds or thousands of people became ill.
Elderly people and infants would die, and the public would lose
The second was that they would move quickly to remove a product
from the market only to find it was not contaminated, causing damage
to the reputations of innocent food growers, manufacturers, and
vendorspossibly even pushing a firm into bankruptcy.
The worst part is, both scenarios repeatedly came true. But no
Using new "genetic fingerprinting" technology, Nelson Moyer, a
public health and environmental microbiologist with the University
Hygienic Laboratory at The University of Iowa, can now identify
many common foodborne illnesses quickly and pinpoint their origin
with great accuracy.
The Hygienic Labwhich was established in 1904 to serve the
state of Iowa as a public health and environmental laboratoryis
a member of PulseNet, the Centers for Disease Control and Preventions
(CDC) network of experts in the surveillance and investigation of
foodborne pathogens such as E. coli and Salmonella. Moyer manages
the testing of samples for the state of Iowa, then he sends information
to the CDC where it is entered into an electronic database and shared
with other states.
In the past, outbreaks of foodborne illness were diagnosed based
upon anecdotal evidencean ill person in North Liberty, three
in Iowa City, a handful in Coralville, all of whom ate at the same
restaurant on Sunday night. Today, all that puzzle-fitting guesswork
is eliminated with the DNA "fingerprint" that tells scientists,
without a doubt, if certain strains of bacteria are related.
"This system leads to much earlier detection," Moyer says. "Now,
were engaged in an active system. We can determine with certainty
which food is to blame. And instead of two months, it takes about
The process sounds deceptively simple. Moyer and his team take
bacteria from human samples and treat them with an enzyme that breaks
the genetic material into fragments. They place these pieces of
DNA in a gel and expose them to an electrical field that causes
the pieces to migrate. Smaller fragments travel quickly and thus
go farther. So at the end of the cycle, scientists have a unique,
banded picture resembling a bar code, which they can compare to
other such pictures. A series of perfect genetic matches signals
an epidemic. A similar code in the DNA of bacteria replicated from
a food product proves it is the source.
"In the past, we always had confusion because there would be ill
people who didnt fit the profile," says Mary Gilchrist, director
of the University Hygienic Lab. "Now, we can tease apart the strains
and identify who was made ill by what food. This allows epidemiologists
to withdraw the food immediately, because they can be comfortable
that it truly is contaminated. And by doing this, we minimize illness,
deaths, and fear in the community."
Five years ago, the Hygienic Lab became one of the first public
health facilities in the United States to test for human calici
viruses, which are responsible for as much as two-thirds of all
foodborne illnesses nationwide. The disease usually lasts 24 to
48 hours, with symptoms of nausea, vomiting, diarrhea, abdominal
pain, low-grade fever, and malaise.
The human calici viruses, also known as Norwalk-like viruses, cannot
be detected by standard methods such as cell culture. And virologists
have not yet identified a cell line that supports growth of these
viruses outside of the human body. But Michael Loeffelholz, chief
of virology, serology, and molecular biology at the Hygienic Lab,
oversees the application of a new polymerase chain reaction (PCR)
test that can amplify and replicate the genetic material from human
samples suspected of being infected with a Norwalk-like strain.
"We put a small segment of the organisms nucleic acid into
a test tube to prime the copying of the RNA or DNA. Then we put
it through a series of repeated temperature cycles to synthesize
new strands of the material," he says. "Two to three hours go by,
and we have literally billions of copies of the nucleic acid. Then
we can analyze the sample and determine whether the genetic material
is that of Norwalk-like viruses."
Now Loeffelholz and his team are educating lab personnel from other
states, such as Minnesota, Wisconsin, and Missouri, to use the PCR
Better, faster detection of foodborne pathogens clearly is good
for individual health. But, Gilchrist points out, it is also a boon
to the economic well-being of communities.
"When we find unrelated cases, we can say with certainty that it
is not a cluster...not an outbreak," Gilchrist says. "This saves
food growers and supplierssuch as the many farmers, manufacturers,
and vendors in Iowamuch economic hardship because we will
not pull their food product from the market in error."