Stopping 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 faith.

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 vendors—possibly even pushing a firm into bankruptcy.

The worst part is, both scenarios repeatedly came true. But no more.

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 Lab—which was established in 1904 to serve the state of Iowa as a public health and environmental laboratory—is a member of PulseNet, the Centers for Disease Control and Prevention’s (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 evidence—an 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, we’re engaged in an active system. We can determine with certainty which food is to blame. And instead of two months, it takes about two weeks."

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 didn’t 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 organism’s 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 method.

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 suppliers—such as the many farmers, manufacturers, and vendors in Iowa—much economic hardship because we will not pull their food product from the market in error."