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Lab Technology Helps Scientists Identify And Track Outbreaks Of Food Poisoning

 

Source: Albuquerque Journal
Publication date: 4/17/2000

At first glance, the slab of clear gel bathed in a translucent liquid doesn't look very impressive. It doesn't even light up or writhe when it gets a series of electrical jolts. But, if the right set of circumstances come together, that gel might help tell you if your bout with food poisoning came from the potato salad at the church picnic or from alfalfa sprouts imported from out of state. Better yet, health officials say, it can help them track a foodborne outbreak faster and prevent more people from opening a can or carton and chowing down on the contaminated contents. And suffering the very unpleasant aftereffects.
"Hopefully, there will be fewer sick people," said David Mills, director of the Scientific Laboratory Division of the state Department of Health. The technology goes by the name pulsed-field gel electrophoresis. But think of it as a fingerprint, a pattern that identifies a bacterium too small to see with your naked eye. If the patterns of a bunch of samples match up, you can be pretty sure they all started in the same place. "We've used it for several outbreaks," said State Epidemiologist C. Mack Sewell. Sometimes the pattern showed a relationship, sometimes it didn't. So far, though, the technology showed a couple of New Mexico cases were related to multistate outbreaks of salmonella from alfalfa sprouts and unpasteurized orange juice. In another instance, a few cases in New Mexico were linked to those in other states but a particular food source was not identified. Cases of shigella in January and February were linked to a five-layer food dip. That last case was particularly satisfying, Mills said. Health officials found the canned dip in New Mexico was sold through a discount chain to which people have to buy memberships, he said. Store owners were able to identify customers who bought the dip and call them and warn them not to eat it (if they hadn't eaten it already), he said.
The technology is not a lot different from the approach used to determine the father of a child, said Paul Torres, a state laboratory scientist who performs the test. You're just looking at how closely the DNA matches up. Of course, first you have to get samples from sufferers. That's sometimes a problem in food poisoning, said Sewell. Many people simply ride out their illnesses at home and never see a doctor, he said. Others might see their doctor and get treated without a sample of the organism causing the illness ever being taken.
Usually, such samples are taken from an infected person's feces. If the clinician takes a sample and sends it to the Department of Health, then state technicians go to work. The bacteria are isolated from the sample, and then special enzymes and detergents are used to digest everything except the bacterial DNA. Then another specific enzyme cuts the DNA at particular spots. DNA is made of four ingredients, and the enzyme targets particular spots where a certain molecule links with another, Torres explained. The result is a collection of DNA snipped into different-sized fragments. The various samples
then are applied to channels on a comblike device, which is plunged into a prepared gelatinous substance. The samples then are left behind in little holes on the gel. The slab of gel is put into a machine with hexagonal patterns of electrodes that emit pulses of electricity. It is covered in a liquid. And then, for 18 or 19 hours, the pulsing continues. The electric pulses send the DNA fragments skittering through the gel, which is made with agar. The smaller fragments, since they can move and change direction faster, end up toward the far end of the slab, Torres said, while the bigger fragments stay closer to the starting point.
The final product is stained with ethidium bromide and put under an ultraviolet light. A digital camera captures the image and sends it into a computer, where a series of bars appears. The pattern is vaguely similar to a bar code read by scanners at a grocery store checkout. If the patterns from different samples match, then it's likely they are part of the same outbreak related to a common source of contamination. Information about matching samples is sent on to the department's epidemiologists, who use the information to help identify common cases and trace the source of an illness. Samples also can be taken from a
suspected source of the bacteria to match against samples from sick people.
When some clients of a restaurant in Truth or Consequences suffered from salmonella last year, for example, the culprits were fingered when samples from patients matched up with samples from the eatery's nachos and burritos, Torres said.
New Mexico has been using the technology since spring 1999, according to Linda Nims, supervisor of genetic microbiology at the state lab. The goal is for all the states to be hooked up through the Centers for Disease Control and Prevention, she said. With the computer patterns shooting into that center daily, scientists can identify organisms around the country that might come from a common source.
In these days of national and international distribution of food, the pooling of information might identify cases with a common source that individual states, with reports of only an isolated illness or two, might not recognize.
In addition to recording patterns of new samples as they come in, the lab also is taking frozen samples from past years and determining their genetic patterns, Nims said. "We are running a pulse field on all organisms so we can get a catalog of the patterns of organisms found in New Mexico," she said.
The CDC, in the meantime, is gathering a database for the entire country. Conceivably, such an approach could help track any organisms used in bioterrorism, Torres added. The lab has been regularly checking patterns of shigella and salmonella, common causes of diarrhea and food poisoning, but also is branching out to record patterns of the bacteria that cause whooping cough and plague.