The Hotdog Test

David Smith
September 01, 2000

Despite the Danger: Biohazard signs posted on the door, John Burg feels comfortable in 118 Borland Lab.


Cook it well. The USDA routinely finds dangerous Listeria monocytogenes in about 2 percent of all commercially produced hotdogs.

"The pathogens in here are deadly, but they're food-borne diseases. You have to eat them to get sick," Burg said. "It's not like I'm working with anthrax or anything that's airborne."

Burg, a microbiology major at Penn State, is working with Listeria monocytogenes. This bacteria, which can be found in ready-to-eat foods such as hotdogs and soft cheeses, milk, and raw meats, cause listeriosis, a disease with such flu-like symptoms as headache, fever, and diarrhea. In the most serious cases, the bacteria attack the nervous system, infecting the brain and causing meningitis. In 1998, hotdogs made at a Michigan plant caused an outbreak of listeriosis that killed 21 people and sickened 80.

Yet it can be several weeks after a person has eaten food contaminated by Listeria monocytogenes before symptoms appear. This long incubation period makes it difficult for doctors to determine the cause of an outbreak and makes accurate testing within the food industry very important.

Burg's project has been to test a test, one that former Penn State graduate student Alex Teo designed to detect Listeria in milk. Teo called his test "oPSU broth," or "optimized PSU broth." The oPSU broth is jelly-like, brownish yellow at the bottom and slightly pink at the top.

If Listeria is present in the sample added to it, the oPSU broth turns black. Here's how it works: A sugar called esculin gets hydrolyzed, split in half, by an enzyme that Listeria produces. One of the products of that split reacts with the iron in oPSU broth; the compound formed is black.

Burg spends most of his time mixing reagents together and putting them through the autoclave, a kind of pressure cooker used to sterilize the mixture. But the heat of the autoclave would destroy some of the amino acids used in the broth, so Burg sterilizes these by putting them through a filter with holes only 0.2 micrometers wide. Those holes are too small for bacteria to pass through.

Once the broth is ready, Burg prepares a sample by taking a hotdog (bought at a local supermarket) and putting it and some water through the Stomacher, a machine that pounds the hotdog for about five minutes. Burg draws 0.5 ml samples from the resulting pulp and adds them to test tubes filled with oPSU broth. He then lets the tubes sit in a warm incubator for anywhere from a day to a few weeks. If the broth turns black in that time, chances are Listeria are present.

In the course of his research Burg has discovered that two other bacteria—ones that are not a type of Listeria—will also turn the broth black. Burg hopes to improve on the broth's design so that it only detects strains of Listeria.

Once he has ruled out these two bacteria, he still can't be sure that the bacteria in the tube are dangerous. The oPSU broth also reacts to the harmless types of Listeria: L. grayi, L. innucua, L. seeligeri, and L. ivanovii. Only by testing the DNA of the bacteria he finds in the hotdog can Burg be sure which strain is present. So far, Burg has only tested the DNA of one sample and the test was positive for L. monocytogenes.

While Burg was surprised to find Listeria monocytogenes in his sample, he adds that it "isn't really news because there are occasional outbreaks. What's important to me is making sure this broth works well with hotdogs and trying to improve on it."

The USDA's Food Safety and Inspection Service (FSIS) laboratory guidebook, explains the "official" method for isolating and identifying Listeria in hotdogs. The FSIS method uses two broths followed by streaking the sample onto a solid medium and seeing if the medium changes color. Burg and his adviser, food scientist Stephen Knabel, believe oPSU broth followed by a DNA test is more sensitive.

"The current methods used in industry don't work as well with low numbers of cells," Knabel said.

No one is sure how much bacteria it takes for a person to get sick but scientists have estimated it at 1000 cells. So to protect himself, Burg always wears gloves while working. Only after Burg had worked for the lab for some time was he permitted to work unsupervised. "We wanted to make sure he knew what he was doing, because we do work with dangerous bacteria," Knabel said. "But once we were confident in him we let him pursue this project on his own."

John Burg is a senior honors student majoring in microbiology. His research adviser is Stephen J. Knabel, Ph.D., associate professor of food science in the College of Agricultural Sciences, 101 Borland Laboratory, University Park, PA 16802; 814-863-1372; Writer David Smith graduated in May 2000 with a B.A. in journalism.

Last Updated September 01, 2000