WEST LAFAYETTE, Ind. – Researchers at Purdue University are developing new electronic sensors that should be able to detect the deadly pathogen Listeria monocytogenes in food processing lines.
In May, President Bill Clinton announced that the federal government would require food processors to cut the rate of Listeria illnesses in half by 2005, instead of 2010 as was previously planned. Later this summer, the U.S. Department of Agriculture is expected to propose that processors be required to test for Listeria monocytogenes in their plants and on equipment. There would be no requirement for the processors to test for the bacterium in food.
Listeria monocytogenes is the most deadly food pathogen of them all. It sickens more than 2,500 people each year in the United States, and one out of five of its victims dies.
Arun Bhunia, associate professor of food science at Purdue, says Listeria monocytogenes is a difficult pathogen to control. "It is very heat resistant, it grows even when refrigerated, and it survives freezing," he says.
Listeria monocytogenes is a common bacterium found in many foods, including meat, poultry, seafood and vegetables. According to research conducted by the U.S. Department of Agriculture in 1999, it was found in 12 percent of the ground beef, 25 percent of the ground chicken, and 30 percent of the ground turkey sold. Six percent of ready-to-eat luncheon meats were found to contain the pathogen.
Bhunia says healthy individuals usually do not suffer from the disease caused by the bacterium. "Not everyone is sensitive to Listeria monocytogenes," Bhunia says. "Individuals who are immunocompromised, very young, old, who are cancer or AIDS patients, or who have had an organ transplant are sensitive to this organism."
A 1998 outbreak of Listeria killed 15 people and sickened another 100 people. Approximately 15 million pounds of hot dogs and luncheon meats were destroyed because of the outbreak.
Listeria monocytogenes can cause diarrhea, headaches, fever and even more extreme conditions such as meningitis, encephalitis, liver abscesses and pregnancy miscarriages.
Unlike Salmonella, which sickens humans no matter what strain is present, only one form of the Listeria causes illness. And although cases of food illness aren't as common with Listeria monocytogenes as with other food pathogens – 1.5 million cases of Salmonella annually vs. an estimated 2,500 cases of Listeria – Listeria monocytogenes is much more deadly. According to 1999 statistics from the Centers for Disease Control and Prevention, less than one half of one percent of the people infected with Salmonella die, but 20 percent of the cases of Listeria monocytogenes are fatal.
Better detection of this fatal food pathogen is a high priority for the food industry, Bhunia says. "At the present time we can only detect a large number of bacteria in a sample. To get a large number, you have to let the sample grow in a laboratory," he says. "It can take as much as five to seven days to grow, test and confirm the presence of a specific pathogen."
That is too long, says Richard Linton, Purdue associate professor of food science. "In a week that product is going to be on the shelves. If there's a major food pathogen outbreak, it's pretty scary to think about what can happen right now."
A team of scientists at Purdue is working to develop an electronic device that could identify Listeria monocytogenes within minutes and be able to detect very low levels of the bacterium in one gram or one milliliter of food, an amount about the size of a pencil eraser. The research coordinator is Michael Ladisch, professor of agricultural and biological engineering and biomedical engineering.
The ability to test for low levels of the bacterium is important, Linton says, because Listeria monocytogenes is infectious at as few as 10 cells. "Compare that with a food illness caused by Salmonella, where it would take about a million cells to cause an illness," Linton says. "That's why it's so important to be able to detect minute amounts of this pathogen in food."
The research on developing Listeria sensors is being funded by the Purdue Food Safety Engineering Project with money from the U.S. Department of Agriculture. The Food Safety Engineering Project is a cooperative agreement between the USDA Agricultural Research Service and Purdue's School of Agriculture. The mission of the project is to develop new knowledge, technologies and systems to prevent chemical and microbial contamination of foods.
"Imagine if a food processor can probe a sample of ground beef," Linton says, using a pencil from his desk as the hypothetical probe, "and know whether that sample of meat contains a harmful pathogen such as the E. Coli 0157:H7 strain. We can do the test in 15 minutes, but right now we're days away from that 15 minutes because it takes us days to build up enough of the bacteria to test to see whether it is the deadly variety."
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