Listeria is not as common as other food pathogens like Salmonella: there are 1.5 million cases of salmonella annually versus an estimated 2,500 cases of listeria. Yet listeria is considered extremely dangerous due to the rate of death caused by the pathogen — about 20 percent. By contrast, less than one half of one percent of the people infected with salmonella die according to 1999 statistics from the Centers for Disease Control and Prevention.

In addition, it does not take very many cells of listeria to cause infection. While it takes millions of salmonella cells to cause illness in humans, as few as 10 listeria cells can make someone sick.

Listeria monocytogenes can cause diarrhea, headaches, fever and even more extreme conditions such as meningitis, encephalitis, liver abscesses and pregnancy miscarriages.

Healthy people do not usually suffer from the complications caused by listeria, according to team member Arun Bhunia, a microbiologist and associate professor of food science at Purdue. Other members of the group are computer, biomedical and biological engineers.

"Individuals who are immuno-compromised, very young, old, who are cancer or AIDS patients, or who have had an organ transplant are sensitive to this organism," he said.

The tiny device under development at Purdue is intended to detect listeria at low levels.

"At the present time we can only detect a large number of bacteria in a sample," said Bhunia. "To get a large number, you have to let the sample grow in a laboratory. It can take as much as five to seven days to grow, test and confirm the presence of a specific pathogen."

Seven days is too long, said Richard Linton, Purdue associate professor of food science who also is a member of the scientific team. "In a week that product is going to be on the shelves," he said. "If there's a major food pathogen outbreak, it's pretty scary to think about what can happen right now."

According to current models, the microchip will contain a specific protein that likes to bind exclusively with the listeria bacteria. Liquid used to wash the surface of a one-gram food sample will be pumped into the microchip. If listeria bacteria are present in the liquid, the protein will bind with it, at which point the chip will send an electronic signal to alert observers that listeria has been detected. This process is expected to take just a few minutes.

The exact implementation of this idea is still in development, said Bhunia, but should involve some sort of "micromachine" that manages all the different steps.

Completion of the project is two or three years away, but this work in progress is "a promising technology," said Bhunia.

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