(April 30) EAST LANSING, Mich. — Listeria monocytogenes is a hardier pathogen than once believed, according to Catherine Donnelly of the University of Vermont Department of Nutrition and Food Science.

The pathogen adapts to acidic conditions formerly thought to be too hostile for its survival, she said. It persists for lengthy periods under refrigeration. Survivors of stressed populations may be more virulent than pathogens killed by the stress.

She recommended that food processors be especially vigilant against listeria contamination of their products.

Donnelly conducted a seminar April 22 on food-safety challenges posed by listeria at Michigan State University’s National Food Safety and Toxicology Center. She is a member of an advisory group that is developing recommendations to both the Department of Health and Human Services and the U.S. Department of Agriculture on “use by” labeling of consumer food products.

Pull dates frequently are thought of as being an assured indicator of safety, but such is not the case, she said. “We are developing a scientific framework for the establishment of safety-based ‘use by’ date labeling,” she said.

That effort is especially needed today, she said.

“Shelf lives are being pushed out many, many days,” she said, citing the example of bagged lettuce processed in California that is shipped to Vermont for what might be an extended retail display and then lengthy storage in home refrigerators before eventual consumption.

Illnesses attributed to listeria are not as common as those caused by other microbial pathogens such as Salmonella and Campylobacter, but outbreaks that do occur tend to have dire consequences, she said. The effects are most serious on persons with underdeveloped or weakened immune systems, such as the young, the elderly and chemotherapy patients; pregnant women also are at risk. Mortalities can run as high as one-third among the infected, she said. An outbreak from a Michigan meat plant four years ago resulted in 21 deaths and six miscarriages, she said.

Donnelly said the incidence of listeria-caused illnesses actually may be higher than statistics indicate because of shortcomings in detection capabilities.

“Positive samples routinely escape detection,” she said. Different federal agencies use different methods to test for the pathogen -- none of them yields as high as a 75% detection rate, and when a combination of methods is used, the rate rises to only 90%, she said.

She cited a positive development in that the various agencies are exhibiting improved cooperation in efforts to control foodborne diseases even though they still have separate jurisdictions. “The good thing is, now they’re sitting at one table,” she said.

One problem in such a long food chain, despite continuous refrigeration, is that Listeria adapts to chilling, Donnelly said. A study of salsa at 40 degrees Fahrenheit showed the pathogen was still alive after 17 days.
The risk is exacerbated by uncertain conditions in home refrigerators, she noted.

“With most refrigerators, we don’t have a clue what the temperature inside is,” she said. A 1999 study of nearly 1,000 home refrigerators disclosed that more than one-quarter had storage temperatures above 41 degrees. More than 1% were at 51 degrees or higher.

"That’s not a refrigerator, it’s an incubator,” she said.