Despite the efforts of food retailers and food-processing plant managers to maintain a clean, safe environment, strains of the deadly pathogen Listeria monocytogenes can persist for up to a year or longer, according to Cornell University food scientists in the latest issue of Journal of Food Protection (July 2004).

"This is disturbing because this points the finger at retail stores and some processors as a continuing source of food contamination," says Brian D. Sauders, a Cornell doctoral candidate in food science, who worked on the study with Martin Wiedmann, D.V.M., Cornell assistant professor of food science.

Sauders and Wiedmann examined specific strains of L. monocytogenes that had been found in 125 foods in 50 retail food stores and seven food-processing plants in New York state examined by inspectors of the New York State Department of Agriculture and Markets. The inspectors found the bacteria during routine surveys, sanitary inspections and as a result of consumer complaints between 1997 and 2002.

Listeria can cause listeriosis, a deadly disease that primarily affects pregnant women, newborn children, and adults with weakened immune systems. Each year in the United States about 2,500 people are infected, of which one-fifth die. Pasteurization and cooking kill the bacterium.

The foods in which Listeria was found included ready-to-eat delicatessen foods like ham, beef bologna, chicken, pastrami, roast beef and smoked fish. It also was found in hummus, imitation crab, cheeses and in foods requiring cooking before consumption, such as hot dogs and raw foods including beef, ground chuck, turkey, lobster tails and shrimp.

The bacterium was found directly on food in 47 of 50 retail food stores, including 20 food stores where the bacterium was found on several foods. When the 50 stores were re-inspected weeks, months or even a year later, about 34 percent had persistence of the same strains of Listeria. Of the seven food-processing plants where Listeria was found, three had persistent strains of the bacterium.

Wiedmann explains that food retailers have a harder time controlling for Listeria than do food processors. Food processors can control for people entering the plant, while retailers cannot always control the pathogens introduced by customers and employees. "Listeria is a very hardy organism. Even if you think you're doing a good job of cleaning and getting rid of Listeria, it is likely to return. Normal cleaning and even super cleaning does not always get rid of it. It's a tribute to Listeria's ability to survive," says Wiedmann.

The study is intended to help state health departments track the origins of listeriosis. "While our understanding of the ecology of [Listeria] has clearly improved over the last decade, considerable gaps still exist in our understanding of the transmission of human listeriosis. For example, our knowledge of the contributions of food contamination with Listeria at retail, at restaurants, and at home is extremely limited," writes Sauders in the study.

In addition to Sauders and Wiedmann, the article (titled "Distribution of Listeria monocytogenes Molecular Subtypes Among Human and Food Isolates from New York State Shows Persistence of Human Disease-Associated Listeria monocytogenes Strains in Retail Environments") was authored by Kurt Mangione, Curtis Vincent, Jon Schermerhorn and Claudette M. Farchione of the New York State Department of Agriculture and Markets; Nellie B. Dumas and Dianna Bopp of the New York State Department of Health; Laura Kornstein of the New York City Department of Health; and Esther Fortes and Katy Windham of Cornell. Funding for the research came from the U.S. Department of Agriculture and the National Institutes of Health.