The bacterium Listeria monocytogenes can be fatal. Eating as few as eight listeria cells has made a person ill.
"This has led the regulatory agencies to enforce a 'zero tolerance' level for Listeria monocytogenes," writes Marisol Municio, a graduate student in food science at Penn State. That, in turn, "has forced several companies to recall all foods containing L. monocytogenes, independent of the number of cells present." Such recalls are costly, Municio notes: Both consumers and the food industry would benefit by knowing which foods are most susceptible to infection by the bacterium, and which processing schemes might kill it.
In 1987, for example, "although a cluster of 31 cases of listeriosis, including 14 deaths in Philadelphia, was epidemiologicaly linked to the consumption of ice cream, all attempts to isolate L. monocytogenes from ice cream or its ingredients were futile. Therefore, ice cream was eventually rejected as the vehicle for infection."
But is ice cream exempt? Can listeria survive freezing and frozen storage?
Municio and her adviser, Stephanie Doores, decided to find out. For her master's degree project, Municio inoculated an ice cream mix, froze it, packaged it, and stored it for one to five months before feeding it to two groups of healthy and immunocompromised mice.
She found that the bacterium retained its virulence even after five months of storage at -19 degrees C. "Freezing apparently did not cause enough damage to prevent the organism from spreading," Municio writes. It grew and multiplied in the mice's stomachs and intestines, spread to their livers and spleens, and in some cases reached their brains.
"Even after 18 months of freezing," notes Doores, "only 53.4 percent of the Listeria were killed."
Avoiding ice cream, however, is not what Doores and Municio recommend. For one thing, says Doores, "Listeria monocytogenes can be found naturally in the environment." More comfortingly, the healthy mice in Municio's study were able to fight off the infection in three to five days. Only in the immunocompromised mice did the quantity of listeria in their organs increase after the third day. And since "infection by L. monocytogenes in mice presents similarities to that in humans both in symptoms and in susceptibility to the disease," as Municio writes, "low doses . . . are unlikely to cause disease in healthy humans."
But Municio does warn that the potential for listeria infection is increasing, as consumers turn more and more to "minimally processed products" prepared with less salt, heat, acids, or preservatives. People with compromised immune systems—those taking immunosuppressant drugs such as steroids, or who have AIDS—she concludes, should be especially careful about their selection and handling of raw and frozen foods.
Marisol Municio received her master's degree in food science in May 1993. Stephanie Doores, Ph.D., her adviser, is associate professor of food science in the College of Agricultural Sciences, 103 Borland Lab, University Park, PA 16802; 814-863-2956. Municio also collaborated with Robert F. Parker, Ph.D., of the department of veterinary sciences at Penn State. Their work was funded by the USDA.