The bacterium Listeria Monocytogenes isn’t the household name its distant cousin E. coli is, but it should be, at least to some segments of the population. While L.Monocytogenes is responsible for less than 1/100 of 1 percent (2,500 out of an estimated 76,000,000 cases annually) of food-borne illness in the United States, it’s responsible for a whopping 28 percent (500) of the annual deaths. Those who have the most serious problems with L. Monocytogenes infections are the elderly, newborns, or people with suppressed immune systems. The bacteria also may cause serious problems for the unborn child of a pregnant woman who becomes infected.
Monica Borucki, research geneticist, USDA-Agricultural Research Service, and adjunct assistant professor, Veterinary and Comparative Anatomy, Pharmacology, and Physiology, came to Washington State University three years ago to start a research program on L. Monocytogenes.
While many species of Listeria are widespread in the environment, only L. Monocytogenes, and only some of its serotypes, appear to cause human epidemics and serious illness, she says.
Serotypes are rather like apple varieties. If the genus Listeria is thought of as fruit, and the species L.Monocytogenes is thought of as apples, then L.Monocytogenes serotypes are like Granny Smith, red delicious, and other apple varieties. Individuals within a serotype also vary and are called strains, just as red delicious may vary from orchard to orchard.
Borucki’s current focus is on developing a fast, reproducible test that will distinguish between various L.Monocytogenes strains and provide genetic information on how the strains differ. Current tests, which may take several days to run, distinguish between strains but provide no information on how they differ. Borucki and colleague Douglas Call, assistant professor, VCAPP, are developing a microarray that will do both.
A typical microarray is composed of hundreds to thousands of DNA or gene fragments that are attached to a small glass slide. The microarrays that Borucki and Call are developing have fragments from several different serotypes and strains of L.Monocytogenes. When DNA from L. Monocytogenes samples are tested on the array, it’s possible to identify gene or DNA fragments that differ between them.
Once they have a microarray that will differentiate between the various L. Monocytogenes serotypes and strains, Borucki and Call hope to identify which genes are associated with serious epidemics and illness. Probes for those genes then will be developed so that bacteria samples from public health or food industry laboratories can be rapidly and accurately screened.
At this time, it can be expensive anytime L.Monocytogenes is identified, both in terms of human health and in terms of product recalls, says Borucki. Public health officials want all food recalled that is thought to be contaminated with L. Monocytogenes recalled because of the potentially high mortality associated with some strains. With rapid testing that could determine whether the genes that cause serious illness are present in a sample, recalls would not be necessary except when those genes are present.
In food processing plants, it will be possible to determine what areas, conditions, or types of foods are the source for dangerous serotypes. Once it’s known where the bacteria get into food, it will be easy to concentrate prevention and clean up to those areas.
“Right now, if it’s Listeria of any kind, they clean it up,” says Borucki.