The Dangers of Listeria in Food

As of October 3, 2011, at least 100 people have been sickened and 18 have died from an outbreak of Listeria monocytogenes associated with cantaloupes from Jensen Farms, Colorado. The grower has voluntarily recalled the produce, and the CDC and other agencies have issued bulletins to alert the public of this potential health hazard.

Listeria is a group (genus) of bacteria that contains six species with Listeria monocytogenes being the most notable in terms of pathogenicity. L. monocytogenes is a rod-shaped, Gram positive bacterium that is the agent of listeriosis, a serious bacterial infection that can occur in humans. Listeriosis usually occurs in people with compromised immune systems, which include the elderly, small children and pregnant women. The disease causes fever, muscle aches and gastrointestinal symptoms and eventually progresses into septicemia, or the presence of bacteria in the blood. Listeriosis is a life-threatening illness with a fatality rate of approximately 25 percent.

L. monocytogenes occurs widely in the environment and can be found in soil, water, sewage, plants and food. The bacterium is killed after exposure to high heat (e.g. pasteurization), but food can become contaminated following cooking or if the food is eaten raw, as in the case of cantaloupes where the bacteria would presumably occur on the outer rind of the fruit. Listeria is especially dangerous in packaged meat products (e.g. hot dogs, lunch meats, etc.) because of the bacteria’s ability to grow and thrive at refrigerator temperatures.

Fortunately, Listeria infections are rare, and food producers generally take extensive precautions to sanitize production plants and test foods and surfaces for this bacterium. With raw foods, careful washing of produce before eating is always advisable to reduce or eliminate the levels of microorganisms on the food.

At LEGEND, testing for L. monocytogenes is routinely performed on swab samples from food production/packaging plants and on bulk samples of food products. The “gold standard” of Listeria testing continues to be direct culture in an enrichment broth followed by plating onto selective agar and performing a series of confirmatory tests. LEGEND provides swab kits to clients needing Listeria testing and can assist in development of a routine monitoring plan.

For more information on Listeria or to request sampling supplies, please contact LEGEND’s Phoenix, Arizona laboratory.