Listeria (Listeriosis)

Causes

How does someone get listeriosis?

You get listeriosis by eating food contaminated with Listeria monocytogenes. Babies can be born with listeriosis if their mothers eat contaminated food during pregnancy. However, healthy persons may consume contaminated foods without becoming ill. Persons at risk can prevent listeriosis by avoiding certain high-risk foods and by handling and storing food properly.

Organism (Etiologic Agent)

Listeria monocytogenes, a gram-positive rod-shaped bacterium.

Reservoir

Listeria monocytogenes is commonly found in soil and water. Animals can carry the bacterium without appearing ill and can contaminate foods of animal origin, such as meats and dairy products.

Transmission

Most human infections follow consumption of contaminated food. Rare cases of nosocomial transmission have been reported.

When Listeria bacteria get into a food processing factory, they can live there for years, sometimes contaminating food products. The bacterium has been found in a variety of raw foods, such as uncooked meats and vegetables, as well as in foods that become contaminated after cooking or processing, such as soft cheeses, processed meats such as hot dogs and deli meat (both products in factory-sealed packages and products sold at deli counters), and smoked seafood. Unpasteurized (raw) milk and cheeses and other foods made from unpasteurized milk are particularly likely to contain the bacterium.

Listeria is killed by pasteurization and cooking; however, in some ready-to-eat foods, such as hot dogs and deli meats, contamination may occur after factory cooking but before packaging. Unlike most bacteria, Listeria can grow and multiply in some foods in the refrigerator.