is a small, gram-positive bacillus that can grow in anaerobic or aerobic conditions. It is found widely in the environment in soil, decaying vegetation and water and may be part of the fecal flora of many mammals, including healthy human adults.1
L. monocytogenes presents a particular concern with respect to food handling because it can grow at refrigerator temperatures (4°C to 10°C), temperatures commonly used to control pathogens in foods. Freezing also has little detrimental effect on the microbe. Although pasteurization is sufficient to kill Listeria, failure to reach the desired temperature in large packages can allow the organism to survive. Food can also be contaminated after processing by the introduction of unpasteurized material, as happens during the preparation of some cheeses. Listeria can also be spread by contact with contaminated hands, equipment and counter tops.
The centralized production of prepared ready-to-eat food products in Canada increases the risk of higher levels of contamination, since it requires that foods be stored for long periods at refrigerated temperatures that favour the growth of Listeria. During the preparation, transportation and storage of prepared foods, the organism can multiply to reach a threshold needed to cause infection.
The approximate infective dose of L. monocytogenes
is estimated to be 10–100 million colony forming units (CFU) in healthy hosts, and only 0.1–10 million CFU in people at high risk of infection ().2
Foods such as raw vegetables, raw (unpasteurized) milk and cheese, and meats (fresh and frozen) may become contaminated with L. monocytogenes
because of where they come from and how they are processed. Ready-to-eat foods such as cold cuts or deli meats, cheeses and other dairy products are ideal sources for contamination.
Although L. monocytogenes
was recognized as an animal pathogen over 80 years ago,3
the first outbreak confirming an indirect transmission from animals to humans was reported only in 1983, in Canada's Maritime provinces.4
In that outbreak, cabbages, stored in the cold over the winter, were contaminated with Listeria
through exposure to infected sheep manure. A subsequent outbreak in California in 1985 confirmed the role of food in disseminating listeriosis. Since then Listeria
has been implicated in many outbreaks of food-borne illness, most commonly from exposure to contaminated dairy products and prepared meat products, including turkey and deli meats, pâté, hot dogs and seafood and fish.5