Listeria monocytogenes is a Gram-positive, non-sporing rod. It is motile and is aerobic and facultatively anaerobic.

Listeriosis is a comparatively rare disease; however, because of the potential severity of the disease (the mortality rate is 25-50%) measures for its control in foods are very important. It is generally agreed that the majority of cases of listeriosis are foodborne and may be preventable. Almost all cases of listeriosis infection in humans have been recognised as being due to Listeria monocytogenes.

Symptoms are typically meningitis or septicaemia and in pregnant women it can cause a flu-like illness, which can result in miscarriage, stillbirth or birth of a severely ill infant.

Listeria is ubiquitous in the environment and so can be transferred to foods from a wide variety of sources. Infection from Listeria can also originate from direct or indirect contact with animals (sheep and cows can both excrete Listeria monocytogenes in faeces and sometimes in milk).

Foods can become contaminated with Listeria at any stage in the food chain, from the farm, through the processing and distribution, to the consumer's kitchen. Numerous studies have now indicated that L. monocytogenes can be found in a very wide range of foods, including milk, soft cheese, raw and pre-cooked chicken and meats, pâté, fermented sausage, raw vegetables, ice cream and seafoods.

Listeria monocytogenes is psychrotrophic i.e. potentially capable of growing , albeit slowly, at refrigeration temperatures as low as 0ºC.

Of all the non-sporing, vegetative food pathogens, Listeria is the most heat resistant. It is, however, generally agreed that milk pasteurisation will destroy normal levels of L. monocytogenes in milk.


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