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Listeria monocytogenes is a baton-shaped bacteria that can cause miscarriages in pregnant women, liver failure in healthy people, and death in people with weaker health.

It grows between 35 and 113 F (2 and 45 C), particularly in the range of 98.6 F (37 C.) It can grow in the refrigerator, and while it won't grow in the freezer, freezing doesn't kill it off: a large percentage of the bacteria can survive freezing. To flourish, it likes a high pH, ideally above 6, though other studies seem to show that a pH above 4 is enough.

It is present in soil everywhere. It can be on organic raw vegetables grown in fields fertilized by animal manure, and in "ensilage" (fodder preserved by allowing it to ferment) fed to animals. It is in bird droppings.

It can enter premises through soil on the soles of people's shoes, or on someone's hands who has been petting a cat or a dog.

When a person is infected with Listeria, the condition is called "listeriosis" ("listériose" in French.) The first identified human infection was in 1929.

After 12 hours of infection, you may feel like you have the flu. Full-blow listeriosis may develop 1 to 6 weeks afterwards. 80 to 90% of the people who contract it require hospitalization.

The more listeria bacteria in the food you eat, the more for your body to have to fight off, and the greater the chance of your getting ill. Sometimes it doesn't affect very healthy people; they are able to fight it off.

The number of infections every year is very small, compared to cases of food poisoning caused by other things, but the mortality rate compared to other causes of food poisoning is much higher. To be infected with salmonella, you have to ingest over 100,000 cells of the bacteria. For Listeria, under 1,000 cells may do it

In food processing plants, Listeria can be introduced into food in 3 ways:
(1) already present in the food, or present in additional already-processed food brought in;
(2) by people coming in or equipment being brought in;
(3) by populations of the bacteria already present in the plant.

Listeria is hard to eliminate by normal cleaning. Chlorine and all-purpose cleaners won't eliminate it, and it tolerates cold temperatures and salt.

It can flourish on door handles, sponges, cloths, under the edges of tables, conveyor belts, chairs, and on light switches. It can thrive in the deli counters that started to become popular in North American grocery stores in the 1980s, in cold cuts, soft moist cheeses, ready-made salads. Even if the Listeria is starved for 6 months, some of the cells can survive in a weakened state, recovering and flourishing once appropriate conditions for them return.

Listeria is killed at 161 F (72 C.) In milk, it's done through pasteurization. Pasteurized milk, though, can have Listeria in it, if through improper handling it was re-infected after pasteurization. Dreyer's Ice Cream Bars, made from pasteurized milk, were infected in October 1997.

It can be in hot dogs and sandwich meat. Contamination of things such as packaged hot-dogs and sandwich meat can occur during handling after it's cooked but not packaged yet. It infected turkey wieners from Sara Lee Bil Mar Foodservice meat-processing plant in Zeeland, Michigan in October, 1998, and deli-style turkey meat in August 2002.

To be safe, treat hot-dogs as though they were raw meat. Disinfect or wash anything that fluid from the hot-dog package touches, and don't eat them cold, heat them thoroughly. Health officials in Canada and America even want consumers to cook packaged sandwich meat.

Health officials are even now advising to avoid refrigerated pâté and meat spreads, and only eat canned ones.

Dried sandwich meats such as salami and pepperoni are generally regarded as being safe from Listeria, because they offer a hostile environment for it. But even on dried salami, though Listeria may not flourish, it may survive.

Some raw milk advocates say that in good cheese making processes, neither Listeria nor E-coli can survive, but soft cheeses are often good breeding grounds for the bacteria, because they are both moister and less acidic than hard cheeses. In soft cheeses, Listeria is mostly on the surface, where the acidity level decreases even further during ripening.

Yet, it is not just soft cheeses that Listeria can infect. In Ireland in 2003 it was found in semi-hard cheeses, and in 2003, in France Listeria was found even in cheeses made from pasteurized milk.

It can also be found in smoked fish, prepacked sandwiches, and can even live in refrigerator dill pickles.

In North America, a zero-tolerance policy towards Listeria has been introduced, though in the United States, policies on pasteurization are enacted at the state level, not the federal level.

France, instead, has adopted the position that given that Listeria is everywhere in the environment, adopting maximum-acceptable amounts is the way forward.

On 10 February 1988 the French Ministère de l’Agriculture called for research into Listeria in dairy products, and on 29 June 1992 the Ministry called for research on charcuterie products as well. On 9 September 1992, the Conseil Supérieur d’Hygiène Publique de France (CSHPF) advised that the maximum threshold should be 100 count of Listeria bacteria per gram.


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Also called:

Listeria monocytogènes (French)


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