Listeria bacteria are widespread in the environment and they are worldwide commonly found in soil and water, as well as in animal digestive tracts. There are more than 15 species of bacteria in this genus, but human cases of Listeria infection are almost always caused by Listeria monocytogenes (L. monocytogenes) while Listeria ivanovii is pathogenic in animals but rarely in humans. The other species are not considered to be pathogenic in humans or animals.
L. monocytogenes are able to form communities of bacteria called biofilms and are therefore resistant to a wide range of stresses. This capacity varies among genotypes . They can tolerate acidic, dry and salty conditions, in the presence or absence of oxygen. Moreover, unlike most pathogenic bacteria, they are able to survive and multiply in refrigerated foods, making ready-to-eat foods of particular concern.
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