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Background


 

 

 Listeria monocytogenes is a gram-positive bacterium, in the phylum Firmicutes, class Bacilli, order Bacillales and family Listeriaceae. Other species in the Listeria genus include L. grayi, L. innocua, L. ivanovii, L. marthii, L. seeligeri, and L. welshmeri. L. monocytogenes is the most pathogenic and virulent species in this genus. It is rod-shaped, unencapsulated, facultatively intracellular, aerobic and facultatively anaerobic, motile by means of flagella, catalase-positive and oxidase-negative. It is found in numerous animal species, including various mammals, birds and fish.

 It is ubiquitous in the environment and can be isolated above all from soil, feed, silage and water. It can be detected in a wide range of animal products. Although it is non-spore forming, it is highly resistant to both high and low temperatures and to dehydration. It multiplies at temperatures between -1.5° and 45° C, pH between 4.3 and 9.6 and aw values of up to 0.90. It is thus also able to multiply in foods stored under refrigeration (4°C). In milk, it is inactivated in 10 seconds at 72.3°C and 1 second at 85°C. It tolerates highly saline conditions: in liquid medium containing NaCl 22%, it maintains its ability to multiply for up to 46 days (Gualanda 1998).

 In humans and animals, it is responsible for a group of food-borne diseases commonly known as “listeriosis”: meningitis, meningoencephalitis and mother-child and perinatal infections, which particularly affect susceptible subjects (infants, children, pregnant women and the elderly). However, healthy adults can also be affected, especially following consumption of highly contaminated foods. In such cases, L. monocytogenes is mainly responsible for febrile gastrointestinal conditions.

 


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