Listeria monocytogenes is a Gram-positive facultative intracellular pathogen. It is commonly found in environments such as surface water, soil, plant and animal tissues. L. monocytogenes is also widely distributed in food, especially in meat, smoked fish, raw milk, cheese, eggs and raw vegetables. These bacteria are able to survive in conditions of processes related to food production, e.g. cooling temperature. That capacity makes contaminated food the main source of human infections. The number of consumed bacteria in food and the health of the human influence the course of the infection. People with intact immune systems usually show harmless symptoms: abdominal pains, diarrhea and increased body temperature. In the case of people with deficient immunity, especially pregnant women and elderly, infection can lead to a serious disease called listeriosis. In the last years listeriosis has become one of the most dangerous food-borne diseases with a high mortality rate: 20-30%. According to the EFSA report the number of cases of the disease in the European Union in 2007 was 1,558. L. monocytogenes is able to produce various virulence factors linked to the pathogenesis that allow the bacteria to avoid or significantly reduce the effects of the host immune responses. Mechanisms responsible for the pathogenic properties of bacteria are objects of research interest. The results of these studies will create more effective ways of preventing and treating the disease, e.g. by producing inhibitory substances for controlling bacteria growth in foods or identifying targets for new antimicrobial agents. Despite molecular biological tools that have contributed to significant progress in research on genes related to the pathogenesis of L. monocytogenes infections several aspects of the disease still need to be researched to understand its processes.
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