Aetiology

Listeriosis is a food-borne disease. Listeria monocytogenes is the only one of 6 Listeria species that causes human infection.[1][27][28]​ It is a gram-positive, non-spore-forming, facultatively anaerobic bacillus.[5][27][29]​ Different serotypes have been discovered with no diagnostic importance in their identification except for epidemiological purposes.[29] The microbe grows in temperatures between 1°C and 37°C (33.8°F and 98.6°F), but optimal growth is at 30°C to 37°C (86°-98.6°F). Listeria grows better at 4°C to 10°C (39.2°-50°F) than other pathogens. This phenomenon is known as cold enrichment and contributes to Listeria identification.[27][28]​ The bacillus is widespread in nature and commonly contaminates many kinds of food, including soft cheeses, unpasteurised milk and its products, unheated deli meats, premade deli salads, refrigerated pate or meat spreads, refrigerated smoked fish, raw or lightly cooked sprouts, and cut melon.[7][30][31][32]

Rare routes of infection are vertical (trans-placentally or through the contaminated birth canal) and through direct contact (veterinarians, farmers) with contaminated animal embryos or poultry.[2][4] Ruminant carriers may be asymptomatic.[33]

Pathophysiology

The inoculum that can cause disease depends on the immunological profile of the patient as well as on the stomach acidification potential.[1][3] Immunocompromised or older individuals, pregnant women, and neonates are more susceptible due to relatively impaired cellular immunity and inadequate stomach acidification. Immunisation and resistance to listeriosis are provided by sensitised lymphocytes and not by serum-specific antibodies. This mechanism leads to the increase of listeriosis cases among patients with AIDS (especially if CD4 <100/mm³), haematological malignancies, or chemotherapy treatment.[1][18][19] Incubation periods range from 10 to 70 days. The pathogen's virulence also determines the gravity of clinical disease and is related to the specific serotype. Serotypes are identified during Listeria epidemics.

Listeria is an intracellular pathogen with the capability of using the host cell nutrients for division and the potential to escape from immune system attack by moving from cell to cell. Iron is necessary for the metabolism and growth of the bacterium; hence, infection can be seen in iron overload states (which are also a form of immunocompromise).[5][29][34] Through blood circulation, Listeria can involve every organ, but most commonly it involves the central nervous system and the placenta.[1][35]

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