Listeria are non-spore-forming, aerobic, microaerophilic or facultatively anaerobic, Gram-positive rods that are motile by means of peritrichous flagella. The organism is ubiquitous in the environment and may be harbored in many animals. Its presence in plant floors, walls, drains, condensed and standing water, and food residues on processing equipment, and its involvement in certain highly fatal (20 30%) foodborne outbreaks, make this a pathogen of major concern. As a psychrotroph, L. monocytogenes can grow at — 1 to 45°C (optimum 30 37°C). Growth occurs at pH 4.4 9.4 and in water activities above 0.92. Foods of concern for transmission of listeriosis include non-shelf-stable, ready-to-eat meat and poultry products, deli-type foods, soft cheeses, seafood, and unpasteurized dairy products. Although there is an enteric form of listeriosis, the major concern is associated with the nonenteric infection, which affects mainly the central nervous system (meningitis, meningo-encephalitis, bacteremia), and may result in stillbirth, fetal death, or spontaneous abortion in pregnant women. The infectious dose is believed to be > 100 cells/g of food, and the incubation period a few days to 2 3 months. The pathogen is estimated to be responsible for approximately 0.02% of the cases of foodborne illness in the United States and 28% of the deaths.
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