Listeria monocytogenes is a gram-positive, facultative anaerobic, non-sporeforming rod, which expresses a typical tumbling motility at 20-25 ° C, but not at 35 ° C. The organism is psychotropic and grows over a temperature range of 0° to 45 ° C, with an optimum around 37 ° C.
Listeria can survive acid, nitrite and salt and can thrive even in the refrigerator. It is well established that any fresh food product of animal or plant origin may harbor varying numbers of Listeria monocytogenes.
In general, the organism has been found in raw milk; soft cheese, fresh and frozen meat, poultry and seafood products; and on fruits and vegetable products.
Its prevalence in milk and dairy products has received much attention because of early outbreaks. Dairy cow that appear healthy can serve as reservoirs for Listeria monocytogenes and secret the organism in milk. Once obtained from the cow, milk may further contaminated through inadvertent contact with feces and silage, both of which often contain Listeria and are normally present in the diary farm environment.
Raw vegetables also have been implicated as sources in multiple listeriosis outbreaks. Vegetables may become increasingly important in human listeriosis transmission since current trend in food consummation patterns reflect increasing consumption of raw and ready-to-eat vegetable.
Food contamination of Listeria monocytogenes
