Listeria monocytogenes is a facultatively anaerobic, gram-positive, rod-shaped coccobacillus bacterium. It is divided into 11 serovars; however, most human and animal cases are caused by serovars 4b, 1/2b, and 1/2a.
L. monocytogenes are ubiquitous in nature and found in soil, water and animal digestive tracts worldwide. Vegetables may be contaminated through soil or the use of manure as fertilizer. L. monocytogenes can survive and multiply at low temperatures usually found in refrigerators. Eating contaminated food with high numbers of L. monocytogenes is the main route of infection.