The bacteria also infect animals, particularly ruminants (Low & Donachie, 1997).
The name Listeria monocytogenes evolved from different names since 1926. Its former names were Erysipelothrix monocytogenes, Cornybacterium parvulum, Cornybacterium infantesipticum, Bacterium monocytogenes and Bacterium monocytogenes hominin (Uniprot Taxonomy, 2009). The genus Listeria was classified under the Cornebacteriaceae family until 1973. Molecular studies classified it as distinct and the Family Listeriaceae was created within the order Bacillales (Todar, 2009).
Listeria contamination can be transmitted through animal feed, manure, mastitic cows, plant and animal products, and through bacterial biofilms. Prevalence of the pathogen in food processing plants is caused by the entry of raw meat and poultry from animals that are infected with Listeria (Dharmarha, 2008). The pathogen can cross to surfaces of floors, sinks, water, equipment, and workers. L. monocytogenes persists with time and contaminate the environment where processing wastes are disposed. Moreover, Listeria is able to survive even under low temperatures.
Infected food sold to consumers can cause major outbreaks. Eating contaminated food allows the entry of L. monocytogenes into the digestive system. After ingestion, the immune system of the host is activated and targets the invading bacteria which are engulfed by phagocytes. Phagocytosis is a major response of cell-mediated immunity (CMI), which is mechanism against L. monocytogenes. CMI can be enough to prevent infection, but in cases where the immune system is compromised or weak, as in young children and the elderly, L. monocytogenes can evade the host defense system and cross the intestinal barrier (Dharmarha, 2008). L. monocytogenes is also able to cross the blood-brain barrier and the placental barrier resulting in infection of the brain (encephalitis and meningitis) and the unborn fetus