Listeria monocytogenes

Listeria monocytogenes is the causative agent of Listeriosis. It is one of the most virulent foodborne pathogens with 20 percent of clinical infections resulting in death^{[citation needed]}. Responsible for approximately 2,500 illnesses and 500 deaths in the United States (U.S.) annually, Listeriosis is the leading cause of death among foodborne bacterial pathogens with fatality rates exceeding even Salmonella and Clostridium botulinum. 

L. monocytogenes is a Gram-positive bacterium, in the division Firmicutes, named for Joseph Lister. Motile via flagella at 30°C and below but usually not at 37°C,L. monocytogenes can instead move within eukaryotic cells by explosive polymerization of actin filaments (known as comet tails or actin rockets). Studies suggest that up to 10% of human gastrointestinal tracts may be colonized by L. monocytogenes. Nevertheless, clinical diseases due to L. monocytogenes are more frequently recognized by veterinarians, especially as meningo-encephalitis in ruminants. See: listeriosis in animals. Due to its frequent pathogenicity causing meningitis in newborns (acquired transvaginally), pregnant mothers are often advised not to eat soft cheeses such as Brie, Camembert, feta and queso blanco fresco, which may be contaminated with and permit growth of L. monocytogenes. It is the third most common cause of meningitis in newborns. More recently, L. monocytogenes has been used as the model organism to illustrate the Patho-biotechnology concept.

L. monocytogenes is a gram positive, non-spore forming, motile, facultatively anaerobic, rod shaped bacterium. It is catalase positive, oxidase negative, and expresses a Beta hemolysin which causes destruction of red blood cells. This bacterium exhibits characteristic tumbling motility when viewed with light microscopy. Although L. monocytogenes is actively motile by means of peritrichous flagella at room temperature (20-25C), the organism does not synthesize flagella at body temperatures (37C). 

The genus Listeria belongs to the Clostridium sub-branch, together with Staphylococcus, Streptococcus, Lactobacillus and Brochothrix. The genus Listeria includes 6 different species (L. monocytogenes, L. ivanovii, L. innocua, L. welshimeri, L. seegligeri, and L. grayi). Both L. ivanovii and L. monocytogenes are pathogenic in mice, but only L. monocytogenes is consistently associated with human illness. There are 13 serotypes of L. monocytogenes which can cause disease, but more than 90 percent of human isolates belong to only three serotypes: 1/2a, 1/2b, and 4b. L. monocytogenes serotype 4b strains are responsible for 33 to 50 percent of sporadic human cases worldwide and for all major foodborne outbreaks in Europe and North America since the 1980s.

Infection by L. monocytogenes causes the disease listeriosis. The manifestations of listeriosis include septicemia, meningitis (or meningoencephalitis), encephalitis,corneal ulcer, pneumonia,and intrauterine or cervical infections in pregnant women, which may result in spontaneous abortion (2nd/3rd trimester) or stillbirth. Surviving neonates of Fetomaternal Listeriosis may suffer granulomatosis infantiseptica - pyogenic granulomas distributed over the whole body, and may suffer from physical retardation. Influenza-like symptoms, including persistent fever, usually precede the onset of the aforementioned disorders. Gastrointestinal symptoms such as nausea, vomiting, and diarrhea may precede more serious forms of listeriosis or may be the only symptoms expressed. Gastrointestinal symptoms were epidemiologically associated with use of antacids or cimetidine. The onset time to serious forms of listeriosis is unknown but may range from a few days to three weeks. The onset time to gastrointestinal symptoms is unknown but probably exceeds 12 hours. An early study suggeseted that L. monocytogenes was unique among Gram-positive bacteria in that it possessed lipopolysaccharide, which served as an endotoxin. A later study did not support these findings.

The infective dose of L. monocytogenes varies with the strain and with the susceptibility of the victim. From cases contracted through raw or supposedly pasteurized milk, one may safely assume that in susceptible persons, fewer than 1,000 total organisms may cause disease. L. monocytogenes may invade the gastrointestinal epithelium. Once the bacterium enters the host's monocytes, macrophages, or polymorphonuclear leukocytes, it becomes blood-borne (septicemic) and can grow. Its presence intracellularly in phagocytic cells also permits access to the brain and probably transplacental migration to the fetus in pregnant women. The pathogenesis of L. monocytogenes centers on its ability to survive and multiply in phagocytic host cells.

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