Listeria

Listeria is a bacterial genus containing six species. Named in honour of Joseph Lister, Listeria species are Gram positive bacilli and are typified by L. monocytogenes, the causative agent of Listeriosis.

Listeria ivanovii is a pathogen of ruminants, and can infect mice in the laboratory, although it is only rarely the cause of human disease.

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Listeria monocytogenes

Listeria monocytogenes is a bacterium commonly found in soil, stream water, sewage, plants, and food.^[1] Each bacterium is Gram-positive and rod-shaped. Listeria are known to be the bacteria responsible for listeriosis, a rare but lethal food-borne infection that has a devastating mortality rate of 25%^[2](Salmonella, in comparison, has a less than 1% mortality rate^[3]). They are incredibly hardy and able to grow in temperatures ranging from 4°C (39°F), the temperature of a refrigerator, to 37°C (99°F), the body's internal temperature^[1]. Furthermore, listerosis's deadliness can be partially attributed to the infection's ability to spread to the nervous system and cause meningitis.^[1] Finally, Listeria has a particularly high occurrence rate in newborns because of its ability to infect the fetus by penetrating the endothelial layer of the placenta.^[2]

Pathogenesis

Listeria uses the cellular machinery to move around inside the host cell: it induces directed polymerization of actin by the ActA transmembrane protein, thus pushing the bacterial cell around.

Listeria monocytogenes for example, encodes virulence genes which are thermoregulated. The expression of virulence factor is optimal at 37 degrees Celsius and is controlled by a transcriptional activator, PrfA, whose expression is thermoregulated by the PrfA thermoregulator UTR element. At low temperatures, the PrfA transcript is not translated due to structural elements near the ribosome binding site. As the bacteria infects the host, the temperature of the host melts the structure and allows translation initiation for the virulent genes.

Mechanism of Infection

The majority of Listeria bacteria are targeted by the immune system before they are able to cause infection. Those that escape the immune system's initial response, however, spread though intracellular mechanisms and are therefore guarded against circulating immune factors (AMI).^[2]

To invade, Listeria induces macrophage phagocytic uptake by displaying D-galactose receptors that are then bound by the macrophage's polysaccharide receptors (Notably, in most bacterial infections it is the host cell, not the bacteria, that displays the polysaccharide). ^[3] Once phagocytosed, the bacteria is encapsulated by the host cell's acidic phagolysosome organelle. ^[1] Listeria, however, escapes the phagolysosome by lysing the vacuole's entire membrane with secreted hemolysin, ^[4] now characterized as the exotoxin listeriolysin O.^[1] The bacteria then replicate inside the host cell's cytoplasm. ^[2]

Listeria must then navigate to the cell's periphery to spread the infection to other cells. Outside of the body, Listeria has flagellar-driven motility. However, at 37°C, flagella cease to develop and the bacteria instead usurps the host cell's cytoskeleton to move. ^[2] Listeria, inventively, polymerizes an actin tail or "comet" ^[4], using host-produced actin filaments ^[5] with the promotion of virulence factor ActA^[2]. The comet forms in a polar manner ^[6] and aids the bacteria's migration to the host cell's outer membrane. Gelsolin, an actin filament severing protein, localizes at the tail of Listeria and accelerates the bacterium's motility.^[6] Once at the cell surface, the actin-propelled Listeria pushes against the cell's membrane to form protrusions called filopods^[1] or "rockets". The protrusions are guided by the cell's leading edge ^[7]to contact adjacent cells which subsequently engulf the Listeria rocket and the process is repeated, perpetuating the infection.^[2] Once phagocytosed, the Listeria is never again extracellular: it is an intracytoplasmic parasite ^[4] like Shigella flexneri and Rickettsia.^[2]

Epidemiology

The Center for Science in the Public Interest has published a list of foods that have sometimes caused outbreaks of Listeria: hot dogs, deli meats, raw milk, cheeses (particularly soft-ripened cheeses like feta, Brie, Camembert, blue-veined, or Mexican-style “queso blanco”), raw and cooked poultry, raw meats, ice cream, raw vegetables, raw and smoked fish and the green lip mussel.^[8]

Prevention

The prevention of Listeria as a food illness involves effective sanitizing of food contact surfaces. Alcohol has proven to be an effective topical sanitizer against Listeria. Quaternary ammonium can be used in conjunction with alcohol as a food contact safe sanitizer with increased duration of the sanitizing action. Nonflammable Alcohol Vapour in carbon dioxide NAV-CO2 systems or sodium hypochlorite are frequently used to sanitize surfaces to prevent Listeria.

Modern Relevance/Future Research

Listeriosis is an opportunistic pathogen: it is most prevalent in the elderly, pregnant mothers, and AIDS patients. With improved healthcare leading to a growing elderly population and extended life expectancies for AIDS patients, physicians are more likely to encounter this otherwise rare infection (only 0.7 per 100,000 healthy people are infected with virulent Listeria each year).^[1] Better understanding the cell biology of Listeria infections, including relevant virulence factors, may help us better treat Listeriosis and other intracytoplasmic parasites. Researchers are now investigating the use of Listeria as a cancer vaccine, taking advantage of its "ability induce potent innate and adaptive immunity."^[5]

Treatment

Antibiotics effective against Listeria species include Ampicillin, vancomycin, ciprofloxacin, linezolid, azithromycin, and cotrimoxazole.

Future treatment options

Intralytix has created a virus spray with bacteriophages to be applied to food for the prevention of Listeriosis by killing six strains of L. monocytogenes bacterium.^[9] EBI Food Safety has created and put a similar product on the market, LISTEX™ P100. LISTEX™ P100 prevents Listerios in food by using bacteriophages for killing Listeria. ^[10]

See also

• List of foodborne illness outbreaks

References