Listeria is a bacterial genus containing six species. Named after the English pioneer of sterile surgery, Joseph Lister, the genus was given its current name in 1940. 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.
Listeria monocytogenes is a bacterium commonly found in soil, stream water, sewage, plants, and food. Each bacterium is Gram-positive and rod-shaped. Listeria are known to be the bacteria responsible for listeriosis, a rare but potentially lethal food-borne infection: the case fatality rate for those with a severe form of infection may approach 25% (Salmonella, in comparison, has a mortality rate estimated at less than 1%). Although Listeria has low infectivity, it is a hardy bacterium 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). Listeriosis is a serious illness, and may manifest itself as a meningitis, or affect the newborn due to its ability to penetrate the endothelial layer of the placenta. Vegetables can become contaminated from the soil, and animals can also be carriers. Listeria has been found in uncooked meats, uncooked vegetables, unpasteurized milk, foods made from unpasteurized milk, and processed foods. Pasteurization and sufficient cooking kill listeria; however, contamination may occur after cooking and before packaging. For example, meat processing plants producing ready-to-eat foods, such as hot dogs and deli meats, must follow extensive sanitation policies and procedures to prevent listeria contamination.
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 infect the host, the temperature of the host melts the structure and allows translation initiation for the virulent genes.
Listeria monocytogenes is a Gram-positive, rod-shaped bacterium. It is the agent of listeriosis, a serious infection caused by eating food contaminated with the bacteria. The disease affects primarily pregnant women, newborns, and adults with weakened immune systems. Listeriosis is a serious disease for humans; the overt form of the disease has a mortality greater than 25 percent. The two main clinical manifestations are sepsis and meningitis. Meningitis is often complicated by encephalitis, a pathology that is unusual for bacterial infections. Microscopically, Listeria species appear as small, Gram-positive rods, which are sometimes arranged in short chains. In direct smears, they may be coccoid, so they can be mistaken for streptococci. Longer cells may resemble corynebacteria. Flagella are produced at room temperature, but not at 37°C. Hemolytic activity on blood agar has been used as a marker to distinguish L. monocytogenes among other Listeria species, but it is not an absolutely definitive criterion. Further biochemical characterization may be necessary to distinguish between the different Listeria species. As Gram-positive, nonsporeforming, catalase-positive rods, the genus Listeria was classified in the family Corynebacteriaceae through the seventh edition of Bergey's Manual. The 16S rRNA cataloging studies of Stackebrandt, et al. demonstrated that L. monocytogenes was a distinct taxon within the Lactobacillus-Bacillus branch of the bacterial phylogeny constructed by Woese. In 2001, the Family Listeriaceae was created within the expanding Order Bacillales, which also includes Staphylococcaceae, Bacillaceae and others. Within this phylogeny there are six species of Listeria. The only other genus in the family is Brochothrix.