Listeria

Listeria is a genus of bacteria. Until 1992, 10 species were known, each containing two subspecies. By 2019, 20 species had been identified. The genus received its current name, after the British pioneer of sterile surgery Joseph Lister, in 1940. Listeria species are Gram-positive, rod-shaped, and facultatively anaerobic, and do not produce endospores. The major human pathogen in the genus Listeria is L. monocytogenes. It is usually the causative agent of the relatively rare bacterial disease listeriosis, an infection caused by eating food contaminated with the bacteria. Listeriosis can cause serious illness in pregnant women, newborns, adults with weakened immune systems and the elderly, and may cause gastroenteritis in others who have been severely infected. Listeriosis is a serious disease for humans; the overt form of the disease has a case-fatality rate of around 20%. The two main clinical manifestations are sepsis and meningitis. Meningitis is often complicated by encephalitis, when it is known as meningoencephalitis, a pathology that is unusual for bacterial infections. L. ivanovii is a pathogen of mammals, specifically ruminants, and has rarely caused listeriosis in humans. The incubation period can vary from three to 70 days. The first documented case of listeriosis was in 1924. In the late 1920s, two researchers independently identified L. monocytogenes from animal outbreaks. They proposed the genus Listerella in honor of surgeon and early antiseptic advocate Joseph Lister, but that name was already in use for a slime mold and a protozoan. Eventually, the genus Listeria was proposed and accepted. All species within the genus Listeria are Gram-positive, catalase-positive rods and do not produce endospores. The genus Listeria was classified in the family Corynebacteriaceae through the seventh edition of Bergey's Manual of Systematic Bacteriology. The 16S rRNA cataloging studies of Stackebrandt, et al. demonstrated that L. monocytogenes is a distinct taxon within the Lactobacillus-Bacillus branch of the bacterial phylogeny constructed by Woese. In 2004, the genus was placed in the newly created family Listeriaceae. The only other genus in the family is Brochothrix. The genus Listeria as of 2019 is known to contain 20 species: L. aquatica, L. booriae, L. cornellensis, L. costaricensis, L. goaensis, L. fleischmannii, L. floridensis, L. grandensis, L. grayi, L. innocua, L. ivanovii, L. marthii, L. monocytogenes, L. newyorkensis, L. riparia, L. rocourtiae, L. seeligeri, L. thailandensis, L. weihenstephanensis, and L. welshimeri. Listeria dinitrificans, previously thought to be part of the genus Listeria, was reclassified into the new genus Jonesia. Under the microscope, Listeria species appear as small rods, which are sometimes arranged in short chains. In direct smears, they may be coccoid, and 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 from other Listeria species, but it is not an absolutely definitive criterion. Further biochemical characterization may be necessary to distinguish between the different species of Listeria. Listeria can be found in soil, which can lead to vegetable contamination. Animals can be carriers. Listeria has been found in uncooked meats, uncooked vegetables, fruits including rockmelon and apples, pasteurized or unpasteurized milk, foods made from 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 monocytogenes is commonly found in soil, stream water, sewage, plants, and food. Listeria is responsible for listeriosis, a rare but potentially lethal foodborne illness. The case fatality rate for those with a severe form of infection may approach 25%. (Salmonellosis, in comparison, has a mortality rate estimated at less than 1%.) Although L. monocytogenes has low infectivity, it is hardy and can grow in temperatures from 4 °C (39.2 °F) (the temperature of a refrigerator) to 37 °C (98.6 °F), (the body's internal temperature). Listeriosis is a serious illness, and the disease may manifest as meningitis, or affect newborns due to its ability to penetrate the endothelial layer of the placenta. 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. L. monocytogenes, for example, encodes virulence genes that are thermoregulated. The expression of virulence factor is optimal at 39 °C, 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 denatures the structure and allows translation initiation for the virulent genes. The majority of Listeria bacteria are attacked by the immune system before they are able to cause infection. Those that escape the immune system's initial response, however, spread through intracellular mechanisms, which protects them from circulating immune factors (AMI).