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 cantaloupe 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. 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 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