- Genus: Listeria
- Species: monocytogenes
- Listeria monocytogenes is the agent responsible for listeriosis, a food-borne disease that can be very serious in humans.
- Disease is generally restricted to the elderly, pregnant women, infants and immunocompromised individuals.
- The major clinical manifestations of listeriosis are sepsis and meningitis.
- Pregnant women who become infected may experience a mild flu-like illness but the fetus may be infected transplacentally, causing miscarriage or stillbirth.
- Listeria are small, Gram-positive bacilli or coccobacilli sometimes seen in chains (cf. Streptococcus) or diphtheriod-like (cf. Corynebacterium).
- The organisms are generally beta-hemolytic, catalase-positive, and grow well at 4°C.
- They are motile at room temperature, but do not produce flagella at 37°C.
- Listeria are intracellular pathogens whose virulence factors (e.g. hemolysin, phospholipase C) are thermoregulated.
- Listeria monocytogenes is ingested with contaminated food.
- Generally, the organisms are destroyed by host defenses but they are able to survive within macrophages via escape from the phagolysosome, allowing intracellular replication. They are, therefore, facultative intracellular parasites.
- The bacteria are capable of polymerizing the host cell's actin to produce motility and this allows spread from cell to cell without an extracellular stage.
- Growth within the macrophages may carry the organisms to the brain or transplacentally, resulting in meningitis or fetal infection, respectively.
- The mortality of listeriosis may exceed 20%.
- Since the organisms are largely intracellular, cell mediated defenses (activated macrophages, CD8+ T-cells) are most important.
- The organisms may be found in a variety of places including animals and soil.
- Contamination leading to disease outbreaks has been associated with hot dogs, various meats, milk, cheese and raw vegetables.
- A recent outbreak (2011) was traced to cantaloupes from Colorado.
- Clinical: Not possible
- Laboratory: The organisms may readily be cultured from blood or CSF. Small, catalase-positive, beta-hemolytic colonies appear after 48 hours. "Cold-enrichment" (placing the samples at 4°C for several months) may help recovery from difficult specimens.
- Sanitary: Hygienic food processing and proper cooking of foods.
- Immunological: No vaccines are available.
- Chemotherapeutic: Ampicillin, vancomycin, ciprofloxin, azithromycin.