- Clinical science
Listeriosis is an infectious disease caused by the gram-positive bacterium Listeria monocytogenes. The bacteria are usually transmitted to humans through ingestion of contaminated food (especially raw milk products). In immunocompetent patients, the disease is mostly asymptomatic, although mild flu-like symptoms or febrile gastroenteritis may occur. Invasive disease due to bacteria spreading beyond the gastrointestinal tract results in most symptoms and generally develops in high-risk groups, including immunocompromised, elderly, and pregnant patients. The clinical manifestation is usually mild in pregnant women, but consequences for the fetus can be very severe (see ). In immunocompromised and elderly patients, invasive disease can lead to sepsis and meningitis. Suspected listeriosis can be differentiated from other causes of infection through blood cultures. Antibiotic therapy is indicated for high-risk groups; ampicillin or penicillin G are the drugs of choice.
- Pathogen: Listeria monocytogenes; a gram-positive, catalase-positive, rod-shaped, facultative intracellular, motile bacterium
- Route of transmission
- Risk factors
Listeria relies on several pathogenic mechanisms to permit infection and evasion of the host immune system:
- Invasion: the bacteria enter host cells via binding of the virulence factor InlA to E-cadherin in cells of the intestinal epithelium, the blood–brain barrier, and the placenta.
- Proliferation: once inside the phagosome, virulence factors (listeriolysin O, phospholipases) enable escape from the phagosome and entry into the cytoplasm.
- Cell to cell mobility: Listeria can move from cell to cell without being exposed to the extracellular environment via actin rocket tails.
- T cells are exposed to Listeria antigens → secrete interferon-γ and TNF-α → macrophages are activated → increase production of reactive oxygen species
- CD8+ T cells release of perforin and granzymes → lyse host cells infected with Listeria
- Phagocytic mechanisms eventually outperform the evasive mechanisms of the bacteria, especially when acting in synergy with antibiotic drugs
Most infections are asymptomatic or mild, especially in immunocompetent individuals.
- Healthy adults: self-limited gastroenteritis, watery diarrhea
- Pregnant women: flu-like illness, chorioamnionitis, spontaneous abortion, sepsis
- Infants: neonatal meningitis or sepsis,
Testing is generally not needed in immunocompetent individuals, as the infection is self-limiting and symptoms will have resolved by the time listeriosis is diagnosed.
- Indications: suspected listeriosis, particularly among high-risk groups (e.g., pregnant women)
- Characteristic tumbling motility when grown in broth
- Ideal growth at refrigerated temperatures
- Typically grow on blood agar with a narrow band of beta hemolysis surrounding the colonies
- Lumbar puncture: indicated for suspected Listeria meningitis
- In immunocompetent patients with gastroenteritis, no treatment is usually necessary.
- Indications for antibiotic treatment: CNS infection; , endocarditis, bacteremia, neonatal infection, or immunocompromise
- First-line treatment: ampicillin or penicillin G, usually in combination with gentamicin for synergistic effect
- Second-line treatment: cotrimoxazole, macrolide
- High-risk individuals: avoid food products made from unpasteurized milk and soft cheeses (e.g., brie, feta, and camembert).
- Properly cook meat prior to consumption
- Obligation to report: Listeriosis is a notifiable disease in the United States.