• 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 congenital listeriosis). 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
    • Contaminated food, especially raw milk products; : Listeria can grow in temperatures as low as -1.5 ºC.
    • Congenital listeriosis: transplacental transmission during pregnancy; or direct contact with infected vaginal secretions during birth
    • Incubation time: 3–70 days (usually within a month)
  • Risk factors



Listeria relies on several pathogenic mechanisms to permit infection and evasion of the host immune system:

Clearance of infection primarily relies on macrophage activation by T cells

  • T cells are exposed to Listeria antigens → secrete interferon-γ and TNF-αmacrophages 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

Clinical features

Most infections are asymptomatic or mild, especially in immunocompetent individuals.



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.

  • Blood cultures
    • 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



  • 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.