Listeriosis

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 individuals, 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, e.g., older adults, pregnant individuals, and those who are immunocompromised. The clinical manifestation is usually mild in pregnant patients, but consequences for the fetus can be very severe (see congenital listeriosis). In immunocompromised and older adults, 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
  • Acquired listeriosis: ingestion of contaminated food (listeria can grow in temperatures as low as -1.5 ºC) ^[1]
    • Raw milk products
    • Raw and smoked meat/fish
    • Industrially processed vegetables such as ready-made salads
  • Congenital listeriosis ^[2]
    • Transplacental transmission during pregnancy
    • Perinatal transmission (via infected vaginal secretions)
• Incubation time: 1–90 days (usually within a month) ^[3]
• Risk factors
  • Immunodeficiency
  • Age > 60
  • Pregnancy
  • Neonates

• Listeria relies on several pathogenic mechanisms for infection and evasion of the host immune system: ^[4]
  • 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, the virulence factor listeriolysin O forms pores to enable the pathogen's escape from the phagosome and its entry into the cytoplasm.
    • Listeriolysin O also inactivates T cell receptors and impairs T cell activation by antigen presenting cells.
  • Cell-to-cell mobility: Actin rocket tails formed by polymerization enable Listeria to move intracellularly and from cell to cell across cell membranes, thus avoiding the antibodies.
• Clearance of infection primarily relies on macrophage activation by T cells. ^[5]
  • T cells that are exposed to Listeria antigens secrete interferon-γ and TNF-α → activation of macrophages → increase in production of reactive oxygen species
  • CD8+ T cells release perforin and granzymes and lyse of 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. ^[6]

• Healthy adults: self-limiting gastroenteritis (watery diarrhea)
• Pregnant women
  • Flu-like illness
  • Chorioamnionitis
  • Spontaneous abortion
  • Sepsis
• Infants
  • Neonatal meningitis
  • Sepsis
  • Granulomatosis infantiseptica
  • For more information, see “Congenital listeriosis” in the articles on congenital TORCH infections.
• Immunocompromised and older individuals
  • Fever, headache, muscle aches, fatigue
  • Nausea, vomiting, watery diarrhea
  • Sepsis
  • Listeria meningitis
    • Neck stiffness
    • Mental status change, impaired consciousness (even coma)
    • Tremor, ataxia
    • Seizures

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 (cold enrichment)
  • Typically grow on blood agar with a narrow band of beta hemolysis surrounding the colonies
• Lumbar puncture: indicated for suspected Listeria meningitis
  • CSF analysis ^[7]
    • Pleocytosis
    • CSF protein levels moderately elevated
    • Decreased CSF glucose
  • Microscopy (low sensitivity)
  • CSF culture

• 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 should avoid food products made from unpasteurized milk and soft cheeses (e.g., brie, feta, and camembert).
• Cook meat thoroughly prior to consumption.
• Listeriosis is a notifiable disease in the United States.