Listeria monocytogenes

Microbiology - The Tumbling Intruder

• Identity: Gram-positive rod, facultative intracellular, and facultative anaerobe.
• Lab Tests: Catalase-positive, oxidase-negative, and displays a narrow zone of beta-hemolysis on blood agar.
• Motility: Exhibits characteristic tumbling motility at 22-28°C via flagella; it is non-motile at 37°C. Intracellularly, it uses host cell actin polymerization for movement.

⭐ Listeria is famous for its characteristic 'tumbling motility' in broth culture at room temperature, which is distinct from the motility of other bacteria.

Pathogenesis - Actin Rocket Evasion

Listeria invades the gastrointestinal tract, using Internalin A/B for entry into host cells. To replicate, it must first escape the phagolysosome.

• Phagosome Escape: Secretes Listeriolysin O (LLO) and phospholipases to lyse the vacuolar membrane.
• Cytosolic Hijacking: Once in the cytoplasm, it polymerizes host actin using the surface protein ActA, forming a propulsive "actin rocket" tail.
  • 📌 ActA polymerizes Actin for motility.
• Cell-to-Cell Spread: The actin rocket pushes the bacterium directly into an adjacent cell, allowing it to spread while avoiding extracellular immunity (e.g., antibodies, complement).

⭐ Listeriolysin O (LLO) is a pore-forming toxin crucial for escaping the phagosome; its activity is maximized at acidic pH, characteristic of the phagolysosomal environment.

Clinical Syndromes - Trouble at the Extremes

• Healthy Adults: Typically a mild, self-limited febrile gastroenteritis; often resolves without treatment.

• High-Risk Groups:

  • Pregnant Women: Presents as a non-specific, flu-like illness. Can cause amnionitis, preterm labor, or stillbirth.
  • Newborns:
    • Early-onset (in utero): "Granulomatosis infantiseptica" - a severe, disseminated disease with abscesses and granulomas in multiple organs.
    • Late-onset (2-3 weeks): Classically presents as meningitis or meningoencephalitis with sepsis.
  • Elderly (>60) & Immunocompromised: High risk for severe, invasive disease, including sepsis, meningitis, and meningoencephalitis.

⭐ In pregnant women, Listeria infection can be a mild, flu-like illness but can lead to devastating fetal consequences like disseminated abscesses and granulomas, known as granulomatosis infantiseptica.

Diagnosis & Treatment - The Cold Case Cure

• Culture from sterile sites (blood, CSF) is definitive.
  • CSF analysis: ↑ neutrophils, ↑ protein, ↓ glucose (mimics bacterial meningitis).
  • Gram stain of CSF is often negative.
• Cold enrichment: Growth at 4°C can isolate the organism from mixed samples.
• Weakly beta-hemolytic on blood agar.
• Treatment: Ampicillin is the drug of choice, often with gentamicin.

⭐ The ability to grow at refrigeration temperatures (4°C), known as cold enrichment, is a key laboratory feature that allows Listeria to multiply in contaminated refrigerated foods.

High‑Yield Points - ⚡ Biggest Takeaways

• Gram-positive rod with characteristic tumbling motility and ability to form actin rockets for cell-to-cell spread.
• Unique in its ability to grow at refrigerated temperatures (cold enrichment), contaminating foods like deli meats and soft cheeses.
• Major cause of meningitis in newborns and the immunocompromised.
• In pregnant women, can lead to amnionitis, septicemia, and spontaneous abortion.
• Treatment of choice is ampicillin.