BACTERIA · Gram-positive rod (facultative intracellular)
Listeria monocytogenes
Listeria monocytogenes — Gram stain
Stain: Gram stainMorphology: Small gram-positive rods, sometimes coccobacillary/coryneform-appearingYield: HIGHDifficulty: MEDIUM
Image: Wikimedia Commons (CC BY-SA 4.0)
Key facts
**Pathogenesis**: Facultative intracellular — uses ActA to polymerize host actin into 'rockets' that propel cell-to-cell, evading antibodies. Listeriolysin O escapes phagolysosome. Grows at 4°C (cold enrichment). **Diagnostic clue**: Tumbling motility in broth at 22°C, umbrella motility in stab culture, beta-hemolytic, catalase-positive. **Virulence**: Internalin (epithelial entry), listeriolysin O, ActA.
Boards buzzwords
- tumbling motility at 22°C
- actin rockets
- cold enrichment
- unpasteurized dairy/deli meats
- neonatal meningitis
- granulomatosis infantiseptica
Associated diseases
- Neonatal meningitis (#3 cause)
- Meningitis in elderly (>60) and immunocompromised
- Spontaneous abortion / granulomatosis infantiseptica
- Gastroenteritis (unpasteurized dairy, deli meats, soft cheeses)
Treatment
Ampicillin (± gentamicin for synergy) — drug of choice; TMP-SMX for penicillin-allergic; ceftriaxone is INACTIVE — always add ampicillin in elderly/immunocompromised empiric meningitis coverage
