Listeria monocytogenes


Gram-positive bacilli
Listeria monocytogenes

  • Gram positive, nonspore-forming, motile (flagella), facultatively anaerobic, rod-shaped bacterium. 
  • Catalase-positive and oxidase-negative, and expresses a beta hemolysin, which causes destruction of red blood cells.
  • Division: Firmicutes.
  • Can grow and reproduce inside the host’s cells and is one of the most virulent food-borne pathogens, with 20 to 30 percent of clinical infections resulting in death.
  • Responsible for an estimated 1,600 illnesses and 260 deaths in the United States (U.S.) annually, listeriosis is the leading cause of death among foodborne bacterial pathogens, with fatality rates exceeding even Salmonella and Clostridium botulinum.
  • Up to 10% of human gastrointestinal tracts may be colonized by L. monocytogenes.
  • Due to its frequent pathogenicity, causing meningitis in newborns (acquired transvaginally), pregnant mothers are often advised not to eat soft cheeses such as Brie, Camembert, feta, and queso blanco fresco, which may be contaminated with and permit growth of L. monocytogenes.
  • Third-most-common cause of meningitis in newborns.
  • Genus Listeria belongs to the class, Bacilli, and the order, Bacillales, which also includes Bacillus and Staphylococcus.
  • Genus Listeria includes six different species (L. monocytogenes, L. ivanovii, L. innocua, L. welshimeri, L. seeligeri, and L. grayi). Both L. ivanovii and L. monocytogenes are pathogenic in mice, but only L. monocytogenes is consistently associated with human illness.


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Important considerations:  The choice of an agent should be based on local antimicrobial sensitivities, site of infection, cost, and comorbid conditions.   Generally, the most common agents/regimens are listed first.  Listed dosages may need to be adjusted for renal dysfunction. 

  1. Ampicillin 2 grams IV every 4-6 hours
  2. Bactrim DS (TMP-SMX) 160/800mg po bid.  Severe infection: 15 to 20 mg/kg/day (based on trimethoprim component)  IV, given in equally divided doses every 6 to 8 hours.

Listeria monocytogenes