Listeria Monocytogenes

Facultative, intracellular parasite

Characteristics/Epidemiology

Widespread among animals in nature

Able to grow at 4 degress celcius.

1-15% human - asymptomatic intestinal carriers

2-3% of processed diary product, 20-30% of ground meats, and majority of poultry are contaminated with L. monocytogenes.

Pathogenesis/Transmission

Transmission usually food-borne.

Infection most common in pregnant women, their foetuses, or newborn, and in immunocomprimsed persons.

Pathogenesis

• Enters (macrophage) cells by normal phagocytosis, but escapes from phagosome via actions of listeriolysin O (membrane-damaging toxin, similar to streptolysin O and pneumolysin).
  -> Mutants lacking listeriolysin O are avirulent.
• -> Grows in cytosol
• -> assembles an actin filament "tail" that pushes the bacterium to the surface of the macrophage
• -> moves directly into a neighboring cell

Clinical significance

Listeria monocytogenes can cause:

• Septicemia (most common)
• Meningitis (most common)
• Granulomatous skin lesion
• Lymphadenitis
• Endocarditis
• Flu-like illness in pregnant women (usually 3rd trimester)
  -> can pass to newborn/foetus (also by asymptomatic vaginal carrier)

Laboratory identification

Blood agar - small colonies with narrow zone of beta hemolysis.

(Common to all listeria) Distinguished by morphology, motility, and catalase-positivity

Treatment

First line: Ampicillin

Second line: Trimethoprim + sulfamethoxazole

Prevention/immunity

Prevention: Proper food preparation and handling

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Things to revise/add later:

Bibliography: LWW microbiology

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