Listeria Summary

Listeria monocytogenes:

  Gram-positve bacilli capable of forming endospores (see WebLinked image)

  Naturally occuring in environment: soil, water, vegetation, intestinal tract of mammals and birds, fish, insects

  Healthy adults and children can be asymptomatic carriers

  Transmitted to man via direct contact with environmental sources or ingestion of contaminated food products, including milk, cheese, cole slaw, fish, chicken, beef, pork, vegetables, etc.

  Primarily infection of neonates and older adults

  Three clinical manifestations dependent on :

    1. Asymptomatic carriage in healthy children and adults
    2. Infection of fetus or newborn from infected mother
    3. Infection of immunocompromised patients

  Organisms passed from mother to fetus either:

    1. In utero (granulomatosis infantiseptica)
    2. During childbirth

  If infection occurs during delivery, disease presents in one of two clinical forms:

    1. Early septicemic (1-5 days after birth)
    2. Delayed meningitic (10-20 days after birth)

  Facultative intracellular pathogen that invades and grows in variety of mammalian cells, including:

    1. Macrophages
    2. Epithelial cells
    3. Fibroblasts

  Normal macrophages readily phagocytose extracellular organisms that have been opsonized with complement or antibody, but these organisms can then survive intracellularly. Once internalized via phagocytosis, organisms are capable of breaking down phagolysosome membrane, multiplying in the cytoplasm, and passing directly into a new host cell, via pseudopod extensions and actin filaments

  Antibodies, i.e., humoral immune response and extracellular bactericidal factors are ineffective against these organisms

  Cell-mediated immune response dependent on macrophage activation by lymphokines (T cell response) eventually are effective at clearing organisms


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