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  • Genus: Legionella
  • Species: pneumophila

  • Legionellae are ubiquitous aquatic saprophytes
  • Legionella is a relatively recent discovery in terms of human disease.
  • L. pneumophila, in particular, is the etiologic agent of Legionnaire's disease, first described in Philadelphia in 1976.
  • Legionnaire's disease is characterized by cough and fever with radiologic evidence of pneumonia.
  • Legionella is a unique, previously unrecognized bacterium. One might say it is a new organism for an old disease.

  • Legionella are motile, catalase-positive, Gram-negative bacilli. Some strains produce a yellow fluorescence under 366 nm ultraviolet light. Most are fastidious, requiring iron and L-cysteine for growth.
  • There are 25 species of Legionella and 42 serogroups. Most (85%) clinical isolates are L. pneumophila serotype 1.
  • The Legionella cell wall is mostly diaminopimelic acid (DAP) and contains little peptidoglycan. The LPS appears to lack the endotoxic lipid A moiety.
  • Several species produce hemolysins and there are some reports of exotoxins.
  • Legionella are capable of intracellular (macrophage) multiplication.

  • Legionella have, in general, a low potential for virulence and most infections occur in persons having compromised immunity or pulmonary function.
  • There are two forms of disease produced by Legionella:

    1. Pontiac fever: An epidemic flu-like condition described in Pontiac Michigan in 1968 was later found to be due to Legionella. This condition was marked by fever, chills, headache and malaise that lasted 2-5 days and resolved.
    2. Legionnaire's disease: A severe pneumonia characterized by fever, chills and a non-productive cough. This multi-organ disease has significant mortality if not treated promptly.

  • The virulence of Legionella is dependent upon their ability to survive and multiply within macrophages.

  • Most healthy individuals resist infection by Legionella but the mechanisms are not well understood.

  • Legionella are typically associated with aerosolized water (central air conditioning, cooling towers, showers, whirlpools).
  • Disease is generally waterborne; transmission occurs via airborne droplets.
  • The organisms exist in nature; humans are an accidental host.


  • Clinical: Symptoms include headache, malaise, rapid fever, nonproductive cough, pneumonia. Generally, disease is difficult to diagnose but might be suspected in middle aged to older men who smoke and drink.
  • Laboratory: Bacteria can be grown on Buffered Charcoal-Yeast Extract (BCYE) agar. Direct immunofluorescent may be used to visualize the organisms. An increase in Legionella-specific serum antibody is evidence of infection.


  • Sanitary: Regular maintenance of air conditioning or the inclusion of biocidal compounds into water cooling towers reduces the reservoir. Similarly, hyperchlorination of the water supply eliminates the source.
  • Immunological: None available.
  • Chemotherapeutic: Erythromycin is the drug of choice.

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