Legionella is a Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease. Legionella is common in many environments, with at least 48 species and 70 serogroups identified.

The side-chains of the cell wall carry the bases responsible for the somatic antigen specificity of these organisms.
The chemical composition of these side chains both with respect to components as well as arrangement of the different sugars determines the nature of the somatic or O antigen determinants, which are essential means of serologically classifying many Gram-negative bacteria.

The most important type for causing human disease is Legionella pneumophila (wikipedia).

In July 1976 Legionella were recognized for the first time in the Bellevue-Standfort Hotel in Philadelphia, when an outbreak of pneumonia occurred among people attending the 58. convention of the American Legion. The mystery disease sickened 180 persons of 4400 delegates, causing 29 deaths.
January 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella.
Sources for legionella are heated water systems where optimized conditions for growth are given. Those are:

  • hot-water systems
  • in "older" swimming pools
  • air washer in air conditioning
  • cooling towers
  • in biofilm
  • in hospitals
  • showers in school building
Infection of humans with legionella is basically possible through contact with tap water. Two ways of legionella transmission are possible:
  • aspiration of minor amount of water (= micro-aspiration)
  • the inhalation of mist droplets containing the bacteriavia (= aerosols)