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Legionella sp. under UV illumination.
Legionella sp. under UV illumination.
Scientific classification
Kingdom: Bacteria
Phylum: Proteobacteria
Class: Gamma Proteobacteria
Order: Legionellales
Family: Legionellaceae
Genus: Legionella
Brenner et al. 1979

Legionellosis Microchapters


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Historical Perspective




Differentiating Legionellosis from other Diseases

Epidemiology and Demographics

Risk Factors

Natural History, Complications and Prognosis


History and Symptoms

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This page is about microbiologic aspects of the organism(s).  For clinical aspects of the disease, see Legionellosis.

Template:Seealso Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Legionella is a Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila.[1][2]

Legionella are common in many environments, with at least 50 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.

Legionella acquired its name after a July, 1976 outbreak among people attending a convention of the American Legion in Philadelphia. The mystery disease sickened 221 persons, causing 34 deaths. In that bicentennial year, a pandemic among U.S. war veterans was widely publicized and produced a national panic [3]. On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella. See Legionnaire's Disease for full details.

Historical Perspective

In 1976, legionella was discovered after an outbreak among people who attended the Philadelphia convention of the American Legion. Those affected had a type of pneumonia that eventually became known as Legionnaires’ disease.


The genus Legionella is made up of 58 species and 3 subspecies (pneumophila, fraseri, and pascullei ). Legionella is a Gram-negative bacteria with strict growth requirements. Monoclonal antibodies against lipopolysaccharide epitopes on the bacterial cell surface detect L pneumophila serogroup 1.


Legionella reside in drinking and surface water and are transmitted in aerosols to humans. It multiplies intracellularly in alveolar macrophages leading to an immune reaction that produces destructive alveolar inflammation. Inoculation of surgical wounds via contaminated tap water has been recorded.


legionnaires disease is caused by a bacterium known as legionella found naturally in freshwater environments, like lakes and streams.

Legionella can become a health concern when it grows and spreads in human-made water systems example

  • Decorative fountains and water features
  • Showerheads and sink faucets
  • Hot water tanks and heaters
  • Cooling towers
  • Large, complex plumbing systems


Legionella is traditionally detected by culture on buffered charcoal yeast extract (BCYE) agar. Legionellae require the presence of cysteine to grow and therefore do not grow on common blood agar media used for laboratory based total viable counts or on site displides. Common laboratory procedures for the detection of Legionella in water[4] concentrate the bacteria (by centrifugation and/or filtration through 0.2 micron filters) before innoculation onto a charcoal yeast extract agar containing antibiotics (e.g. glycine vancomycim polymixin cyclohexamide, GVPC) to suppress other flora in the sample. Heat or acid treatment are also used to reduce interference from other microbes in the sample.

After incubation for up to 10 days, suspect colonies are confirmed as Legionellae if they grow on BCYE containing cysteine, but not on agar without cysteine added. Immunological techniques are then commonly used to establish the species and/or serogroups of bacteria present in the sample.

New techniques for the rapid detection of Legionella in water samples are emerging including the use of polymerase chain reaction (PCR)[5] and rapid immunological assays[6]. These technologies can typically provide much faster results.


Legionella have been known for some time to live within amoebae in the natural environment.[7] Legionella species are the causative agent of the human Legionnaires' disease and the lesser form, Pontiac fever. Legionella transmission is via aerosols—the inhalation of mist droplets containing the bacteria. Common sources include cooling towers, domestic hot-water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources of Legionella include freshwater ponds and creeks. Person-to-person transmission of Legionella has not been demonstrated.[8]

Once inside a host, incubation may take up to two weeks. Initial symptoms are flu-like, including fever, chills, and dry cough. Advanced stages of the disease cause problems with the gastrointestinal tract and the nervous system and lead to diarrhea and nausea. Other advanced symptoms of pneumonia may also present.

However, the disease is generally not a threat to most healthy individuals, and tends to lead to harmful symptoms only in those with a compromised immune system and the elderly. Consequently, it is actively checked for in the water systems of hospitals and nursing homes. In the United States, the disease affects between 8,000 to 18,000 individuals a year.

Molecular biology

With the application of modern molecular genetic and cell biological techniques, the mechanisms used by Legionella to multiply within macrophages are beginning to be understood. The specific regulatory cascades that govern differentiation as well as the gene regulation are being studied. The genome sequences of four L. pneumophila strains have been published and it is now possible to investigate the whole genome by modern molecular methods. The molecular studies are contributing to the fields of clinical research, diagnosis, treatment, epidemiology, and prevention of disease.[2]

Controlling potential sources of Legionella

Common sources of Legionella include cooling towers used in industrial cooling water systems as well as in large central air conditioning systems, domestic hot water systems, fountains, and similar disseminators that draw upon a public water supply. Natural sources include freshwater ponds and creeks.

Recent research in the Journal of Infectious Diseases provides evidence that Legionella pneumophila, the causative agent of Legionnaires disease, can travel at least 6 km from its source by airborne spread. It was previously believed that transmission of the bacterium was restricted to much shorter distances. A team of French scientists reviewed the details of an epidemic of Legionnaires disease that took place in Pas-de-Calais in northern France in 2003–2004. There were 86 confirmed cases during the outbreak, of whom 18 perished. The source of infection was identified as a cooling tower in a petrochemical plant, and an analysis of those affected in the outbreak revealed that some infected people lived as far as 6–7 km from the plant.[9]

Several European countries established a working group known as the European Working Group for Legionella Infections (EWGLI)[10] to share knowledge and experience about monitoring potential sources of Legionella. That group has published guidelines about the actions to be taken to limit the number of colony forming units (i.e. live bacteria that are able to multiply) of Legionella per litre

Legionella bacteria cfu/litre Action required - 35 samples per facility is required, 20 water / 10 swabs
1000 or less System under control. (150+ CFU/ml in healthcare facilities or nursing homes require immediate action)
more than 1000
up to 10,000
Review program operation. The count should be confirmed by immediate re-sampling. If a similar count is found again, a review of the control measures and risk assessment should be carried out to identify any remedial actions.
more than 10,000 Implement corrective action. The system should immediately be re-sampled. It should then be ‘shot dosed’ with an appropriate biocide, as a precaution. The risk assessment and control measures should be reviewed to identify remedial actions.

Temperature affects the survival of Legionellae as follows:

  • 70 to 80 °C (158 to 176 °F) - Disinfection range
  • At 66 °C (151 °F) - Legionellae die within 2 minutes
  • At 60 °C (140 °F) - Legionellae die within 32 minutes
  • At 55 °C (131 °F) - Legionellae die within 5 to 6 hours
  • 50 to 55 °C (122 to 131 °F) - They can survive but do not multiply
  • 20 to 50 °C (68 to 122 °F)- Legionellae growth range
  • 35 to 46 °C (95 to 115 °F) - Ideal growth range
  • Below 20 °C (68 °F) - Legionellae can survive but are dormant

The above data can be confirmed in an online article by Reliance World Wide.[11]

Control of Legionella growth can be through : A. Chemical Treatment 1. Short term - Cl2, must be repeated every 3 to 5 weeks, corrosion factors 2. Long term - ClO2, takes up to 1 month for system saturation B. Non-Chemical Treatment 1. Short term - Thermal eradication - must be repeated every 3 to 5 weeks 2. Long term - Industrial size copper silver ionisation (Ionization) technology such as 1-AquaLyse.ca, 2-Liquitech or 3-TarnPure.

Units have provisional EPA approval. No copper-silver has yet had efficacy data approved by EPA or received final EPA approval.

Guidelines for control of Legionella in cooling towers

Many governmental agencies, cooling tower manufacturers and industrial trade organizations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers. Below is a list of sources for such guidelines:


  1. Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th Edition ed.). McGraw Hill. ISBN 0-8385-8529-9.
  2. 2.0 2.1 Heuner K, Swanson M (editors). (2008). Legionella: Molecular Microbiology. Caister Academic Press. ISBN 978-1-904455-26-4 .
  3. Lawrence K. Altman (09-01-2006). "In Philadelphia 30 Years Ago, an Eruption of Illness and Fear". New York Times. Check date values in: |date= (help)
  4. ISO 11731
  5. Biologie moléculaire, diagnostic moléculaire, microbiologie, biotechnologies
  6. Legionella Field Test - Products - Hydrosense
  7. Swanson M, Hammer B (2000). "Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages". Annu Rev Microbiol. 54: 567–613. PMID 11018138.
  8. Winn, W.C. Jr. (1996). Legionella (In: Baron's Medical Microbiology, Baron, S. et al, eds (4th Edition ed.). University of Texas Medical Branch. ISBN 0-9631172-1-1. (via NCBI Bookshelf)
  9. Nguyen T, Ilef D, Jarraud S, Rouil L, Campese C, Che D, Haeghebaert S, Ganiayre F, Marcel F, Etienne J, Desenclos J (2006). "A community-wide outbreak of legionnaires disease linked to industrial cooling towers--how far can contaminated aerosols spread?". J Infect Dis. 193 (1): 102–11. PMID 16323138.
  10. European Working Group for Legionella Infections
  11. What is Legionnaires' Disease?

See also

External links