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Legionella
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Legionella is a Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila.
Legionella is 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.
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Encyclopedia
Legionella is a Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila.
Legionella is 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
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On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella. See Legionnaire's Disease for full details.
Detection
Legionella is traditionally detected by culture on buffered charcoal yeast extract (BCYE) agar. Legionella requires the presence of cysteine to grow and therefore does 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 concentrate the bacteria (by centrifugation and/or filtration through 0.2 micrometre filters) before inoculation 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 Legionella 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.
Many hospitals use the Legionella Urinary Antigen test for initial detection when Legionella pneumonia is suspected. Some of the advantages offered by this test is that the results can be obtained in a matter of hours rather than the five days required for culture, and that a urine specimen is generally more easily obtained than a sputum specimen. One disadvantage is that the urine antigen test only detects anti-bodies towards Legionella pneumophila; only a culture will detect infection by the other Legionella species.
New techniques for the rapid detection of Legionella in water samples are emerging including the use of polymerase chain reaction (PCR) and rapid immunological assays. These technologies can typically provide much faster results.
Pathogenesis
Legionella live within amoebae in the natural environment. 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.
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 should be actively checked for in the water systems of hospitals and nursing homes. The Texas Department of State Health services provides recommendations for hospitals to detect and prevent the spread of nosocomial infection due to legionella. According to the journal "Infection Control and Hospital Epidemiology," Hospital-acquired Legionella pneumonia has a fatality rate of 28%, and the source is the water distribution system.
In the United States, the disease affects between 8,000 to 18,000 individuals a year.
Possible use as a weapon
It has been suggested that Legionella could be used as a weapon, and indeed genetic modification of Legionella pneumophila has been shown where the mortality rate in infected animals can be increased to near 100%.
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.
Source control
Common sources of Legionella include cooling towers (used in industrial cooling water systems), 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. Many governmental agencies, cooling tower manufacturers, and industrial trade organisations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers.
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, northern France, in 2003–2004. There were 86 confirmed cases during the outbreak, of which 18 resulted in death. 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.
Several European countries established the European Working Group for Legionella Infections (EWGLI) to share knowledge and experience about monitoring potential sources of Legionella. The EWGLI has published guidelines about the actions to be taken to limit the number of colony-forming units (CFU, that is, live bacteria that are able to multiply) of Legionella per litre:
| Legionella bacteria CFU/litre | Action required (35 samples per facility are required, including 20 water and 10 swabs) |
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| 1000 or less | System under control. | 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. (150+ CFU/ml in healthcare facilities or nursing homes require immediate action.) |
Temperature affects the survival of Legionella, as follows:
Legionella dies instantly - pasteurisation occurs. At 55 °C (131 °F) - 95% die 50 to 55 °C (122 to 131 °F) - Can survive but do not multiply 35 to 46 °C (95 to 115 °F) - Ideal growth range 20 to 50 °C (68 to 122 °F) - Growth range Below 20 °C (68 °F) - Can survive but are dormant, even below freezing
From published Health and Safety Executive guidance for employers in the United Kingdom
The above data are recognised industry standard ranges within the United Kingdom, taken from ACOP L8.
Control of Legionella growth can occur through chemical or thermal methods. Copper-silver ionization is a chemical process that disperses and destroys biofilms and slimes that can harbor Legionella over the long term. Hyperchlorination with chlorine dioxide or monochloramine is a similarly dispersive alternative treatment. Ultraviolet light, thermal eradication, and ozone are short-term (nondispersive) treatments.
Chlorine
A short-term chemical treatment, chlorine must be repeated every 3–5 weeks. Corrosion factors apply.
Copper-silver ionization
Industrial-size copper-silver ionization has been approved by the U.S. Environmental Protection Agency for Legionella control and prevention. This long-term treatment takes 30–45 days for complete control.
Chlorine dioxide
Chlorine dioxide is an approved potable water disinfectant with few environmental and health issues. When applied properly, it can significantly reduce Legionella levels within weeks. It can take as long as 17 months for complete control of Legionella through an entire system. Monochloramine is an alternative.
Thermal eradication
Thermal eradication (superheating to and flushing) is a nonchemical treatment that typically must be repeated every 3–5 weeks.
See also
External links
Images
Maintenance guidelines
- Procedure for Cleaning Cooling Towers and Related Equipment (pages 239 and 240 of 249)
- Best Practices for Control of Legionella
- Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants
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