The genus Legionella is a pathogenic group of gram negative bacterium, that includes the species L. pneumophila, which causes Legionnaires Disease and L.longbeachae which causes Pontiac Fever. It may be readily visualized with a silver stain. Legionella is common in many environments, including soil and aquatic systems, 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 of a then-unknown "mystery disease" sickened 221 persons, causing 34 deaths. The outbreak was first noticed among people attending a convention of the American Legion—an association of U.S. military veterans. The convention in question occurred in Philadelphia during the U.S. Bicentennial year. This epidemic among U.S. war veterans, occurring in the same city as—and within days of the 200th anniversary of—the signing of the Declaration of Independence, was widely publicized and caused great concern in the United States. On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella. See Legionnaires' Disease for full details.
Legionella is traditionally detected by culture on buffered charcoal yeast extract (BCYE) agar. Legionella requires the presence of cysteine and iron to grow and therefore does not grow on common blood agar media used for laboratory based total viable counts or on site dipslides. 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.
Legionella live within amoebae in the natural environment. Upon inhalation the bacteria can infect alveolar macrophages. subverting the normal host cell machinery to create a niche where the bacteria can replicate. This results in Legionnaires' disease and the lesser form, Pontiac fever. Legionella transmission is airborne via respiratory droplets containing the bacteria. Common sources include cooling towers, swimming pools (especially in Scandinavian countries), 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. Prodromal 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.
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 nearly 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 six L. pneumophila strains have been published and it is now possible to investigate the whole genome by modern molecular methods. It has been discovered that Legionella is a genetically diverse species with 7-11% of genes strain specific. The molecular function of some of the proven virulence factors of Legionella have been discovered by some researchers. Molecular studies are contributing to the fields of clinical research, diagnosis, treatment, epidemiology, and prevention of disease.
Source control 
The most common sources of Legionella and Legionnaires' disease outbreaks are cooling towers (used in industrial cooling water systems), domestic hot water systems,and spas. Additional sources include large central air conditioning systems, fountains, domestic cold water, swimming pools (especially in Scandinavian countries and northern Ireland) 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)|
|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.)|
- Above 70 °C (158 °F) - Legionella dies almost instantly
- At 60 °C (140 °F) - 90% die in 2 minutes (Decimal reduction time (D) = 2)
- At 50 °C (122 °F) - 90% die in 80–124 minutes, depending on strain (Decimal reduction time (D) = 80-124)
- 48 to 50 °C (118 to 122 °F) - Can survive but do not multiply
- 32 to 42 °C (90 to 108 °F) - Ideal growth range
- 25 to 45 °C (77 to 113 °F) - Growth range
- Below 20 °C (68 °F) - Can survive but are dormant, even below freezing
- 60 to 70 °C (140 to 158 °F) to 80 °C (176 °F) - Disinfection range
- 66 °C (151 °F) - Legionella die within 2 minutes
- 60 °C (140 °F) - Legionella die within 32 minutes
- 55 °C (131 °F) - Legionella die within 5 to 6 hours
- 20 °C (68 °F) to 45 °C (113 °F) - Legionella multiply
- 20 °C (68 °F) & below - Legionella are dormant
Control of Legionella growth can occur through chemical or thermal methods. The least expensive and most effective control method is keeping all cold water below 25°C (78°F) and all hot water above 51°C (124°F). Copper-silver ionization is a heavy metal, systemic toxin that destroys biofilms and slimes that can harbor Legionella over the long term. To date no copper-silver system has had EPA approved efficacy studies resulting in final EPA approval as a biocide. Chlorination with chlorine dioxide or monochloramine are extremely effective oxidizing biocides. Ultraviolet light is an excellent treatment but it does not leave a residual in the bulk water system. Thermal eradication is a short term marginally effective and expensive method. Ozone is extremely effective oxidizing biocide for cooling towers, fountains and spas treatment.
A very effective chemical treatment is chlorine. For systems with marginal issues chlorine will provide effective results at 0.5 ppm residual in the hot water system. For systems with significant Legionella problems a residual of as much as 3 ppm free chlorine is required in the hot water system. This level of chlorine will destroy copper piping within 7 to 10 years.
Copper-silver ionization 
Industrial-size copper-silver ionization is recognized by the U.S. Environmental Protection Agency and WHO for Legionella control and prevention. Any biocide that is approved by the EPA will have an MSDS and an EPA approved biocide label. Purchasers are responsible for ensuring they have these labels for all biocides they use. As long as levels of copper and silver ions are sufficient to perform their disinfection function, disinfection can occur as rapidly as one week.
Ionization is an effective industrial control and prevention process to control Legionella in potable water distribution systems at or just below high risk levels and cooling towers found in health facilities, hotels, nursing homes and most large buildings. In 2003 researchers that heavily support ionization developed a four validation process that supports their research on ionization. Ionization became the first such hospital disinfection process to have fulfilled a proposed four-step modality evaluation; by then it had been adopted by over 100 hospitals. Additional studies indicate ionization is superior to thermal eradication.
Chlorine dioxide 
Chlorine dioxide has been EPA approved as a primary potable water disinfectant since 1945. It does not produce any carcinogenic byproducts like chlorine and is not a restricted heavy metal like copper. It has proven excellent control of Legionella in cold and hot water systems and its ability as a biocide is not impacted by pH, or any water corrosion inhibitors like silica or phosphate. Monochloramine is an alternative. Like chlorine and chlorine dioxide, monochloramine is EPA approved as a primary potable water disinfectant. EPA registration requires an EPA biocide label which lists toxicity and other data required by the EPA for all EPA registered biocides. If the product is being sold as a biocide then the manufacturer is legally required to supply a biocide label. And the purcharser is legally required to apply the biocide per the biocide label. When first applied to a system chlorine dioxide can be added at disinfection levels of 2 ppm for 6 hours to clean up a system. This will not remove all biofilm but will effectively remediate the system of Legionella.
Vaccine research 
There is no vaccine for legionellosis, and antibiotic prophylaxis is not effective. Any licensed vaccine for humans in the US is most probably still many years away. Vaccination studies using heat-killed or acetone-killed cells have been carried out, and guinea pigs were challenged intraperitoneally or by using the aerosol model of infection. Both vaccines were shown to give moderately high levels of protection. Protection was found to be dose dependent and correlated with antibody levels as measured by enzyme-linked immunosorbent assay to an outer membrane antigen and by indirect immunofluorescence to heat-killed cells.
Moist heat sterilization 
Minimal monitoring guidelines are stated in ACOP L8 in the UK. These are not mandatory however are widely regarded as so. An ACOP is an Approved Code of Practice which an employer or property owner must follow, or achieve the same result. Failure to show monitoring records to at least this standard has resulted in several high profile prosecutions, e.g. Nalco + Bulmers - Both could not prove a sufficient scheme to be in place whilst investigating an outbreak, therefore both were fined in the region of £300,000GBP. Important case law in this area is R v Trustees of the Science Museum 3 All ER 853, (1993) 1 WLR 1171
Any building within the UK which is subject to HASAW 1974 is required under COSHH and ACOP L8 to have a legionella risk assessment carried out. The report should include a detailed narrative of the site, asset register, simplified schematic drawings (if none available on site), recommendations on compliance and a proposed monitoring scheme.
Log books should be held on site for a minimum of 5 years. E-logbooks are available, however issues can arise if a site audit is carried out and the auditor cannot access the server for any reason (User isn't set up, someone is on holiday/ill, etc.). Electronic logbooks are generally more useful when managing large portfolios, however a duplication is advisable because of the 5 year 'on site / available for inspection' requirement, and therefore kills the 'no paper' argument.
The requirements of the L8 ACOP and regulations says that the legionnaires risk assessment should be reviewed at least every 2 years and whenever there is reason to suspect it is no longer valid, such as if you have added to, or modified, your water systems, or if the use of the water system has changed, or if your legionella control measures are no longer working. For a simple system, the risk assessment review may be fairly straight forward, but for more complex systems it may be best to carry out a full legionnaires risk assessment from scratch. Some of the key things to look out for when completing a risk assessment (when there are no cooling towers or evaporative condensors on a site are dead legs, the disinfection of shower heads, sentinel taps that should have a weekly flush, the use of anti stganation pumps on calorifiers etc.
See also 
- Copper-silver ionization
- Environmental microbiology
- European Working Group for Legionella Infections
- Nosocomial infection
- Ryan KJ; Ray CG (editors) (2004). Sherris Medical Microbiology (4th ed.). McGraw Hill. ISBN 0-8385-8529-9.
- Heuner K, Swanson M (editors). (2008). Legionella: Molecular Microbiology. Caister Academic Press. isbn = 978-1-904455-26-4.
- Lawrence K. Altman (August 1, 2006). "In Philadelphia 30 Years Ago, an Eruption of Illness and Fear". New York Times.
- ISO 11731-2:2004 Water quality -- Detection and enumeration of Legionella -- Part 2: Direct membrane filtration method for waters with low bacterial counts
- Trends in legionnaires disease, 1980-1998: declining mortality and new patterns of diagnosis. Benin AL; Benson RF; Besser RE. Clin Infect Dis November 1, 2002;35(9):1039-46. Epub October 14, 2002.
- Swanson M, Hammer B (2000). "Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages". Annu Rev Microbiol 54: 567–613. doi:10.1146/annurev.micro.54.1.567. PMID 11018138.
- Winn, W.C. Jr. (1996). Legionella (In: Baron's Medical Microbiology, Baron, S. et al., eds. (4th ed.). University of Texas Medical Branch. ISBN 0-9631172-1-1. (via NCBI Bookshelf)
- Report of the Texas Legionnaires' Disease Task Force, Texas Department of State Health Services 
- Infection Control and Hospital Epidemiology, July 2007, Vol. 28, No. 7, "Role of Environmental Surveillance in Determining the Risk of Hospital-Acquired Legionellosis: A National Surveillance Study With Clinical Correlations" 
- Gilsdorf et al., Clinical Infectious Diseases 2005; 40 p1160–1165 "New Considerations in Infectious Disease Outbreaks: The Threat of Genetically Modified Microbes" http://cid.oxfordjournals.org/content/40/8/1160.full
- Raychaudhury S, Farelli JD, Montminy TP, Matthews M, Ménétret JF, Duménil G, Roy CR, Head JF, Isberg RR, Akey CW (Apr 2009). "Structure and function of interacting IcmR-IcmQ domains from a Type IVb secretion system in Legionella pneumophila". Structure 17 (4): 590–601. doi:10.1016/j.str.2009.02.011. PMC 2693034. PMID 19368892.
- 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?". Journal of Infectious Diseases 193 (1): 102–11. doi:10.1086/498575. PMID 16323138.
- "European Working Group for Legionella Infections".
- "LEGIONELLA and the prevention of legionellosis".
- "Safe Hot Water Temperature".
- "Controlling Legionella in Domestic Hot Water Systems".
- "Employers Guide to the control of Legionella".
- Hayes, John. "Copper/silver ionization gaining approval". Professional Carwashing & Detailing 25 (12).
- Stout & Yu 2003 "(1) Demonstrated efficacy of Legionella eradication in vitro using laboratory assays, (2) anecdotal experiences in preventing legionnaires’ disease in individual hospitals, (3) controlled studies in individual hospitals, and (4) validation in confirmatory reports from multiple hospitals during a prolonged time."
- Block 2001.
- Legionella Risk Assessment ACOP L8
- CDC Division of bacterial and Mycotic Diseases: Legionellosis
- Directors of Health Promotion and Education page on Legionellosis
- European Working Group for Legionella Infections
- Legionnaires' disease outbreaks
- Legionella Information
- Legionella Risk management
- Legionella case studies
Maintenance guidelines 
- Centers for Disease Control and Prevention - Procedure for Cleaning Cooling Towers and Related Equipment (pages 239 and 240 of 249)
- Cooling Technology Institute - Best Practices for Control of Legionella
- California Energy Commission - Cooling Water Management Program Guidelines For Wet and Hybrid Cooling Towers at Power Plants
- ASHRAE Guideline
- Guidelines for Control of Legionella in Ornamental Fountains
- Employers Guidelines for prevention of Legionella