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Legionella

From Wikipedia, the free encyclopedia
Legionella
Legionella sp. under UV light
Scientific classification Edit this classification
Domain: Bacteria
Phylum: Pseudomonadota
Class: Gammaproteobacteria
Order: Legionellales
Family: Legionellaceae
Brenner et al. 1979
Genus: Legionella
Brenner et al. 1979
Species

Legionella is a type of bacteria (gram-negative bacteria that includes the species L. pneumophila) that can be seen using a silver stain or grown in a special media that contains cysteine, an amino acid. It is known to cause legionellosis[3] (all illnesses caused by Legionella) including a pneumonia-type illness called Legionnaires' disease and a mild flu-like illness called Pontiac fever.[3] These bacteria are common in many places, like soil and water. There are over 50 species and 70 types (serogroups) identified. It is important to note that Legionella does not spread from person-to-person.[4] Most individuals who are exposed to the bacteria do not get sick.[5] Most outbreaks result from poorly maintained cooling towers.

The cell wall of the Legionella bacteria has parts that determine its specific type. The structural arrangement and building blocks (sugars) in the cell wall help classify the bacteria.

Legionella got its name after a 1976 outbreak of a then-unknown "mystery disease" at a convention of the American Legion, an association of U.S. military veterans, in Philadelphia. This outbreak happened within days of the 200th anniversary of the signing of the Declaration of Independence, which led to it being highly publicized and caused great concern in the U.S.[6] On January 18, 1977, the causative agent was identified as a previously unknown bacterium subsequently named Legionella.

Detection

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Legionella is traditionally detected by culture on buffered charcoal yeast extract agar. It requires the presence of cysteine and iron to grow, so 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[6] concentrate the bacteria (by centrifugation and/or filtration through 0.2-μm filters) before inoculation onto a charcoal yeast extract agar containing selective agents (e.g. glycine, vancomycin, 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.[7]

After incubation for up to 10 days, suspect colonies are confirmed as Legionella if they grow on buffered charcoal yeast extract agar containing cysteine, but not on agar without cysteine added. Immunological techniques are then commonly used to determine the species and/or serogroups of bacteria present in the sample.[4]

Although the plating method is quite specific for most species of Legionella, one study has shown that a coculture method that accounts for the close relationship with amoebae may be more sensitive, since it can detect the presence of the bacteria even when masked by their presence inside the amoebae.[8] Consequently, the clinical and environmental prevalence of the bacteria are likely to be underestimated due to the current laboratory methodology.[9]

Many hospitals use the Legionella urinary antigen test for initial detection when Legionella pneumonia is suspected. Some of the advantages offered by this test are that the results can be obtained in hours rather than the several days required for culture, and that a urine specimen is generally more easily obtained than a sputum specimen. Disadvantages are that the urine antigen test only detects antigen of Legionella pneumophila serogroup 1 (LP1); only a culture will detect infection by non-LP1 strains or other Legionella species and that isolates of Legionella are not obtained, which impairs public health investigations of outbreaks.[10]

A black lateral flow test showing a negative result for Serogroup 1
An example of a LFICA test showing a Negative result for Serogroups 1.

New techniques for the rapid detection of Legionella in water samples have been developed, including the use of polymerase chain reaction and rapid immunological assays. These technologies can typically provide much faster results.[11] Some Lateral Flow Immunochromatographic Assay tests (LFICA) now show results in 25 minutes (for Serogroups 1-15[12]). This allows for a more rapid response and treatment of systems (where required) helping to minimize the exposure to end users.

Government public-health surveillance has demonstrated increasing proportions of drinking water–associated outbreaks, specifically in healthcare settings.[13]

Analyses of genome sequences from Legionellales have identified 24 conserved signature indels (CSIs) in diverse proteins including 30S ribosomal protein S8, periplasmic serine endoprotease DegP precursor, DNA polymerase I, and ABC transporter permease, that are specifically present in different species from the genus Legionella.[14] These molecular signatures provide novel and reliable means for distinguishing members of the genus Legionella from all other bacteria and for their diagnostics.[14]

Pathogenesis

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A Legionella pneumophila bacterium (green) caught by a Vermamoeba vermiformis amoeba (orange)

In the natural environment, Legionella lives within amoebae such as Acanthamoeba spp., Naegleria spp., Vermamoeba spp., or other protozoa such as Tetrahymena pyriformis.[15]

Upon inhalation, the bacteria can infect alveolar macrophages, where they can replicate. This results in Legionnaires' disease and the less severe illness Pontiac fever. Legionella transmission is via inhalation of water droplets from a contaminated source that has allowed the organism to grow and spread (e.g., cooling towers). Transmission also occurs less commonly via aspiration of drinking water from an infected source. Person-to-person transmission has not been demonstrated,[4] though it could be possible in rare cases.[16]

Once inside a host, the incubation period may be 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 be present; however, the disease is generally not a threat to most healthy individuals, and tends to lead to severe symptoms more often in immunocompromised hosts and the elderly. Consequently, the water systems of hospitals and nursing homes should be periodically monitored. The Texas Department of State Health Services provides recommendations for hospitals to detect and prevent the spread of hospital-acquired disease due to Legionella infection.[17] According to Infection Control and Hospital Epidemiology, hospital-acquired Legionella pneumonia has a fatality rate of 28%, and the source is the water distribution system.[18]

Legionella species typically exist in nature at low concentrations, in groundwater, lakes, and streams. They reproduce after entering man-made equipment, given the right environmental conditions.[19] In the United States, the disease affects between 8,000 and 18,000 individuals a year.[20]

Sources

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Documented sources include cooling towers,[21] swimming pools (especially in Scandinavian countries), domestic water systems and showers, ice-making machines,[22] refrigerated cabinets, whirlpool spas,[23][24] hot springs,[25] fountains,[26] dental equipment,[27] soil,[28] automobile windshield washer fluid,[29] industrial coolant,[30] and waste water treatment plants.

Airborne transmission

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The largest[31] and one of the most common sources of Legionnaires' disease outbreaks are cooling towers (heat rejection equipment used in air conditioning and industrial cooling water systems) primarily because of the risk for widespread circulation. Many governmental agencies, cooling tower manufacturers, and industrial trade organizations have developed design and maintenance guidelines for controlling the growth and proliferation of Legionella within cooling towers.[32][33]

Research in the Journal of Infectious Diseases (2006) provided evidence that L. 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. Of 86 confirmed cases during the outbreak, 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.[34]

Because of these risks, a UK legal mandate requires owners to notify the local authorities of any cooling towers a company operates.[35]

Recreational exposure

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As outlined above, cooling towers are well established as sources of Legionella that may have an effect on community exposure to the bacterium and Legionnaires' disease epidemics.[36] In addition to cooling towers, use of swimming pools, spa pools, and other recreational water bodies has also been shown to increase risk of exposure to Legionella, though this differs by species of Legionella.[37] In a review of disease caused by recreational exposure to Legionella, most exposures occurred in spas or pools used by the public (hotels or recreational centers) or in natural settings (hot springs or thermal water).[37]

Hotels and other tourist destinations have contributed to Legionella exposure.[38] Relative danger at commonly used facilities with heating and cooling water systems depends on several factors, such as the water source, how much Legionella is present (if there is any), if and how the water system is treated, how people are interacting with this water, and other factors that make the water systems so dynamic.[38]

In addition to tourists and other recreators, gardeners may be at increased risk for exposure to Legionella.[39] In some countries (like Australia), Legionella lives in soil and compost.[39] Warmer temperatures and increased rainfall in some regions of the world due to climate change may impact Legionella in soil, gardeners' seasonal exposure to contaminated soil, and complex water systems used by the public.[39]

[edit]

Not only are Legionella spp. present in man-made water systems and infrastructure, but also this bacteria lives in natural bodies of water, such as lakes and rivers.[37] Weather patterns and other environmental factors may increase risk of Legionella outbreaks; a study in Minnesota, USA, using outbreak information from 2011 to 2018 showed precipitation as having the greatest effect of increasing risk of Legionella exposure when taking into account other environmental factors (temperature, relative humidity, land use and age of infected person).[40] Weather patterns heavily relate to the established infrastructure and water sources, especially in urban settings. In the US, most cases of Legionella infection have occurred in the summertime, though they were likely more associated with rainfall and humidity than summer temperatures.[39] Severe rain patterns can increase risk of water source contamination through flooding and unseasonable rains; therefore, natural disasters, especially those associated with climate change, may increase risk of exposure to Legionella.[39]

Vaccine research

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No vaccine is available for legionellosis.[41][42] Vaccination studies using heat-killed or acetone-killed cells have been carried out in guinea pigs, which were then given Legionella intraperitoneally or by aerosol. Both vaccines were shown to give moderately high levels of protection. Protection was dose-dependent and correlated with antibody levels as measured by enzyme-linked immunosorbent assay (ELISA) to an outer membrane antigen and by indirect immunofluorescence to heat-killed cells.[43]

Molecular biology

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Legionella has been discovered to be a genetically diverse species with 7-11% of genes being strain-specific. The molecular function of some of the proven virulence factors of Legionella have been discovered.[44]

Legionella control and biomonitoring

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Control of Legionella growth can occur through chemical, thermal or ultraviolet treatment methods.[45]

Heat

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One option is temperature control—i.e., keeping all cold water below 25 °C (77 °F) and all hot water above 51 °C (124 °F).[3]

Temperature affects the survival of Legionella as follows:[3]

  • Above 70 °C (158 °F) – Legionella dies almost instantly
  • At 60 °C (140 °F) – 90% die in 2 minutes (Decimal reduction time (D) = 2 minutes)
  • At 50 °C (122 °F) – 90% die in 80–124 minutes, depending on strain (D = 80–124 minutes)
  • 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

Other temperature sensitivity:[46][47]

  • 60 to 70 °C (140 to 158 °F) to 80 °C (176 °F) – Disinfection range
  • 66 °C (151 °F) – Legionella dies within 2 minutes
  • 60 °C (140 °F) – Legionella dies within 32 minutes
  • 55 °C (131 °F) – Legionella dies within 5 to 6 hours

Water temperature can be monitored in real-time with electronic devices.[48]

Controlling Legionella in potable water systems

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Potable water refers to hot or cold water that is intended for drinking. The CDC recommends that hot water is kept between 140 °F (60 °C) and 120 °F (49 °C) and that cold water is stored below 68 °F (20 °C). It is also recommended to flush infrequently used fixtures regularly.[49]

In building water systems:

[edit]

Chlorine

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A very effective chemical treatment is chlorine. For systems with marginal issues, chlorine provides effective results at >0.5 ppm[50] residual in the hot water system. For systems with significant Legionella problems, temporary shock chlorination—where levels are raised to higher than 2 ppm for a period of 24 hours or more and then returned to 0.5 ppm—may be effective.[51] Hyperchlorination can also be used where the water system is taken out of service and the chlorine residual is raised to 50 ppm or higher at all distal points for 24 hours or more. The system is then flushed and returned to 0.5 ppm chlorine prior to being placed back into service. These high levels of chlorine penetrate biofilm, killing both the Legionella bacteria and the host organisms. Annual hyperchlorination can be an effective part of a comprehensive Legionella preventive action plan.[52]

Copper-silver ionization

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Copper-silver ionization is recognized by the U.S. Environmental Protection Agency and WHO for Legionella control and prevention.[53][54] It is a popular method used in building water systems to control Legionella bacteria, mainly because it is affordable and does not require much maintenance. In this method, the copper ions weaken the bacteria's cell wall, allowing silver ions to then disrupt the bacteria's DNA and proteins, preventing further proliferation.[55] Copper and silver ion concentrations must be maintained at optimal levels, taking into account both water flow and overall water usage, to control Legionella.

Copper-silver ionization is an effective process to control Legionella in potable water distribution systems found in health facilities, hotels, nursing homes, and most large buildings. However, it is not intended for cooling towers because of pH levels greater than 8.6, that cause ionic copper to precipitate. Furthermore, tolyltriazole, a common additive in cooling water treatment, could bind the copper making it ineffective. 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.[56] Copper-silver ionization works slower than other disinfectants and is affected by the water's chemical makeup.[55]

Chlorine dioxide

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Chlorine dioxide has been approved by the U.S. Environmental Protection Agency as a primary disinfectant of potable water since 1945. Chlorine dioxide does not produce any carcinogenic byproducts like some other chlorine sources when used in the purification of drinking water that contains natural organic compounds such as humic and fulvic acids; trihalomethanes may be formed. Drinking water containing such molecules has been shown to increase the risk of cancer.

Since chlorine dioxide stays as a gas, it is easier for it to enter microorganisms to disrupt their internal functions. It works better than chlorine when it comes to disrupting biofilms and is effective across a wider pH range. Testing has demonstrated that low levels of chlorine dioxide reduced Legionella bacteria to undetectable levels in 6 days; however, its effectiveness can be reduced when amoebae are present.[57] It is also not widely distributed in water systems due to concerns regarding its toxicity, unpleasant odors, and harmful byproducts, as stated above, that it can create.

Monochloramine is an alternative. It is created by mixing chlorine and ammonia and valued for its stability and ability to penetrate biofilms better than chlorine. Like chlorine and chlorine dioxide, monochloramine is approved by the U.S. Environmental Protection Agency as a primary potable water disinfectant. Environmental Protection Agency registration requires a biocide label, which lists toxicity and other data required for all registered biocides.

It does work slower than chlorine and requires precise chemical management due to concerns for toxicity.[57]

Ultraviolet

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Ultraviolet light, in the range of 200 to 300 nm, can inactivate Legionella. According to a review by the US EPA, three-log (99.9%) inactivation can be achieved with a dose of less than 7 mJ/cm2.[58]

Biomonitoring

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A Legionella-specific aptamer has been discovered[59] and in 2022 was developed into an assay for detecting to a limit of 104.3 cells/mL with no processing steps.[60]

European standards

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Several European countries established the European Working Group for Legionella Infections[61] to share knowledge and experience about monitoring potential sources of Legionella. The working group has published guidelines about the actions to be taken to limit the number of colony-forming units (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 resampling. 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 resampled. 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.)

Monitoring guidelines are stated in Approved Code of Practice L8 in the UK. These are not mandatory, but are widely regarded as so. An employer or property owner must follow an Approved Code of Practice, 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 – neither could prove a sufficient scheme to be in place while investigating an outbreak, therefore both were fined about £300,000GBP. Important case law in this area is R v Trustees of the Science Museum 3 All ER 853, (1993) 1 WLR 1171[62]

Employers and those responsible for premises within the UK are required under Control of Substances Hazardous to Health to undertake an assessment of the risks arising from Legionella. This risk assessment may be very simple for low risk premises, however for larger or higher risk properties may include a narrative of the site, asset register, simplified schematic drawings, recommendations on compliance, and a proposed monitoring scheme.[63]

The L8 Approved Code of Practice recommends that the risk assessment should be reviewed at least every 2 years and whenever a reason exists to suspect it is no longer valid, such as water systems have been amended or modified, or if the use of the water system has changed, or if there is reason to suspect that Legionella control measures are no longer working.[64]

Weaponization

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Legionella can be used as a weapon, and indeed genetic modification of L. pneumophila has been shown where the mortality rate in infected animals can be increased to nearly 100%.[65][66][67] A former Soviet bioengineer, Sergei Popov, stated in 2000 that his team experimented with genetically enhanced bioweapons, including Legionella.[67] Popov worked as a lead researcher at the Vector Institute from 1976 to 1986, then at Obolensk until 1992, when he defected to the West. He later divulged much of the Soviet biological weapons program and settled in the United States.[68]

See also

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References

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