legionella legionella legionella is a genus of pathogenic gram-negative bacteria that includes the species l. pneumophila, causing legionellosis (all illnesses caused by legionella) including a pneumonia-type illness called legionnaires' disease and a mild flu-like illness called pontiac fever. legionella legionella sp. under uv light scientific classification domain: bacteria phylum: pseudomonadota class: gammaproteobacteria order: legionellales family: legionellaceaebrenner et al. 1979 genus: legionellabrenner et al. 1979 species legionella adelaidensislegionella anisalegionella beliardensislegionella birminghamensislegionella bozemanaelegionella brunensislegionella busanensislegionella cardiacalegionella cherriilegionella cincinnatiensislegionella clemsonensislegionella donaldsoniilegionella drancourtiilegionella dresdenensislegionella drozanskiilegionella dumoffiilegionella erythralegionella fairfieldensislegionella falloniilegionella feeleiilegionella geestianalegionella genomospecies 1legionella gormaniilegionella gratianalegionella gresilensislegionella hackeliaelegionella impletisolilegionella israelensislegionella jamestowniensiscandidatus legionella jeoniilegionella jordanislegionella lansingensislegionella londiniensislegionella longbeachaelegionella lyticalegionella maceacherniilegionella maioricensislegionella massiliensislegionella micdadeilegionella monrovicalegionella moravicalegionella nagasakiensislegionella nautarumlegionella norrlandicalegionella oakridgensislegionella parisiensislegionella pittsburghensislegionella pneumophilalegionella quateirensislegionella quinlivaniilegionella rowbothamiilegionella rubrilucenslegionella sainthelensilegionella santicrucislegionella shakespeareilegionella spiritensislegionella steeleilegionella steigerwaltiilegionella saoudiensislegionella taurinensislegionella thermalislegionella tucsonensislegionella tunisiensislegionella wadsworthiilegionella waltersiilegionella worsleiensislegionella yabuuchiae legionella may be visualized with a silver stain or cultured in cysteine-containing media such as buffered charcoal yeast extract agar. it is common in many environments, including soil and aquatic systems, with at least 50 species and 70 serogroups identified. these bacteria, however, are not transmissible from person to person; furthermore, most people exposed to the bacteria do not become ill. most outbreaks are traced to poorly maintained cooling towers. 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 and 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 an outbreak in 1976 of a then-unknown "mystery disease" sickened 221 people, causing 34 deaths. the outbreak was first noticed among attendees at a convention of the american legion—an association of u.s. military veterans. the convention occurred in philadelphia during the u.s. bicentennial year on july 21–24, 1976. 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. detection 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 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. 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. 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. consequently, the clinical and environmental prevalences of the bacteria are likely to be underestimated due to the current laboratory methodology. 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. 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. government public-health surveillance has demonstrated increasing proportions of drinking water–associated outbreaks, specifically in healthcare settings. 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, etc. that are specifically present in different species from the genus legionella. these molecular signatures provide novel and reliable means for distinguishing members of the genus legionella from all other bacteria and for their diagnostics. pathogenesis 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. 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, though it could be possible in rare cases. 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 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. 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. 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. in the united states, the disease affects between 8,000 and 18,000 individuals a year. sources documented sources include cooling towers, swimming pools (especially in scandinavian countries), domestic water systems and showers, ice-making machines, refrigerated cabinets, whirlpool spas, hot springs, fountains, dental equipment, soil, automobile windshield washer fluid, industrial coolant, and waste water treatment plants. airborne transmission the largest and one of the most common source 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 organisations have developed design and maintenance guidelines for controlling the growth and proliferation of legionella within cooling towers. 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. because of these risks, a uk legal mandate requires owners to notify the local authorities of any cooling towers a company operates. cooling tower notification recreational exposure 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. 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. 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). hotels and other tourist destinations have contributed to legionella exposure. 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. in addition to tourists and other recreators, gardeners may be at increased risk for exposure to legionella. in some countries (like australia), legionella lives in soil and compost. 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. exposure related to natural disasters and climate change not only are legionella spp. present in man-made water systems and infrastructure, but this bacteria also lives in natural bodies of water, such as lakes and rivers. 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). 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. 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. vaccine research no vaccine is available for legionellosis. 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 to an outer membrane antigen and by indirect immunofluorescence to heat-killed cells. however, a licensed vaccine is most likely still many years away. molecular biology legionella has been discovered to be 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. legionella control a biomonitoring solution exists in a legionella-specific aptamer-based assay. control of legionella growth can occur through chemical, thermal or ultraviolet treatment methods. heat the more expensive option is temperature control—i.e., keeping all cold water below 25 °c (77 °f) and all hot water above 51 °c (124 °f). the high cost incurred with this method arises from the extensive retrofitting required for existing complex distribution systems in large facilities and the energy cost of chilling or heating the water and maintaining the required temperatures at all times and at all distal points within the system. temperature affects the survival of legionella as follows: 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, even below freezing, but are dormant other temperature sensitivity 60 to 70 °c