Legionella
Legionella is a genus of Gram-negative bacteria 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 including a pneumonia-type illness called Legionnaires' disease and a mild flu-like illness called Pontiac fever. These bacteria are common in many places, like soil and water. There are over 50 species and 70 types identified. Legionella does not spread from person-to-person. Most individuals who are exposed to the bacteria do not get sick. 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 in the cell wall help classify the bacteria.
Etymology
Legionella was named 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. On January 18, 1977, the causative agent was identified as a previously unknown bacterium subsequently named Legionella.Detection
The detection of Legionella typically requires growing them on buffered charcoal yeast extract agar. As Legionella growth requires cysteine and iron, it cannot grow on other common lab media.To detect Legionella in water, it is first concentrated, then inoculated into charcoal yeast extract agar containing selective agents that prevent the growth of other organisms. Heat or acid treatments are sometimes used to eliminate other microbes in a sample.
After incubation for up to ten days, the presence of Legionella can be confirmed if colonies grow on agar with cysteine but not on agar without it. Immunological techniques are then commonly used to determine the species and/or serogroups of bacteria present in the sample.
Some hospitals use the Legionella urinary antigen test when Legionella pneumonia is suspected. This test is faster and uses a urine instead of a sputum sample, giving results in hours compared to days. However, it only detects one type of Legionella: Legionella pneumophila serogroup 1. Non-LP1 strains can only be detected through culturing.
Methods like polymerase chain reaction and rapid immunological tests can detect Legionella in water much faster.
Government health surveillance reports have shown an increase in the proportion of water-related Legionella outbreaks, particularly in healthcare settings.
Genomic analyses of Legionella has resulted in the identification of 24 conserved signature indels 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 of the Legionella. These markers can help distinguish Legionella from other types of bacteria, improving diagnosis.
Airborne transmission
The bacteria can spread through tiny droplets of water that get into the air. People can breathe in these droplets, which then infect cells in the airways, resulting in illness. This is the most common way Legionella spreads.Recreational exposure
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 or in natural settings.Hotels and other tourist destinations have contributed to Legionella exposure. The 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 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, 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 artificial water systems and infrastructure, but also these bacteria live 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. 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.Molecular biology
Legionella is 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.''Legionella'' disease manifestation
Signs and symptoms
, often called atypical pneumonia, is the most common form of legionellosis. The early symptoms are general, including fever, muscle pain, headache, shortness of breath, and a dry or productive cough. Patients with pneumonia who also have neurological or gastrointestinal symptoms like loss of appetite, nausea, or vomiting may be more likely to have legionellosis. A physical examination may reveal abnormal lungs sounds such as rales or rhonchi, and if consolidation is present, there may be signs like egophony or dullness to percussion. Laboratory tests might show either a high or low white blood cell count, low platelets, elevated liver enzymes, low sodium levels, and possibly decreased kidney function.Another form of legionellosis is Pontiac fever, which resembles the flu and includes symptoms like fever, headache, muscle pain, chills, dizziness, nausea, vomiting, and diarrhea. This form is milder than Legionella pneumonia and typically resolves on its own.
In some cases, Legionella can cause infections outside the lungs, including skin and soft tissue infections similar to cellulitis. This is especially a concern if contaminated water comes into contact with surgical wounds. It can also lead to heart infections, such as prosthetic valve endocarditis, myocarditis, and pericarditis. In rare cases, Legionella species have been linked to joint infections and sinusitis.
Pathogenesis
In nature, Legionella bacteria live inside tiny organisms, like amoebae. These amoebae are found in water and soil. They are found in low amounts in natural water sources like lakes and streams, but can grow quickly in man-made water systems under the right conditions.Legionella is spread through inhaling contaminated water droplets, which can come from mists, sprays, or other sources that release tiny droplets into the air. In homes, the most common sources of exposure are shower heads and sinks. The incubation period, or the time it takes for symptoms to appear, is usually two to ten days for Legionella pneumonia and one to three days for Pontiac fever. In rare cases, infection can also happen if people accidentally breathe in drinking water. Person-to-person spread has not been proven, but could be possible in rare situations.
Most healthy people don't get severely sick. The risk of Legionella infection is higher in adults, especially those over 40 years old. People with certain health conditions, like kidney or liver disease, chronic lung disease, or heart disease, are at a greater risk. Those with weakened immune systems, such as cancer patients or organ transplant recipients, are at risk as well. People with chronic illnesses, like autoimmune disease treated with TNF inhibitors, also face a higher risk of infection. Men are about three times more likely than women to get infected, while children are less likely to develop severe cases. Smoking, including cannabis smoking, is strongly linked to increased risk due to damage to the airway lining.
Hospitals and nursing homes are especially concerned about water system safety because vulnerable patients are at a higher risk. For example, the Texas Department of State Health Services, has guidelines for hospitals to stop the spread of Legionella.
In the United States, Legionella infects about 8,000 to 18,000 people each year. Preventing exposure to contaminated water droplets remains key to reducing spread.