Legionella are rod-shaped bacteria of the family Legionellaceae that are flagellated at one of the poles. The bacteria are nearly omnipresent and are found primarily in freshwater reservoirs, although they have also been detected in saltwater. They are the causative agents of Legionnaires’ disease (also known as legionellosis), which is associated with severe pneumonia, and of so-called Pontiac fever, a milder course of legionellosis without pneumonia.
What are legionella?
Legionella bacteria of the family Legionellaceae are rod-shaped, gram-negative bacteria that vary in length from about 2 to 5 micrometers. Of the more than 48 known species, the bacterium Legionella pneumophila is most prominent as the causative agent of Legionnaires’ disease and Pontiac fever. In about 90 percent of cases of Legionnaires’ disease or legionellosis, this bacterium is the causative pathogen. The aerobic, non-spore-forming bacteria are usually flagellated monopolar with one or more flagella. This means that they can actively move around. Legionella are also capable of forming biofilms that provide protection from adverse environmental conditions and from biocides. The bacteria depend on amino acids for energy production, as they cannot metabolize sugars, for example. To process amino acids, the presence of cysteine and ferric ions is required. The bacteria are sensitive to desiccation and can survive for only a few minutes at temperatures above 60 degrees Celsius.
Occurrence, distribution, and characteristics
Bacterial species from the Legionellaceae family are found worldwide. Mostly, aerobic bacteria colonize surface waters and water reservoirs. Some species also occur in soil. In usually low concentrations, they can even be detected in groundwater. Some Legionella are resistant to salt water, as has now been proven. Some species, such as Legionella pneumophila, are pathogenic. They are the main causative agents of Legionnaires’ disease or legionellosis, which was first described in 1976 during a Legionnaires’ meeting in Philadelphia, USA. At that time, the transmission route was mainly the contaminated air conditioning system of the hotel where the meeting was held. In the case of acute pneumonia as a result of infection with Legionella, further spread and infection usually occurs via droplet infection, as in the case of many other lung diseases. There is an increased risk of infection in the warm water area of swimming pools, i.e. under showers and in whirlpools, because the bacteria find optimal conditions at temperatures in the range of 30 to 50 degrees Celsius. Since Legionella depend on amino acids for their energy supply as well as on the presence of the sulfur-containing amino acid cysteine and ferric ions, the bacteria are often associated with autotrophic iron–manganese bacteria. Amoebae also play some role in the spread of Legionella pneumophila. The bacteria are phagocytosed by the amoebae but escape lysis. They can multiply inside the amoeba and are relatively well protected against toxins and disinfectants. Amoebae, known to cause amoebic dysentery, form so-called cysts as permanent survival forms that are excreted in the stool and may also contain Legionella. Legionella find the best survival conditions in infectious amoebic cysts because they are relatively well protected from desiccation and contaminants. Survival in the cysts provides a good guarantee for a permanent population of the bacteria, because the bacteria do not form spores or other persistent forms. If the cysts are ingested by humans or animals, the bacteria are released in the digestive tract and can cause a recurrence of legionellosis. The disease is more or less independent of amoebic dysentery, which is caused by the amoebic cysts. It is, so to speak, a kind of double infection with two different pathogens. Analogous to survival in the amoebae, the pathogen also knows how to escape dissolution in the phagocytes after ingestion by producing certain enzymes and exotoxins, and instead benefits from protection by the phagocytes and from their onward transport.
Diseases and ailments
Legionella bacteria are nearly ubiquitous, but the hazards they pose are mainly limited to a few species such as Legionella pneumophila.They pose a risk of contracting Legionnaires’ disease or the less dangerous Pontiac fever. The risk of infection depends very much on the bacterial density and the status of one’s immune system. People with weakened immune systems or with artificially suppressed immune systems are particularly at risk, which may be necessary, for example, after an organ transplant in order to avoid a rejection reaction. Chemotherapy or radiation therapy for concomitant cancer treatment also temporarily weakens the immune system, so that the risk of infection temporarily increases. Experience has shown that infections with Legionnaires’ disease occasionally occur endemically in spatially restricted areas. The endemic infections are usually due to a selectively high concentration of the pathogen. For example, containers and pipes for hot water supply are predestined for an accumulation of Legionella if they are handled improperly, if the water is not heated to values above 60 degrees Celsius, and if there are also periods when the water reservoirs are dormant. This is true, for example, for schools, whose hot water supply is not only not used on weekends, but the temperature is also reduced to values that provide optimal development opportunities for the pathogens in order to save costs. In the past, endemic infections from air conditioning systems in public buildings and hotels also occurred when the system’s water separators turned out to be breeding grounds for infectious Legionella bacteria. The germs were then evenly distributed throughout the buildings by the air conditioning system. Precautionary handling of technical systems that does not provide Legionella with an opportunity for exceptional multiplication therefore provides effective protection against infection.
