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Legionella is a waterborne bacterium that causes Legionnaire’s disease. It grows in slow-moving or still warm water and can be found in plumbing, showerheads, and water storage tanks. Outbreaks of Legionella pneumonia have been attributed to evaporative condensers and cooling towers.

Legionella pneumophila is a thin, aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of legionellosis or Legionnaires’ disease.

L. pneumophila is an acid-fast, non-sporulating, and morphologically a non-capsulated rod-like bacteria often characterized as being a coccobacillus. aerobic and unable to hydrolyze gelatin or produce urease, they are also non-fermentative. L. pneumophila is neither pigmented nor does it autofluorescence. It is oxidase- and catalase-positive, and produces beta-lactamase. L. pneumophilahas a colony morphology that is grey-white with a textured cut-glass appearance; it also requires cysteine and iron to thrive.

Cell membrane structure

While L. pneumophila is categorized as a Gram-negative organism, it stains poorly due to its unique lipopolysaccharide-content in the outer leaflet of the outer cell membrane. The bases for the somatic antigen specificity of these organisms are located on the side-chains of its cell wall.

The chemical composition of these side chains both with respect to components as well as the arrangement of the different sugars determines the nature of the somatic or O-antigen determinants, which are important means of serologically classifying many Gram-negative bacteria. At least 35 different serovars of L. pneumophila have been described as well as several other species being subdivided into a number of serovars.

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What is Legionnaire’s disease?

Legionnaires’ disease is a potentially fatal form of pneumonia that can affect anybody, but which principally affects those who are susceptible because of age, illness, poor immune systems, smoking, etc.

It is caused by the bacterium L. pneumophila and related bacteria that can be found naturally in environmental water sources such as rivers, lakes, and reservoirs, usually in low numbers. As they are commonly found in environmental sources they may also be found in purpose-built water systems such as cooling towers, evaporative condensers, and whirlpool spas.

If conditions are favorable the bacterium may grow to create conditions in which the risk from Legionnaires’ disease is increased. It is therefore important to control the risks by introducing measures outlined in the Approved Code of Practice & guidance document Legionnaires’ disease – The control of legionella bacteria in water systems (L8).

Legionnaires’ disease is a type of pneumonia. It was named after an outbreak of severe pneumonia in Philadelphia in the USA that affected a meeting of American Legionnaires in 1976.

It is actually one of a group of similar diseases collectively known as legionellosis. The other forms, e.g. Pontiac Fever and Lochgoilhead Fever, have similar symptoms but are not as serious as Legionnaires’ disease.

Legionnaires’ disease occurs more frequently in men than in women. It usually affects middle-aged or elderly people, and it more commonly affects smokers or people with another chest

Contracting Legionnaires’ Disease

The agent that causes Legionnaires’ disease is a bacterium called L. pneumophila. People catch Legionnaires’ disease by inhaling small droplets of water suspended in the air, which contain the bacteria. Certain conditions increase the risk of legionella:

  • a suitable temperature for growth, 20 to 45 degrees centigrade;
  • a source of nutrients for the organism, e.g. sludge, scale, rust, algae, and other organic matter.
  • a way of creating and spreading breathable droplets, eg the aerosol created by a cooling tower or spa pool.

However, remember that most people exposed to legionella do not become ill, and Legionnaires’ disease does not spread from person to person.


The symptoms of Legionnaires’ disease are similar to those similar to flu:

  • high temperature, fever, and chills;
  • cough;
  • muscle pains; and Headaches
  • In a bad case, there may also be pneumonia, and occasionally diarrhea, and signs of mental confusion.

Sources of legionella

Legionella bacteria are widespread in nature, mainly living in natural water systems, e.g. rivers, and ponds. However, the conditions are rarely right for people to catch the disease from these sources.

Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where the water is maintained at a temperature high enough to encourage growth, eg cooling towers, evaporative condensers, spa pools, and hot water systems used in all sorts of premises (work and domestic).

Most community outbreaks in the UK have been linked to installations such as cooling towers, which can spread droplets of water over a wide area. These are found as part of air-conditioning and industrial cooling systems.

Fatal cases of Legionnaires’ disease have also been associated with spa pool demonstrations

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