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Legionella is a Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila. Legionella are common in many environments, with at least 50 species and 70 serogroups identified. 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 as well as 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.
Additional recommended knowledge
Legionella is traditionally detected by culture on buffered charcoal yeast extract (BCYE) agar. Legionellae require the presence of cysteine to grow and therefore do not grow on common blood agar media used for laboratory based total viable counts or on site displides. Common laboratory procedures for the detection of Legionella in water concentrate the bacteria (by centrifugation and/or filtration through 0.2 micron filters) before innoculation onto a charcoal yeast extract agar containing antibiotics (e.g. glycine vancomycim polymixin cyclohexamide, GVPC) to suppress other flora in the sample. Heat or acid treatment are also used to reduce interefernece from other microbes in the sample.
After incubation for up to 10 days, suspect colonies are confirmed as Legionellae if they grow on BCYE containing cysteine, but not on agar without cysteine added. Immunological techniques are then commonly used to establish the species and/or serogroups of bacteria present in the sample.
New techniques for the rapid detection of Legionella in water samples are emerging including the use of polymerase chain reaction (PCR) and rapid immunological assays. These technologies can typically provide much faster results.
Legionella have been known for some time to live within amoebae in the natural environment. Legionella species are the causative agent of the human Legionnaires' disease and the lesser form, Pontiac fever. Legionella transmission is via aerosols—the inhalation of mist droplets containing the bacteria. Common sources include cooling towers, domestic hot-water systems, fountains, and similar disseminators that tap into a public water supply. Natural sources of Legionella include freshwater ponds and creeks. Person-to-person transmission of Legionella has not been demonstrated.
Once inside a host, incubation may take up to two weeks. Initial 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 harmful symptoms only in those with a compromised immune system and the elderly. Consequently, it is actively checked for in the water systems of hospitals and nursing homes. In the United States, the disease affects between 8,000 to 18,000 individuals a year.
Controlling potential sources of Legionella
Common sources of Legionella include cooling towers used in industrial cooling water systems as well as in large central air conditioning systems, domestic hot water systems, fountains, and similar disseminators that draw upon a public water supply. Natural sources include freshwater ponds and creeks.
Recent research in the Journal of Infectious Diseases provides evidence that Legionella 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 in northern France in 2003–2004. There were 86 confirmed cases during the outbreak, of whom 18 perished. 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.
Several European countries established a working group known as the European Working Group for Legionella Infections (EWGLI) to share knowledge and experience about monitoring potential sources of Legionella. That group has published guidelines about the actions to be taken to limit the number of colony forming units (i.e. live bacteria that are able to multiply) of Legionella per litre
Temperature affects the survival of Legionellae as follows:
The above data can be confirmed in an online article by Reliance World Wide.
Control of Legionella growth can be through :
A. Chemical Treatment 1. Short term - Cl2, must be repeated every 3 to 5 weeks, corrosion factors 2. Long term - ClO2, takes up to 17 months for system saturation B. Non-Chemical Treatment 1. Short term - Thermal eradication - must be repeated every 3 to 5 weeks 2. Long term - Industrial size copper silver ionisation (Ionization) technology such as 1-AquaLyse.ca, 2-Liquitech or 3-TarnPure.
Guidelines for control of Legionella in cooling towers
Many governmental agencies, cooling tower manufacturers and industrial trade organizations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers. Below is a list of sources for such guidelines:
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Legionella". A list of authors is available in Wikipedia.|