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.
Legionella acquired its name after a July, 1976 outbreak among people attending a convention of the American Legion in Philadelphia. The mystery disease sickened 221 persons, causing 34 deaths. In that bicentennial year, a pandemic among U.S. war veterans was widely publicized and produced a national panic. On January 18, 1977 the causative agent was identified as a previously unknown bacterium, subsequently named Legionella. See Legionnaire's Disease for full details.
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 micrometre filters) before inoculation 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 interference 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.
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.
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
|Legionella bacteria cfu/litre||Action required - 35 samples per facility is required, 20 water / 10 swabs|
|1000 or less||System under control. (150+ CFU/ml in healthcare facilities or nursing homes require immediate action)|
|more than 1000|
up to 10,000
|Review program operation. The count should be confirmed by immediate re-sampling. If a similar count is found again, a review of the control measures and risk assessment should be carried out to identify any remedial actions.|
|more than 10,000||Implement corrective action. The system should immediately be re-sampled. It should then be ‘shot dosed’ with an appropriate biocide, as a precaution. The risk assessment and control measures should be reviewed to identify remedial actions.|
Temperature affects the survival of Legionellae as follows:
- 70 to 80 °C (158 to 176 °F) - Disinfection range
- At 66 °C (151 °F) - Legionellae die within 2 minutes
- At 60 °C (140 °F) - Legionellae die within 32 minutes
- At 55 °C (131 °F) - Legionellae die within 5 to 6 hours
- 50 to 55 °C (122 to 131 °F) - They can survive but do not multiply
- 20 to 50 °C (68 to 122 °F)- Legionellae growth range
- 35 to 46 °C (95 to 115 °F) - Ideal growth range
- Below 20 °C (68 °F) - Legionellae can survive but are dormant
The above data can be confirmed in an online article by Reliance World Wide.
Control of Legionella growth can be through : A. Chemical Treatment
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:
Images of Legionella bacteria: