Legionella in the environment

Legionella bacteria are widely distributed, and grow naturally in fresh water environments. They can live in all types of water, including both natural sources such as rivers and streams, and artificial water sources such as water towers associated with cooling systems, hot and cold water systems and spa pools (see here for more detail on potential sources). They only become a risk to health when the temperature and micronutrients allow the Legionella to grow rapidly, such as in water systems which are not properly designed, installed and/or maintained. As the bacteria often reproduce within a protozoan, mammalian cell or biofilm [1] they have unusual nutrient requirements, and it is only when conditions conspire to mimic these environments and encourage accelerated growth that outbreaks tend occur. Legionella bacteria multiply at temperatures in the range 25 - 42ºC, with an optimal temperature of 35ºC [2; 3] and most outbreaks can be traced to a man-made water environment where these temperatures have been reached (see here for more detail on environmental impact on Legionella bacteria).

Because of this, Legionnaires' disease could be considered a preventable illness, since controlling or eliminating the bacteria in certain reservoirs will prevent cases of the disease [1, 4]. Typical sites likely to encourage Legionella growth, and aerosolise the bacteria, are industrial cooling towers and wet air ventilation systems, see here for a more comprehensive list of potential sources. If they are poorly maintained, accumulated sediments and corrosion of the minerals, used in the construction of such systems, can provide the nutrients required by the Legionella bacteria to grow. This can also lead to a build-up of scale within the pipes, which can provide a greater surface area for colonisation, thus further encouraging growth.

Reference List

1. FIELDS B. S., BENSON R. F. & BESSER R. E. (2002) Legionella and Legionnaires' disease: 25 years of investigation Clinical Microbiology Reviews 15(3) pp.506 - 526 http
2. KATZ S. M. & HAMMEL J. M. (1987) The effect of drying, heat and pH on the survival of Legionella pneumophila Annals of Clinical and Laboratory Science 17, pp.150 - 156 http
3. MAUCHLINE W. S., JAMES B. W., FITZGEORGE R. B., DENNIS P. J. & KEEVIL C. W. (1994) Growth temperature reversibly modulates the virulence of Legionella pneumophila Infection and Immunity 62, pp.2995 - 2997 http
4. BARTRAM J. (2007) Legionella and the prevention of legionellosis WHO, Geneva ISBN 92 4 156297 http pdf