Conditions that Promote Growth


L. pneumophila bacteria are widely distributed in water systems. They tend to grow in biofilms or slime on the surfaces of lakes, rivers and streams, and they are not eradicated by the chlorination used to purify domestic water systems. Low and even nondetectable levels of the organism can colonize a water source and grow to high concentrations under the right conditions.

Conditions that promote growth of the organism include heat, sediment, scale, and supporting (commensal) microflora in water. Common water organisms including algae, amoebae, and other bacteria appear to amplify Legionella growth by providing nutrients or harboring the organism. Because of its ability to remain viable in domestic water systems, it is capable of rapid multiplication under the proper conditions.

Water conditions that tend to promote the growth of Legionella include:


temperatures between 20° and 50°C (68° - 122°F) (The optimal growth range is 35° - 46°C [95° - 115°F]);

pH between 5.0 and 8.5;

sediment that tends to promote growth of commensal microflora; and

micro-organisms including algae, flavobacteria, and Pseudomonas, which supply essential nutrients for growth of Legionella or harbor the organism (amoebae, protozoa).

Inspection and Maintenance. Visual inspection and periodic maintenance of the system are the best ways to control growth of Legionella and related organisms. Good maintenance is necessary both to control Legionella growth and for effective operation. The system should be properly monitored and maintained to prevent buildup of scale and sediment and bio-fouling, all of which support Legionella growth and reduce operating efficiency.

Biocide. Unfortunately, measurements of water quality such as total bacterial counts, total dissolved solids, and pH have not proven to be good indicators of Legionella levels in cooling towers. Periodic use of biocides is needed to ensure control of Legionella growth.