Legionnaires’ disease (Legionellosis) has been described as “the most significant waterborne-disease risk associated with drinking water in the United States”. Legionellosis is caused by Legionella, bacteria that often proliferate in biofilm on the inner surfaces of water systems. Legionella can grow in water temperatures up to 140° or more. Therefore they are especially problematic in recirculating systems such as cooling towers, ornamental fountains and hot water systems in hospitals, hotels, nursing homes and other institutions.
Biofilm is a layer of bacteria, protozoa, algae, and other organisms in a matrix of polysaccharide “glue” secreted by bacteria. The biofilm binds the organisms together and protects them from most biocides. As biofilm grows thicker, pieces of the biofilm slough off and are carried away by the stream of water. Legionella sometimes infect protozoa in the biofilm and are carried out of the biofilm in the bodies of the protozoa, which then die and release Legionella into the water stream.
Humans usually contract Legionellosis by breathing aerosols from showers and other bathroom fixtures or by breathing aerosols from a cooling tower or fountain. Biofilm also restricts water flow in pipes and tubing and inhibits heat transfer in heat exchangers.