Presence of Legionella in London's water supplies.

Legionella occurs frequently (52 to 54%) in domestic water and cooling water inside commercial, industrial and health care buildings, and these types of water systems are now regarded as a normal habitat for Legionella. The factors that predispose a particular water system to colonization by these organisms are ill-defined, although it is fairly certain that biological and physicochemical environmental factors play an important role in allowing Legionella to multiply in the circulating water. It has been postulated that the organism may gain access to water systems inside buildings by one of three routes: contact with air through open points such as uncovered storage tanks or vents, ingress of soil or surface water during construction or repair, or intermittent seeding with organisms present in low numbers in the public water supply. Three studies in the USA have found Legionella in 0.4 to 8.8% of drinking-water samples, but these were not representative of the public supply network as a whole. The aim of this study was to determine, over a period of 1 year, the frequency of Legionella in London's drinking water--from the treatment plant through to the consumer's tap. To date, Legionella has not been isolated from raw river water entering London's treatment works or from treated water entering the distribution network. Sixty-two monitoring taps in buildings located in 21 supply areas have been sampled twice for Legionella; only 2 (2.4%) have proved positive during the autumn and winter of 1985/86. The strain found was L. pneumophila serotype 1, subgroup Olda, and the numbers ranged from 10(2) to 10(4)/l. Although the survey is incomplete, it is already clear that the public water supplies in London are not a source of strains of Legionella associated with disease.