Copper and silver ions more effective against legionellae than against mycobacteria in a hospital warm water system

We studied the influence of electrolytically released copper and silver ion s on the microbiological quality in a warm water system of a hospital. The concentration of nontuberculous mycobacteria was followed for three, and th at of legionellae and other heterotrophic bacteria in the water for four ye ars. The highest concentrations of copper and silver ions were 220 and 68 m ug/l, respectively. Silver ion concentration of about 3 mug/l was sufficien t to control the growth of legionellae in circulating warm water. The resul ts showed that it is more difficult to eradicate legionellae from taps and showers: these points were colonized by a small number of legionellae after the metal ion concentrations were increased in the circulating water. A re gular use of water eradicated legionellae from the shower. One tap was stil l used irregularly, and this may be a reason why it still contained small c oncentrations of legionellae also in the last years of the study. Mycobacte ria were occasionally isolated from the circulating water and repeatedly fr om the shower, even when the metal concentrations were high. To control leg ionella bacteria in warm water systems, silver concentrations of only 3 mug /l are needed if all taps and showers of the system are regularly used. Suc h low copper and silver concentrations, however, are not efficient against nontuberculous mycobacteria or other heterotrophic bacteria. (C) 2001 Elsev ier Science Ltd. All rights reserved.