Copper and silver ions more effective against Legionellae than against mycobacteria in a hospital warm water system
Introduction
Legionellae and nontuberculous mycobacteria are common heterotrophic bacteria in water distribution systems. These bacteria are a problem especially in water systems of hospitals, where they may cause nosocomial infections, usually in patients with other underlying diseases (Edelstein, 1988; Wallace et al., 1998). A severe nosocomial Legionnaires’ disease case occurred in a 460-bed hospital caused by Legionella pneumophila serogroup 6, and the probable source of infection was a warm water system. The concentrations of legionella bacteria detected in water samples varied from 3.5×102 to 2.0×104 cfu/l. Warm water systems were cleaned with flushing 75°C water twice for 30 min, and all tanks of warm water were removed. The temperature of the water leaving from heat exchanger was set to 60°C. Control samples, however, still contained high concentrations of legionellae. Also many previous studies have shown the difficulties of thermal eradication in large water systems (Fischer-Hoch et al., 1984; Muraca et al., 1990).
Several species of nontuberculous mycobacteria may cause nosocomial infections. Most cases are local wound infections or abscesses caused by rapidly growing species (Wallace et al., 1998). In immunocompromised patients, the infections may be disseminated. The occurrence of mycobacteria in hospital water systems has been known for a long time (Bailey et al., 1970; Bullin et al., 1970; McSwiggan and Collins, 1974). With the recent molecular techniques the hospital water supplies have been able to verify as the source of mycobacterial infections acquired in hospitals (von Reyn et al., 1994; Kauppinen et al., 1999). Resistance of mycobacteria to disinfectants and high temperatures (Carson et al., 1978; Merkal and Crawford, 1979; Best et al., 1988; Schulze-Röbbecke and Buchholtz, 1992) makes them extremely difficult to eradicate from tap water. Chlorination (Lockwood et al., 1989) and temporary heating of the hot water combined with flushing of faucets and shower heads (Sniadick et al., 1993) have been used to control mycobacteria in hospital water systems. Long-term efficacy of these preventive measures against mycobacteria is not known.
Copper and silver ions electrolytically released in water is a new and promising prevention method against legionella bacteria (Liu et al., 1994). Silver ions inhibit bacterial growth by interfering with electron transport, binding DNA and interacting with cell membrane (Tilton and Rosenberg, 1978). Copper ions may function by enhancing displacement reactions, disrupting enzyme structure and binding thiol or other groups on protein molecules (Thurman and Gerba, 1989). Landeen et al. (1989) have reported enhanced inactivation of legionella bacteria when 400 μg/l of copper, 40 μg/l of silver and 0.4 mg/l of chlorine were combined in their in vitro experiments. Legionellae have been eliminated from a hospital water distribution system in situ at copper/silver concentrations of 400/40 μg/l or greater (Liu et al., 1994). More than 30 hospitals in USA are using copper–silver ionisation to control the growth of legionellae in their water systems (Lin et al., 1998a). A laboratory study has suggested that mycobacteria could also be eliminated from water systems by copper and silver ions (Lin et al., 1998b), but there are no field studies relating to this matter.
In Finland, the technical and aesthetic limit concentration for copper in drinking water is 1000 μg/l and that for silver is 10 μg/l (Anon, 1994). The aim of this study was to investigate if legionellae, nontuberculous mycobacteria and other heterotrophic bacteria could be eliminated from a warm water system of a Finnish hospital by low concentrations of copper and silver ions.
Section snippets
Water treatment system
A water treatment unit (TPU4-4/3, Tarn-Pure System, Pan-Ionic Ltd., UK) was installed in a warm water system of a hospital, at the site where warm water returns to the heat exchanger (Fig. 1). Copper and silver ions were continuously released in the water by the treatment unit, which could handle 4.5 m3 water in a day. The daily warm water consumption of the system was 4 m3. The copper–silver electrodes contained 70% of copper and 30% of silver. Before installation the original plastic cover of
Metal concentrations
Before the use of the treatment unit (the first 3 sampling times) copper concentrations in the warm water varied from 37 to 110 μg/l and silver concentrations from 0 to 1 μg/l. There was an increase in the silver concentrations at all warm water sampling sites after the 7th samples when the output current of the unit was raised to 350 mA. The silver concentrations, however, decreased at the end of the 2nd year because of build-up of scale on electrodes. Since then the electrodes were cleaned every
Discussion
The achieved copper and, especially, silver concentrations lowered the legionella counts at every previously legionella-positive sampling site. In the circulating warm water, silver ion concentration higher than 3 μg/l diminished legionella content to undetectable. The results of the shower show that also at peripheral sites such a low silver concentration would be high enough to eradicate legionellae, if the site is used regularly. However, when there was no regular use, even a high silver
Conclusions
Even very low concentrations of metal, especially silver, ions can be effective against legionellae everywhere in the water system. Good prevention efficacy requires, however, regular use of all points in water system.
Nontuberculous mycobacteria and other heterotrophic bacteria were more tolerant to copper and silver ions than legionellae.
Properly maintained copper–silver ionisation method may be a solution to the serious problem of nosocomial legionella pneumonia.
Acknowledgements
This study was financed by The National Research and Development Centre for Welfare and Health, Ministry of Social Affairs and Health, Central Hospital of Vaasa, The National Agency for Medicines, The Association of Finnish Local Authorities and National Public Health Institute. The expert help of Marjo Tiittanen, Pirkko Karakorpi, Kauko Mäki and Mikko Vahteristo is gratefully acknowledged. Thanks are also due to the staff of the hospital of Vaasa and the culture media units of National Public
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