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Applied and Environmental Microbiology, June 2002, p. 2711-2715, Vol. 68, No. 6
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.6.2711-2715.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

Negative Effect of High pH on Biocidal Efficacy of Copper and Silver Ions in Controlling Legionella pneumophila

Yu-sen E. Lin,¹ Radisav D. Vidic,² Janet E. Stout,^3,4 and Victor L. Yu^3,4*

Graduate Institute of Environmental Education, National Kaohsiung Normal University, Kaohsiung, Taiwan,¹ Department of Civil and Environmental Engineering,² Department of Medicine, University of Pittsburgh, Pittsburgh, Pennsylvania 15261,³ VA Pittsburgh Healthcare System, Pittsburgh, Pennsylvania 15240⁴

Received 2 October 2001/ Accepted 13 March 2002

Copper-silver (Cu-Ag) ionization has effectively controlled Legionella spp. in the hot water systems of numerous hospitals. However, it was ineffective at controlling Legionella in one Ohio hospital despite the confirmation of adequate total concentrations of copper and silver ions. The pH of the water at this hospital was found to be 8.5 to 9.0. The purpose of this study was to investigate the impact of pH and other water quality parameters, including alkalinity (HCO₃^-), hardness (Ca²⁺ and Mg²⁺), and amount of dissolved organic carbon (DOC), on the control of Legionella by Cu-Ag ionization. Initial concentrations of Legionella and copper and silver ions used in batch experiments were 3 x 10⁶ CFU/ml and 0.4 and 0.08 mg/liter, respectively. Changes in bicarbonate ion concentration (50, 100, and 150 mg/liter), water hardness (Ca²⁺ at 50 and 100 mg/liter; Mg²⁺ at 40 and 80 mg/liter), and level of DOC (0.5 and 2 mg/liter) had no significant impact on the efficacy of copper and silver ions in killing Legionella at a neutral pH. When the pH was elevated to 9 in these experiments, copper ions achieved only a 10-fold reduction in the number of Legionella organisms in 24 h, compared to a millionfold decrease at pH 7.0. Silver ions were able to achieve a millionfold reduction in 24 h at all ranges of water quality parameters tested. Precipitation of insoluble copper complexes was observed at a pH above 6.0. These results suggest that pH may be an important factor in the efficacy of copper-silver ionization in controlling Legionella in water systems.

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* Corresponding author. Mailing address: Infectious Disease Section, VA Medical Center, University Dr. C, Pittsburgh, PA 15240. Phone: (412) 688-6179. Fax: (412) 688-6950. E-mail: vly+{at}pitt.edu.

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Applied and Environmental Microbiology, June 2002, p. 2711-2715, Vol. 68, No. 6
0099-2240/02/$04.00+0     DOI: 10.1128/AEM.68.6.2711-2715.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

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