Individual and combined effects of copper and silver ions on inactivation of Legionella pneumophila

Copper/silver ionization is a new disinfection method that is being used to eradicate Legionella pneumophila from hospital hot water recirculating systems. The objective of this study was to determine the susceptibility of L. pneumophila serogroup 1 to copper and silver ions alone and in combination. L. pneumophila serogroup 1 (L. p. sg(-1)) was completely inactivated (6-log reduction) at copper concentration of 0.1 mg/l within 2.5 h, whereas more than 24 h was required to achieve a similar reduction at the highest silver ion concentration tested (0.08 mg/l). Checkerboard method and Gard additive model prediction demonstrated that copper and silver ions in combination could result in additive or synergistic effect depending on the concentration of copper and silver ions. Under the experimental conditions used in this study, synergism of copper/silver ions in eradicating L. p. sg(-1) was observed at higher concentration combinations of copper/silver ions (e.g. 0.04/0.04 mg/l) while only an additive effect was observed at lower concentration combinations (e.g. 0.02/0.02 mg/l). This study suggested that both copper and silver ions are effective in inactivating L. pneumophila and the combined effect is greater than that seen with either ion alone. Copyright (C) 1996 Elsevier Science Ltd