In vitro efficacy of copper and silver ions in eradicating Pseudomonas aeruginosa, Stenotrophomonas maltophilia and Acinetobacter baumannii: Implications for on-site disinfection for hospital infection control
Introduction
Pseudomonas aeruginosa, Stenotrophomonas maltophilia and Acinetobacter baumannii (“waterborne pathogens”) are Gram-negative bacteria commonly present in chlorinated potable water. These organisms are opportunistic pathogens which do not pose a threat to the general public. However, these organisms have been epidemiologically linked to hospital-acquired respiratory infections in the intensive care units (Squier et al., 2000; Lee et al., 1998; Rusin et al., 1997) which affect millions of hospitalized patients. The hospital potable water system can be the reservoir responsible for these hospital-acquired infections. It has been suggested that waterborne pathogen-related infections could be prevented by avoidance of non-sterile potable water among high-risk patients as well as disinfection of the water distribution systems (von Reyn et al., 1994).
Control of hospital-acquired Legionnaires’ disease has been accomplished by disinfection of the hospital supply system (Alyssa et al., 1995; Blanc et al., 2005; Chen et al., 2005). Copper–silver ionization is a robust technology that has been used in more than 300 hospitals in the United States and the Europe to control Legionella in hot water systems. Copper and silver ions (Cu=0.2–0.4 mg/L, Ag=0.02–0.04 mg/L) are introduced into hospital water distribution systems via electrolysis. These positively charged metallic ions attach to the negatively charged bacterial cell wall and cause cell lysis and death (Bitton and Freihofer, 1977; Friedman and Dugan, 1968; Slawson et al., 1990). Copper–silver ionization has been successful in preventing outbreaks of hospital-acquired Legionnaires’ disease (Stout and Yu, 2003). In vitro efficacies of copper and silver ions have also been demonstrated including Legionella (Landeen et al., 1989; Lin et al., 1996), Naegleria fowleri (Cassells et al., 1995), Coliphage MS-2 and Poliovirus (Yahya et al., 1992) and Pseudomonas cepacia (Pyle et al., 1992).
Given the efficacy of ionization against Legionella, it would be cost-effective if ionization is capable of eradicating other waterborne pathogens. However, no data are currently available. Thus, the objective of this study was to determine the in vitro efficacy of copper and silver ions in eradicating P. aeruginosa, S. maltophilia and A. baumannii. Furthermore, the efficacy of the combination of copper and silver ions was also determined as whether the combination demonstrated synergistic effect.
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Test organisms
The environmental isolates of P. aeruginosa, S. maltophilia and A. baumannii were selected as the test organisms. These isolates were transferred from −80 °C stock, inoculated on blood agar plate (BAP) media and incubated at 37 °C in a humidified incubator for 48 h. Inoculation was repeated overnight. The inocula were removed and suspended in 30 mL of sterile deionized water. The cells were washed twice by centrifugation at 1000g (2500 rpm) for 10 min. Ten milliliter of the suspension was removed and
Efficacy of ions on P. aeruginosa
Copper ion was effective in eradicating P. aeruginosa. All copper concentrations tested (0.1–0.8 mg/L) achieved more than 99.999% reduction of P. aeruginosa in 1.5 h (Fig. 1). This inactivation rate is similar to the rate of Legionella eradication (Lin et al., 1996). Silver concentrations of 0.04 and 0.08 mg/L also achieved more than 99.999% reduction of P. aeruginosa in 72 and 12 h, respectively (Fig. 2). The silver concentration of 0.02 mg/L achieved 99.999% reduction in 96 h. Silver concentration
Discussion
P. aeruginosa, S. maltophilia and A. baumannii are waterborne pathogens which can be found easily in chlorinated finish water. These bacteria are opportunistic pathogens which do not affect healthy people. However, they can infect immunocompromised patients in the hospitals (especially patients in intensive care units) with infections such as pneumonia, bacteremia and urinary tract infections. Therefore, the presence of these bacteria in water may increase colonization with subsequent
Conclusion
The presence of waterborne pathogens in domestic finish water can cause opportunistic infections in hospitalized patients. On-site supplemental disinfection of hospital water systems might be one of the approaches to prevent these infections. Copper and silver ions are effective in eradicating P. aeruginosa, S. maltophilia and A. baumannii in vitro. Copper–silver ionization may have the potential to eradicate major waterborne pathogens in hospital distribution systems. The eradication efficacy
Acknowledgment
This study was supported by Career Development Grant (NHRI-EX94-9206PC) from National Health Research Institute, Taiwan.
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