Copper and Silver Biocidal Mechanisms, Resistance Strategies, and Efficacy for Legionella Control

Abstract

Legionella pneumophila, regularly found within premise plumbing distribution systems of many large and complex buildings, is responsible for 90% of legionellosis cases. When properly applied and maintained, onsite water treatment offers options for pathogen inactivation and health protection. The addition of copper and silver ions by copper–silver ionization (CSI) is reviewed here. A literature survey was conducted to assess current knowledge about the biocidal mechanisms of copper and silver ions against Legionella spp. and their survival responses and resistance mechanisms. Understanding how copper and silver ions affect bacteria, how bacteria defend against metal ions, and environmental conditions that affect CSI efficacy may help better define conditions under which CSI, alone or in combination with other treatment(s), might offer reliable control of Legionella. Elevated dissolved oxygen, the sodium content of the water, combined disinfectants, and higher temperature may provide conditions under which Legionella spp. may be more susceptible to metal ions.

Critical Review: Propensity of Premise Plumbing Pipe Materials to Enhance or Diminish Growth of Legionella and Other Opportunistic Pathogens

“…In potable water, copper passively released from plumbing materials can be present in the germicidal range for Legionella of 0.1–0.8 mg/L [ 62 , 119 , 120 , 140 ], even in some parts of plastic pipe systems connected with brass fittings [ 141 , 142 ]. Passive release or purposeful dosing that results in copper concentrations of 0.05–0.8 mg/L are thought to limit Legionella growth [ 62 , 83 , 119 , 120 , 143 ].…”

Examining the efficacy of copper‐silver ionization for management of Legionella: Recommendations for optimal use

“…A search of the relevant literature was performed using the Microsoft Bing and Google Chrome Scholar search engines. Search terms included combinations of the following: “ Legionella ,” “copper,” “silver,” “ionization,” “CSI,” “disinfection,” “resistance,” and “regulations.” A number of excellent reviews also served as a starting point for background information (Cachafeiro et al, 2007; June & Dziewulski, 2018; Lin et al, 2011; Sicairos‐Ruelas et al, 2019; USEPA, 2016). Discussions with a number of colleagues were also helpful, including experts at LiquiTech, Lombard, IL (http://www.liquitech.com) who provided access to their case study data.…”

“…L. pneumophila possesses the ATPase LpCopA protein with 45% identical amino acid sequence with the E. coli CopA (Bondarczuk & Piotrowska‐Seget, 2013; Kim et al, 2009). Although L. pneumophila is capable of counteracting copper ROS production with superoxide dismutase and copper efflux resistance mechanisms have been identified, it is important to note that copper resistance genes found in other gram‐negative bacteria have not been located in Legionella species (June & Dziewulski, 2018). Bédard et al (2021) isolated a strain of L. pneumophila from biofilms in a heat exchanger that was resistant to 5 mg/L of copper, although a similar strain isolated from the water was sensitive to copper.…”

“…E. coli and Salmonella resistance to silver is controlled by two genes: SilS, an ATP kinase sensor, and SilR, a DNA‐binding activator responder (Silver, 2003; Silvestry‐Rodriguez et al, 2007). To date, the literature has not demonstrated the presence of genomic silver resistance (i.e., sil operon) in Legionella species (June & Dziewulski, 2018; Sütterlin, 2015).…”

“…The conductivity of water is influenced by the concentration of dissolved salts and other inorganic chemicals and affects the ability of water to pass an electrical current (Dziewulski et al, 2015; June & Dziewulski, 2018). Conductivity is measured in micromhos per centimeter (μmhos/cm) or micro‐Siemens per centimeter (μS/cm).…”