Antibacterial activity of copper
Copper as an antibacterial surface agent
Copper can easily be used in public health in its solid form in hospitals and medical environments, for example, doors, knobs, piping material or other inanimate surfaces in different facilities (Casey et al. 2010; Mikolay et al. 2010; Karpanen et al. 2012; Schmidt et al. 2012; Inkinen et al. 2017) or for medical devices (Goudarzi et al. 2017; Schmidt et al. 2017). To test its antimicrobial surface activity, researchers mostly use coupons with varying concentrations of copper.
Noyce et al. (2006) tested copper alloys (61–95% Cu) for their antibacterial activity on E. coli O157 at different temperatures (22°C and 4°C). They demonstrated an antibacterial effect in all conditions, especially at 22°C, but only the high copper alloys (95% Cu) completely killed E. coli (Noyce et al. 2006). Similarly, Wilks et al. (2005) demonstrated copper antibacterial activities on E. coli O157 at different temperatures (20°C and 4°C). They also showed antibacterial activities in all conditions, especially at 20°C and when copper concentration in alloys was superior to 85% (Wilks et al. 2005).
Additionally, the effect of copper alloy surfaces (65–100% Cu) against vegetative and sporal Clostridium difficile was studied and it was demonstrated that copper alloys (> 70% Cu) provide a significant reduction in survival of C. difficile vegetative cells and spores with a complete killing after 24–48 h (Weaver et al. 2008). The antibacterial effect of copper coupons (C19700, 99% Cu) on C. difficile was also highlighted and a complete killing of C. difficile vegetative cells within 30 min and a 99·8% reduction in the viability of C. difficile spores after 3 h was observed (Wheeldon et al. 2008).
A study investigated the antibacterial activity of copper coupons (99% Cu and 63% Cu) on clinical isolates of E. coli, Enterobacter spp., Klebsiella pneumoniae, Pseudomonas aeruginosa and Acinetobacter baumannii all being potent multidrug-resistant Gram-negative pathogens responsible for nosocomial infections. Copper surfaces showed antimicrobial activity in all tested strains, especially with copper coupons containing 99% Cu which had a bactericidal effect within 2, 3, 5 and 6 h for A. baumannii, Enterobacter spp., K. pneumoniae and P. aeruginosa and E. coli respectively (Souli et al. 2013).
In summary, the copper antimicrobial activity increases proportionally to its concentration, in accordance with several studies (Mehtar et al. 2008; Elguindi et al. 2009; Zhu et al. 2012; Souli et al. 2013) and pure copper coupons present higher antibacterial efficacy. These studies also highlight that the experimental temperature directly impacts the ‘contact killing’ process, in agreement with other previously published studies (Faúndez et al. 2004; Elguindi et al. 2009; Michels et al. 2009). Furthermore, it was shown that copper ions released from coupons were media dependent (Molteni et al. 2010). The authors compared four different media: 0·1 mol l−1 Tris-Cl pH 7, M17 (Terzaghi and Sandine 1975), water and 100 mmol l−1 phosphate-buffered saline pH 7 (NaPi), and demonstrated that in Tris-Cl and M17 media complete killing of E. hirae was obtained in 12 and 90 min, respectively, vs 6 h using the other media. They concluded that the application of bacteria to copper surfaces in Tris-Cl buffers enhanced ‘contact killing’ through a higher release of copper ion and established a causal link between these dissolved copper concentrations and the rates of killing. In the same way, a phosphate buffer with addition of HEPES has been previously shown to increase dramatically the bactericidal effect of the Cu-H2O2 mixture on E. coli (Hartemann et al. 1995).