Several metals, such as manganese, zinc, magnesium, iron and copper, are essential elements required for the survival and normal function of microorganisms. For example, some of these metals are enzyme cofactors that without their precise interaction these enzymes cannot function. Since metals are highly reactive, they are closely controlled by the microorganisms via, for example, intra- and extra-cellular sequestration by cell envelopes, exclusion by permeability barriers, active transport membrane efflux pumps, and extracellular chelation or precipitation by secreted metabolites. However, above a certain threshold and time of exposure, which differs between the microorganisms, they are overwhelmed by the metals overload and die.  Similarly, exposure of microorganisms to non-essential metals, such as silver or gold, can result in the microorganism’s death.

Copper and silver are the two main metals in use today as active biocidal agents in different no-touch products for the reduction of bioburden in medical environments. For example, silver and copper ionization devices have been shown to reduce environmental reservoirs of Legionella in hospitals (1), and metallic copper and copper oxide impregnated hard and soft surfaces have been shown to reduce bioburden and nosocomial infections (2,3).

Copper biocidal efficacy is achieved through several parallel mechanisms. It is believed that the first site that copper damages is the microorganisms’ envelope, where copper ions cause membrane permeabilization and membrane lipid peroxidation. Significant copper-induced disruption of membrane integrity inevitably leads to loss of cell viability. However, even with intact membranes, the intracellular copper ions may displace essential metals from their native binding sites in the proteins, or via direct interactions with the proteins and nucleic acids inhibiting their biological assembly and activity (4). In addition, redox between Cu+ and Cu2+ results in formation of hydroxyl radicals that may attack lipids, proteins, DNA and other biomolecules, also leading the microorganisms death (5).

Similarly to copper, silver ions can also cause the disruption of the bacterial, fungal, and protozoa cell membrane membranes (e.g. ref 6) and also following their intracellular absorption thorough pinocytosis, they can denaturate and inactivate proteins and essential enzymes (7).

 

  1. Almeida D, Cristovam E, Caldeira D, Ferreira JJ, Marques T. (2016) Are there effective interventions to prevent hospital-acquired Legionnaires’ disease or to reduce environmental reservoirs of Legionella in hospitals? A systematic review. Am J Infect Control. 44(11):e183-e188.
  2. Borkow, G., and Monk, A.B. (2012) Fighting nosocomial infections with biocidal non-intrusive hard and soft surfaces. World J Clin Infect Dis 2(4):77-90.
  3. Salgado CD, Sepkowitz KA, John JF, Cantey JR, Attaway HH, Freeman KD, Sharpe PA, Michels HT, Schmidt MG. (2013) Copper surfaces reduce the rate of healthcare-acquired infections in the intensive care unit. Infect Control Hosp Epidemiol. 34(5):479-86.
  4. Borkow G, Gabbay J. (2005) Copper as a biocidal tool. Curr Med Chem 12:2163-75.
  5. Valko M, Morris H, Cronin MT. (2005) Metals, toxicity and oxidative stress. Curr Med Chem 12:1161-208.
  6. Jung WK, Koo HC, Kim KW, Shin S, Kim SH, Park YH. (2008) Antibacterial activity and mechanism of action of the silver ion in Staphylococcus aureus and Escherichia coli. Appl Environ Microbiol. 74(7):2171-8.
  7. Russell AD, Hugo WB (1994) Antimicrobial activity and action of silver. Progress in Medicinal Chemistry; 31: 351-370.