Silver has a long and intriguing history as an antibiotic in human health care. It has been developed for use in water purification, wound care, bone prostheses, reconstructive orthopaedic surgery, cardiac devices, catheters and surgical appliances. Advancing biotechnology has enabled incorporation of ionizable silver into fabrics for clinical use to reduce the risk of nosocomial infections and for personal hygiene. The antimicrobial action of silver or silver compounds is proportional to the bioactive silver ion (Ag(+)) released and its availability to interact with bacterial or fungal cell membranes. Silver metal and inorganic silver compounds ionize in the presence of water, body fluids or tissue exudates. The silver ion is biologically active and readily interacts with proteins, amino acid residues, free anions and receptors on mammalian and eukaryotic cell membranes. Bacterial (and probably fungal) sensitivity to silver is genetically determined and relates to the levels of intracellular silver uptake and its ability to interact and irreversibly denature key enzyme systems. Silver exhibits low toxicity in the human body, and minimal risk is expected due to clinical exposure by inhalation, ingestion, dermal application or through the urological or haematogenous route. Chronic ingestion or inhalation of silver preparations (especially colloidal silver) can lead to deposition of silver metal/silver sulphide particles in the skin (argyria), eye (argyrosis) and other organs. These are not life-threatening conditions but cosmetically undesirable. Silver is absorbed into the human body and enters the systemic circulation as a protein complex to be eliminated by the liver and kidneys. Silver metabolism is modulated by induction and binding to metallothioneins. This complex mitigates the cellular toxicity of silver and contributes to tissue repair. Silver allergy is a known contra-indication for using silver in medical devices or antibiotic textiles.
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Metallic silver and silver compounds are used widely in medical devices and health care products to provide antibacterial and antifungal action. Experience has shown that they are generally safe in use and effective in controlling pathogenic organisms. They do not achieve a ‘germ-free’ state in wounds, device-related infections or biofilm formation however. Biofilms are silver resistant, and silver-resistant bacteria have been isolated from burn wounds, chronic ulcers and nosocomial isolates . It is expected that where silver has been unsuccessful in limiting infections, much of the free silver ion released for antimicrobial purposes has been mopped up by albumins, globulins, free anions and protein residues on cell membranes. At the moment, we do not know the minimal levels of silver ion necessary in any situation to clear infections, although recent research suggests that concentrations of free ion equivalent to 0.5–1.0M silver nitrate will be adequate [Lansdown and Philip, unpubl.]. Manufacturers of new products should envisage providing a balance between the silver ion released for antibacterial purposes and the minimal toxic threshold. Although much initial research is still conducted in the laboratory with types of bacterial/fungal strains, further clinical and pharmacological studies are urgently required to examine the safety and efficacy of silver in human patients and volunteers with close attention to ethical considerations.
Lansdown AB. Silver in health care: antimicrobial effects and safety in use. Curr Probl Dermatol. 2006 October Imperial College Faculty of Medicine, Charing Cross Hospital, London, UK