Other Antimicrobials: Silver
Silver is a heavy metal with a long history of use in medicine and public health. Dating back 6,000 years, silver has been used for wound healing, treatment of infection, water purification and beverage preservation. In high concentration, silver can be a broad spectrum antimicrobial; the challenge has been to sustain this concentration on a material or biologic surface. Toxicity due to silver is low; silver is poorly soluble and has limited penetration through skin. To be effective, silver must be used in an ionized (oxidized) form rather than its pure metallic form.
Mechanism of Action
Silver targets the bacterial plasma and cytoplasmic membrane. It binds to the electron donor groups in enzymes that are located on the membrane and involved in cellular replication and energy transduction. By binding to these enzymes, silver causes inactivation of the enzymes, membrane disintegration and a release of potassium ions from the bacterial cell resulting in cell death.
Silver-based technologies have been successfully used in various devices and pharmaceutical applications such as chronic hemodialysis catheters, endotracheal tubes, urinary catheters and wound care products. Evidence of the efficacy of silver as an effective antimicrobial in medical devices is limited, specifically for urinary and vascular catheters. In the CDC Catheter-Associated Urinary Tract Infection (CAUTI) Guidelines (2009), the evidence for the efficacy of silver when used in urinary catheters was termed to be "low-quality" and use of these catheters received a 1B recommendation.28,31 In the CDC Guidelines for the Prevention of Intravascular Catheter-Related Infections (2011), vascular platinum/silver catheters were listed as having mixed evidence (one study showed a reduction in the incidence of catheter colonization and CRBSI, and another study found no difference in catheter colonization or CRBSI) resulting in no recommendations for use to prevent intravascular catheter-related infection.20
A clinical trial investigating the use of silver-coated endotracheal tubes showed a reduction in microbial colonization with no significant reduction in morbidity for the duration of intubation, duration of ICU or hospital lengths of stay or frequency and severity of adverse effects. In fact, compared to patients with a non-coated endotracheal tube, those in the silver-coated group showed a higher mortality rate secondary to respiratory failure.32
Silver has only been shown to be effective in short duration in in vitro studies, but its efficacy in a clinical performance is questionable. Proteins, carbohydrates and lipids can bind readily to ionic silver present in clinical application therby inactivating its antimicrobial affect. Silver tested in a broth mixture without proteins and low in chloride can provide misleading results.31,47 There is minor risk for silver sensitivity or local skin discoloration.