In metallic (elemental) form, silver is unreactive and cannot kill bacteria. Silver atoms (denoted as Ag or Ag0) must lose an electron and become positively charged silver ions (Ag+) to become bactericidal. Elemental silver ionizes in the air but more readily when exposed to an aqueous environment such as wound exudate. In contrast, silver compounds contain positive silver ions bound to negatively charged ions or molecules. Some silver ions become detached from the compound when exposed to aqueous environments.
Silver ions are highly reactive and affect multiple sites within bacterial cells, ultimately causing bacterial cell death. They bind to bacterial cell membranes, disrupting the bacterial cell wall and cell leakage. Silver ions transported into the cell disrupt cell function by binding to proteins and interfering with energy production, enzyme function, and cell replication. Silver ions are active against many bacteria, fungi, and viruses, including many antibiotic-resistant bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin-resistant Enterococci (VRE).
Studies of the effects of silver dressings on experimental models of biofilms have
suggested that silver may reduce bacterial adhesion, destabilize the biofilm matrix, kill bacteria within the matrix, and increase the susceptibility of bacteria to antibiotics.
Some laboratory studies have suggested that silver may benefit wound healing in addition to controlling bioburden alone. For example, silver nitrate, nanocrystalline silver, and some silver-containing dressings have been found to have anti-inflammatory effects and encourage blood vessel formation (neovascularization).
Only a small proportion of silver presented to a wound site in a dressing is involved in
antimicrobial action. Most rest remains within the dressing or binds to proteins in the
wound or debris. Very little is systemically absorbed.
Even if absorbed systemically, silver is excreted mainly via the biliary route in feces. Some are excreted in the urine. Silver is not absorbed into the central or peripheral nervous systems.