Silver

There is no conclusive evidence that silver performs any essential function in living organisms. Similar to other heavy metals, the silver ion reacts with nucleophilic amino acid residues in proteins, attaching to sulfhydryl, amino, imidazole, phosphate, and carboxyl groups of membrane or enzyme proteins, generally resulting in protein denaturation (1). As for a number of other transition metals, this accounts for silver’s antibacterial activity. The only significant therapeutic use of silver [as the nitrate or its sulfonamides (2,3)] is the disinfection of the skin of thermal injury patients, where the antiseptic effect is primarily due to the metal’s reactivity with viral and bacterial proteins (4). This protein-denaturing action of the silver ion makes the metal an effective sterilant for water and other beverages, an effect that has been recognized and put to use over centuries in one form or another (e.g., in the use of silver drinking and storage vessels). Currently, finely divided silver metal is incorporated in commercial filters for domestic water purification, or in cosmetic formulations as a silver chloride/titanium dioxide composite, where at levels of 50-500 ppm it is an effective preservative against microbial contamination in the pH range 3-11.5. The only limiting factor there is the reducing potential of the medium (5,6).