Silver biocides are based on solutions of water soluble silver salts such as silver chloride or citrate, or in the case of one manufacturer, silver chloride adsorbed onto titanium dioxide, which produces a product with greater longevity of activity. Silver is effective at extremely low concentrations (ppb levels of silver ions) and has a very broad spectrum kill. At the recommended use levels silver is considered nontoxic to humans.
Quaternary biocides are based on alkyl, alkyl aryl or dialkyl quaternized tertiary amines. Effective broad range kill can be optimized by varying the alkyl(dialkyl) chain length(s).They also have detergent properties, which means they clean as well as disinfect. For this reason they have become the primary antibiotics used in the cleaning industry; however, there are questions about their biodegradability and toxicity.
These two types of biocides differ in fundamental ways. Silver has a three-pronged attack of binding and disrupting the cell wall, binding to enzymes in the microbe and causing denaturation, which incapacitates the energy source of the cell thus leading to rapid cell death. It also binds to DNA and stops replication.
Quats kill by binding to the “anionic” cell wall, defusing through and binding to the cytoplastic membrane causing disorganization, leakage and cell death.
There is cause to believe that because of over-use, bacteria are adapting to and building up immunity to quat-based biocides. In the case of silver, due to its modes of action, the cells cannot adapt to become immune, arguably making silver the biocide of choice for the future.
