The oligodynamic effect is a biocidal effect of metals, especially heavy metals, that occurs even in low concentrations. In modern times, the effect was observed by Karl Wilhelm von Nägeli, although he did not identify the cause. Scholarly texts from ancient India promoted the use of brass and silver in ritual cleansing practice as well as in consumption of food and drink. The ancient Indian medical text Sushruta Samhita promoted the use of specific metals in surgical procedures as a measure to prevent infection. Brassdoorknobs and silverware both exhibit this effect to an extent.
Mechanism
The metals react with thiol or amine groups of proteins, a mode of action to which microorganisms may develop resistance. Such resistance may be transmitted by plasmids.
For many decades, arsenic was used medicinally to treat syphilis. It is still used in sheep dips, rat poisons, wood preservatives, weed killers, and other pesticides. Arsenic is also still used for murder by poisoning, for which use it has a long and continuing history in both literature and fact.
Bismuth compounds have been used because of their astringent, antiphlogistic, bacteriostatic, and disinfecting actions. In dermatology bismuth subgallate is still used in vulnerary salves and powders as well as in antimycotics. In the past, bismuth has also been used to treat syphilis and malaria.
Boron
esters derived from glycols are being used for the control of microorganisms in fuel systems containing water.
Copper
Brass vessels release a small amount of copper ions into stored water, thus killing fecal bacterial counts as high as 1 million bacteria per milliliter. Copper sulfate mixed with lime is used as a fungicide and antihelminthic. Copper sulfate is used chiefly to destroy green algae that grow in reservoirs, stock ponds, swimming pools, and fish tanks. Copper 8-hydroxyquinoline is sometimes included in paint to prevent mildew. Paint containing copper is used on boat bottoms to prevent barnacle growth.
Gold
Gold is used in dental inlays and inhibits the growth of bacteria.
Lead
Physicians prescribed various forms of lead to heal ailments ranging from constipation to infectious diseases such as the plague. Lead was also used to preserve or sweeten wine. Lead arsenate is used in insecticides and herbicides. Some organic lead compounds are used as industrial biocides: thiomethyl triphenyllead is used as an antifungal agent, cotton preservative, and lubricant additive; thiopropyl triphenyllead as a rodent repellant; tributyllead acetate as a wood and cotton preservative; tributyllead imidazole as a lubricant additive and cotton preservative.
Mercury
and acetate were used for disinfecting mucous membranes at an effective concentration of 0.07% in aqueous solutions. Due to toxicological and ecotoxicological reasons phenylmercury salts are no longer in use. However, some surgeons use mercurochrome despite toxicological objections. Dental amalgam used in fillings inhibits bacterial reproduction. Organic mercury compounds have been used as topical disinfectants and preservatives in medical preparations and grain products. Mercury was used in the treatment of syphilis. Calomel was commonly used in infant teething powders in the 1930s and 1940s. Mercurials are also used agriculturally as insecticides and fungicides.
Nickel
The toxicity of nickel to bacteria, yeasts, and fungi differs considerably.
Silver
The metabolism of bacteria is adversely affected by silver ions at concentrations of 0.01–0.1 mg/L. Therefore, even less soluble silver compounds, such as silver chloride, also act as bactericides or germicides, but not the much less soluble silver sulfide. In the presence of atmospheric oxygen, metallic silver also has a bactericidal effect due to the formation of silver oxide, which is soluble enough to cause it. Even objects with a solid silver surface have a bactericidal effect. Silver drinking vessels were carried by military commanders on expeditions for protection against disease. It was once common to place silver foil or even silver coins on wounds for the same reason. Silver sulfadiazine is used as an antiseptic ointment for extensive burns. An equilibrium dispersion of colloidal silver with dissolved silver ions can be used to purify drinking water at sea. Silver is incorporated into medical implants and devices such as catheters. Surfacine is a relatively new antimicrobial for application to surfaces. Silver-impregnated wound dressings have proven especially useful against antibiotic-resistant bacteria. Silver nitrate is used as a hemostatic, antiseptic and astringent. At one time, many states required that the eyes of newborns be treated with a few drops of silver nitrate to guard against an infection of the eyes called gonorrheal neonatal ophthalmia, which the infants might have contracted as they passed through the birth canal. Silver ions are increasingly incorporated into many hard surfaces, such as plastics and steel, as a way to control microbial growth on items such as toilet seats, stethoscopes, and even refrigerator doors. Among the newer products being sold are plastic food containers infused with silver nanoparticles, which are intended to keep food fresher, and silver-infused athletic shirts and socks, which claim to minimize odors.
Thallium
Thallium compounds such as thallium sulfate have been used for impregnating wood and leather to kill fungal spores and bacteria, and for the protection of textiles from attack by moths. Thallium sulfate has been used as a depilatory and in the treatment of venereal disease, skin fungal infections, and tuberculosis.
is used as a weak antiseptic, and in paints as a white pigment and mold-growth inhibitor. Zinc chloride is a common ingredient in mouthwashes and deodorants, and zinc pyrithione is an ingredient in antidandruff shampoos. Galvanized fittings on roofs impede the growth of algae. Copper- and zinc-treated shingles are available. Zinc iodide and zinc sulfate are used as topical antiseptics.
Safety
Besides the individual toxic effects of each metal, a wide range of metals are nephrotoxic in humans and/or in animals. Some metals and their compounds are carcinogenic to humans. A few metals, such as lead and mercury, can cross the placental barrier and adversely affect fetal development. Several can induce special protein complexes called metallothioneins.
