DISINFECTION USING COPPER, SILVER AND ZINC IONS
Introduction
The purpose of this short study of literature available on the microbicidal mechanisms of copper
(Cu), silver (Ag) and zinc (Zn) ions is to provide the reader with a clearer understanding and
background information. It is by no means complete, and should be supplemented with new
data as it becomes available. Also, the quest for information would largely be determined by the
technology sector in which the reader operate e.g. a great deal of research has been done on
the use on zinc in the medical field – from the treatment of colds to soar throats, and influences
on the immune system. Fundamentally though, is that these ions exert microbicidal influences
over a range of cellular targets, and various conditions will determine which of these influences
constitute the major microbicidal effect.
Copper and silver ions have been used for centuries for disinfecting water. The early Greeks
used copper and silver vessels to store water and Greek royalty stored their water in silver
flagons and drank from silver goblets. The low solubility of these metals in water served as a
natural controlled-release mechanism which added trace amounts of these ions to the water in
the vessel. The amounts were high enough to purify the water without causing objectionable
taste or health problems to the users.
Copper and silver ion disinfection of swimming pool water has several advantages over chlorine.
The ions are chemically stable and do not undergo the destructive reactions that aqueous
chlorine does, thus it is easier to maintain an effective residual dose. They do not form
objectionable by-products such as chloramines or THM's as chlorine does. However,
maintaining their concentrations in the ppb range using standard metering techniques would
prove difficult to implement reproducibly and conveniently. Also, their relatively slower rates of
inactivation would make it difficult to keep up with the high bio-burden levels which occur during
times of peak bather load. The problem of maintaining ppb concentrations of the ions in a
convenient and reproducible manner can be solved by using electrolytic generation of the ions,
or by using the ions of the metal salt solution of AquaCare.
The more recent use of copper and silver ions to inactivate microorganisms is well documented.
In addition to bacteria, they also control viruses, algae, and fungi. They are as effective at the
parts per billion (ppb) levels. Current EPA maximum levels for these metals are 1 ppm for copper
and 50 ppb for silver. At these levels, their inactivation rates are lower than that of chlorine.
Extremely small amounts of silver have significant effects on bacteria, a phenomenon referred to
as oligodynamic activity. Silver has the most powerful oligodynamic activity of the metals,
followed closely by copper. The oligodynamic activity of metals provides a valuable alternative to
the use of systemic antibiotics and/or disinfectants in certain situations.
Mechanism of copper and silver disinfection
Researchers have performed a number of studies which have shed light on the mechanism of
copper injury to bacterial cells. It was initially observed that copper-induced damage through the
copper ion concentrations in the system. Laboratory experiments confirmed that levels of
copper as low as 25 and 50 ppb caused 90% bacterial cell injury in 6 and 2 days respectively.
Copper-injured E. coli cells were subjected to physiology studies in which oxygen uptake was
monitored. It was found that the injured cells had significantly lower oxygen uptake than healthy
cells and associated the damage to the respiratory chain. It can therefore be hypothesized that
the damage was caused by copper binding to the sulfhydryl-groups of respiratory enzymes in
the cell membrane.
A series of experiments to study the metabolism of copper-injured E. coli were also done. C-13
NMR and Gas Chromatography (GC) were used to study differences in metabolism between
injured and healthy bacteria. The NMR and GC studies of metabolism were carried out on E.
coli cells grown under aerobic and anaerobic conditions, using glucose and succinate as
nutrients. These studies showed inhibition of glycolysis and tricarboxylic acid activity in the
copper-treated cells in comparison to healthy cells. Again, damage to cell surface enzymes by
copper was implicated as the cause of reduced metabolic activity.
Studies on mice to determine the survival and virulence of chlorine and copper-injured Yersinia
enterocolitica bacteria were done. It was found that fewer of the copper-damaged cells in orally-
inoculated mice survived attack by low gastric pH than did the chlorine-damaged cells. The
virulence of the copper-damaged cells was therefore significantly lower than the chlorine-
damaged cells, which were similar in virulence to undamaged controls.
Legionella pneumophila has been shown to be killed within 6 h on exposure to a silver ion
solution of 0,05 ppm. In hospitals, silver has been combined with copper ions for the control of
Legionella in water systems. Evaluations have been made of the efficacy of Cu–Ag ionization in
eradicating L. pneumophila from hospital water supplies. Legionella species have been shown to
persist in the distribution system containing < 0,3 ppm Cu and < 0,03 ppm Ag. However, when
Cu and Ag concentrations were > 0,4 ppm and > 0,04 ppm, respectively, significant decreases
in Legionella occurred. Copper and silver ions have been found to be superior to thermal
treatment for the removal of Legionella. Fewer sites throughout the distribution system were
positive for Legionella when using the Ag and Cu ions. Fewer nosocomial cases of legionellosis
were also observed when Ag/Cu ions were implemented.
In a study conducted on the copper/silver/iodine system it was found that there were two
populations of bacteria in the system exposed to iodine alone; the majority of the bacteria which
were inactivated by the treatment and a small population that was resistant to it. This latter
population was responsible for regrowth after the iodination was stopped. The bacteria exposed
to the copper and silver metal ions did not exhibit regrowth. The authors described this as being
due to different modes of action of the iodine and the metal ion systems.
Three possible mechanisms have been proposed for inhibition of microorganisms by silver:
interference with electron transport, binding to DNA, and interaction with the cell membrane. The
formation of complexes with sulfhydryl groups can inactivate cell surface enzymes and interfere
with respiration at the cell membrane.
Copper is known to attack respiratory enzymes in bacteria, presumably by binding to groups
containing; sulfhydryl, amine, and carboxyl moieties. Copper is also thought to facilitate
hydrolysis or nucleophilic displacement reactions in peptide chains or nucleic acids. Finally,
copper is able to chelate with phosphate groups and this could result in the opening of the DNA
double helices.
Although a number of studies exist which document changes in metabolism and inactivity of
microorganisms which have been damaged by metal ions, there has not yet been a definitive
work which links the changes caused by the metal ions on a molecular level. There is an even
greater scarcity of data on changes in metabolism and infectivity of microorganisms damaged by
synergistic combinations of various metal ions.
Little is known or has been proposed about the changes occurring on the molecular level
caused by metal ion combinations as well. The most probable molecular mechanism is a two-
step mechanism to explain how two (or more) different chemical molecules may result in more
efficient disinfection when used together or sequentially. It was proposed that attack by copper
on the cell walls may make them more permeable to other charged metal ions such as silver or
zinc which are normally excluded by healthy cells.
Mechanism of zinc disinfection
The health beneficial effect of zinc on humans is multi-factorial and is based on (1) direct antiviral
effects of zinc ions, (2) amplification and maintenance of immunity, (3) augmentation of interferon
activity and (4) a natural defense mechanism at the cell membrane level.
Our bodies cannot store zinc and modern diets are too low in zinc and as such zinc deficiency is
a worldwide public health problem, therefore it is worth considering zinc supplementation as a
prophylactic and therapeutic measure against flu, even the avian flu virus H5N1. A healthy
immune system is essential for good health and well-being especially when viruses can put the
immune system under stress.
A skin attachment model was used to determine if ZnCl2 would reverse or inhibit Salmonella
attachment to a broiler’s skin. Skin micrographs indicated that 25 mM and 50 mM ZnCl2
reduced Salmonella attachment by 69% and 99.9%, respectively, in the reversal experiments. In
the inhibition experiments, 25 and 50 mM ZnCl2 reduced firmly attached cells by 82 and 91%,
respectively. Reduction of Salmonella may be attributed, in part, to the bactericidal activity of
ZnCl2 in addition to bacterial cell detachment.
Despite limited evidence of efficacy from well-designed clinical trials, complementary and
alternative natural products such as Echinacea, feverfew leaf, ascorbic acid, garlic, and zinc salts
continue to gain popularity in the self-care approach to treating colds. One of the most
controversial and popular natural remedies for the common cold is zinc salts. Zinc lozenges and
lollipops are widely available in drug stores and supermarkets and are heavily promoted to the
public.
At concentrations of 0,1 mM/l, zinc is able to inhibit in vitro replication of several viruses known
to cause the common cold. However, zinc’s exact mode of action in vivo is unknown. Several
mechanisms have been postulated and are reviewed. Proposed mechanisms by which zinc may
exert its therapeutic effect include inhibiting viral capsid protein production, inducing the
production of gamma interferon, and stabilizing and protecting plasma membranes against lysis
by cytotoxic agents. Another suggested mechanism involves zinc inhibiting rhino viral interaction
with intercellular adhesion molecules - the site where the virus initially binds to epithelial cells.
Some have also proposed that zinc may interfere with the release of histamine and other
inflammatory mediators from mast cell granules. Still others have noted that zinc does have
immune-enhancing properties, at least at the deficiency end of the nutritional spectrum.
Individuals who are zinc-deficient, such as children from economically disadvantaged
populations, adolescents with low dietary intake of zinc, and adults with subclinical zinc
deficiency, may benefit from zinc supplementation through an enhancement of cellular immunity.
The properties of the bactericidal action of silver as affected by inorganic salts and ion chelators
suggest that when silver ions come into contact with bacterial cells, the silver ions are
transferred into the cells, and the subsequent generation of reactive oxygen species in the cells
are involved in the bactericidal activity of the silver ions.
The bactericidal activity of silver ion has been known since ancient times, and its spectrum is
rather broad. Silver ion reacts with the thiol-group in vital enzymes and inactivates them or
interacts with DNA, resulting in marked enhancement of pyrimidine dimerization by
photodynamic reaction and possible prevention of DNA replication. Structural changes in the cell
envelope and the presence of some small electron-dense granules formed with silver and sulfur
have also been demonstrated in bacterial cells.
It is proposed that two possible successive processes may be involved in the action of silver.
First, bacterial cells that make contact with silver take up silver ions, which inhibit several
functions in the cell and consequently damage the cells. The second is the generation of reactive
oxygen species, which are produced possibly through the inhibition of a respiratory enzyme(s) by
silver ions and attack the cell itself.
It is hoped that this information provides a guide into the possible mechanisms of microbicidal
efficacy of copper, silver and zinc ions.
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