Ions, not particles, make silver toxic to bacteria

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Ions, not particles, make silver toxic to bacteria

Post by Cr6 on Sun Feb 18, 2018 2:31 am

Some research on why silver is so effective at preventing infections on open wounds.

Terminal oxidases and reductases

When bacteria grow in aerobic environments, the terminal electron acceptor (O2) is reduced to water by an enzyme called an oxidase. When bacteria grow in anaerobic environments, the terminal electron acceptor is reduced by an enzyme called a reductase.

In mitochondria the terminal membrane complex (Complex IV) is cytochrome oxidase. Aerobic bacteria use a number of different terminal oxidases. For example, E. coli does not have a cytochrome oxidase or a bc1 complex. Under aerobic conditions, it uses two different terminal quinol oxidases (both proton pumps) to reduce oxygen to water.

Anaerobic bacteria, which do not use oxygen as a terminal electron acceptor, have terminal reductases individualized to their terminal acceptor. For example, E. coli can use fumarate reductase, nitrate reductase, nitrite reductase, DMSO reductase, or trimethylamine-N-oxide reductase, depending on the availability of these acceptors in the environment.

Most terminal oxidases and reductases are inducible. They are synthesized by the organism as needed, in response to specific environmental conditions.

Most complete coverage is found here:

Ionized silver

In order for silver to have any antimicrobial properties, it must be in its ionized form (Lok et al., 2007; Rai et al., 2009). Silver in its non-ionized form is inert (Guggenbichler et al., 1999), but contact with moisture leads to the release of silver ions (Radheshkumar and Munstedt, 2005). Thus, all forms of silver or silver containing compounds with observed antimicrobial properties are in one way or another sources of silver ions (Ag+); these silver ions may be incorporated into the substance and released slowly with time as with silver sulfadiazine, or the silver ions can come from ionizing the surface of a solid piece of silver as with silver nanoparticles.

Observed effects of silver exposure

Feng et al. (2000) conducted a study to observe the effects of silver ions on gram-positive and gram-negative bacteria, namely Staphylococcus aureus and Escherichia coli. They treated cells with AgNO3, which is a source of Ag+ in aqueous environments, and looked at the structural and morphological effects of these silver ions on the cells. The cells were exposed to AgNO3 for 4-12 hours before being prepared for microscopy. The cell were then fixed and sliced with an ultramicrotome to produce ultrathin sections for transmission electron microscopy (TEM). They observed that cells exposed to the Ag+ ions seemed to have activated a stress response that led to the condensation of DNA in the center of the cell. They also observed cell membrane detachment from the cell wall, cell wall damage, and electron dense granules outside and, in some instances, inside the cell (Figure 3). It was proposed that condensation of DNA occurred as a protective measure in order to protect the genetic information of the cell (Feng et al., 2000), however condensation of DNA could also prevent cell replication by preventing the DNA from being accessed by transcriptional enzymes such as DNA polymerase. The electron dense granules that formed inside and outside the cell were extracted and subjected to X-ray microanalysis to determine their composition. It was discovered that the granules were in part composed of silver and sulfur. This finding supports the idea that silver inactivates proteins by binding to sulfur-containing compounds (Klueh et al., 2000). It was also observed that when treated with Ag+, E. coli, a gram-negative bacterium, sustained more structural damages than the gram-positive S. aureus (Feng et al., 2000).

It has also been shown that treating cells with silver leads to cell shrinkage and dehydration (Figure 4) (Guggenbichler et al., 1999). The TEM images from Feng et al. (2000) (Figure 4) show that cells that sustained extensive damage eventually ended up with cell wall and cell membrane damage. Damage to the cell membrane could lead to the leaking of cytoplasm from the cell, which would result in dehydrated and shrunken cells as shown by the SEM images from Guggenbichler et al. (1999).
Attack on Gram-positive vs. Gram-negative

There are two explanations as to why gram-positive bacteria are less susceptible to Ag+ than gram-negative bacteria. The first involves the charge of peptidoglycan molecules in the bacterial cell wall. Gram-positive bacteria have more peptidoglycan than gram-negative bacteria because of their thicker cell walls, and because peptidoglycan is negatively charged and silver ions are positively charged, more silver may get trapped by peptidoglycan in gram-positive bacteria than in gram-negative bacteria (Kawahara et al., 2000). The decreased susceptibility of gram-positive bacteria can also simply be explained by the fact that the cell wall of gram-positive bacteria is thicker than that of gram-negative bacteria.

Silver sulfadiazine

Fox and Modak (1974) explored the mechanism of prevention of burn wound infections by silver sulfadiazine. At the time of publication, it had been known for quite a while that silver sulfadiazine (Figure 5) delivered in the form of a topical cream was effective at preventing infections in burn wounds, however it was not known if the antimicrobial activity was due entirely to the silver ions or if the sulfadiazine anion also contributed to the bactericidal effect. Tests from the study showed that sulfadiazine from silver sulfadiazine does not get transported into cells as much as silver. Silver isotopes (110Ag+) were also used to show that silver ions that enter cells complex with DNA. Additionally, the rate at which certain silver containing compounds release silver ions into solution was measured by adding silver compounds to human serum and measuring the amount of unreacted silver compound with increasing time (Figure 6). Notice in Figure 6 how silver sulfadiazine (Ag sulfadiazine) gradually releases the majority of its silver ions into solution over an extended period of time whereas silver nitrate immediately released all of its silver ions into solution (Fox and Modak, 1974). Therefore, silver sulfadiazine's effectiveness as an antimicrobial agent for preventing burn wound infections is due to its tendency to dissociate in solution: silver sulfadiazine provides a steady supply of silver ions over a long period of time where as other silver salts such as silver nitrate release a large amount of silver ions all at once. If silver is employed as the primary antimicrobial agent in burn wound creams, the burn wound needs a steady supply of silver ions over a long period of time to kill off any microbes that could possibly infect the wound until it heals. Compounds that release silver ions all at once would need to be applied very frequently in order to kill off invading bacteria and prevent infection, and sometimes highly frequent application isn't always practical or possible for individuals, so compounds that constantly release a bactericidal amount of silver ions, such as silver sulfadiazine, are the most effective at preventing burn wound infections.

It has also been shown that when silver treatment is combined with other antimicrobial methods such as UV light, copper ions, or oxidizers, a synergistic effect is observed, that is bacterial growth is inhibited more by treatment with silver and an additional antimicrobial method than would be expected if the inhibition effects of silver and that additional antimicrobial method were summed (Silvestry-Rodriguez et al., 2007). Because silver can inflict a fair amount of damage to the cell only once it gains access to the cytoplasm, it is believed that if some other antimicrobial method can give silver ions access to the cytoplasm sooner than if silver ions were working alone, a synergistic effect of the two methods would be observed (Silvestry-Rodriguez et al., 2007).

Ions, not particles, make silver toxic to bacteria
Rice University researchers report too small a dose may enhance microbes’ immunity

HOUSTON – (July 11, 2012) – Rice University researchers have settled a long-standing controversy over the mechanism by which silver nanoparticles, the most widely used nanomaterial in the world, kill bacteria.

Their work comes with a Nietzsche-esque warning: Use enough. If you don’t kill them, you make them stronger.

Scientists have long known that silver ions, which flow from nanoparticles when oxidized, are deadly to bacteria. Silver nanoparticles are used just about everywhere, including in cosmetics, socks, food containers, detergents, sprays and a wide range of other products to stop the spread of germs.

But scientists have also suspected silver nanoparticles themselves may be toxic to bacteria, particularly the smallest of them at about 3 nanometers. Not so, according to the Rice team that reported its results this month in the American Chemical Society journal Nano Letters.

In fact, when the possibility of ionization is taken away from silver, the nanoparticles are practically benign in the presence of microbes, said Pedro Alvarez, George R. Brown Professor and chair of Rice’s Civil and Environmental Engineering Department.

“You would be surprised how often people market things without a full mechanistic understanding of their function,” said Alvarez, who studies the fate of nanoparticles in the environment and their potential toxicity, particularly to humans. “The prefix ‘nano’ can be a double-edged sword. It can help you sell a product, and in other cases it might elicit concerns about potential unintended consequences.”

He said the straightforward answer to the decade-old question is that the insoluble silver nanoparticles do not kill cells by direct contact. But soluble ions, when activated via oxidation in the vicinity of bacteria, do the job nicely.

To figure that out, the researchers had to strip the particles of their powers. “Our original expectation was that the smaller a particle is, the greater the toxicity,” said Zongming Xiu, a Rice postdoctoral researcher and lead author of the paper. Xiu set out to test nanoparticles, both commercially available and custom-synthesized from 3 to 11 nanometers, to see whether there was a correlation between size and toxicity.

“We could not get consistent results,” he said. “It was very frustrating and really weird.”

Xiu decided to test nanoparticle toxicity in an anaerobic environment – that is, sealed inside a chamber with no exposure to oxygen — to control the silver ions’ release. He found that the filtered particles were a lot less toxic to microbes than silver ions.

Working with the lab of Rice chemist Vicki Colvin, the team then synthesized silver nanoparticles inside the anaerobic chamber to eliminate any chance of oxidation. “We found the particles, even up to a concentration of 195 parts per million, were still not toxic to bacteria,” Xiu said. “But for the ionic silver, a concentration of about 15 parts per billion would kill all the bacteria present. That told us the particle is 7,665 times less toxic than the silver ions, indicating a negligible toxicity.”

“The point of that experiment,” Alvarez said, “was to show that a lot of people were obtaining data that was confounded by a release of ions, which was occurring during exposure they perhaps weren’t aware of.”

Alvarez suggested the team’s anaerobic method may be used to test many other kinds of metallic nanoparticles for toxicity and could help fine-tune the antibacterial qualities of silver particles. In their tests, the Rice researchers also found evidence of hormesis; E. coli became stimulated by silver ions when they encountered doses too small to kill them.

“Ultimately, we want to control the rate of (ion) release to obtain the desired concentrations that just do the job,” Alvarez said. “You don’t want to overshoot and overload the environment with toxic ions while depleting silver, which is a noble metal, a valuable resource – and a somewhat expensive disinfectant. But you don’t want to undershoot, either.”

He said the finding should shift the debate over the size, shape and coating of silver nanoparticles. “Of course they matter,” Alvarez said, “but only indirectly, as far as these variables affect the dissolution rate of the ions. The key determinant of toxicity is the silver ions. So the focus should be on mass-transfer processes and controlled-release mechanisms.”

“These findings suggest that the antibacterial application of silver nanoparticles could be enhanced and environmental impacts could be mitigated by modulating the ion release rate, for example, through responsive polymer coatings,” Xiu said.


The toxic effect of silver ions and silver nanoparticles towards bacteria and human cells occurs in the same concentration range
Christina Greulich,a  Dieter Braun,b  Alexander Peetsch,c  Jörg Diendorf,c  Bettina Siebers,*b  Matthias Epple*c  and  Manfred Köller*a
Author affiliations


Silver is commonly used both in ionic form and in nanoparticulate form as a bactericidal agent. This is generally ascribed to a higher toxicity towards prokaryotic cells than towards mammalian cells. Comparative studies with both silver ions (such as silver acetate) and polyvinylpyrrolidone (PVP)-stabilized silver nanoparticles (70 nm) showed that the toxic effect of silver occurs in a similar concentration range for Escherichia coli, Staphylococcus aureus, human mesenchymal stem cells (hMSCs), and peripheral blood mononuclear cells (PBMCs), i.e. 0.5 to 5 ppm for silver ions and 12.5 to 50 ppm for silver nanoparticles. For a better comparison, bacteria were cultivated both in Lysogeny broth medium (LB) and in Roswell Park Memorial Institute medium (RPMI)/10% fetal calf serum (FCS) medium, as the state of silver ions and silver nanoparticles may be different due to the presence of salts, and biomolecules like proteins.

The effective toxic concentration of silver towards bacteria and human cells is almost the same.

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Re: Ions, not particles, make silver toxic to bacteria

Post by Cr6 on Sun Feb 18, 2018 2:44 am

(Suprisingly, the actual "mechanism" of this effect is not really described all that well... how does this actually work?   )

Oligodynamic effect
Silver spoons self-sanitize due to the oligodynamic effect

The oligodynamic effect (from Greek oligos "few", and dynamis "force") is a biocidal effect of metals, especially heavy metals, that occurs even in low concentrations. The effect was discovered by Karl Wilhelm von Nägeli, although he did not identify the cause.[1] Brass doorknobs and silverware both exhibit this effect to an extent.



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. Bactericidal concentrations are reduced rapidly by adding colloidal silver, which has a high surface area. Even objects with a solid silver surface (e.g., table silver, silver coins, or silver foil) 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.[21]

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.[2] Silver is incorporated into medical implants and devices such as catheters. Surfacine (silver iodide) 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 nanoparticies, which are intended to keep food fresher, and silver-infused athletic shirts and socks, which claim to minimize odors.[14]


Oligodynamic Effect of Brass

Brass fish kohl applicator on fish spine chain

Especially heavy metals show this effect. The exact mechanism of action is still unknown. Data from silver suggest that these ions denature proteins (enzymes) of the target cell or organism by binding to reactive groups resulting in their precipitation and inactivation. Silver inactivates enzymes by reacting with the sulfhydryl groups to form silver sulfides. Silver also reacts with the amino-, carboxyl-, phosphate-, and imidazole-groups and diminish the activities of lactate dehydrogenase and glutathione peroxidase. Bacteria (Gram-positive and Gram-negative) are in general affected by the oligodynamic effect, but they can develop a heavy-metal resistance, or in the case of silver a silver-resistance. Viruses in general are not very sensitive. The toxic effect is fully developed often only after a long time (many hours).

Certain metals, e.g. brass and copper are known to be far more poisonous to bacteria than others (such as stainless steel and aluminum), which is why they are used in mineral sanitizers for swimming pools and spas. Brass is used to manufacture cosmetic containers, especially those used to apply kohl or kajal to the eye lids, in the Middle east and Asia.

Many infections can spread by doorknobs. Brass doorknobs automatically disinfect themselves in about eight hours, while stainless steel and aluminum knobs never do. Brass doorknobs therefore tend to be more sanitary than stainless or aluminum doorknobs. The effect is important in hospitals, and useful in any building.[1]


"The exact mechanism of this action is still unknown but some data suggest that the metal ions denature protein of the target cells by binding to reactive groups resulting in their precipitation and inactivation. The high affinity of cellular proteins for the metallic ions results in the death of the cells due to cumulative effects of the ion within the cells (Benson 2002). Similarly, silver inactivates enzymes by binding with sulfhydryl groups to form silver sulfides or sulfhydryl-binding propensity of silver ion disrupts cell membranes, disables proteins and inhibits enzyme activities (Thurman & Gerba 1988; Semikina & Skulacher 1990). The study also suggest that positively charged copper ion distorts the cell wall by bonding to negatively charged groups and allowing the silver ion into the cell (Hambidge 2001). Silver ions bind to DNA, RNA, enzymes and cellular proteins causing cell damage and death."


Structure and bonding

Thiols and alcohols have similar connectivity. Because sulfur is a larger element than oxygen, the C–S bond lengths, typically around 180 picometers in length, is about 40 picometers longer than a typical C–O bond. The C–S–H angles approach 90° whereas the angle for the C-O-H group is more obtuse. In the solid or liquids, the hydrogen-bonding between individual thiol groups is weak, the main cohesive force being van der Waals interactions between the highly polarizable divalent sulfur centers.

Due to the lesser electronegativity difference between sulfur and hydrogen compared to oxygen and hydrogen, an S–H bond is less polar than the hydroxyl group. Thiols have a lower dipole moment relative to the corresponding alcohol.

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Re: Ions, not particles, make silver toxic to bacteria

Post by Cr6 on Sun Feb 18, 2018 3:21 am

Antibacterial potential of silver nanoparticles synthesised by electron beam irradiation
by Rani M. Pattabi, Kandikere R. Sridhar, Srinath Gopakumar, Bhat Vinayachandra, Manjunatha Pattabi
International Journal of Nanoparticles (IJNP), Vol. 3, No. 1, 2010


Antibacterial activity of silver nanoparticles synthesised by irradiating an aqueous solution of AgNO3 in a bio-compatible polymer polyvinyl alcohol (PVA) with electrons from a microtron, at doses of 1 kGy and 2 kGy, has been evaluated against selected Gram-positive and Gram-negative bacteria. Nanoparticles produced by electron beam irradiation (EBI) at 1 kGy showed significantly higher inhibitory effects than 2 kGy on Gram-negative bacteria (Escherichia coli and Pseudomonas aeruginosa) and Gram-positive bacteria (Staphylococcus aureus and Streptococcus pneumoniae). The inhibition was highest on P. aeruginosa, while it was least on S. aureus. The particles generated at 1 kGy at 1,000 μg/ml concentration showed significantly lower number of colonies of P. aeruginosa against 100 μg/ml. Our study demonstrates that silver nanoparticles generated at 1 kGy irradiation dosage will be of immense value to produce broad spectrum bacteriostatic or bactericidal agents and needs further studies for clinical applications.

Online publication date: Thu, 13-May-2010


Cons of using copper:
The kids used to get what is called 'Indian Childhood Cirrhosis (ICC)', a fatal liver disorder,cause and treatment of same remained unknown till late seventies.
Now stainless steel has replaced copper utensils, ICC has virtually disappeared.


Wishing well

Oligodynamic effect

Another theory is that people may have unknowingly discovered the biocidal properties of both copper and silver;[citation needed] the two metals traditionally used in coins. Throwing coins made of either of these metals could help make the water safer to drink. Wells that were frequented by those that threw coins in may have been less affected by a range of bacterial infections making them seem more fortunate and may have even appeared to have cured people suffering from repeated infections.

In November 2006 the "Fountain Money Mountain" reported that tourists throw just under 3 million pounds sterling per year into wishing wells.[4]


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