Antimicrobial-resistant infections are a leading cause of global death, killing nearly 1.3 million people yearly and outpacing malaria and HIV, according to a 2022 study published in The Lancet. In January 2023, a study published in Frontiers in Microbiology confirmed mounting evidence that adding silver nanoparticles to certain antibiotics boosts their ability to kill resistant bacteria.
Knowledge of silver’s antimicrobial value has been around since ancient times. Silver nanoparticles (AgNP), however, with their tiny spheres interacting at the cellular level, signify a new frontier in fighting bacteria, gaining more attention over the past two decades.
This new study adds to mounting evidence that adding AgNPs to antibiotics could make them better able to disable drug-resistant bacteria and reduce the worldwide burden of antimicrobial resistance (AMR).
What Are Silver Nanoparticles?
Silver nanoparticles are small spheres between 1 and 100 nanometers in diameter. Because these particles are so small, they have a much larger total surface area than a similar amount of silver made up of larger particles. The large surface area increases their ability to interact with cells and molecules.
Science is still uncovering how silver fights bacteria, but current research shows it likely uses several weapons to weaken and overcome microbes, including targeting their membranes, DNA, and ribosomes. Antibiotics, on the other hand, typically offer only one way of destroying bacteria. The multifaceted approach of AgNPs seems to support antibiotics’ targeted line of attack while also bringing new weapons to the fight.
The Study’s Findings
This particular study in Frontiers in Microbiology examined the antimicrobial activity of AgNPs and their safety in mammalian cells.
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Using different commercially-available silver nanoparticles, scientists tested the power of AgNPs alone against the gram-negative bacteria Escherichia coli, Pseudomonas aeruginosa, and Acinetobacter baumannii, as well as gram-positive, methicillin-resistant Staphylococcus aureus, commonly known as MRSA.
The researchers also tested AgNPs mixed with different antibiotics against the same bacteria, including four that are leading pathogenic killers worldwide.
While AgNPs did not improve the effectiveness of some antibiotics, they strongly empowered others.
When scientists added AgNPs to the generic antimicrobial drug, colistin, the required dose to kill bacteria shrank nearly sevenfold. This finding is significant since colistin is a harsh antibiotic used as a last resort against certain resistant bacterial infections.
The study found the greatest synergy when mixing AgNPs with aminoglycoside antibiotics—ribosome-targeting antibiotics often used to treat Pseudomonas aeruginosa, which causes various infections. In particular, the effective dose of the generic aminoglycoside, amikacin, was reduced 22-fold, a robust finding since bacteria are growing increasingly resistant to aminoglycosides.
In summary, adding AgNps to certain antibiotics in the lab reduces the amount of the antibiotics needed to kill bacteria and makes them effective against some multi-drug resistant bacteria.
Silver Toxicity May Be a Concern
Before researchers test new drugs on humans, they must uncover possible toxicity. One goal of the study highlighted here was to test the safety of AgNPs on mammalian cells and a commonly used worm, Caenorhabditis elegans, for early drug studies.
At doses effective for killing bacteria, the study showed no toxicity to mammalian cell lines, including stem cells, skin cells, and immune cells called macrophages. When the researchers treated the worms with antibiotics mixed with AgNPs, their lifespan remained normal. However, they moved less, suggesting a possible side effect on muscles or nerves.
Scientists are unsure if this same effect would occur in humans, making it a concern requiring further research.
Rather than a silver bullet, however, AgNPs may be a “silver spoon” to feed multi-drug resistant bacteria their medicine, as stated by a research article published in 2013 in Science Translational Medicine. This study found that adding AgNPs to the antibiotic vancomycin disabled gram-negative bacteria. They also reported that silver boosted antimicrobial power against biofilms, which are colonies of tenacious bacteria living together to promote their growth and survival.
Another study published in 2020, found mixing AgNPs with eleven different antibiotics significantly inhibited the bacterial growth of multiple resistant species. At the same time, the study discussed concerns about silver’s possible toxicity to cells, known as cytotoxicity.
While scientists are optimistic that more studies will lead to novel treatments for antimicrobial resistance, concerns and unknowns remain about precisely how AgNPs mixed with antibiotics will interact with human bodies.
Concerns About Silver’s Increasingly Widespread Use
From 2007 to 2017 alone, governments worldwide awarded nearly 5,000 patents to companies registering silver-containing products. A 2018 review published in the journal Antibiotics lists many of those 5,000 patents.
According to the review, the medical industry offers numerous devices coated or infused with silver, like surgical needles, implants, wound treatments, and dentures. One company even patented a silver coating for walls.
For personal care, consumers can buy silver-bathed shaving devices, toothbrushes, deodorants, sanitary pads, and cosmetic lotions, to name a few toiletry items. A multitude of home products containing silver are also on the market, like air purifiers, computer keyboards, food storage containers, latex gloves, surface cleaners, laundry detergent, and a cleaning system for your bathtub’s pipes.
Experts are concerned with the vast amount of silver creeping into various aspects of daily living. In medieval times, silverware was used throughout the day among the wealthier classes, causing argyria, a largely harmless condition turning one’s skin to blue from too much silver.
The therapeutic use of silver goes back about 2,000 years, but not to the widespread level we see today. Concerns are also growing that silver may leech from commercial products into streams and waterways, possibly changing the microbial structure of diverse ecosystems.
Many experts recommend using silver only when necessary to reduce the risk of toxicity and to reduce the possibility of creating the same problem we did with the overuse of antibiotics, namely creating bacteria resistant to silver’s antimicrobial powers. These experts argue we need to learn more about how AgNPs interact with bacteria and the human body and why they are so potent.
The questions are compelling. Do we need silver in everything? Do we know what the risks are?
If further research confirms silver’s ability to fight the global crisis of antimicrobial resistance safely, it will do a world of good.