Palacký University scientists have described the mechanism by which bacteria develop resistance to silver nanoparticles – which means antibiotics can now restore the power they once had. The experts found out that bacteria exposed to silver nanoparticles produce abundant amounts of substances that force the silver nanoparticles to clump together – and by doing so, they lose their antibacterial effect. The researchers have also found a way to prevent the bacteria from gaining resistance to the silver nanoparticles. These new findings could greatly help in the ongoing efforts to combat the growing resistance of microorganisms to antibiotics, which severely complicates the treatment of bacterial infections. The results of the Olomouc scientists’ work were published in the journal Communications Biology.
The long-term over-prescribing of antibiotics, typical of the end of the last century in particular, has resulted in the increasing resistance of bacteria to antibiotics. In recent years, chemists, microbiologists, and physicians have therefore become increasingly interested in the antibacterial effects of silver nanoparticles, which have gradually become part of many commercial products with antibacterial effects. Scientists from the Department of Physical Chemistry at the UP Faculty of Science and from the Department of Microbiology at the UP Faculty of Medicine and Dentistry were once among the first researchers in the world to demonstrate the great antimicrobial effect of silver nanoparticles as well as their ability to restore the effectiveness of inactive conventional antibiotics against resistant bacteria.
Bacteria versus nanosilver
However, bacterial resistance may not only be a problem for antibiotics, but also for the silver nanoparticles themselves. Scientists from the Faculties of Science and Medicine in Olomouc have been working on this challenge for the last ten years. In 2018, they were the first in the world to describe how the mechanism of resistance of Gram-negative bacterium E. coli to silver nanoparticles emerges and develops. After repeated administration of nanosilver, this bacterium increases its production of a protein called flagellin, which causes the nanoparticles to aggregate and subsequently lose their antibacterial properties.
The fact is that the flagellin protein has adhesive properties and thus acts as a kind of glue that holds the silver particles together. This results in large clusters of nanoparticles that lose all of their antibacterial activity. The Olomouc scientists published this groundbreaking discovery in the journal Nature Nanotechnology in 2018. With almost 700 citations, their work has received enormous acclaim among experts.
However, the research didn’t stop there. Subsequently, scientists from the UP Faculty of Science, in collaboration with their colleagues from the UP Faculty of Medicine and Dentistry and the Czech Advanced Technology and Research Institute (CATRIN), investigated the mechanism of resistance of Gram-positive bacterium Staphylococcus aureus to silver nanoparticles, which also induced the aggregation of nanoparticles.
“In this case, the aggregation of silver nanoparticles could not have been triggered by the flagellin protein, as S. aureus does not possess it. After extensive and challenging research, we discovered and described the mechanism by which this bacterium also resists the effects of nanosilver. This time it was the excessive formation of bacterial biofilm, which has a similar impact on silver nanoparticles as flagellin,” said Lucie Hochvaldová from the Department of Physical Chemistry at the UP Faculty of Science.
The power of pomegranates
The UP research team also started looking for a way to overcome this bacterial resistance. Once they described the resistance mechanism in detail, they became able to find a way to prevent the aggregation of nanoparticles around the bacteria. “We achieved this by adding substances to the nanosilver that simultaneously inhibit both the flagellin production in E. coli and the bacterial biofilm formation in S. aureus. Such properties are exhibited by substances contained, for example, in pomegranate rind extract,” said Aleš Panáček from the Department of Physical Chemistry at the UP Faculty of Science.
The moment this extract was applied together with silver nanoparticles, the bacteria stopped forming flagellin or biofilm to a sufficient extent, thus losing the ability to make nanosilver particles aggregate and develop resistance to their effects. However, the researchers have found yet another way to overcome the bacterial resistance to silver nanoparticles – this time through increasing their stability by binding them to graphene. The firm bond that binds the silver nanoparticles to the surface of graphene stabilises them so much that the flagellin and/or biofilm produced by the bacteria cannot aggregate the particles, and thus they preserve their high antibacterial activity.
“Our research will certainly improve our understanding of the bacterial resistance mechanisms to nanostructured materials, which differ from resistance mechanisms to conventional antibiotics. It also provides potential strategies to combat bacteria causing serious infectious diseases, counteract rising bacterial resistance complicating treatments, and develop more effective antimicrobial treatments. We believe that this work will stimulate a series of detailed experimental investigations involving research on the development of bacterial resistance to antibacterial nanoparticulate materials as well as further search for ways to fight it,” added Milan Kolář from the Department of Microbiology at the UP Faculty of Medicine and Dentistry.