As ‘superbug’ bacteria resistant to commonly prescribed antibiotics have become a global threat to public health in recent years, noble metals, such as silver, in various forms have been attracting broad attention due to their antimicrobial activities. While it is still a mystery as to exactly how silver kills bacteria, a team of researchers at the University of Arkansas has studied the antimicrobial effect of silver ions at the nanoscale on the dynamics of proteins in live bacteria.
Sadoon et al studied the antimicrobial effect of silver ions at the nanoscale on the diffusive dynamics of histone-like nucleoid-structuring proteins in live Escherichia coli bacteria using single-particle-tracking photoactivated localization microscopy. Image credit: Arek Socha.
“Traditionally, the antimicrobial effects of silver have been measured through bioassays, which compare the effect of a substance on a test organism against a standard, untreated preparation,” said senior author Dr. Yong Wang, a scientist in the Department of Physics at the University of Arkansas.
“While these methods are effective, they typically produce only snapshots in time.”
Instead, Dr. Wang and his colleagues used an advanced imaging technique, called single-particle-tracking photoactivated localization microscopy, to watch and track a particular protein found in Escherichia coli bacteria over time.
They were surprised to find that silver ions actually sped up the dynamics of the protein, opposite of what they thought would happen.
“It is known that silver ions can suppress and kill bacteria; we thus expected that everything slowed down in the bacteria when treated with silver. But, surprisingly, we found that the dynamics of this protein became faster,” Dr. Wang said.
The study authors observed that silver ions were causing paired strands of DNA in the bacteria to separate, and the binding between the protein and the DNA to weaken.
“Then the faster dynamics of the proteins caused by silver can be understood,” Dr. Wang said.
“When the protein is bound to the DNA, it moves slowly together with the DNA, which is a huge molecule in the bacteria. In contrast, when treated with silver, the proteins fall off from the DNA, moving by themselves and thus faster.”
The observation of DNA separation caused by silver ions came from earlier work that Dr. Wang and co-authors had done with bent DNA.
Their approach was to put strain on DNA strands by bending them, thus making them more susceptible to interactions with other chemicals, including silver ions.
“The study validated the idea of investigating the dynamics of single proteins in live bacteria,” Dr. Wang said.
“What we want to do eventually is to use the new knowledge generated from this project to make better antibiotics based on silver nanoparticles.”
The results were published in the journal Applied and Environmental Microbiology.
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Asmaa A. Sadoon et al. Silver Ions Caused Faster Diffusive Dynamics of Histone-Like Nucleoid-Structuring Proteins in Live Bacteria. Applied and Environmental Microbiology, published online March 2, 2020; doi: 10.1128/AEM.02479-19
