The Antimicrobial Properties of Silver Nanoparticles in Bacillus subtilis Are Mediated by Released Ag+ Ions
In this clinical study, published in the journal PLOS-One, researchers found that silver nanoparticles exhibit significant antimicrobial action against the Bacillus subtilis bacterium, and that the effectiveness is due largely to the release of silver ions. Bacillus subtilis, though a relatively benign soil bacteria, is extremely difficult to kill due to its ability to produce endospores – a kind of “armor plating” which makes the microbe virtually impervious to temperature variations, desiccation, ultra-violet light, starvation and some disinfectants.
Because of that unusual characteristic, Bacillus subtilis is often used in clinical testing as a substitute for its cousin bacillus anthracis (i.e., anthrax). The researchers treated B. subtilis cultures with 0-50 ppm concentrations of silver nanoparticles, and found that while a concentration of 5 ppm inhibited bacterial growth for 12 hours, concentrations of 10 ppm and higher were lethal to the bacterium. The researchers found that reactive oxygen species triggered by the release of positively charged silver ions from the silver nanoparticles contributed to the permeability of the cell membrane of the microbe, and that the ionic silver then penetrated the cell membrane and disrupted the integrity of the microbe’s chromosomal DNA.
The researchers concluded, “To the best of our understanding, this is the first study to directly analyze silver particles present within bacterial cells treated with silver nanoparticles, and the results indicate that positively charged silver ions are primarily responsible for silver nanoparticle microbial toxicity… our results support the theory that silver nanoparticles exert microbial toxicity through the release of positively charged silver ions that subsequently penetrate into bacterial cells.”
