The Molecular Mechanisms of the Antibacterial Effect of Picosecond Laser Generated Silver Nanoparticles and Their Toxicity to Human Cells

Abstract

Silver nanoparticles (Ag NPs) are known to have antibacterial properties. They are commonly produced by chemical synthesis which involves the use of harmful reducing agents. Contras, the laser technique is able to generate high-purity Ag NPs in water with specified surface charge characteristics. In the past, the molecular mechanisms contributing to the bactericidal effects of Ag NPs have been investigated extensively, but little is known of the antibacterial and toxic effects and mechanisms involved in laser-generated Ag NPs. In the current study Ag NPs were generated by picosecond laser ablation. Their antibacterial activity was determined on the gram-negative bacteria E. coli and Pseudomonas aeruginosa, and the gram positive bacteria Staphylococcus aureus including the methicillin resistant strain MRSA. Results showed that the laser generated Ag NPs exhibited strong dose-dependent antibacterial activity against all the three bacterial strains tested. Using E.coli as a model system, the laser Ag NPs treatment induced significantly high levels of reactive oxygen species (ROS). These ROS did not include detectable hydroxyl radicals, suggesting for the first time the selective ROS induction in bacterial cells by laser generated Ag NPs. The increased ROS was accompanied by significantly reduced cellular glutathione, and increased lipid peroxidation and permeability, suggesting ROS related bacterial cell damage. The laser generated Ag NPs exhibited low toxicity (within 72 hours) to five types of human cells although a weak significant decrease in cell survival was observed for endothelial cells and the lung cells. We conclude that picosecond laser generated Ag NPs have a broad spectrum of antibacterial effects against microbes including MRSA with minimal human cell toxicity. The oxidative stress is likely the key mechanism underlying the bactericidal effect, which leads to lipid peroxidation, depletion of glutathione, DNA damages and eventual disintegration of the cell membrane.

Antibacterial mechanisms of a novel type picosecond laser-generated silver-titanium nanoparticles and their toxicity to human cells

“…NP sample solution 50 µL at different concentrations was added into each well and was incubated at 37°C for 18 hours. The zones of inhibition (ZOIs) which reflect the susceptibility of microbes to the NPs were then measured 10 and presented as the average value from 2-4 measurements for each well.…”

“…First of all, E. coli bacterial cells in a density of 10 4 cfu/mL were incubated with laser-generated Ag-TiO 2 NPs at different concentrations (10,30 and 50 µg/mL) for 5 hours at 37°C in triplicate with shaking at 225 rpm. The bacterial cells were then pelleted by centrifugation at 1,000 rpm for 5-10 minutes.…”

“…9 The molecular and cellular mechanisms that underline the antibacterial properties of either Ag NPs or TiO 2 NPs separately have been studied extensively. 10,11 Under UV light, the electron-holes in TiO 2 NPs interact with water and oxygen to generate reactive oxygen species (ROS), especially hydroxyl radicals, 12 achieving antibacterial effects. Multiple mechanisms, including ROS generation contribute to the bactericidal effect of Ag NPs.…”

“…Multiple mechanisms, including ROS generation contribute to the bactericidal effect of Ag NPs. 10 However, the molecular mechanisms by which the composite Ag-TiO 2 NPs kill bacteria have been largely underinvestigated. In addition, the toxicity of the AgTiO 2 composite NPs to human cells are awaiting to be fully characterized.…”

“…10,22 The process is carried out in pure water, free of any chemical contaminations. Composite NPs can be rapidly generated by simultaneous ablation of bulk metal blocks in the same reaction using a laser beam.…”