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.
Highlights
The fast appearance of new bacterial strains resistant to current available antibiotics has become a growing obstacle to public healthcare
We identified multiple molecular and cellular mechanisms that contribute to the bactericidal effects of laser Ag NPs, and determined their toxicity to different types of human cells originating from the lungs, skin, blood vessels, kidneys and the liver
Results showed that after co-incubation with bacteria at 37°C for 24 hours, the laser generated Ag NPs created clear zones of inhibition (ZOI) against E. coli which was in a concentration dependent manner (Fig 1C-a and 1D), indicating a significant antibacterial effect of the laser Ag NPs against E. coli
Summary
The fast appearance of new bacterial strains resistant to current available antibiotics has become a growing obstacle to public healthcare. Almost 5,000 hospital deaths per year were caused by multi-drug resistant bacteria (MDR) such as methicillin resistant Staphylococcus aureus (MRSA) in the UK alone [1]. This increase in the pathogenic bacterial resistance to drugs motivated the search for new antimicrobial therapeutic agents [2], and nanoparticles (NPs) are considered to be good candidates for this purpose. In the past decade numerous types of NPs have been developed for antibacterial applications. Efforts have largely been devoted to the development of drug carrier platforms using mainly organic NPs, metallic NPs still stand out as promising therapeutic agents due to their direct antimicrobial activities. Several metallic NPs including silver (Ag), copper (Cu), titanium (Ti), Zinc (Zn), and their oxide derivatives were identified to exhibit antimicrobial effects, among which Ag NPs are the most popular and widely used in both clinical practice and domestic consumables [3,4]
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