Green synthesis of silver nanoparticles: biomolecule-nanoparticle organizations targeting antimicrobial activity

Abstract

Since discovery of the first antibiotic drug, penicillin, in 1928, a variety of antibiotic and antimicrobial agents have been developed and used for both human therapy and industrial applications. However, excess and uncontrolled use of antibiotic agents has caused a significant growth in the number of drug resistant pathogens. Novel therapeutic approaches replacing the inefficient antibiotics are in high demand to overcome increasing microbial multidrug resistance. In the recent years, ongoing research has focused on development of nano-scale objects as efficient antimicrobial therapies. Among the various nanoparticles, silver nanoparticles have gained much attention due to their unique antimicrobial properties. However, concerns about the synthesis of these materials such as use of precursor chemicals and toxic solvents, and generation of toxic byproducts have led to a new alternative approach, green synthesis. This eco-friendly technique incorporates use of biological agents, plants or microbial agents as reducing and capping agents. Silver nanoparticles synthesized by green chemistry offer a novel and potential alternative to chemically synthesized nanoparticles. In this review, we discuss the recent advances in green synthesis of silver nanoparticles, their application as antimicrobial agents and mechanism of antimicrobial mode of action.

Fig. 1. Schematic representation of the procedure for green synthesis of silver nanoparticles using various biological entities.

Fig. 2. Nitrate reductase mediated synthesis of AgNPs (this figure has been adapted from ref. 119 with permission from Springer).

Fig. 3. Bactericidal effect of green synthesized AgNPs on different bacterial strains. Dose-dependent activity of AgNPs synthesized using Allophylus cobbe leaves. The bacterial strains were incubated at various AgNP concentrations ranging from 0.1 to 1.0 μg ml⁻¹ and bacterial rate survival was estimated by colony forming unit (CFU) assay at 4 h (this figure has been adapted from ref. 22 with permission from Springer).

Fig. 4. General mechanisms for antimicrobial mode of action of silver nanoparticles (this figure has been adapted from ref. 159 with permission from Frontiers).

Fig. 5. Time-dependent TEM images of E. coli cells treated with biosynthesized AgNPs using Aspergillus niger extract. (a) The untreated cell; (b) adhesion of AgNPs on the E. coli cell wall at 1 h; (c) disruption of the cell membrane by formation of pits and gaps at 5 h; (d) complete disruption of the cell wall and membrane; (e) penetration of AgNPs into the E. coli cell at 8 h; (f) disintegration of the cell and cell lysis at 12 h (this figure has been adapted from ref. 176 with permission from Hindawi).

Fig. 6. FE-SEM images of B. cereus, S. aureus, E. coli and P. aeruginosa cells untreated (A–D) and treated (E–H) with AgNPs. After 60 min of treatment with AgNPs biosynthesized using soil derived Pseudomonas putida, all cells were subjected to severe membrane damage (this figure has been adapted from ref. 164 with permission from Elsevier).

Fig. 7. Agarose gel electrophoresis of biosynthesized AgNP-treated plasmid DNA. Lane 1, DNA marker. Lane 2, untreated plasmid DNA in supercoiled form. Lane 3, plasmid DNA treated with 0.51 μg of AgNPs showing a decrease in supercoiled DNA form. Lane 4, plasmid DNA treated with 1.02 μg of AgNPs. Lane 5, plasmid DNA treated with 2.55 μg of AgNPs. Lane 6 plasmid DNA treated with 0.51 μg of AgNPs showing degradation of DNA bands. Lane 7, plasmid DNA treated with 5.1 μg of AgNPs showing the highest degree of DNA degradation (this figure has been adapted from ref. 189 with permission from Springer).

Fig. 8. Synergistic effect of biosynthesized AgNPs and standard antibiotic drugs on ROS generation. Results of ROS measurement after 12 h treatment of the bacterial cells with AgNPs alone, antibiotics alone, and combinations of AgNPs with antibiotics (this figure has been adapted from ref. 22 with permission from Springer).

Fig. 9. (A) H₂-DCFDA assay for investigation biosynthesized AgNP-induced intracellular ROS generation; (B) the dose-dependent ROS generation in AgNP-treated in E. coli and P. aeruginosa; (C) relative GSH concentration in the treated cells as compared to positive control (PC), 1 mM H₂O₂; (D) fluorescence microscopy images of untreated, AgNP-treated, and gentamicin-treated E. coli and P. aeruginosa cells. SYTO 9 (green fluorescent) stained cells are intact and live, while PI (red fluorescent) stained cells are dead due to disruption of the cell membrane. As control, gentamicin, a standard antibiotic drug known to cause cell membrane damage, treated cell also displayed red fluorescent colour (this figure has been adapted from ref. 168 with permission from American Chemical Society).

Anupam Roy

Onur Bulut

Sudip Some

Amit Kumar Mandal

M. Deniz Yilmaz