Size-dependent antimicrobial properties of sugar-encapsulated gold nanoparticles synthesized by a green method

Abstract

The antimicrobial properties of dextrose-encapsulated gold nanoparticles (dGNPs) with average diameters of 25, 60, and 120 nm (± 5) and synthesized by green chemistry principles were investigated against both Gram-negative and Gram-positive bacteria. Studies were performed involving the effect of dGNPs on the growth, morphology, and ultrastructural properties of bacteria. dGNPs were found to have significant dose-dependent antibacterial activity which was also proportional to their size. Experiments revealed the dGNPs to be bacteriostatic as well as bactericidal. The dGNPs exhibited their bactericidal action by disrupting the bacterial cell membrane which leads to the leakage of cytoplasmic content. The overall outcome of this study suggests that green-synthesized dGNPs hold promise as a potent antibacterial agent against a wide range of disease-causing bacteria by preventing and controlling possible infections or diseases.

Figure 1

Effect of different sizes of dextrose-encapsulated GNPs on the growth of E. coli. Growth analysis curves were measured by monitoring the optical density (OD) at 600 nm, and the E. coli was treated with dGNPs of sizes: 25 ± 5; 60 ± 5. and 120 ± 5 nm at different concentrations (NPs/mL).

Figure 2

Plate assay showing the number of viable cells recovered after treatment of E. coli with dGNPs. (A) The bacterial cells were treated with dGNPs in the liquid media for 12 h and then were spread on the agar plates and incubated for further 12 h. (B) Plot of the number of E. coli colonies recovered against the number of dGNPs.

Figure 3

Visualization of dGNP induced morphological changes in E. coli cell membranes under the TEM. (A) Morphology of the untreated E. coli cell at 0 h (a); cross section of the untreated E. coli cell after 12 h (b). (B) Interaction of dGNPs with E. coli cell at 0 h (a); cross section of the dGNPs treated E. coli cell after 6 h showing the initiation of the cell disruption by the formation of outer membrane vesicles (OMVs) (b); cross section of the lysed E. coli cell after 12 h of treatment with dGNPs (c); the magnified view of OMV formation, which represents the initiation of disruption of cell membrane (d).

Figure 4

Monitoring dGNPs-induced permeability of E. coli cell membranes and leakage of nucleic acids. (A) The left half shows an image in the differential interference contrast mode, while the right half shows the corresponding fluorescence image. (B) The percentage of cells with permeable membranes from five or more fields of view obtained by two independent experiments.

Figure 5

Spread plate assays of S. epidermidis after treatment with dGNPs at different concentrations. (A) Plate assay showing the number of viable cells recovered after the treatment of S. epidermidis without (control) or with dGNPs). (B) Graphs plotted for the number of S. epidermidis colonies recovered against the number of dGNPs.

Figure 6

Visualizing dGNP-induced morphological changes of S. epidermidis cell membranes via TEM. (A) Morphology of the untreated S. epidermidis cells at 0 h (a); cross section of the untreated S. epidermidis cell after 12 h (b). (B) Interaction of dGNPs with S. epidermidis cell at 0 h (a); cross section of the dGNPs-treated S. epidermidis cell after 6 h showing the initiation of the cell wall disruption (b); cross section of the lysed S. epidermidis cell after 12 h of treatment with dGNPs (c).

Figure 7

Monitoring of dGNPs-induced permeability of S. epidermidis cell membranes via propidium iodide. (A) For each image the left half shows an image in the differential interference contrast mode, while the right half shows the corresponding fluorescence image. (B) The percentage of cells with permeable membranes from five or more fields of view obtained by two independent experiments.