Does the antibacterial activity of silver nanoparticles depend on the shape of the nanoparticle? A study of the Gram-negative bacterium Escherichia coli

Abstract

In this work we investigated the antibacterial properties of differently shaped silver nanoparticles against the gram-negative bacterium Escherichia coli, both in liquid systems and on agar plates. Energy-filtering transmission electron microscopy images revealed considerable changes in the cell membranes upon treatment, resulting in cell death. Truncated triangular silver nanoplates with a {111} lattice plane as the basal plane displayed the strongest biocidal action, compared with spherical and rod-shaped nanoparticles and with Ag(+) (in the form of AgNO(3)). It is proposed that nanoscale size and the presence of a {111} plane combine to promote this biocidal property. To our knowledge, this is the first comparative study on the bactericidal properties of silver nanoparticles of different shapes, and our results demonstrate that silver nanoparticles undergo a shape-dependent interaction with the gram-negative organism E. coli.

FIG. 1.

(a) Absorption spectra of solutions containing spherical silver nanoparticle recorded immediately after precipitation and after 1 week. (b) Absorption spectra of reaction mixtures containing elongated (rod-shaped) and truncated triangular silver nanoplates and other anisotropic silver nanoparticles and the first and second wash liquor (centrifuge). (Top inset) Absorption spectrum of solution containing purified truncated triangular silver nanoplates. (Bottom inset) UV-visible spectrum of the solution containing rod-shaped particles with larger aspect ratios.

FIG. 2.

EFTEM images of silver nanoparticles. (A) Spherical nanoparticles synthesized by citrate reduction. (B) Silver nanoparticles of different shapes. (C) Purified rod-shaped nanoparticles.

FIG. 3.

OPML-XRD pattern of truncated triangular silver nanoplates. (Inset) TEM image of the purified truncated triangular particles.

FIG. 4.

(A) Petri dishes initially supplemented with 10⁷ CFU/ml of E. coli and incubated with different forms of silver nanoparticles at (a) 1, (b) 12.5, (c) 50, and (d) 100 μg. (B) Number of E. coli colonies, expressed as log(1 + number of colonies grown on plates under the conditions used for panel A), as a function of the amount of silver nanoparticles in agar plates.

FIG. 5.

Petri dishes initially supplemented with 10⁵ CFU/ml of E. coli and incubated with silver at (a) 1, (b) 6, (c) 12.5, (d) 50, and (e) 100 μg and corresponding graphs showing log(1 + number of colonies grown on plates) as a function of the concentration of silver in agar plates. (A) Ag⁺ (in the form of AgNO₃); (B) spherical silver nanoparticles.

FIG. 6.

Growth curve of E. coli in 100 ml NB with 10⁷ CFU/ml in the presence of different concentrations of different silver nanoparticles. (A) AgNO₃; (B) spherical nanoparticles; (C) truncated triangular nanoparticles; (D) spherical nanoparticles with an initial bacterial concentration of 10⁵ CFU/ml. Amounts (in micrograms) of silver nanoparticles are given in each panel.

FIG. 7.

(a) Killing activity for E. coli (in 100 ml NB supplemented with 10⁷ CFU/ml) in the presence of a mixture of triangular, rod-shaped, and polyhedral silver nanoparticles (before purification), containing different amounts of total silver (Ag⁺ and Ag nanoparticles) and CTAB. Total silver amounts are shown in each panel. (b) Decay curve of E. coli growth in 100 ml NB with 10⁷ CFU/ml treated with the mother solution of truncated triangular and rod-shaped nanoparticles (total amount of silver, 12 μg). (Inset) Comparative graph of the dynamics of E. coli growth in the presence of 12 μg of spherical silver nanoparticles, 10 μg of triangular nanoparticles, and 12 μg of ionic silver (AgNO₃) in 100 ml NB supplemented with 10⁷ CFU/ml. Axes and units for the inset are same as those for the main graph and have been omitted for simplicity.

FIG. 8.

EFTEM images of E. coli cells. (A) Untreated E. coli. Flagella can be seen. (B) E. coli grown on agar plates supplemented with Ag⁺ (AgNO₃). Arrows indicate partially damaged membranes. These cells are viable. (C) E. coli treated with triangular silver nanoplates. Silver nanoparticles appear as dark irregular pits on the cell surface. (D) E. coli treated with spherical silver nanoparticles. (E) Enlarged image of part of the bacterial cell membrane treated with triangular silver nanoparticles. The cell membrane is damaged in multiple locations.