Silver-Protein Nanocomposites as Antimicrobial Agents

Abstract

The use of nanomaterials alone or in composites with proteins is a promising alternative to inhibit pathogenic bacteria. In this regard, this study used seed proteins from both fenugreek (Trigonella foenum-graecum L.) (FNP) and mung bean (Viga radiate) (MNP), with silver nanoparticles (Ag-NPs) and nanocomposites of either Ag-NPs plus FNP (Ag-FNP) or Ag-NPs plus MNP (Ag-MNP) as inhibitory agents against pathogenic bacteria. FNP and MNP were isolated from fenugreek seeds and mung bean seeds, respectively, and fractionated using Sodium Dodecyl Sulfate-Polyacrylamide Gel Electrophoresis (SDS-PAGE). Both FNP and MNP were immobilized with Ag-NPs to synthesize the nanocomposites Ag-FNP and Ag-MNP, respectively. The physicochemical characteristics of Ag-NPs and their composites with proteins were studied by X-ray Diffraction (XRD), dynamic light scattering (DLS), the zeta potential, Scanning and Transmission Electron Microscopy (SEM and TEM, respectively), Atomic Force Microscopy (AFM), and the Brunauer-Emmett-Teller isotherm (BET), elucidating their structural parameters, size distribution, size charges, size surface morphology, particle shape, dimensional forms of particles, and specific surface area, respectively. The sole proteins, Ag-NPs, and their nanocomposites inhibited pathogenic Gram-positive and Gram-negative bacteria. The inhibitory activities of both nanocomposites (Ag-FNP and Ag-MNP) were more than those obtained by either Ag-NPs or proteins (FNP, MNP). Minimum inhibitory concentrations (MICs) of Ag-FNP were very low (20 and 10 µg mL^-1) against Salmonellatyphimurium and Pseudomonasaerugenosa, respectively, but higher (162 µg mL^-1) against E. coli and Listeriamonocytogenes. MICs of Ag-MNP were also very low (20 µg mL^-1) against Staphylococcusaureus but higher (325 µg mL^-1) against Listeriamonocytogenes. TEM images of Staphylococcusaureus and Salmonellatyphimurium, treated with Ag-FNP and Ag-MNP, at their MIC values, showed asymmetric, wrinkled exterior surfaces, cell deformations, cell depressions, and diminished cell numbers.

Keywords: antibacterial activity; fenugreek; mung bean; nanocomposite; seed proteins; silver nanoparticles.

Figure 1

SDS–PAGE of mung bean protein (MNP) (lane 1) and fenugreek protein (FNP) (lane 2) compared to protein standard (St).

Figure 2

Powder X-ray Diffraction (XRD) pattern for (A) MNP and FNP, (B) Ag-FNP, and (C) Ag-MNP.

Figure 3

DLS for (A) silver nanoparticles (AgNPs); (B) silver mung bean protein nanocomposite (Ag-MNP), and (C) silver fenugreek protein nanocomposite (Ag-FNP).

Figure 4

Zeta potential value for (A) silver nanoparticles (AgNPs), (B) silver mung bean protein nanocomposite (Ag-MNP), and (C) silver fenugreek protein nanocomposite (Ag- FNP).

Figure 5

Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) images of (A) silver nanoparticles (AgNPs), (B) silver-fenugreek seed protein nanocomposite (Ag-FNP), and (C) silver-mung bean seed protein nanocomposite (Ag-MNP).

Figure 6

The three-dimensional form of Atomic Force Microscope (AFM) images of 50 × 50 nm of (A) FNP, (B) Ag-FNP, (C) MNP, and (D) Ag-MNP.

Figure 7

TEMs of S. typhimurium affected by MIC of (A) Ag-FNP and (B) Ag-MNP at (30,000×, 80,000×). MICs of Ag-FNP and Ag-MNP against S. typhimurium were 20 and 40 µg/mL, respectively.

Figure 8

TEMs of S. aureus as affected by MIC of (A) Ag-MNP and (B) Ag-FNP at (30,000×, 80,000×). The MIC of Ag-FNP and Ag-MNP against S. aureus was 20 µg/mL.