Antibacterial Efficacy of Green Synthesized Silver Nanoparticles Using Salvia nubicola Extract against Ralstonia solanacearum, the Causal Agent of Vascular Wilt of Tomato

Abstract

Ralstonia solanacearum is a phytopathogen causing bacterial wilt diseases of tomato and affecting its productivity, which leads to prominent economic losses annually. As an alternative to conventional pesticides, green synthesized nanoparticles are believed to possess strong antibacterial activities besides being cheap and ecofriendly. Here, we present the synthesis of silver nanoparticles (Sn-AgNPs) from medicinally important aqueous plant extracts of Salvia nubicola. Characterization of biologically synthesized nanoparticles was performed through UV-vis spectrophotometry, Fourier transform infrared spectroscopy (FTIR), energy-dispersive X-ray spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy (TEM), and thermogravimetric analysis. The antibacterial activity of the biosynthesized silver nanoparticles was tested against the phytopathogen R. solanacearum through in vitro experiments. Preliminary phytochemical analysis of the plant extracts revealed the presence of substantial amounts of flavonoids (57.08 mg GAE/g), phenolics (42.30 mg GAE/g), tannins, and terpenoids. The HPLC phenolic profile indicated the presence of 25 possible bioactive compounds. Results regarding green synthesized silver nanoparticles revealed the conformation of different functional groups through FTIR analysis, which could be responsible for the bioreduction and capping of Ag ions into silver NPs. TEM results revealed the spherical, crystalline shape of nanoparticles with the size in the range of 23-63 nm, which validates SEM results. Different concentrations of Sn-AgNPs (T1 (500 μg/mL) to T7 (78.1 μg/mL)) with a combination of plant extracts (PE-Sn-AgNPs) and plant extracts alone exhibited an efficient inhibition of R. solanacearum. These findings could be used as an effective alternative preparation against the bacterial wilt of tomato.

Figure 1

HPLC chromatogram of the methanolic extract of S. nubicola.

Figure 2

Studying the effect of AgNO₃ salt on the biosynthesis of S. nubicola silver nanoparticles and UV–vis absorption spectra of the Sn-AgNPs by using S. nubicola.

Figure 3

(a) Absorption spectra of Sn-AgNPs at various concentrations of S. nubicola plant extract and silver salt (9:1, 8:2, 7:3, 6:4, 5:5, 4:6, 3:7, 2:8, and 1:9). (b) Different volumetric ratios of Sn-AgNPs showing color changes.

Figure 4

FTIR analysis of the biosynthesized Sn-AgNPs.

Figure 5

EDX analysis of the biosynthesized nanoparticles by S. nubicola.

Figure 6

XRD patterns of the synthesized silver NPs of S. nubicola.

Figure 7

Scanning electron micrographs of S. nubicola-mediated silver NPs (Sn-AgNPs).

Figure 8

TEM images of green synthesized silver nanoparticles from S. nubicola plant extract at 100× magnification.

Figure 9

Microtiter plate showing in vitro activity.

Figure 10

Effect of different concentrations (twofold dilution) of PE + NPs, NPs, and PE against R. solanacearum. Plant extract refers to an aqueous extract of the aerial parts of S. nubicola. Columns of the same treatments sharing the same alphabets are not significantly different. The HSD test was applied for each treatment, separately.