In this study, metabolites involved in the free-biomass filtrates for three endophytic actinomycetes of Streptomyces capillispiralis Ca-1, Streptomyces zaomyceticus Oc-5, and Streptomyces pseudogriseolus Acv-11 were used as biocatalysts for green synthesis of silver nanoparticles (Ag-NPs). Characterization of biosynthesized Ag-NPs was accomplished using UV-Vis spectroscopy, X-ray diffraction patterns (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM-EDX), transmission electron microscopy (TEM), and particle size analyzer. The biosynthesized Ag-NPs showed maximum surface plasmon resonance (SPR) at 440 for strain Ca-1 and 450 for both strains of OC-5 and Acv-11. Nanoparticle spherical shape was recorded with size ranging from 23.77 to 63.14 nm, 11.32 to 36.72 nm, and 11.70 to 44.73 nm for Ca-1, Oc-5, and Acv-11, respectively. SEM-EDX analysis exhibited the weight percentages of 17.3, 22.3, and 48.7% for Ag-NPs synthesized by strains Ca-1, Oc-5 and Acv-11, respectively. The activities of biosynthesized Ag-NPs were concentration dependent and the obtained results confirmed the efficacy of Ag-NPs as antimicrobial agents against Gram-positive and Gram-negative bacteria as well unicellular and multicellular fungi. The MIC for Gram-positive bacteria, Gram-negative bacteria (E. coli), and eukaryotic microorganisms was 0.25 mM with clear zone ranging from 10.3 to 14.6 mm, while MIC for Pseudomonas aeruginosa was 1.0 mM for Ag-NPs synthesized by strain Ca-1 and 0.25 mM for those synthesized by strains Oc-5 and Acv-11. Moreover, Ag-NPs exhibited antimicrobial activity against four plant pathogenic fungi represented by Alternaria alternata, Fusarium oxysporum, Pythium ultimum, and Aspergillus niger at 2.0, 1.5, 1.0, and 0.5 mM of Ag-NPs with different degree. In vitro assessment of the antioxidant efficacy of biosynthesized Ag-NPs was achieved by scavenging assay of H2O2, reducing power of Fe3+, or total antioxidant assay. The results showed that antioxidant activities of Ag-NPs were concentration dependent with the highest activity at Ag-NP concentration of 2.0 mM. Furthermore, the biosynthesized NPs have prospective bioinsecticidal activity against Culex pipiens and Musca domestica. Green synthesis of NPs could be quite potential for the development of new bioactive compounds used in different biomedical applications.
Keywords: Antimicrobial activity; Antioxidant activity; Bioinsecticidal; Endophytic Streptomyces spp.; Phytopathogenic fungi; Silver nanoparticles.