Synthesis of Silver Nanoparticles by Bacillus stearothermophilus Using Gamma Radiation and Their Antimicrobial Activity

Abstract

Abstract: Bacillus stearothermophilus culture supernatant was used for preparation of silver nanoparticles (AgNPs) by reduction of silver ions via nitrate reductase enzyme which was optimized using Response Surface Methodology (RSM).The optimized conditions for nitrate reductase activity were as follows; medium containing; (%) glucose: 0.1, peptone: 1, yeast extract: 0.4, KNO 3: 0.4 also, pH: 7.5, temp: 25°C and incubation period 3 days with maximum nitrate reductase activity of (731.24 U/ml). AgNPs were characterized by UV-Vis spectroscopy, Transmission Electron Microscopy (TEM), Dynamic Light Scattering (DLS) and Fourier Transform Infrared Spectroscopy (FT-IR). Effect of gamma radiation on bacterial free filtrate before and after mixing with Ag+ in aqueous solutions was investigated. Increasing of gamma radiation doses from 0.25 up to 3 kGy enhanced the formation of AgNPs. The antimicrobial activity of AgNPs was investigated against some pathogenic multidrug-resistant strains of yeast, Gram-negative and Gram-positive bacteria using the Kirby-Bauer method. AgNPs effectively inhibited microbial growth in the order of Candida spp>Gram-positive bacteria>Gram-negative bacteria. Minimum inhibitory concentrations (MICs) and minimum lethal concentrations (MLCs) of AgNPs were determined for selected strains by a broth dilution method. The results showed that the lowest MIC and MLC of AgNPs was 100 and 200ppm for Candida parapsilosis, respectively. This in vitro study demonstrated the effectiveness of the synthesized AgNPs as a new tool in combating against infectious diseases.