Microbial biosynthesis of silver nanoparticles and their antimicrobial activity

Abstract

Biological synthesis of silver nanoparticles is a developing branch in biotechnology. This branch involves the usage of plants and microorganisms as a green alternative to toxic chemicals and expensive physical methods. In this study, we isolated thirty-seven bacterial isolates that showed variable degrees of tolerance toward AgNO3. Two bacterial isolates showed the highest tolerance up to 30 mM AgNO3. Both showed capability to reduce AgNO3 and synthesize AgNPs. These two bacterial isolates (Ism37 and Ism26) were identified using 16S rRNA gene as Bacillus cereus and Enterobacter cloacae and were submitted to Genbank given accession numbers KP988025 and KP988024, respectively. Preliminary characterization for AgNPs was carried out using UV-Vis spectrometry and showed intense peaks at 450 and 440 nm for Bacillus cereus Ism37 and Enterobacter cloacae Ism26, respectively. AgNPs synthesis was only possible in the presence of light. Synthesized AgNPs were further characterized using X-ray diffraction (XRD) and showed (fcc) face cubic centered crystal silver nanoparticles. Fourier transform infrared (FTIR) indicated the involvement of different organic compounds in the AgNPs synthesis. Dynamic light ABSTRACT 2 scattering (DLS) and TEM (transmission electron microscopy) revelled oval to spherical shaped AgNPs with size range 10-80 and 5-50nm for Bacillus cereus Ism37 and Enterobacter cloacae Ism26, respectively. Different physcio-chemical factors influencing the AgNPs synthesis were tested; Incubation periods, pH, temperature, AgNO3 concentration and different volumes of bacterial lysate supernatant. Both organisms showed optimum pH at 7, temperature at 35°C and the concentration of AgNO3 was 1 mM. The variation was observed in the incubation period and the volume of bacterial lysate supernatant, while Bacillus cereus Ism37 required 24 h and 30 ml, Enterobacter cloacae Ism26 needed 20 h and 40 ml, respectively.