Abstract
Objective: The present goals of our study were biological synthesis, characterizations of silver nanoparticles, and evaluation of its antimicrobial activity against microbial pathogens like Escherichia coli, Enterococcus faecalis, Streptococcus pneumoniae and Staphylococcus aureus.
 Methods: The bacterial Strain NS-24 was isolated on nutrient agar medium and was selected for the synthesis of silver nanoparticles based on its gram-negative characteristics. The characterizations of silver nanoparticles were done by UV-Visible spectroscopy, Atomic Force Microscopy (AFM), High Resolution-Transmission Electron Microscopy (HR-TEM), Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDX), X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Later, the molecular characterization of the Strain NS-24 was done by DNA extraction and 16S rRNA gene sequencing.
 Results: The UV-visible spectrophotometric observation of the Strain NS-24 supernatant and AgNO3 solution showed maximum absorbance at 423 nm. The AFM data confirmed that the particles were polydispersed and spherical in shape. Additionally, the FTIR analysis revealed the IR spectral band patterning and TEM analyzes showed the size of biological AgNPs was in the range of 12.56 nm to 27.32 nm, with an average of 18.06 nm in size. Further, the 16S rRNA gene sequencing revealed the identity of Strain NS-24 as Stenotrophomonas maltophilia. The antimicrobial activity of AgNPs was studied on different gram-negative and gram-positive bacterial strains like Escherichia coli (MTCC 40), Enterococcus faecalis (MTCC 6845), Streptococcus pneumoniae (MTCC 8874) and Staphylococcus aureus (MTCC 2825), which showed good inhibition of their growth at varying concentrations of AgNPs against all the pathogens.
 Conclusion: Our findings showed that the synthesized AgNPs from the isolated bacterium was small in size and had profound antibacterial activity against pathogenic micro-organisms.
Highlights
Nanotechnology is an experimental field of applied science and technology
The bacterial Strain NS-24 was chosen based on its gramnegative nature
Biological synthesis of AgNPs was successfully performed by bacterial supernatant
Summary
Nanotechnology is an experimental field of applied science and technology. The prefix ‘nano’ means dwarf or extremely small. Nanotechnology is the study of controlling the matter on an atom and molecular scale. It deals with structures between 1 to 100 nm in at least one dimension and it involves in modifying or developing materials within that size. Microorganisms are universally present in air, soil, and water and play an important role in restoring the physical, chemical and biological properties of soil. Their role in agriculture is vital for nitrogen fixation, phosphate solubilisation, and mobilization of nutrients. Such beneficial microbes are called bio-fertilizers, which include both free-living and symbiotic forms [3]
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