In vitro Antimicrobial Activity of Green Synthesized Nanoparticles of Leaf extracts of Medicinally Important Orchid Vanda spathulata (L.) Spreng

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Background: In the current study, an easy, expeditious, environment-friendly and economical method was developed through a green synthesis of silver nanoparticles (AgNPs) using Vanda spathulata (L.) Spreng. leaf extract as an antimicrobial agent.

Objectives: To determine the antimicrobial activity of green synthesized AgNPs against a diverse range of bacterial and fungal pathogens by using disc diffusion method.

Methods: The synthesized AgNPs were characterized by UV-VIS spectroscopy, X-ray diffraction (XRD), Fourier Transform Infrared (FTIR) spectroscopy and Transmission Electron Microscopy (TEM). The antimicrobial activity of AgNPs against the selected Gram positive and Gram-negative pathogens was carried out using Kirby-Bauer Disc Diffusion Susceptibility Test Method.

Results: The UV Visible spectrum analysis of synthesized nanoparticles absorbance occurs at 422nm. Appearance of this peak, allotted to a surface plasmon, is well-documented for numerous metal nanoparticles with size starting from a one to hundred nanometre. FTIR analysis of synthesized nanoparticles confirmed the presence of functional groups regarding alcohols, phenols, aromatic and carboxylic acids. SEM and TEM analysis were carried out to observe the topology and AgNPs size, that showed the synthesis of numerous sizes of polydispersed spherical AgNPs. SEM analysis showed the ranges from 16-102 nm, while TEM study shows in the range between 16–67nm as well spherical and crystalline nature of the nanoparticles. Most of the nanoparticles aggregated and only a few of them were scattered, when observed under SEM. XRD pattern showed that the synthesized silver nanoparticles formed as crystalline in nature. The average crystalline size according to Debye–Scherrer equation calculated is found to be 83.449nm. Energy Dispersive Spectroscopy (EDS) studies revealed the presence of pure silver in the synthesized nanoparticles. Silver (Ag 93.70%) was the major constituent element compared to chloride (6.30%). Antimicrobial activity of the biosynthsized AgNPs was studied against selective bacterial and fungal pathogens. Among the various bacteria, the highest zone was observed at higher dose (150µl) of AgNPs against gram-negative bacteria Escherichia coli (10.15 mm) while lowest zone was observed at lower dose (50µl) against Escherichia coli (3.90 mm). The growth inhibition was highest zone in higher dose (150µl) of AgNPs against gram-positive bacteria like Staphylococcus aureus (9.85mm). Among the various fungi, the highest zone was observed at higher dose (150µl) of AgNPs against Candida albicans (8.10 mm) while lowest zone was observed at lower dose (50µl) against Candida albicans (3.10 mm).

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