Green synthesis of sliver nanoparticles using different bacteria: Do the synthesized nanoparticles differ in their antimicrobial activity

Objectives: Biosynthesis and characterization of silver nanoparticles (AgNPs) from Escherichia coli, Acinetobacter baumannii and Staphylococcus aureus and determination of antimicrobial activity against selected pathogens. Methods: E. coli (ATCC 25922), A. baumannii (clinical strain), S. aureus (ATCC 25923) were cultured in nutrient broth medium and used for biosynthesis of AgNPs. AgNO3 concentration, pH, incubation time and temperature were optimized for AgNP biosynthesis. Antimicrobial activity of the synthesized AgNPs was studied using the well diffusion assay. Results: All the selected bacteria produced silver nanoparticles at alkaline pH when the concentration of AgNO3 was greater than 0.3 g/L. The optimum reaction temperature was 60oC. UV-Visible spectroscopy with a maximum absorbance of approximately 420 - 430 nm confirmed the presence of AgNPs. AgNPs produced by S. aureus resulted in larger zone of inhibition (ZOI) against the selected pathogens where AgNPs produced by E. coli showed comparatively smaller ZOI. Gram negative bacteria (E.coli, P. aeruginosa) were more sensitive to AgNPs compared to gram positive bacteria (Methicillin Resistant S. aureus, S. aureus) and fungal species (Candida albicans). Conclusion: AgNPs produced by S. aureus are the most effective among the tested AgNPs while E. coli produced the least effective AgNPs.