Green silver nanoparticles from bacteria- antioxidant, cytotoxic and antifungal activities

Silver nanoparticles (AgNPs) have gained significant attention in recent years due to their unique physicochemical properties and wide-ranging applications. This study investigates the green synthesis of silver nanoparticles using the pathogen Shigella flexneri 29508 and evaluates their efficacy as both antioxidant and antifungal agents. The pure strain of the bacterium was identified as a potential nanoparticle producer based on its ability to reduce silver ions to nanoparticles. The formation of silver nanoparticles was confirmed by characteristic colour changes and further confirmed by UV–visible spectroscopy with SPR around 415 nm. The synthesized silver nanoparticles were characterized using various techniques, including TEM, SEM and DLS. DLS confirmed the nano-size, homogeneity and good stability of the fabricated particles TEM analysis revealed the spherical morphology of the nanoparticles, with an average size of 50 nm. SEM analysis also confirmed and supported the data. The antioxidant activity of the green-synthesized silver nanoparticles was evaluated using standard assays, and results demonstrated significant antioxidant potential, indicating the ability of the nanoparticles to neutralize free radicals and protect against oxidative stress. The bio-synthesised silver nanoparticles were also tested for their antifungal properties against two clinically relevant fungal strains. Resazurin-based micro-dilution viability assays, agar well diffusion, and spread plate assay methods were employed to determine the MIC and MBC of the nanoparticles, assessing their inhibitory effects on fungal growth. The results revealed potent antifungal activity, with varying degrees of efficacy against the tested fungal pathogens. Besides bioactivity, the cytotoxicity of the nanoparticles was also evaluated using primary cell cultures of peripheral blood cells. The bio-fabricated structures exhibited minimal toxicity and mortality, indicating their benign and eco-friendly nature in biological systems. This study highlights the successful green synthesis of silver nanoparticles using bacteria and elucidates their antioxidant, cytotoxicity, and antifungal activities. These findings contribute to the development of eco-friendly nanoparticle synthesis methods and suggest potential applications in the fields of medicine, agriculture, and environmental remediation.