Mechanistic evaluation of antibacterial and photocatalytic properties of biosynthesized silver nanoparticles against pathogenic bacteria and synthetic dyes

Hazardous materials and bacteria are known to cause life-threatening issues to people with compromised immune systems. Another global threat of manufacturing industries; synthetic dyes are one of the key factors of water pollution; it is difficult to treat this wastewater in an efficient and economical way. Thus, there is an urgent need for developing new antibacterial and photocatalytic agents to combat harmful microorganisms and biohazardous compounds (dyes). Therefore, in present research work, biosynthesized silver nanoparticles (AgNPs) were evaluated as a novel antibacterial and photocatalytic agent. The cyanobacterium (Anabaena variabilis) cell-free extract was used to synthesis AgNPs by following the optimized conditions, which included a 1:9 cell extract to 1 mM AgNO₂ ratio, pH 7.4, 30 °C, and characterize by UV-visible spectroscopy. Pathogenic bacteria like Gram-positive Staphylococcus aureus, Gram-negative including Klebsiella pneumoniae, Escherichia coli and Pseudomonas aeruginosa were used to evaluate the antibacterial properties of AgNPs. The minimum inhibitory concentrations (MICs) were found 25 ± 2.7 µg/mL for S. aureus, 12.5 ± 1.7 µg/mL for K. pneumoniae, 12.5 ± 1.7 µg/mL for E. coli, and 6.25 ± 1.3 µg/mL for P. aeruginosa. Transmission electron microscopic study confirms the physical disruption of AgNPs treated bacteria, while propidium iodide staining based fluorescence microscopy, and flow cytometric study confirm the qualitative and quantitative antibacterial activity of AgNPs. Additionally, biosynthesized AgNPs were evaluated for photocatalytic degradation in methylene blue (MB) and acridine orange (AO). After 120 min of AgNPs treatment, MB degradation efficiency was observed 83.78 ± 1.21 with a degradation rate constant 0.0035 min^{- 1}, while AO, percentage degradation efficiency was observed 92.24 ± 1.32 with a degradation rate constant 0.0164 min^{- 1}.