Background: Biosynthesis of nanoparticles using microorganisms, enzymes, and plant extracts is regarded as an alternativeto chemical methods. Microalgae appear to be an efficient biological platform for nanoparticle synthesis as they grow rapidly and produce large biomass at lower cost. Objectives: The possibility of silver nanoparticles biosynthesisby freshwater green microalgae, Chlamydomonas reinhardtii, was evaluated. Furthermore, antibacterial properties of the synthesized nanoparticles were investigated via analysis of growth and toxin production of Listeria monocytogenes. Materials and Methods: Silver nanoparticles were synthesized by incubating 47.5 mL of fresh C. reinhardtii culture with 2.5 mL of 200 mM AgNO3 solution for 48 h. Characterization of the synthesized nano particles was performed by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometry (EDS) and X-ray diffraction analysis (XRD). Concentration of biosynthesized silver nanoparticles was measured by high resolution ICP-OES spectrometer. Inhibitory effect of silver nanoparticles on L. monocytogenes growth was measured. Further, the expression of listeriolysin O was investigated by serial microdilution method and Real-Time PCR assay. Results: Spherical silver nanoparticles with average size of about 10 nm were formed. The particles had inhibitory effects on bacterial growth and antagonist activity on the expression of listeriolysin O. Conclusions: C. reinhardtii has the potential to be used as an effective platform for production of silver and other nanoparticles. Silver nanoparticles had potent antibacterial properties.

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