en 28706606 <p>Background: It is well documented that Silver Nanoparticles (SNPs) are potent antimicrobial agents. However, little is known about antimicrobial effects of biologically synthesized SNPs at molecular level. In the present study, efficacy of the green microalgae <em>Chlorella vulgaris</em> in biosynthesis of silver nanoparticles and inhibitory effect of the biosynthesized SNPs on growth and virulence of <em>Staphylococcus aureus (S. aureus)</em> were investigated.<br /> Methods: Algal suspension was incubated in the presence of silver nitrate to induce formation of nanoparticles. The experiment was conducted under a pH range to evaluate pH effect on the shape and properties of nanoparticles. Characterization was performed by Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), Energy Dispersive Spectrometry (EDS) and X-ray diffraction analysis (XRD). Moreover, concentration of biosynthesized SNPs was measured by high resolution ICP-OES spectrometer. Antibacterial effect of SNPs on growth of <em>S. aureus</em> was evaluated by broth micro-dilution method. Inhibitory effect of SNPs on alpha hemolysin, a well-known virulence factor of <em>S. aureus</em> was investigated through real time PCR assay.<br /> Results: Spherical SNPs were produced with characteristic monodispersity at low and neutral pHs; however, in alkaline condition, nanorod structures were formed. SNPs inhibited growth of <em>S. aureus</em> at concentration of 50 <em>&mu;g/ml</em>. Alpha hemolysin expression was also effectively inhibited by SNPs treatment.<br /> Conclusion: In general, results revealed formation of spherical silver nanoparticles with inhibitory effects on bacterial growth and antagonist activity on the expression of alpha hemolysin. Moreover, increase in pH to basic condition resulted in aggregation of nanoparticles and formation of rod-like nanostructures.</p> Biosynthesis, Chlorella vulgaris, Nanoparticles, Silver 120 125 https://www.ajmb.org/En/Article.aspx?id=278 https://www.ajmb.org/PDF/En/FullText/278.pdf MohammadSoleimaniDepartment of Microbiology, Faculty of Medicine, AJA University of Medical Sciences, Tehran, Iran133MaziarHabibi-PirkoohiDepartment of Microbiology, Faculty of Medicine, AJA University of Medical Sciences, Tehran, Iran1129