The aim of this study was to assay the antibacterial activity of Scenedesmus obtusus on some pathogenic bacteria. Microalgae S. obtusus were obtained from Algae Bank of Iran, and the methanol, ethyl acetate, chloroform and n-hexane extract of the microalgae were prepared using the maceration method. Bacterial strains were cultured in a solid culture medium, and antibacterial activity of the the different extracts of S. obtusus, were investigated using well diffusion method, and the determination of minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC), against Staphylococcus aureus, Streptococcus agalactiae as gram-positive bacteria and Escherichia coli and Klebsiella pneumonia as gram-negatives. According to the results, methanolic and ethyl acetate extracts of S. obtusus, showed growth inhibitory effects against all examined bacteria at concentrations of 200, 300 and 400 mg/ml. The chloroform extract was effective only against S. aureus and E. coli, but n-hexane extract had no effect against any of the bacteria. The highest inhibitory effect of S. obtusus extract was observed on methanolic extract against E. coli (16 mm), while the least inhibitory effect belonged to ethyl acetate extract against S. aureus (6.8 mm). The results also showed that the extract of S. obtusus contains antibacterial compounds, and the solvent which used for microalgae extraction significantly affected the antibacterial potential of the extract.
