Cyanobacteria are rich source of primary and secondary metabolites which are important in biotechnology and pharmaceutical industries. The Cyanobacteria plays important role for forming the earth's oxygen atmosphere and their contribution to many other attributes important to human life. Due the importance of this groups of bacteria, the main aim of the present study was the screening and the identification of the bacterial strains belonging to the cyanobacteria, isolated from water of lake of Sidi Mohamed Benali located in the region of Sidi Bel Abbes West of (Algeria)and the evaluation of their antagonistic activity against pathogenic bacteria responsible for nosocomial infection. The antagonistic activity of the isolated, selected cyanobacteria against pathogenic bacteria strains such as Escherichia coli ATCC 25922, Salmonella typhimurium ATCC 6538, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATCC 43300, Enterobacter aerogenes ATCC 35029, Proteus vulgaris ATCC 6896, Klebsiella pneumonia ATCC 10031 was evaluated by the using agar well diffusion method and the study of the bacterial growth in the in the absence and in the presence of the supernatant of Ps. aeruginosa ATCC 27853.Furthermore, the obtained antagonistic activity of the selected cyanobacteria, extracted by the using of different organic solvents such as ethanol, acetone, diethyl ether and methanol was compared. The obtained results showed that the diethyl ether extract of Lyngbya. sp, the methanol extract of Oscillatoria sp and the diethyl ether extract of Phormidium.sp, indicated an important antagonistic activity against Aspergilus niger and Candida albicans. Where, the methanol and the acetone extracts of Oscillatoria sp have manifested the highest antagonistic activity against S. aureus, K. pneumonia respectively. Furthermore, the investigated methanol and the acetone extracts of Oscillatoria sp yielded the highest antagonistic activity against S.aureus, K. pneumonia by methanol extract and against E. aerogenese, Acetone extract of Phormidium sp showed a high antagonistic activity against S. aureus, P. vulgaris, K. pneumonia.
