JOURNAL OF PURE & APPLIED MICROBIOLOGY, Nov. 2014. Vol. 8(Spl. Edn. 2), p. 221-230 * To whom all correspondence should be addressed. E-mail: younesrashad@yahoo.com Streptomyces griseorubens E44G: A Potent Antagonist Isolated from Soil in Saudi Arabia Abdulaziz A. Al-Askar 1 , Waleed M. Abdulkhair 2 , Younes M. Rashad 3 , Elsayed E. Hafez 3 , Khalid M. Ghoneem 4 and Zakaria A. Baka 5 1 Botany and Microbiology Department, Faculty of Science, King Saud University, Riyadh, Saudi Arabia. 2 General Department of Basic Medical Sciences, Microbiology Department, National Organization for Drug Control and Research, Giza, Egypt. 3 Plant Protection and Biomolecular Diagnosis Department, Arid Lands Cultivation Research Institute, City of Scientific Research and Technology Applications, Alexandria, Egypt. 4 Department of Seed Pathology Research, Plant Pathology Research Institute, Agricultural Research Center, Giza, Egypt. 5 Botany Department, College of Science, Damietta University, Damietta, Egypt. (Received: 10 August 2014; accepted: 29 October 2014) In this study, a wide survey was conducted along Saudi Arabia. One hundred soil samples were collected from different 18 governorates representing different climatic conditions. Five hundred and seventy strains of actinomycetes were isolated from the collected soil samples. Among them, 225 were found to be antagonistic to the pathogenic fungus (Fusarium oxysporum f. sp. lycopersici) with varying degrees. Only one isolate designated E44G, had the strongest antagonistic activity against the tested fungus. The taxonomic status of this isolate was established using Phenotypic and molecular methods. The optimum nutritional and environmental conditions were studied to produce the maximum yield of antifungal activity. The highest antifungal activity was obtained at 2 nd day of incubation, 7.5 pH, and 30 °C. Glucose was the best carbon source and yeast extract was the best nitrogen source. In the present study, we report on a potential Streptomyces strain which has antifungal and antibacterial activities and holds the potential for use in studies of bioactive compounds as well as for possible use in biological control of fungal and bacterial diseases of important crops. Key words: Antibacterial, Antifungal, Optimization, Streptomycetes. Fusarium wilt disease, caused by Fusarium oxysporum f. sp. lycopersici, is one of the most important tomato diseases in the world. It can be a serious disease causing a considerable yield loss in tomato affecting its growth and productivity. Chemical control may be available to reduce the effects of this disease effectively and extensively. However, field application of these fungicides may not always be desirable. The persistent, injudicious use of chemicals has been discouraged owing to their toxic effects on non- target organisms and due to the undesirable changes they inflict upon the humans, animals and environment 1 . However, recent researches led to the possibility of biological control as an increasingly realistic option for tomato disease management 2 . Streptomycetes are one of the most abundant microorganisms in the soil. They are of universal occurrence in nature, living and multiplying in both cold and tropical zones, and