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The aim of this study was to isolate and optimize the growth conditions of thermophilic microorganism from hot springs. The isolation was conducted by using the mineral salt basal medium supplemented with 0.6% yeast extract at 500C. Totally, 33 isolates of thermophilic microorganism were isolated from hot springs at Truong Xuan (Khanh Hoa province) and Binh Chau (Ba Ria - Vung Tau province). The effects of temperature (45 - 800C), pH (pH 6 - 9) and carbon sources (malate, pyruvate, acetate, glucose, fructose, or carbon dioxide) on the growth of isolates were examined. In addition, the isolate morphology was also investigated by Gram and spore staining. The isolated thermophilic microorganism showed the diversity in colony morphology and color appearance. Most of them were rod shaped, spore-forming and most grew well at 500C and pH 7. The highest growth of all isolates was observed under malate, glucose, or fructose, as an organic carbon source and unable to use carbon dioxide. Six out of 33 thermophilic microorganism isolates (namely BM7, BS5, NS1, NS3, NS4, and NW6) grew rapidly under high temperatures from 50 - 550C and their morphology characteristics showed high similarity to Bacillus sp. The study evidenced the polymorphic diversity of thermophiles in the geothermal hot spring ecosystems.
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