A strictly anaerobic, thermophilic, gram-positive, spore-forming eubacterium designated strain SEBR 5268 (T = type strain) was isolated from an oil field at a depth of 2,100 m, where the temperature was 92°C. The cells of this organism were gram-positive, straight, motile rods (0.5 by 2 to 3 μm) with peritrichous flagella. The cells occurred singly or in pairs during the logarithmic growth phase, but were pleomorphic and filamentous (length, 15 μm) in old cultures. Growth occurred at temperatures of 40 to 75°C, and optimum growth occurred at temperatures between 55 and 60°C. The fermentable substrates included glucose, fructose, galactose, mannose, cellobiose, maltose, sucrose, lactose, d-xylose, d-ribose, mannitol, pyruvate, and starch. The products of fermentation of glucose were lactate, acetate, ethanol, H, and CO. The DNA base composition was 35 mol% G+C. The results of 16S rRNA sequence comparisons indicated that strain SEBR 5268 was closely related to and , and these three organisms exhibited levels of ribosomal DNA sequence homology of 98 to 99%. The results of DNA-DNA hybridization studies performed with the three organisms confirmed this close affiliation, and as base pairing values of > 70% were obtained, these organisms belong to the same species. Therefore, we propose that should be reclassified as a subspecies of subsp. comb. nov. This automatically creates subsp. . We also propose that strain SEBR 5268 should be classified as a member of a new subspecies of subsp. . The latter differs from subsp. and subsp. by its 16S rRNA sequence, DNA sequence diversity, lower temperature optimum, G+C content, and carbohydrate utilization spectrum. Strain SEBR 5268 has been deposited in the Deutsche Sammlung von Mikroorganismen as strain DSM 9801.


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