An anaerobic thermophilic bacterium, designated strain JW/SA-NV4T, was isolated from a xylan-supplemented enrichment culture from Trego hot spring located within the Black Rock Desert (NV, USA). Cells were generally straight or slightly bent rod-shaped, 0.4–0.8 μm in width and 3–6 μm in length during exponential growth. Cells from stationary phase were variable in size and shape, showing curved or bent morphology. Motility was not seen and flagella were not observed in electron micrographs. Sporulation was not observed. Strain JW/SA-NV4T stained Gram-negative but is phylogenetically Gram-type positive. Growth occurred at pH25 °C 6.8–8.8, with optimum growth at pH 8.4; no growth occurred at pH 9.0 or above or at 6.5 or below. With glucose or xylose as the carbon source, strain JW/SA-NV4T grew at 44–74 °C; no growth occurred at 76 °C or above or at 42 °C or below. However, the optimum temperature was 62 and 66 °C when grown on glucose and xylose, respectively. The shortest doubling time observed with glucose was approximately 4 h, and with xylose approximately 3.4 h. Strain JW/SA-NV4T tolerated an atmosphere containing up to 0.1 % O2; no growth occurred at a gas atmosphere of 0.2 % O2. Chemo-organotrophic growth occurred with xylose, glucose, mannose, xylan, pyruvate, fructose, ribose, Casamino acids, mannitol, tryptone, peptone, cellobiose and yeast extract. When grown in mineral media containing 1 g yeast extract l−1 as an electron donor, thiosulfate and sulfur were reduced to sulfide. The G+C content of the DNA was 38.6 mol% (HPLC). 16S rRNA gene sequence analysis placed strain JW/SA-NV4T within the order Thermoanaerobacterales and within the Thermoanaerobacterales Incertae Sedis Family III, specifically between taxa classified within the genera Thermosediminibacter and Thermovenabulum. The closest phylogenetic neighbours were Thermosediminibacter oceani JW/IW-1228PT (94.2 % 16S rRNA gene sequence similarity) and Thermosediminibacter litoriperuensis JW/YJL-1230-7/2T (94.0 %) [Lee, Y.-J., Wagner, I. D., Brice, M. E., Kevbrin, V. V., Mills, G. L., Romanek, C. S. & Wiegel, J. (2005). Extremophiles9, 375–383]. Based on physiological and genotypic characteristics, strain JW/SA-NV4T (=DSM 18802T=ATCC BAA-1454T) is proposed to represent the type strain of a novel species in a novel genus, Caldanaerovirga acetigignens gen. nov., sp. nov.
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