Abstract
An anaerobic, extremely thermophilic, xylanolytic nonspore-forming bacterium, strain X6B, was isolated from a 70°C Icelandic hot spring sediment. The bacterium was rod-shaped, 3.6–5.9 μm long and 0.7 to 1.0 μm wide, and cells grew singly, in pairs, and occasionally formed chains. The bacterium was nonmotile with no flagella. Cells from mid-to late exponential gowth-phase cultures stained gram-negative but had a gram-positive like cell wall structure in transmission electron photomicrographs. The bacterium grew between 50°C and 78°C with an optimum temperature at about 65°C to 68°C. Growth occurred between pH 5.2 and 8.5 with an optimum pH close to 7. During growth on beech wood xylan, glucose and d-xylose, the isolate produced CO2, acetate and H2 as major fermentation products, and a small amounts of ethanol; lactate was not produced. X6B did not reduce acetone to isopropanol or sulphate or thiosulfate to sulfide. The base composition of X6B's cellular DNA was 35.7 mol% guanine + cytosine. The properties of this strain do not fit any previously described species. The name proposed for the isolated bacterium was Thermoanaerobium acetigenum, spec. nov.
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