Isolation and characterization of Caldicellulosiruptor lactoaceticus sp. nov., an extremely thermophilic, cellulolytic, anaerobic bacterium

An anaerobic, extremely thermophilic, cellulolytic, non-spore-forming bacterium, strain 6A, was isolated from an alkaline hot spring in Hveragerði, Iceland. The bacterium was non-motile, rod-shaped (1.5–3.5x0.7 μm) and occurred singly, in pairs or in chains and stained gram-negative. The growth temperature was between 50 and 78°C with a temperature optimum near 68°C. Growth occurred between pH 5.8 and 8.2 with an optimum near 7.0. The bacterium fermented microcrystalline cellulose (Avicel) and produced lactate, acetate and H2 as the major fermentation products, and CO2 and ethanol occurred as minor fermentation products. Only a restricted number of carbon sources (cellulose, xylan, starch, pectin, cellobiose, xylose, maltose and lactose) were used as substrates. During growth on Avicel, the bacterium produced free cellulases with carboxymethylcellulase and avicelase activity. The G+C content of the cellular DNA of strain 6A was 35.2±0.8 mol%. Complete 16S rDNA sequence analysis showed that strain 6A was phylogenetically related to Caldicellulosiruptor saccharolyticus. It is proposed that the isolated bacterium be named Caldicellulosiruptor lactoaceticus sp. nov.