A novel anaerobic, thermophilic, Gram-positive, spore-forming, and sugar-fermenting bacterium (strain TLO) was isolated from a geothermal spring in Ayaş, Turkey. The cells were straight to curved rods, 0.4-0.6 microm in diameter and 3.5-10 microm in length. Spores were terminal and round. The temperature range for growth was 40-80 degrees C, with an optimum at 70 degrees C. The pH optimum was between 6.3 and 6.8. Strain TLO has the capability to ferment a wide variety of mono-, di-, and polysaccharides and proteinaceous substrates, producing mainly lactate, next to acetate, ethanol, alanine, H(2), and CO(2). Remarkably, the bacterium was able to grow in an atmosphere of up to 25% of CO as sole electron donor. CO oxidation was coupled to H(2) and CO(2) formation. The G + C content of the genomic DNA was 35.1 mol%. Based on 16S rRNA gene sequence analysis and the DNA-DNA hybridization data, this bacterium is most closely related to Thermoanaerobacter thermohydrosulfuricus and Thermoanaerobacter siderophilus (99% similarity for both). However, strain TLO differs from Thermoanaerobacter thermohydrosulfuricus in important aspects, such as CO-utilization and lipid composition. These differences led us to propose that strain TLO represents a subspecies of Thermoanaerobacter thermohydrosulfuricus, and we therefore name it Thermoanaerobacter thermohydrosulfuricus subsp. carboxydovorans.