A biologically pure isolate of a selected bacterium derived from Clostridium autoethanogenum is described which has improved efficiency in the production of ethanol by anaerobic fermentation of substrates comprising carbon monoxide. The bacterium can produce ethanol and acetate at an ethanol to acetate ratio of at least 1.0 and has a productivity of at least 1.2 g of ethanol/l of fermentation broth per day. The bacterium is also characterized in that it has substantially no ability to sporulate.