Glycerol conversion to 1,3-propanediol by newly isolated clostridia

Summary From pasteurized mud and soil samples glycerol-fermenting clostridia that produced 1,3-propanediol, butyrate and acetate were obtained. The isolates were taxonomically characterized and identified as Clostridium butyricum. The most active strain, SH1 = DSM 5431, was able to convert up to 110 g/l of glycerol to 56 g/l of 1,3-propanediol in 29 h. A few Clostridium strains from culture-collections (3 out of 16 of the C. butyricum group) and some isolates of Kutzner from cheese samples were also able to ferment glycerol, but the final concentration and the productivity of 1,3-propanediol was lower than in strain SH1. Strain SH1 grew well in a pH range between 6.0 and 7.5, with a weak optimum at 6.5, and was stimulated by sparging with N₂. Best overall productivity was obtained in fed-batch culture with a starting concentration of 5% glycerol. In all fermentations the yield of 1,3-propanediol in relation to glycerol was higher than expected from NADH production by acid formation. On the other hand the H₂ production was lower than expected, if per mole of acetyl coenzyme A one mole of H₂ is released. The observations point to a substantial transfer of reducing potential from ferredoxin to NAD, which finally results in increased 1,3-propanediol production.