Fermentation of mannitol by Clostridium butyricum: role of acetate as an external hydrogen acceptor

The continuous fermentation of mannitol (pH 6, dilution rate (D)=0.087 h-1) by Clostridium butyricum LMG 1213t1 was investigated under several conditions. Mannitol was readily fermented when glucose or acetate were added in the in-flow medium as co-substrate. Butyrate, CO2 and H2 were the major fermentation products. In mannitol-glucose mixtures (ratios 4 or 8) the amount of mannitol fermented depended upon the amount of glucose in the in-flow medium. In mannitol-acetate mixtures, 1 mol of acetate was needed for the fermentation of approximately 5.5 mol mannitol. We detected d-mannitol-1-phosphate dehydrogenase activity, responsible for the generation of supplementary reduced nicotine adenine dinucleotide (NADH) as a source for extra H2 gas. Fermentation of mannitol-acetate in the presence of [14C]-labelled acetate revealed butyrate as the only labelled fermentation end-product.