Fermentation of triacetin and glycerol by Acetobacterium sp. : no energy is conserved by acetate excretion

Two strains of homoacetogenic bacteria similar to Acetobacterium carbinolicum were enriched and isolated from freshwater and marine sediment samples with triacctin (glycerol triacetylester) as sole carbon and energy source. Also the type strains of A. carbinolicum and A. woodii were found to be able to grow with triacetin, and to convert it nearly exclusively to acetate. The triacetin- hydrolyzing enzyme was inducible, and was localized in the cytoplasmic fraction of both species at an activity of 0.21-0.26 U mg protein -1. During fermentation of glycerol, varying amounts of 1,3-propranediol were produced which could be kept at a minimum in a glycerol-limited chemostat. Growth yields in batch and continuous culture experiments varied between 9.2 and 10.9 g mol glycerol- 1 and 6.5 and 7.6 g tool triacetin- a with five strains of homoacetogenic bacteria tested. These results indicate that excretion of acetate across the cytoplasmic membrane does not contribute to the energy conservation budget of these homoacctogenic bacteria