Summary. Anaerobic growth of Escherichia coli was studied with glycerol as electron source and a three-electrode poised-potential system with potassium ferricyanide as mediator. Similar to fumarate, potassium ferricyanide was used as electron acceptor in batch-culture experiments. In experiments with regulated electrodes, glycerol was degraded completely, and an electron flow of 3.0 to 4.0 mA was obtained. The electron balances were in the range of 98%-107%, and a growth rate of 0.095 h -1 was calculated. These results are discussed with respect to the energetics of iron reduction by E. coli and by other bacteria.

Abstract. Growth of Propionibacterium freudenreichiiwas studied with glycerol, lactate, and propionate as energy sources and a three-electrode poised-potential amperometric electrode system with hexacyanoferrate (III) as mediator. In batch culture experiments with glycerol and lactate as substrates, hexacyanoferrate (III) was completely reduced. Growth yields increased and the fermentation patterns were shifted towards higher acetate formation with increasing hexacyanoferrate (III) concentrations ( 0 . 2 5 -8.0 raM). In experiments with regulated electrodes, glycerol, lactate, and propionate were oxidized to acetate and CO2, and the electrons were quantitatively transferred to the working electrode. Growth yields of 29.0, 13.4 and 14.2 g cell material per mol were calculated, respectively. The high cell yield obtained during propionate oxidation cannot be explained solely by substrate level phosphorylation indicating that additional energy was conserved via electron transport phosphorylation. Furthermore, this result indicated complete reversibility of the methyl-malonyl-CoA pathway in propionic acid bacteria.

In order to influence the fermentation pattern of Propionibacterium freudenreichii towards enhanced propionate formation, growth and product formation with glucose and lactate as energy sources were studied in a three-electrode poised-potential amperometric culture system. With anthraquinone 2,6-disulfonic acid (Eo' =-184 mV; poised electron potential =-224 mV) or cobalt sepulchrate (Eo' =-350 mV;-390 mV) as mediator and an activated platinum working electrode, reduction of bacterially oxidized mediator occurred fast enough to keep more than 50% of the respective mediator (in minimum 0.4 mM) in the reduced state, up to a current of 2 mA. With glucose as substrate, 90.0 or 97.3% propionate was formed during exponential growth in the presence of 0.5 mM anthraquinone 2,6-disulfonic acid or 0.4 mM cobalt sepulchrate, respectively. Growth yields of 56.3 or 53.8 g of cell material per mol of substrate degraded were calculated, respectively, and the electrons were transferred quantitatively from the working electrode to the bacterial cells. With L-lactate, only 68.6 or 72.9% propionate was formed with the same mediators. The results are discussed with respect to energetics, electron transfer potentials, and potential application of the new technique in technical propionate production.

Abstract. 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.