Growth of Propionibacterium acidi-propionici was studied on lactose as substrate and in acid whey permeate in a three-electrode poised-potential system with cobalt sepulchrate as artificial electron donor. In batch culture experiments in a stirred-tank reactor the substrate was fermented completely to propionic acid up to 6.5 g1-1 lactose in a supplemented whey permeate medium. No acetic acid was produced during the growth of P. acidi-propionici. An electron flow of 80-100 mA was obtained and the electron balance was 101%. In continuously growing cultures with 3 g 1-1 of lactose as the substrate, propionate was formed as the only fermentation product up to a dilution rate (D) of 0.04 h-1. With D>0.04h-1 the bacteria immobilized on the working electrode surface. It was examined whether an electron transfer occurred between the platinum working electrode and the immobilized cells.

Abstract. Three strains of new mesophilic homoacetogenic bacteria were enriched and isolated from sewage sludge and from marine sediment samples with methoxyacetate as sole organic substrate in a carbonate-buffered medium under anoxic conditions. Two freshwater isolates were motile, Gram-positive, non-sporefonning rods. The marine strain was an immotile, Gram-positive rod with a slime capsula. All strains utilized only the methyl residue ofmethoxyacetate and released glycolic acid. They also fermented methyl groups of methoxylated aromatic compounds and of betaine to acetate with growth yields of 6 -1 0 g dry matter per mol methyl group. H2/CO2, formate, methanol, hexamethylene tetramine, as well as fructose, numerous organic acids, glycerol, ethylene glycol, and glycol ethers were fermented to acetate as well. High activities of carbon monoxide dehydrogenase (0.4-2.2 U x mg protein -1) were detected in all three isolates. The guanine-plus-cytosine-content of the DNA of the freshwater isolates was 42.7 and 44.4 mol %, with the marine isolate it was 47.7 mol %. The freshwater strains were assigned to the genus Acetobacterium as new strains of the species A. carbinolicum. One freshwater isolate, strain KoMacl, was deposited with the Deutsche Sammlung yon Mikroorganismen GmbH, Braunschweig, under the number DSM 5193.