GROWTH-INHIBITION AND PYRUVATE OVERFLOW DURING GLUCOSE-METABOLISM OF EUBACTERIUM-LIMOSUM ARE RELATED TO A LIMITED CAPACITY TO REASSIMILATE CO2 BY THE ACETYL-COA PATHWAY

The growth characteristics of Eubacterium limosum, an anaerobic acetogen, have been described kinetically for fermentation of glucose. A short-lived exponential growth phase was associated with a homoacetogenic fermentation in which all catabolic CO2 liberated was reassimilated via the acetyl-CoA pathway. During this phase no additional products were observed. A major shift in the metabolism leading to a mixed-acid fermentation pattern (acetate and butyrate) occurred coincident with the onset of the linear growth phase. Significantly diminished yields of acetate in this decelerating growth period were attributed to the limited capacity of the enzymes responsible for reductive generation of methyl donor within the acetyl-CoA pathway: CO2 and H-2 accumulated in the gas phase. Pyruvate overflow (and to a lesser extent lactate, glycerate and carbon monoxide accumulation) were also observed, though pyruvate-ferredoxin oxidoreductase specific activity remained constant. In all experiments specific biomass formation rates were directly related to the rate of acetyl-CoA formation. The exponential growth phase was prolonged and the production of all compounds other than acetate was diminished when the medium was supplemented with tungsten, suggesting that formate dehydrogenase may be the enzyme limiting the correct functioning of the acetyl-CoA pathway.