Acetyl-CoA cleavage pathway in a syntrophic propionate oxidizing bacterium growing on fumarate in the absence of methanogens

A syntrophic propionate-oxidizing bacterium was able to grow in the absence of methanogens by fermentation of fumarate to succinate and CO2. The pathway involved in the conversion of fumarate was investigated. During fermentation of unlabelled fumarate cells incorporated [1-13C]acetate, [2-13C]acetate and [1,2-13C]acetate into C2 and C3 of succinate, but not into C1 and C4. This observation, together with the absence of essential enzymes, showed that the citric acid cycle is not used for the oxidation of fumarate to CO2. All the enzymes of the acetyl-CoA cleavage pathway were detected, indicating that this pathway is involved in fumarate oxidation to CO2. During growth on propionate this acetyl-CoA cleavage pathway may have an anaplerotic function.