The relative contribution of propionate, and long-chain even-numbered fatty acids to the production of long-chain odd-numbered fatty acids in rumen bacteria.

1. The relative contribution of propionate, and long-chain even-numbered fatty acids to the production of long-chain odd-numbered fatty acids in six pure cultures of rumen bacteria were studied, using single and double isotope procedures. 2. Both propionate and even-numbered fatty acids were converted to odd-numbered fatty acids, presumably by elongation of 2-C atoms and alpha-oxidation reactions, but even-numbered fatty acids were much more effective. 3. The occurrence of alpha-oxidation was further confirmed by relatively large quantities of 14CO2 produced when [1-14C]palmitate was used in the incubation medium. 4. The incorporation of propionate into odd-numbered fatty acids was markedly reduced in the presence of palmitate, or stearate. 5. The significance of the operation of alpha-oxidation is discussed in relation to the energy conservation in rumen bacteria.