An Efficient New Process for the Selective Production of Odd-Chain Carboxylic Acids by Simple Carbon Elongation Using Megasphaera hexanoica

The caproate-producing bacterium, Megasphaera hexanoica, metabolizes fructose to produce C(2)~C(8) carbon-chain carboxylic acids using various electron acceptors. In particular, odd-chain carboxylic acids (OCCAs) such as valerate (C(5)) and heptanoate (C(7)), were produced at relatively high concentrations upon propionate supplementation. Using a statistical experimental design method, the optimal culture medium was established for the selective production of OCCAs among the total produced acids. In a medium containing 2.42 g L(−1) sodium acetate and 18.91 g L(−1) sodium propionate, M. hexanoica produced 9.48 g L(−1) valerate, 2.48 g L(−1) heptanoate, and 0.12 g L(−1) caproate. To clarify the metabolism of the exogenous added propionate for OCCAs production, (13)C tracer experiments were performed by supplementing the culture broth with [1,2,3-(13)C(3)] propionate. The metabolites analysis based on mass spectrometry showed that the propionate was only used to produce valerate and heptanoate without being participated in other metabolic pathways. Furthermore, the carbon elongation pathway in M. hexanoica was explained by the finding that the incorporation of propionate and acetate in the produced valerate occurred in only one orientation.