THE EFFECT OF FORMATE ON THE ACIDOGENIC FERMENTATION OF METHANOL BY EUBACTERIUM LIMOSUM

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During growth of the anaerobic acidogenic bacterium, Eubacterium limosum, on methanol-CO₂, formate supplements were shown to influence both rates of growth and the ratio of organic acid fermentation end- products. Formate interrupted the oxidation of methanol to CO₂ thereby lowering the yield of reducing equivalents. A consequence of this was a progressive shift towards acetic acid production at the expense of butyric acid, proportionate to the initial formate concentration. Such a finding supports the physiological model of this organism's methylotrophic metabolism, in which butyric acid production is related directly to the flux of methanol oxidation. At high formate concentrations, reversal of this catabolic pathway (i.e. assimilation of formate as methyl donor for acetyl- CoA formation) resulted in ATP-limited growth. E. limosum could not be acclimatized to growth on formate alone though the cells remained able to transform formate to CO₂ after several transfers.