Lactate formation from fructose or C1 compounds in the acetogen Acetobacterium woodii by metabolic engineering.

Anaerobic, acetogenic bacteria are promising biocatalysts for a sustainable bioeconomy since they capture and convert carbon dioxide to acetic acid. Hydrogen is an intermediate in acetate formation from organic as well as C1 substrates. Here, we analyzed mutants of the model acetogen Acetobacterium woodii in which either one of the two hydrogenases or both together were genetically deleted. In resting cells of the double mutant, hydrogen formation from fructose was completely abolished and carbon was redirected largely to lactate. The lactate/fructose and lactate/acetate ratios were 1.24 and 2.76, respectively. We then tested for lactate formation from methyl groups (derived from glycine betaine) and carbon monoxide. Indeed, also under these conditions lactate and acetate were formed in equimolar amounts with a lactate/acetate ratio of 1.13. When the electron-bifurcating lactate dehydrogenase/ETF complex was genetically deleted, lactate formation was completely abolished. These experiments demonstrate the capability of A. woodii to produce lactate from fructose but also from promising C1 substrates, methyl groups and carbon monoxide. This adds an important milestone towards generation of a value chain leading from CO2 to value-added compounds. KEY POINTS: • Resting cells of the ΔhydBA/hdcr mutant of Acetobacterium woodii produced lactate from fructose or methyl groups + CO • Lactate formation from methyl groups + CO was completely abolished after deletion of lctBCD • Metabolic engineering of a homoacetogen to lactate formation gives a potential for industrial applications.