Metabolic engineering of Klebsiella oxytoca M5a1 to produce optically pure d-lactate in mineral salts medium

Klebsiella oxytoca strains were constructed to produce optical pure d-lactate by pH-controlled batch fermentation in mineral salts medium. The alcohol dehydrogenase gene, adhE, and the phospho-transacetylase/acetate kinase A genes, pta-ackA, were deleted from the wild type. KMS002 (ΔadhE) and KMS004 (ΔadhE Δpta-ackA) exhibited d-lactate production as a primary pathway for the regeneration of NAD+. Both strains produced 11–13 g/L of d-lactate in medium containing 2% (w/v) glucose with yields of 0.64–0.71 g/g glucose used. In sugarcane molasses, KMS002 and KMS004 produced 22–24 g/L of d-lactate with yields of 0.80–0.87 g/g total sugars utilized. Both strains also utilized maltodextrin derived from cassava starch and produced d-lactate at a concentration of 33–34 g/L with yields of 0.91–0.92 g/g maltodextrin utilized. These d-lactate yields are higher than those reported for engineered E. coli strains.