Abstract
Different engineered organisms have been used to produce L-lactate. Poor yields of lactate at low pH and expensive downstream processing remain as bottlenecks. Aspergillus niger is a prolific citrate producer and a remarkably acid tolerant fungus. Neither a functional lactate dehydrogenase (LDH) from nor lactate production by A. niger is reported. Its genome was also investigated for the presence of a functional ldh. The endogenous A. niger citrate synthase promoter relevant to A. niger acidogenic metabolism was employed to drive constitutive expression of mouse lactate dehydrogenase (mldhA). An appraisal of different branches of the A. niger pyruvate node guided the choice of mldhA for heterologous expression. A high copy number transformant C12 strain, displaying highest LDH specific activity, was analyzed under different growth conditions. The C12 strain produced 7.7 g/l of extracellular L-lactate from 60 g/l of glucose, in non-neutralizing minimal media. Significantly, lactate and citrate accumulated under two different growth conditions. Already an established acidogenic platform, A. niger now promises to be a valuable host for lactate production.
Highlights
Lactic acid is a versatile organic acid used in various industrial applications
Marginal but reproducible lactate oxidation activity was observed in the parent strain suggesting that an endogenous lactate dehydrogenase (LDH)-like enzyme may be present in A. niger
While A. niger may contain an endogenous LDH activity, an ldhA gene coding for such an activity could not be identified
Summary
Lactic acid is a versatile organic acid used in various industrial applications. It is a commonly used acidulant and preservative in food, leather, textile industries and helps in controlled drug delivery. Industrial scale biosynthesis of L-lactic acid is achieved through fermentation of microorganisms belonging to Lactobacillus, Bacillus and Rhizopus genera. Lactobacillus species were the first promising candidate for lactic acid production. They are fastidious with respect to growth requirements and cannot utilize starchy raw materials. The pretreatment of raw materials raises the cost of production. Their growth is inhibited at low pH during lactate fermentation and the addition of neutralizing agents makes downstream processing difficult [1]. PLOS ONE | DOI:10.1371/journal.pone.0145459 December 18, 2015
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