Enhanced acetoin production by Serratia marcescens H32 with expression of a water-forming NADH oxidase

Cofactor engineering was employed to enhance production of acetoin by Serratia marcescens H32. 2,3-Butanediol was a major byproduct of acetoin fermentation by S. marcescens H32. In order to decrease 2,3-butanediol formation and achieve a high efficiency of acetoin production, nox gene encoding a water-forming NADH oxidase from Lactobacillus brevis was expressed. Batch fermentations suggested the expression of the NADH oxidase could increase the intracellular NAD+ concentration (1.5-fold) and NAD+/NADH ratio (2.9-fold). Meanwhile, 2,3-butanediol was significantly decreased (52%), and the accumulation of acetoin was enhanced (33%) accordingly. By fed-batch culture of the engineered strain, the final acetoin titer up to 75.2 g/l with the productivity of 1.88 g/(l h) was obtained. To the best of our knowledge, these results were new records on acetoin fermentation ever reported.