Biosynthesis of D-lactic acid from lignocellulosic biomass.

D-lactic acid is a versatile and important industrial chemical that can be applied in the synthesis of thermal-resistant poly-lactic acid. Biosynthesis of D-lactic acid can be achieved by a variety of microorganisms, including lactic acid bacteria, yeast, and fungi; however, the final product yield, optical purity, and the utilization of both glucose and xylose are restricted. Consequently, engineered microbial systems are essential to attain high titer, productivity, and complete utilization of sugars. Herein, we critically evaluate the promising wild-type microorganisms, as well as genetically modified microorganisms to produce enantiomerically pure D-lactic acid, particularly from renewable lignocellulosic biomass. In addition, innovative bioreactor operation, metabolic flux analysis, and recent genetic engineering methods for targeted microbial D-lactic acid synthesis will be discussed.