Converting a D-/L lactic acid bacteria to its d-type counterpart via a combined chemical mutagenesis and biosensor screening method, and its application in lignocellulosic biorefinery

Abstract

Optically pure lactic acid is essential for poly(lactic acid) manufacturing, nonetheless, most wild-type lactic acid bacteria inherently produce D-/L- mixed lactic acid. In this study, an elaborate system was designed for developing d-lactic acid producing mutants from a wild-type D-/L- lactic acid producing Lactobacillus by combining chemical mutagenesis with ethyl methane sulfonate as the mutagen and high throughput screening with a l-lactate dehydrogenase based biosensor. Mechanistic analysis revealed that the loss of l-lactate dehydrogenase activity via C → T transitions in the corresponding gene is responsible for generation of d-lactic acid producing mutants. The obtained d-lactic acid producing mutant exhibited excellent performance in different types of lignocellulosic hydrolysates and produced 128.3 g/L d-lactic acid from corn stover hydrolysate in 3-L bioreactor fermentation, with optical purity higher than 98 %. In conclusion, this study provided a practical framework for obtaining optically pure lactic acid producers without genetic engineering operations, advancing the sustainable production of cellulosic bioplastics.