Metabolic engineering coupled with adaptive evolution strategies for the efficient production of high-quality L-lactic acid by Lactobacillus paracasei

[Display omitted] •CRISPR-Cas9 gene editing platform was established for Lb. paracasei.•High-temperature resistant strain was obtained via adaptive evolution.•L-lactic acid titer and productivity was above 220 g/L and 7.5 g/L/h respectively.•Both L-lactic acid optical and chemical purity exceeded 99.0%•Open fermentation produced high-quality L-lactic acid with high sugar concentration. The main indicators for industrial production of high-quality lactic acid at elevated temperatures are high titer, productivity, yield, and optical purity. However, no such strains have been reported to meet all these requirements simultaneously. In this study, a high optical purity L-lactic acid producing strain is developed through the CRISPR-Cas9 gene editing platform. Further, adaptive evolution was used to breed and select a high-performance strain (NCBIO01-M2-ldhL1-HT) that could efficiently produce L-lactic acid at a high temperature of 45℃. This strain produced 221.0 g/L of L-lactic acid in open fermentation with high initial glucose concentration. Also, L-lactic acid productivity and yield was above 7.5 g/L/h and 0.96 g/g respectively, as well as the optical purity of L-lactic acid in the fermentation broth exceeded 99.1%. In short, this breeding strain possess high potential to be considered for the commercial production of polymer-grade L-lactic acid.