Engineering Bacillus coagulans with high osmotic tolerance for enhancing L-lactic acid production using sweet sorghum juice coupled with acid-pretreated soybean meal under unsterile conditions

Bacillus coagulans provides the economic potential for fermenting non-food sweet sorghum into lactic acid with high titer/productivity, but its production capacity is limited by osmotic pressure tolerance. Herein, a B. coagulans with significantly elevated osmotic pressure tolerance was screened after heavy ion beam mutagenesis combined with automation and high-throughput microdroplet enrichment, showing improved L-lactic acid production and tolerance performance compared to the parental strain. Moreover, combining vitamin B supplementation and acid-pretreated soybean meal increased the conversion yield and productivity. Finally, the engineered strain produced 113.5 g/L of L-lactic acid from sweet sorghum juice after 23-hour fermentation with the high optical purity of 99.8 % in a single batch fermentation under open unsterile conditions, highlighting the potential for more efficient polymer-grade L-lactic acid biomanufacturing from sweet sorghum. • An automated microdroplet enrichment platform was established. • A robust L-lactic acid producing Bacillus coagulans strain was obtained. • The VB supplementation improved production performance. • Batch fermentation produced superior lactic acid titer and productivity.