Screening and bioreactor cultivation of wild-type lactic acid bacteria for high purity D-lactic acid production

Biodegradable polymers are the green alternative to conventional oil-based plastics and have a key role in the achievement of the Sustainable Development Goals of Agenda 2030. Nowadays, polylactic acid (PLA) is one of the most common bioplastics present in the global market. Optically pure D-lactic acid (D-LA) is a valuable monomer for the production of high-performance PLA materials. This study aims to select wild-type lactic acid bacteria capable of producing D-LA with high optical purity and to define an optimized fermentation process in bioreactors to maximize production. A total of 150 LAB strains from the University of Parma Culture Collection (UPCCO) were screened using enzymatic assays and HPLC analysis. Among them, Lactobacillus delbrueckii UPCCO 2214 and Leuconostoc citreum UPCCO 4516 were selected for fed-batch cultivations for their high D-LA production and purity (>97 %). After optimization of fermentation parameters, L. delbrueckii UPCCO 2214 was found to be the most efficient strain for D-LA production, with a yield of 0.74 g/g and a volumetric productivity of 0.96 g/L/h, outperforming L. citreum UPCCO 4516. Bioreactors cultivation has helped to understand microbial production and boost their potentialities. This work supports further investigations and improvements in D-LA production processes to advance the field of biomaterials with benefits for both industry and the environment.