Efficient Fermentative Production of Polymer-Grade D-Lactate By an Engineered Alkaliphilic Bacillus

Polylactic acid (PLA) is one important chemical building block that is well known as a biodegradable and a biocompatible plastic. The traditional lactate fermentation processes need CaCO3 as neutralizer to maintain the desired pH, which results in an amount of insoluble CaSO4 waste during the purification process. In this study, we developed a process for high-optical-purity D-lactate production using an engineered alkaliphilic Bacillus strain. The native L-lactate dehydrogenase gene (ldh) was knocked out, and the D-lactate dehydrogenase gene from Lactobacillus delbrueckii was introduced to construct a D-lactate producer. A fed-batch fermentation under non-sterile conditions was conducted using low-priced peanut meal as a nitrogen source and NaOH as a green neutralizer. The D-lactate titer reached 143.99 g/l, with a yield of 96.09%, an overall productivity of 1.674 g/l/h including with the highest productivity at 16h of 3.04 g/l/h. Moreover, high optical purities (approximately 99.85%) of D-lactate were obtained under both conditions. Given the use of a cheap nitrogen source and a non-sterile green fermentation process, this study provides a more valuable and favorable fermentation process for future polymer-grade D-lactate production.