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Background  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 CaCO 3  as neutralizer to maintain the desired pH, which results in an amount of insoluble CaSO 4  waste during the purification process. To overcome such environmental issue, alkaliphilic organisms have the great potential to be used as an organic acid producer under NaOH-neutralizing agent based fermentation. Additionally, high optical purity property in  d -lactic acid is now attracting more attention from both scientific and industrial communities because it can improve mechanical properties of PLA by blending  l - or  d -polymer together. However, the use of low-price nitrogen source for  d -lactate fermentation by alkaliphilic organisms combined with NaOH-neutralizing agent based process has not been studied. Therefore, our goal was the demonstrations of newly simplify high-optical-purity  d -lactate production by using low-priced peanut meal combined with non-sterile NaOH-neutralizing agent based fermentation.    Results  In this study, we developed a process for high-optical-purity  d -lactate production using an engineered alkaliphilic  Bacillus  strain. First, 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. The key gene responsible for exopolysaccharide biosynthesis ( epsD ) was subsequently disrupted to increase the yield and simplify the downstream process. Finally, 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 16 h of 3.04 g/l/h, which was even higher than that of a sterile fermentation. Moreover, high optical purities (approximately 99.85 %) of  d -lactate were obtained under both conditions.    Conclusions  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.

Efficient fermentative production of polymer-grade d-lactate by an engineered alkaliphilic Bacillus sp. strain under non-sterile conditions