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

Abstract

BackgroundPolylactic 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. 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.ResultsIn 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.ConclusionsGiven 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 Conversion of Agroindustrial Waste into D(-) Lactic Acid by Lactobacillus delbrueckii Using Fed-Batch Fermentation

“…Lactic acid production was not favored by ammonium nitrate, ammonium sulfate, or urea, as these substances are inorganic nitrogen sources, and considering that the base medium does not contain any type of vitamin, which is important to the maintenance of microbial cells. Assavasirijinda et al [9] report similar results in an investigation of D(-) lactic acid production by an engineered Bacillus sp. strain, in which inorganic nitrogen sources did not lead to satisfactory levels of production, whereas the highest lactate production was achieved using peanut meal as the nitrogen source.…”

“…According to Nakano et al [52], the pH of the medium is more efficiently neutralized by a divalent cation (Ca 2+ ) compared to a monovalent cation (Na + , NH 3 + ). Some authors consider NaOH-based fermentation to be an environmentally friendly process that avoids generating precipitated waste [9,53]. However, this agent could increase the osmotic pressure of the medium, causing stress to the microbial cells.…”

“…In contrast, chemical synthesis always leads to the production of racemic DL-lactic acid [6,7]. The optical purity of lactic acid is an important physical property that has attracted the attention of different industries that blend L(+) or D(-) polymers to polymerize high-crystalline PLA, thereby improving the mechanical properties of this polymer [8,9].…”

Environmentally Friendly Production of D(−) Lactic Acid by Sporolactobacillus nakayamae: Investigation of Fermentation Parameters and Fed-Batch Strategies

“…No residual sugar was found in this experiment, which is interesting from the standpoint of further lactic acid purification and polymerization. Moreover, NaOH used as a pH controller in fermentation processes does not generate precipitated waste, making it environmentally friendly [ 23 ]. In contrast, the ammoniacal solution was not an appropriate neutralizing agent, as demonstrated by the low lactic acid production (16.05 g/L).…”

Effective Production of Lactic Acid by a Newly Isolated Alkaliphilic Psychrobacter maritimus BoMAir 5 Strain

“…Alkaliphilic strains can also considered good producers for organic acids [11]. Very few alkaliphilic strains have been reported for LA production from glucose such as Halolactibacillus halophilus that produced 65.8g/l L-LA in batch fermentation at pH 9.0 with a low yield of 0.76g/g [10], Exiguobacterium 8-11-1 strain produced 125g/l of L-LA with a yield of 0.98g/g in Fed batch fermentation at pH 8.5 [12], Enterococcus casseliflavus 79w3 that produced 103g/l of L-LA with a low yield of 0.80g/g at pH 8.0 in batch fermentation [13] and genetically modified Bacillus N16-5 that produced 143.9g/l of D-LA with a yield of 96.1g/g of glucose consumed in Fed batch fermentation [14]. These studies either reported a low concentration of LA with low yield or performed at slightly alkaline conditions that increase the risk of contamination, in addition to the expected instability of genetically modified organisms.…”