Efficient production of polymer-grade L-lactic acid from corn stover hydrolyzate by thermophilic Bacillus sp. strain XZL4

Kinetic characteristics of long-term repeated fed-batch (LtRFb) l-lactic acid fermentation by a Bacillus coagulans strain

Application of degradable plastics is the most critical solution to plastic pollution. As the precursor of biodegradable plastic PLA (polylactic acid), efficient production of l-lactic acid is vital for the commercial replacement of traditional plastics. Bacillus coagulans H-2, a robust strain, was investigated for effective production of l-lactic acid using long-term repeated fed-batch (LtRFb) fermentation. Kinetic characteristics of l-lactic acid fermentation were analyzed by two models, showing that cell-growth coupled production gradually replaces cell-maintenance coupled production during fermentation. With the LtRFb strategy, l-lactic acid was produced at a high final concentration of 192.7 g/L, on average, and a yield of up to 93.0% during 20 batches of repeated fermentation within 487.5 h. Thus, strain H-2 can be used in the industrial production of l-lactic acid with optimization based on kinetic modeling.

Metabolic Engineered Biocatalyst: A Solution for PLA Based Problems

Polylactic acid (PLA) is a biodegradable thermoplastic polyester. In 2010, PLA became the second highest consumed bioplastic in the world due to its wide application. Conventionally, PLA is produced by direct condensation of lactic acid monomer and ring opening polymerization of lactide, resulting in lower molecular weight and lesser strength of polymer. Furthermore, conventional methods of PLA production require a catalyst which makes it inappropriate for biomedical applications. Newer method utilizes metabolic engineering of microorganism for direct production of PLA through fermentation which produces good quality and high molecular weight and yield as compared to conventional methods. PLA is used as decomposing packaging material, sheet casting, medical implants in the form of screw, plate, and rod pin, etc. The main focus of the review is to highlight the synthesis of PLA by various polymerization methods that mainly include metabolic engineering fermentation as well as salient biomedical applications of PLA.

Engineering Bacillus licheniformis as a thermophilic platform for the production of l-lactic acid from lignocellulose-derived sugars

BACKGROUND

Bacillus licheniformis MW3 as a GRAS and thermophilic strain is a promising microorganism for chemical and biofuel production. However, its capacity to co-utilize glucose and xylose, the major sugars found in lignocellulosic biomass, is severely impaired by glucose-mediated carbon catabolite repression (CCR). In this study, a "dual-channel" process was implemented to engineer strain MW3 for simultaneous utilization of glucose and xylose, using l-lactic acid as a target product.

RESULTS

A non-phosphotransferase system (PTS) glucose uptake route was activated via deletion of the glucose transporter gene ptsG and introduction of the galactose permease gene galP. After replacing the promoter of glucokinase gene glck with the strong promoter P_als, the engineered strain recovered glucose consumption and utilized glucose and xylose simultaneously. Meanwhile, to improve the consumption rate of xylose in this strain, several measures were undertaken, such as relieving the regulation of the xylose repressor XylR, reducing the catabolite-responsive element, and optimizing the rate-limiting step. Knockout of ethanol and acetic acid pathway genes further increased lactic acid yield by 6.2%. The resultant strain, RH15, was capable of producing 121.9 g/L l-lactic acid at high yield (95.3%) after 40 h of fermentation from a mixture of glucose and xylose. When a lignocellulosic hydrolysate was used as the substrate, 99.3 g/L l-lactic acid was produced within 40 h, with a specific productivity of 2.48 g/[L h] and a yield of 94.6%.

CONCLUSIONS

Our engineered strain B. licheniformis RH15 could thermophilically produced l-lactic acid from lignocellulosic hydrolysate with relatively high concentration and productivity at levels that were competitive with most reported cases of l-lactic acid-producers. Thus, the engineered strain might be used as a platform for the production of other chemicals. In addition to engineering the B. licheniformis strain, the "dual-channel" process might serve as an alternative method for engineering a variety of other strains.

Highly efficient production of optically pure l-lactic acid from corn stover hydrolysate by thermophilic Bacillus coagulans

A thermophilic strain Bacillus coagulans (NBRC 12714) was employed to produce l-lactic acid from corn stover hydrolysate in membrane integrated continuous fermentation. The strain NBRC 12714 metabolized glucose and xylose by the Embden-Meyerhof-Parnas pathway (EMP) and the pentose phosphate pathway (PPP), producing l-lactic acid with optical purity >99.5%. The overall l-lactic acid titer of 92g/l with a yield of 0.91g/g and a productivity of 13.8g/l/h were achieved at a dilution rate of 0.15h(-1). The productivity obtained was 1.6-fold than that of conventional continuous fermentation without cell recycling, and also was the highest among the relevant studies ever reported. These results indicated that the process developed had great potential for economical industrial production of l-lactic acid from lignocellulosic biomass.

New application of Bacillus strains for optically pure l-lactic acid production: general overview and future prospects

Members of the genus Bacillus are considered to be both, among the best studied and most commonly used bacteria as well as the most still unexplored and the most wide-applicable potent bacteria because novel Bacillus strains are continuously being isolated and used in various areas. Production of optically pure l-lactic acid (l-LA), a feedstock for bioplastic synthesis, from renewable resources has recently attracted attention as a valuable application of Bacillus strains. l-LA fermentation by other producers, including lactic acid bacteria and Rhizopus strains (fungi) has already been addressed in several reviews. However, despite the advantages of l-LA fermentation by Bacillus strains, including its high growth rate, utilization of various carbon sources, tolerance to high temperature, and growth in simple nutritional conditions, it has not been reviewed. This review article discusses new findings on LA-producing Bacillus strains and compares them to other producers. The future prospects for LA-producing Bacillus strains are also discussed.

Production of optically pure L-lactic acid from lignocellulosic hydrolysate by using a newly isolated and D-lactate dehydrogenase gene-deficient Lactobacillus paracasei strain

The use of lignocellulosic feedstock for lactic acid production with a difficulty is that the release of inhibitory compounds during the pretreatment process which inhibit the growth of microorganism. Thus we report a novel lactic acid bacterium, Lactobacillus paracasei 7 BL, that has a high tolerance to inhibitors and produced optically pure l-lactic acid after the interruption of ldhD gene. The strain 7 BL fermented glucose efficiently and showed high titer of l-lactic acid (215 g/l) by fed-batch strategy. In addition, 99 g/l of l-lactic acid with high yield (0.96 g/g) and productivity (2.25-3.23 g/l/h) was obtained by using non-detoxified wood hydrolysate. Rice straw hydrolysate without detoxification was also tested and yielded a productivity rate as high as 5.27 g/l/h. Therefore, L. paracasei 7 BL represents a potential method of l-lactic acid production from lignocellulosic biomass and has attractive application for industries.

Bacillus sp. strain P38: an efficient producer of L-lactate from cellulosic hydrolysate, with high tolerance for 2-furfural

In this study, efficient polymer-grade L-lactic acid production was achieved with the strain Bacillus sp. P38 by using cellulosic hydrolysate as the sole carbon source. In fed-batch fermentation, 180 g L(-1)L-lactic acid was obtained with a volumetric productivity of 2.4 g L(-1)h(-1) and a yield of 0.96 g g(-1) total reducing sugars. No D-isomer of lactic acid was detected in the broth. Strain P38 tolerated up to 10 g L(-1) 2-furfural, and lactate production was sharply inhibited only when the 2-furfural concentration was higher than 6 g L(-1). Moreover, strain P38 also tolerated high concentrations (>6 g L(-1)) of other fermentation inhibitors in cellulosic hydrolysate, such as vanillin and acetic acid, although it was slightly sensitive to formic acid. The efficient L-lactic acid production, combined with high inhibitor tolerance and efficient pentose utilization, indicate that Bacillus sp. P38 is a promising producer of polymer-grade L-lactic acid from cellulosic biomass.