Dry pretreatment of lignocellulose with extremely low steam and water usage for bioethanol production

Competition between biodetoxification fungus and lactic acid bacterium in the biorefinery processing chain for production of cellulosic L-lactic acid

Biodetoxification fungus selectively degrades toxic inhibitors generated from pretreatment of lignocellulose without consuming fermentable sugars. However, one barrier for practical application is the sustained cell viability in the consequent fermentation step to compete the fermentable sugars with fermenting strains, resulting in sugar loss and reduced target product yield. This study investigated the competitive growth property between the biodetoxification fungus Paecilomyces variotii FN89 and the L-lactic acid bacterium Pediococcus acidilactici ZY271 under varying temperature and lactic acid osmatic stress. The results show that the L-lactic acid bacterium Ped. acidilactici ZY271 showed less thermotolerance to Pae. variotii FN89 at high temperature of 45 °C to 50 °C in both synthetic medium and wheat straw hydrolysate. In the higher temperature environment, the growth of the biodetoxification strian failed to compete with the lactic acid fermentation strain and was quickly eliminated from the fermentation system. The high temperature fermentation facilitated a fast transition from the detoxification stage to the fermentation stage for higher production of L-lactic acid.

Upgrading dry acid pretreatment by post-hydrolysis for carbon efficient conversion of lignocellulose

Dry acid pretreatment (DAP) as a promising process for industrial biorefinery provide an efficient bioconversion of cellulose without free wastewater, although the partial xylan and lignin degrade to inhibitors or recondense. A biorefinery strategy for carbon efficient conversion of lignocellulose into bioethanol, xylose, and reactive lignin was developed by upgrading DAP with post-hydrolysis. The results showed that lignocellulose after mild DAP (175 °C, acid dosage of 15 mg/g dry material) obtained higher xylan recovery and lower inhibitors than that of general DAP. Subsequently, post-hydrolysis, simultaneous saccharification and ethanol fermentation were performed at solids loading of 20 wt% without detoxification and sterilization, resulting in xylose and ethanol yield of 71.8 % and 67.6 %. The fractionated lignin presented more reactive β-aryl ether linkages and less condensation than that from DAP. 66 % of lignocellulose carbon was recovered as ethanol, xylose and reactive lignin. This upgrading biorefinery strategy provided an easy-to-operate process for integrated utilization of lignocellulose.

Aqueous amine enables sustainable monosaccharide, monophenol, and pyridine base coproduction in lignocellulosic biorefineries

Thought-out utilization of entire lignocellulose is of great importance to achieving sustainable and cost-effective biorefineries. However, there is a trade-off between efficient carbohydrate utilization and lignin-to-chemical conversion yield. Here, we fractionate corn stover into a carbohydrate fraction with high enzymatic digestibility and reactive lignin with satisfactory catalytic depolymerization activity using a mild high-solid process with aqueous diethylamine (DEA). During the fractionation, in situ amination of lignin achieves extensive delignification, effective lignin stabilization, and dramatically reduced nonproductive adsorption of cellulase on the substrate. Furthermore, by designing a tandem fractionation-hydrogenolysis strategy, the dissolved lignin is depolymerized and aminated simultaneously to co-produce monophenolics and pyridine bases. The process represents the viable scheme of transforming real lignin into pyridine bases in high yield, resulting from the reactions between cleaved lignin side chains and amines. This work opens a promising approach to the efficient valorization of lignocellulose.

Cellulase mediated stress triggers the mutations of oleaginous yeast Trichosporon cutaneum with super-large spindle morphology and high lipid accumulation

Accumulation of intracellular lipid bodies in oleaginous yeast cells is highly restricted by their natural intracellular space. Here we show a cellulase mediated adaptive evolution with ultra-centrifugation fractionation of oleaginous yeast Trichosporon cutaneum to obtain the favorable cell structure for lipid accumulation. Cellulase was added to the wheat straw hydrolysate during long-term adaptive evolution for disruption of cell wall integrity of T. cutaneum cells. The cellulase, together with ultracentrifugation force, triggered multiple mutations and transcriptional expression changes of the functional genes associated with cell wall integrity and lipid synthesis metabolism. The fractionated mutant T. cutaneum YY52 demonstrated the heavily weakened cell wall and high lipid accumulation by the super-large expanded spindle cells (two orders of magnitude greater than the parental). A record-high lipid production by T. cutaneum YY52 was achieved (55.4 ± 0.5 g L-1 from wheat straw and 58.4 ± 0.1 g L-1 from corn stover). This study not only obtained an oleaginous yeast strain with industrial application potential for lipid production but also provided a new method for generation of mutant cells with high intracellular metabolite accumulation.

Cold plasma pretreatment reinforces the lignocellulose-derived aldehyde inhibitors tolerance and bioethanol fermentability for Zymomonas mobilis

BACKGROUND

Lignocellulose-derived aldehyde inhibitors seriously blocked the biorefinery of biofuels and biochemicals. To date, the economic production of lignocellulose-based products heavily relied on high productivities of fermenting strains. However, it was expensive and time-consuming for the achievable rational modification to strengthen stress tolerance robustness of aldehyde inhibitors. Here, it aimed to improve aldehyde inhibitors tolerance and cellulosic bioethanol fermentability for the chassis Zymomonas mobilis ZM4 pretreated using energy-efficient and eco-friendly cold plasma.

RESULTS

It was found that bioethanol fermentability was weaker in CSH (corn stover hydrolysates) than that in synthetic medium for Z. mobilis, and thus was attributed to the inhibition of the lignocellulose-derived aldehyde inhibitors in CSH. Convincingly, it further confirmed that the mixed aldehydes severely decreased bioethanol accumulation through additional aldehydes supplementary assays in synthetic medium. After assayed under different processing time (10-30 s), discharge power (80-160 W), and working pressure (120-180 Pa) using cold atmosphere plasma (CAP), it achieved the increased bioethanol fermentability for Z. mobilis after pretreated at the optimized parameters (20 s, 140 W and 165 Pa). It showed that cold plasma brought about three mutation sites including ZMO0694 (E220V), ZMO0843 (L471L) and ZMO0843 (P505H) via Genome resequencing-based SNPs (single nucleotide polymorphisms). A serial of differentially expressed genes (DEGs) were further identified as the potential contributors for stress tolerance via RNA-Seq sequencing, including ZMO0253 and ZMO_RS09265 (type I secretion outer membrane protein), ZMO1941 (Type IV secretory pathway protease TraF-like protein), ZMOr003 and ZMOr006 (16S ribosomal RNA), ZMO0375 and ZMO0374 (levansucrase) and ZMO1705 (thioredoxins). It enriched cellular process, followed by metabolic process and single-organism process for biological process. For KEGG analysis, the mutant was also referred to starch and sucrose metabolism, galactose metabolism and two-component system. Finally, but interestingly, it simultaneously achieved the enhanced stress tolerance capacity of aldehyde inhibitors and bioethanol fermentability in CSH for the mutant Z. mobilis.

CONCLUSIONS

Of several candidate genetic changes, the mutant Z. mobilis treated with cold plasma was conferred upon the facilitated aldehyde inhibitors tolerance and bioethanol production. This work would provide a strain biocatalyst for the efficient production of lignocellulosic biofuels and biochemicals.

Simultaneous and rate-coordinated conversion of lignocellulose derived glucose, xylose, arabinose, mannose, and galactose into D-lactic acid production facilitates D-lactide synthesis

D-lactide is the precursor of poly(D-lactide) (PDLA) or stereo-complex with poly(L-lactide) (PLLA). Lignocellulosic biomass provides the essential feedstock option to synthesize D-lactic acid and D-lactide. The residual sugars in D-lactic acid fermentation broth significantly blocks the D-lactide synthesis. This study showed a simultaneous and rate-coordinated conversion of lignocellulose derived glucose, xylose, arabinose, mannose, and galactose into D-lactic acid by adaptively evolved Pediococcus acidilactici ZY271 by simultaneous saccharification and co-fermentation (SSCF) of wheat straw. The produced D-lactic acid achieved minimum residual sugars (∼1.7 g/L), high chirality (∼99.1%) and high titer (∼128 g/L). A dry acid pretreatment eliminated the wastewater stream generation and the biodetoxification by fungus Amorphotheca resinae ZN1 removed the inhibitors from the pretreatment. The removal of the sugar residues and inhibitor impurities in D-lactic acid production from lignocellulose strongly facilitated the D-lactide synthesis. This study filled the gap in cellulosic D-lactide production from lignocellulose-derived D-lactic acid.

Cellulosic hydrocarbons production by engineering dual synthesis pathways in Corynebacterium glutamicum

Background

Lignocellulose provides the only practical carbohydrates feedstock for sustainable bioproduction of hydrocarbons as future alternative of fossil fuels. Production of hydrocarbons from lignocellulose is achieved by a biorefinery process chain including pretreatment to breakdown the crystalline structure for cellulase-catalyzed hydrolysis, detoxification of inhibitory compounds generated during pretreatment, enzymatic hydrolysis to fermentable monosaccharide sugars, and fermentation to hydrocarbon products. The major barriers on fermentative production of hydrocarbons from lignocellulose include two aspects: one is the inherent stress of pretreatment-derived inhibitors on microbial cells, the other is the toxicity of hydrocarbons to cell membranes. The microbial cell factory should be tolerant to both inhibitor stress and hydrocarbons toxicity.

Results

Corynebacterium glutamicum was selected as the starting strain of hydrocarbons synthesis since it is well adapted to lignocellulose hydrolysate environment. The dual hydrocarbon synthesis pathways were constructed in an industrial C. glutamicum S9114 strain. The first pathway was the regular one in microalgae composed of fatty acyl-acyl carrier protein (fatty acyl-ACP) reductase (AAR) and aldehyde deformylating oxygenase (ADO) with fatty acyl-ACP as precursor. The second pathway was the direct decarboxylation of free fatty acid by fatty acid decarboxylase (OleT) using the rich fatty acids from the disruption of the transcriptional regulator fasR gene. The transmembrane transportation of hydrocarbon products was avoided by secretively expressing the fatty acid decarboxylase (OleT) to the extracellular space. The hydrocarbons generation from glucose reached 29.2 mg/L, in which the direct decarboxylation pathway contributed more than 70% of the total hydrocarbons generation, and the AAR–ADO pathway contributed the rest 30%.

Conclusion

The dual hydrocarbon synthesis pathways (OleT and AAR–ADO pathways) were constructed in the inhibitors tolerant C. glutamicum S9114 strain for hydrocarbon production using lignocellulose feedstock as the starting feedstock. When corn stover was used for hydrocarbons production after dry acid pretreatment and biodetoxification, the hydrocarbons generation reached 16.0 mg/L. This study provided a new strategy for hydrocarbons synthesis using microbial cell factory suitable for lignocellulose feedstock.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02129-7.

Cyclic L-lactide synthesis from lignocellulose biomass by biorefining with complete inhibitor removal and highly simultaneous sugars assimilation

Cyclic chiral lactide is the monomer chemical for polymerization of high molecular weight polylactic acid (PLA). The synthesis of cyclic L-lactide starts from poly-condensation of L-lactic acid to a low molecular weight pre-polymer and then depolymerized to cyclic L-lactide. Lignocellulose biomass is the most promising carbohydrate feedstock for lactic acid production, but the synthesis of cyclic L-lactide from L-lactic acid produced from lignocellulose has so far not been successful. The major barriers are the impurities of residual sugars and inhibitors in the crude cellulosic L-lactic acid product. Here we show a successful cyclic L-lactide synthesis from cellulosic L-lactic acid by lignocellulose biorefining with complete inhibitor removal and coordinated sugars assimilation. The removal of inhibitors from lignocellulose pretreatment was accomplished by biodetoxification using a unique fungus Amorphotheca resinae ZN1. The non-glucose sugars were completely and simultaneously assimilated at the same rate with glucose by the engineered L-lactic acid bacterium Pediococcus acidilactici. The L-lactic acid production from wheat straw was comparable to that from corn starch with high optical pure (99.6%), high L-lactic acid titer (129.4 g/L), minor residual total sugars (~2.2 g/L), and inhibitors free. The cyclic L-lactide was successfully synthesized from the regularly purified L-lactic acid and verified by detailed characterizations. This study paves the technical foundation of carbon neutral production of biodegradable PLA from lignocellulose biomass. This article is protected by copyright. All rights reserved.

Overexpression of mGDH in Gluconobacter oxydans to improve D-xylonic acid production from corn stover hydrolysate

Background

d-Xylonic acid is a versatile platform chemical with broad potential applications as a water reducer and disperser for cement and as a precursor for 1,4-butanediol and 1,2,4-tributantriol. Microbial production of d-xylonic acid with bacteria such as Gluconobacter oxydans from inexpensive lignocellulosic feedstock is generally regarded as one of the most promising and cost-effective methods for industrial production. However, high substrate concentrations and hydrolysate inhibitors reduce xylonic acid productivity.

Results

The d-xylonic acid productivity of G. oxydans DSM2003 was improved by overexpressing the mGDH gene, which encodes membrane-bound glucose dehydrogenase. Using the mutated plasmids based on pBBR1MCS-5 in our previous work, the recombinant strain G. oxydans/pBBR-R3510-mGDH was obtained with a significant improvement in d-xylonic acid production and a strengthened tolerance to hydrolysate inhibitors. The fed-batch biotransformation of d-xylose by this recombinant strain reached a high titer (588.7 g/L), yield (99.4%), and volumetric productivity (8.66 g/L/h). Moreover, up to 246.4 g/L d-xylonic acid was produced directly from corn stover hydrolysate without detoxification at a yield of 98.9% and volumetric productivity of 11.2 g/L/h. In addition, G. oxydans/pBBR-R3510-mGDH exhibited a strong tolerance to typical inhibitors, i.e., formic acid, furfural, and 5-hydroxymethylfurfural.

Conclusion

Through overexpressing mgdh in G. oxydans, we obtained the recombinant strain G. oxydans/pBBR-R3510-mGDH, and it was capable of efficiently producing xylonic acid from corn stover hydrolysate under high inhibitor concentrations. The high d-xylonic acid productivity of G. oxydans/pBBR-R3510-mGDH made it an attractive choice for biotechnological production.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01763-y.

Increasing Acid Tolerance of an Engineered Lactic Acid Bacterium Pediococcus acidilactici for L-Lactic Acid Production

Acid tolerance of the lactic acid bacterium (LAB) is crucially important for the production of free lactic acid as a chemical monomer by simplified purification steps. This study conducts both metabolic modification and adaptive evolution approaches on increasing the acid tolerance of an engineered Pediococcus acidilactici strain. The overexpression of the genes encoding lactate dehydrogenase, recombinase, chaperone, glutathione and ATPase did not show the observable changes in acid tolerance. On the other hand, the low pH adaptive evolution showed clear improvement. The L-lactic acid generation and cell viability of the adaptively evolved P. acidilactici were doubled at low pH up to 4.0 when wheat straw was used as carbohydrate feedstock. However, the further decrease in pH value close to the pKa (3.86) of lactic acid led to a dramatic reduction in L-lactic acid generation. This result shows a partially successful approach on improving the acid tolerance of the lactic acid bacterium P. acidilactici.

Engineering Corynebacterium glutamicum for synthesis of poly(3-hydroxybutyrate) from lignocellulose biomass

Lignocellulose is the only feasible carbohydrates feedstock for commercial scale and carbon neutral production of poly(3-hydroxybutyrate) (PHB) biopolymer by its great abundance and availability. Microbial cell factories for fermentative PHB synthesis are highly restricted by the growth suppression of inhibitors from lignocellulose pretreatment. This study targeted on a potential PHB-producing cell factory Corynebacterium glutamicum owing to its strong inhibitors tolerance. A systematic metabolic engineering was conducted starting with the stable PHB synthesis pathway construction from glucose and xylose, followed by the enhancement of PHB synthesis on PHA synthase activity and stability, cell morphology modification, and growth factors regulation. The relocation of the PHA synthase on the cell membrane guided by secrete signal peptides and cell membrane display motifs increased the PHB content by 2.4 folds. Excessive nitrogen preferentially promoted the PHB synthesis capacity and resulted in the PHB content increased by 13.3 folds. Modification of the genes responsible for cell division changed the cell morphology but the cell size was not enlarged to a PHB accumulation favorable environment. The metabolic engineering of C. glutamicum resulted in a high fermentative production of PHB using wheat straw as feedstock. This study provided an important microbial cell factory choice for PHB production using lignocellulose feedstock. This article is protected by copyright. All rights reserved.

Ultra-centrifugation force in adaptive evolution changes the cell structure of oleaginous yeast Trichosporon cutaneum into a favorable space for lipid accumulation

Microbial lipid production from lignocellulose biomass provides an essential option for sustainable and carbon neutral supply of future aviation fuels, biodiesel, as well as various food and nutrition products. Oleaginous yeast is the major microbial cell factory but its lipid producing performance is far below the requirements of industrial application. Here we show an ultra-centrifugation fractionation in adaptive evolution (UCF) of Trichosporon cutaneum based on the minor cell density difference. The lightest cells with the maximum intracellular lipid content were isolated by ultra-centrifugation fractionation in the long-term adaptive evolution. Significant changes occurred in the cell morphology with a fragile cell wall wrapping and enlarged intracellular space (two orders of magnitude increase in cell size). Complete and coordinate assimilations of all non-glucose sugars derived from lignocellulose were triggered and fluxed into lipid synthesis. Genome mutations and significant transcriptional regulations of the genes responsible for cell structure were identified and experimentally confirmed. The obtained Trichosporon cutaneum MP11 cells achieved a high lipid production of wheat straw, approximately five-folds greater than that of the parental cells. The study provided an effective method for screening the high lipid containing oleaginous yeast cells as well as the intracellular products accumulating cells in general. This article is protected by copyright. All rights reserved.

Enhanced enzymatic saccharification of sorghum straw by effective delignification via combined pretreatment with alkali extraction and deep eutectic solvent soaking

Hydrogen bond donor (HBD) in ChCl-based deep eutectic solvent (DESs) had significant influence on the Sorghum straw (SS) pretreatment. Lactic acid (LAC) was chosen as the appropriate HBD for preparing ChCl-based DES to pretreat Sorghum straw (SS). Furthermore, sequential pretreatment with dilute sodium hydroxide (0.75 wt%) for 1 h at 121 °C and ChCl:LAC soaking at 140 °C for 40 min was applied to pretreat SS for removing lignin (78.4%) and xylan (67.6%). Hydrolysis for 72 h, the reducing sugar yield reached 94.9%. Moreover, relationships of delignification and xylan removal with saccharification were explored after pretreatment. Finally, the fermentability of SS-hydrolysates was verified by bioethanol fermentation by S. cerevissiae with the yield of 0.45 g ethanol/g glucose. No significant inhibition was observed on ethanol fermentation. Obviously, establishment of high-efficient combination pretreatment with alkali extraction and ChCl:LAC soaking was successfully demonstrated for enhancing enzymatic saccharification of SS.

Acid-based lignocellulosic biomass biorefinery for bioenergy production: Advantages, application constraints, and perspectives

The production of chemicals and fuels from renewable biomass with the primary aim of reducing carbon footprints has recently become one of the central points of interest. The use of lignocellulosic biomass for energy production is believed to meet the main criteria of maximizing the available global energy source and minimizing pollutant emissions. However, before usage in bioenergy production, lignocellulosic biomass needs to undergo several processes, among which biomass pretreatment plays an important role in the yield, productivity, and quality of the products. Acid-based pretreatment, one of the existing methods applied for lignocellulosic biomass pretreatment, has several advantages, such as short operating time and high efficiency. A thorough analysis of the characteristics of acid-based biomass pretreatment is presented in this review. The environmental concerns and future challenges involved in using acid pretreatment methods are discussed in detail to achieve clean and sustainable bioenergy production. The application of acid to biomass pretreatment is considered an effective process for biorefineries that aim to optimize the production of desired products while minimizing the by-products.

Tolerance of Trichosporon cutaneum to lignin derived phenolic aldehydes facilitate the cell growth and cellulosic lipid accumulation

Phenolic aldehydes are the major inhibitors from lignocellulose pretreatment. Previous studies show that oleaginous yeasts are difficult to survive in lignocellulosic hydrolysates even after the removal of furan aldehydes and organic acids inhibitors. This study investigated the cell viability, sugar consumption and lipid accumulation of the major oleaginous yeasts including Trichosporon cutaneum, Rhodosporidium toruloides, Rhodotorula glutinis, Yarrowia lipolytica in wheat straw hydrolysate containing only phenolic aldehydes after furan aldehydes and organic acids were selectively degraded by microorganisms. The results confirmed that the existence of residual phenolic aldehydes was the major reason for poor cell growth and metabolism of oleaginous yeasts. Only T. cutaneum demonstrated the higher tolerance by biodegrading phenolic aldehydes and the satisfactory cell growth and lipid production were obtained. This study revealed that T. cutaneum might be one of the promising cell factories for microbial lipid production from lignocellulosic feedstock.

Transformation of lignocellulose to starch-like carbohydrates by organic acid-catalyzed pretreatment and biological detoxification

Corn dry milling provides a mature model for lignocellulose biorefinery process. To copy this technical success, a crucial step is to transform lignocellulose into starch-like carbohydrates (SLC), similar to milled corn grain and in a similar fashion to corn dry milling. The transformation process should be zero wastewater generation and sufficient fermentable sugar conservation; the product should be in solid particle form, free of toxic residues, and high enzymatic hydrolysis yield and fermentability. Here we designed and verified a SLC transformation process by (i) biodegradable oxalic acid-catalyzed pretreatment, and (ii) simultaneous biodegradation of inhibitors and oxalic acid catalyst. The oxalic acid catalyst was effective on disrupting the lignocellulose structure and also biodegradable at low pH value. The biodetoxification fungus Paecilomyces variotii FN89 was capable of degrading the furan/phenolic aldehydes and oxalic acid simultaneously and ultimately, while the fermentable sugars were well preserved. The obtained SLC from wheat straw and corn stover were similar to dry milled corn meal in terms of morphological properties, fermentable sugar contents, enzymatic hydrolysis yield, elemental contents, and free of inhibitors and acid catalyst. The bioconversion of starch-like wheat straw and corn stover produced 78.5 and 75.3 g/L of ethanol (9.9% and 9.5%, v/v) with the yield of 0.47 and 0.45 g ethanol/g cellulose/xylose, respectively, compared with 78.7 g/L (10.0%, v/v) from corn meal and the yield of 0.48 g ethanol/g starch. Mass balances suggest that the ethanol yield, wastewater generation, and elemental recycling of the SLC from lignocellulose were essentially the same as those of corn meal.

Increasing cellulosic ethanol production by enhancing phenolic tolerance of Zymomonas mobilis in adaptive evolution

Cellulosic ethanol fermentability of ethanologenic strain Zymomonas mobilis is severely inhibited by phenolic aldehydes generated from lignocellulose pretreatment. Here, a 198 days' laboratory adaptive evolution of Z. mobilis 8b in corn stover hydrolysate was conducted to increase its phenolic aldehydes tolerance and ethanol fermentability. The obtained Z. mobilis Z198 demonstrated a significantly improved conversion of the most toxic phenolic aldehyde (vanillin) by 6.3-fold and cellulosic ethanol production by 21.6%. The transcriptional analysis using qRT-PCR revealed that the gene ZMO3_RS07160 encoding SDR family oxidoreductase in Z. mobilis Z198 was significantly up-regulated by 11.7-fold. The overexpression of ZMO3_RS07160 in the parental Z. mobilis increased the ethanol fermentability to that of the adaptively evolved strain Z. mobilis Z198. This study provided a practical method to obtain a robust cellulosic ethanol fermenting strain, and a candidate gene for synthetic biology of biorefinery strains with strong phenolic aldehydes tolerance.

Increasing sodium lactate production by enhancement of Na+ transmembrane transportation in Pediococcus acidilactici

Fermentative production of sodium lactate generally is a low efficient process because of the high Na⁺ osmatic stress on lactic acid bacterium cells. In this study, the homogeneous genes encoding Na⁺/H⁺ antiporters were screened and overexpressed in Pediococcus acidilactici for the enhancement of Na⁺ transmembrane transportation. The function of the gene RS02775 was identified and its overexpressing in P. acidilactici resulted in the significantly improved sodium lactate production. The recombinant not only accelerated the sugar consumption, but also achieved the record high titer of sodium lactate by 121.1 g/L using pure sugars and 132.4 g/L using wheat straw. The transcription analysis shows that the overexpression of Na⁺/H⁺ antiporter significantly upregulated the transcription of the sugar phosphorylation genes of P. acidilactici under high Na⁺ stress. This study provides an effective method for high titer production of sodium lactate using both pure sugars and lignocellulose feedstocks.

Lysine Production by Dry Biorefining of Wheat Straw and Cofermentation of Corynebacterium glutamicum

A preliminary study shows that lysine production from lignocellulose feedstock is feasible, but the conversion of xylose in lignocellulose to lysine remains unsolved. Two technical barriers are responsible for the remaining xylose conversion: one is the xylose loss into the wastewater stream of the biorefinery processing chain, and the other is the lack of efficient lysine-producing strain with xylose utilization. Here, we conducted a new biorefinery approach of consequent dry acid pretreatment and biodetoxification, resulting in zero wastewater generation and then well-preserved xylose. To provide the lysine-producing strain with xylose utilization, we modified the Corynebacterium glutamicum by establishing the xylose assimilation pathway and improving the NADPH cofactor regeneration. The combinational modification of biorefinery processing and strain development led to 31.3 g/L of lysine production with a yield of 0.23 g lysine per gram of wheat straw derived sugars. This study provides a practical method for upgraded lysine production from lignocellulose for future industrial applications.

Potential for reduced water consumption in biorefining of lignocellulosic biomass to bioethanol and biogas

Increasing ethanol demand and public concerns about environmental protection promote the production of lignocellulosic bioethanol. Compared to that of starch- and sugar-based bioethanol production, the production of lignocellulosic bioethanol is water-intensive. A large amount of water is consumed during pretreatment, detoxification, saccharification, and fermentation. Water is a limited resource, and very high water consumption limits the industrial production of lignocellulosic bioethanol and decreases its environmental feasibility. In this review, we focused on the potential for reducing water consumption during the production of lignocellulosic bioethanol by performing pretreatment and fermentation at high solid loading, omitting water washing after pretreatment, and recycling wastewater by integrating bioethanol production and anaerobic digestion. In addition, the feasibility of these approaches and their research progress were discussed. This comprehensive review is expected to draw attention to water competition between bioethanol production and human use.

Process scale-up of an efficient acid-catalyzed steam pretreatment of rice straw for xylitol production by C. Tropicalis MTCC 6192

The present study focuses on operational parameters for the efficient acid catalyzed rice straw pretreatment process for xylitol production. 75.77 % xylose yield was attained when the 24 h presoaked rice straw (≤10 mm or ≤ 15 mm) in 1.5 % (v/v) H₂SO₄ was pretreated in the same reactor at 121 °C for 30 min. Neutralization with barium hydroxide produced insoluble salt and noticeably reduced HMF and furfurals. Xylitol yield of 0.6 g/g of xylose, was achieved by fermenting rice straw hydrolysate medium with C. tropicalis MTCC 6192. This two-step process of production of xylitol from xylose rich hydrolysate is much simpler and produced minimal inhibitors including organic acids such as acetic acid. This process is modified for upscaling at optimized parameters and will simultaneously minimize the pollution problem caused by rice straw and is also promising for commercial scale.

Lipid Accumulation by Xylose Metabolism Engineered Mucor circinelloides Strains on Corn Straw Hydrolysate

Previously, we presented a novel approach for increasing the consumption of xylose and the lipid yield by overexpressing the genes coding for xylose isomerase (XI) and xylulokinase (XK) in Mucor circinelloides. In the present study, an in-depth analysis of lipid accumulation by xylose metabolism engineered M. circinelloides strains (namely Mc-XI and Mc-XK) using corn straw hydrolysate was to be explored. The results showed that the fatty acid contents of the engineered M. circinelloides strains were, respectively, increased by 19.8% (in Mc-XI) and 22.3% (in Mc-XK) when compared with the control strain, even though a slightly decreased biomass in these engineered strains was detected. Moreover, the xylose uptake rates of engineered strains in the corn straw hydrolysate were improved significantly by 71.5% (in Mc-XI) and 68.8% (in Mc-XK), respectively, when compared with the control strain. Maybe the increased utilization of xylose led to an increase in lipid synthesis. When the recombinant M. circinelloides strains were cultured in corn straw hydrolysate medium with the carbon-to-nitrogen ratio (C/N ratio) of 50 and initial pH of 6.0, at 30 °C and 500 rpm for 144 h, a total biomass of 12.6-12.9 g/L with a lipid content of 17.2-17.7% (corresponding to a lipid yield of 2.17-2.28 g/L) was achieved. Our study provides a foundation for the further application of the engineered M. circinelloides strains to produce lipid from lignocelluloses.

An oxidoreductase gene ZMO1116 enhances the p-benzoquinone biodegradation and chiral lactic acid fermentability of Pediococcus acidilactici

p-Benzoquinone (BQ) is a lignin-derived inhibitor to microbial strains. Unlike the furan inhibitors, p-benzoquinone is recalcitrant to traditional detoxification methods. This study shows a biological degradation of p-benzoquinone and a simultaneous D-lactic acid fermentation by an engineered Pediococcus acidilactici strain. The overexpression of an oxidoreductase gene ZMO1116 from Zymomonas mobilis encoding oxidoreductase was identified to improve the D-lactic acid fermentability of P. acidilactici against p-benzoquinone. The gene ZMO1116 was integrated into the genome of P. acidilactici and enabled the engineered P. acidilactici to convert p-benzoquinone into less toxic hydroquinone (HQ), resulting in the improved p-benzoquinone tolerance. Simultaneous saccharification and co-fermentation (SSCF) was conducted using the pretreated and biodetoxified corn stover containing p-benzoquinone, the D-lactic acid production of the engineered strain (123.8 g/L) was 21.4 % higher than the parental strain (102.0 g/L). This study provides a practical method on robust p-benzoquinone tolerance and efficient cellulosic chiral lactic acid fermentation from lignocellulose feedstock.

A short-chain dehydrogenase plays a key role in cellulosic D-lactic acid fermentability of Pediococcus acidilactici

Phenolic aldehydes from lignocellulose pretreatment are strong inhibitors of cell growth and metabolism of cellulosic lactic acid bacteria. Their low solubility and recalcitrance highly reduce the removal efficiency of various detoxification methods. This study shows a simultaneous conversion of phenolic aldehydes and fermentation of D-lactic acid by Pediococcus acidilactici using corn stover feedstock. Vanillin was found to be the strongest phenolic aldehyde inhibitor to P. acidilactici. The overexpression of a short-chain dehydrogenase encoded by the gene CGS9114_RS09725 from Corynebacterium glutamicum was identified to play a key role in D-lactic acid fermentability of P. acidilactici. The engineered P. acidilactici with the genome integration of CGS9114_RS09725 showed the accelerated vanillin reduction and improved cellulosic D-lactic acid production. This study reveals that vanillin conversion is crucial for D-lactic acid fermentation, and the direct expression of a specific vanillin reduction gene in lactic acid bacterium efficiently improves cellulosic D-lactic acid production.

Mechanism of Tolerance to the Lignin-Derived Inhibitor p-Benzoquinone and Metabolic Modification of Biorefinery Fermentation Strains

p-Benzoquinone (BQ) is a lignin-derived inhibitor of biorefinery fermentation strains produced during pretreatment of lignocellulose. Unlike the well-studied inhibitors furan aldehydes, weak acids, and phenolics, the inhibitory properties of BQ, the microbial tolerance mechanism, and the detoxification strategy for this inhibitor have not been clearly elucidated. Here, BQ was identified as a by-product generated during acid pretreatment of various lignocellulose feedstocks, including corn stover, wheat straw, rice straw, tobacco stem, sunflower stem, and corncob residue. BQ at 20 to 200 mg/liter severely inhibited the cell growth and fermentability of various bacteria and yeast strains used in biorefinery fermentations. The BQ tolerance of the strains was found to be closely related to their capacity to convert BQ to nontoxic hydroquinone (HQ). To identify the key genes responsible for BQ tolerance, transcription levels of 20 genes potentially involved in the degradation of BQ in Zymomonas mobilis were investigated using real-time quantitative PCR in BQ-treated cells. One oxidoreductase gene, one hydroxylase gene, three reductase genes, and three dehydrogenase genes were found to be responsible for the conversion of BQ to HQ. Overexpression of the five key genes in Z. mobilis (ZMO1696, ZMO1949, ZMO1576, ZMO1984, and ZMO1399) accelerated its cell growth and cellulosic ethanol production in BQ-containing medium and lignocellulose hydrolysates.IMPORTANCE This study advances our understanding of BQ inhibition behavior and the mechanism of microbial tolerance to this inhibitor and identifies the key genes responsible for BQ detoxification. The insights here into BQ toxicity and tolerance provide the basis for future synthetic biology to engineer industrial fermentation strains with enhanced BQ tolerance.

Adaptive evolution of Gluconobacter oxydans accelerates the conversion rate of non-glucose sugars derived from lignocellulose biomass

Gluconobacter oxydans is capable of oxidizing various lignocellulose derived sugars into the corresponding sugar acids including glucose, xylose, arabinose, galactose and mannose, but simultaneous utilization of these sugars is difficult. This study attempted an adaptive evolution of G. oxydans by alternate transfer in inhibitors containing hydrolysate and inhibitors free hydrolysate for intensifying sugars simultaneous utilization. After 420 days' continuous culture, the conversion rate of all non-glucose sugars significantly improved by several folds and achieved complete conversion of lignocellulose-derived sugars to the corresponding sugar acids. The significant up-regulation of mGDH gene in the adapted G. oxydans strain (more than 40-fold greater than the parental) was considered as the decisive factor for the improvement of strain performance. This evolution adaptation strategy also could be used to accelerate robust sugars utilization for other fermented strains in lignocellulose biorefinery.

Microbial extraction of biotin from lignocellulose biomass and its application on glutamic acid production

Biotin (vitamin B₇) is an important nutrient for various fermentations. It is abundant in agricultural lignocellulose biomass and maintains stable in biorefinery processing chain including acid pretreatment, biodetoxification and saccharification. Here we show a microbial extraction of biotin from biotin-rich corn leaves hydrolysate. Corynebacterium glutamicum was found to have the highest biotin uptake capacity among different biotin auxotrophic microorganisms, and it was further significantly increased by overexpressing the bioYMN gene cluster encoding biotin transporter. Finally 250 folds greater biotin was extracted by recombinant C. glutamicum (303.8 mg/kg dry cell) from virgin corn leaves (1.2 mg/kg), which was far higher than that in commonly used fermentation additives including yeast extract (∼2 mg/kg), molasses (∼1 mg/kg) and corn steep liquor (∼0.75 mg/kg). The biotin extracted from corn leaves was successfully applied to glutamic acid fermentation. This is the first report on microbial extraction of biotin from lignocellulose biomass and fermentation promotion application.

Facilitation of l-Lactic Acid Fermentation by Lignocellulose Biomass Rich in Vitamin B Compounds

Vitamins are important nutrients for many fermentations, but they are generally costly. Agricultural lignocellulose biomass contains considerable amounts of vitamin B compounds, but these water-soluble vitamins are easily lost into wastewater discharge during pretreatment or detoxification of lignocellulose in biorefinery processes. Here, we showed that the dry acid pretreatment and biodetoxification process allowed the preservation of significant amounts of vitamin B, which promoted l-lactic acid fermentation efficiency significantly. Supplementation with specific vitamin B compounds, VB₃ and VB₅, into corn stover hydrolysate led to further increases of cellulosic l-lactic acid yield and fermentation rates. This study provided a new solution for the enhancement of biorefinery fermentation efficiency by using vitamin B compounds in lignocellulose biomass.

Heterozygous diploid structure of Amorphotheca resinae ZN1 contributes efficient biodetoxification on solid pretreated corn stover

BACKGROUND

Fast, complete, and ultimate removal of inhibitory compounds derived from lignocellulose pretreatment is the prerequisite for efficient production of cellulosic ethanol and biochemicals. Biodetoxification is the most promising method for inhibitor removal by its unique advantages. The biodetoxification mechanisms of a unique diploid fungus responsible for highly efficient biodetoxification in solid-state culture was extensively investigated in the aspects of cellular structure, genome sequencing, transcriptome analysis, and practical biodetoxification.

RESULTS

The inborn heterozygous diploid structure of A. resinae ZN1 uniquely contributed to the enhancement of inhibitor tolerance and conversion. The co-expression of gene pairs contributed to the enhancement of the degradation of lignocellulose-derived model inhibitors. The ultimate inhibitors degradation pathways and sugar conservation were elucidated by microbial degradation experimentation as well as the genomic and transcriptomic sequencing analysis.

CONCLUSIONS

The finding of the heterozygous diploid structure in A. resinae ZN1 on biodetoxification took the first insight into the global overview of biodetoxification mechanism of lignocellulose-derived inhibitors. This study provided a unique and practical biodetoxification biocatalyst of inhibitor compounds for lignocellulose biorefinery processing, as well as the synthetic biology tools on biodetoxification of biorefinery fermenting strains.

Microbial Lipid Production from Corn Stover by the Oleaginous Yeast Rhodosporidium toruloides Using the PreSSLP Process

Dry acid pretreatment and biodetoxification (DryPB) has been considered as an advanced technology to treat lignocellulosic materials for improved downstream bioconversion. In this study, the lipid production from DryPB corn stover was investigated by the oleaginous yeast Rhodosporidium toruloides using a new process designated prehydrolysis followed by simultaneous saccharification and lipid production (PreSSLP). The results found that prehydrolysis at 50 °C and then lipid production at 30 °C improved lipid yield by more than 17.0% compared with those without a prehydrolysis step. The highest lipid yield of 0.080 g/g DryPB corn stover was achieved at a solid loading of 12.5%. The fatty acid distribution of lipid products was similar to those of conventional vegetable oils that are used for biodiesel production. Our results suggested that the integration of DryPB process and PreSSLP process can be explored as an improved technology for microbial lipid production from lignocellulosic materials.

Biochemical engineering in China

Chinese biochemical engineering is committed to supporting the chemical and food industries, to advance science and technology frontiers, and to meet major demands of Chinese society and national economic development. This paper reviews the development of biochemical engineering, strategic deployment of these technologies by the government, industrial demand, research progress, and breakthroughs in key technologies in China. Furthermore, the outlook for future developments in biochemical engineering in China is also discussed.

Constructing super large scale cellulosic ethanol plant by decentralizing dry acid pretreatment technology into biomass collection depots

Commercial cellulosic ethanol plants require mature and year-round biomass feedstock supply. Decentralizing pretreatment operation from central ethanol plant into local regional biomass collection depots provides an important solution to reach this goal. In this study, we introduced a newly established pretreatment technology, dry acid pretreatment, into the decentralized pretreatment operation by its advantages on zero wastewater generation and high volumetric density. Collection radius of crop residues feedstock is extended to nearly 100 km by decentralizing dry acid pretreatment, and biorefinery scale for cellulosic ethanol production is increased to the scale of modern petroleum refining factories in the densified agricultural regions in China and USA with the minimum ethanol selling price of below $2/gal. The technology overcomes the barrier of cellulosic ethanol cost increase with increasing biomass collection range, and provides a methodology for optimal supply method of large biorefinery plants in agricultural countries.

A preliminary study on l-lysine fermentation from lignocellulose feedstock and techno-economic evaluation

l-Lysine is a commodity amino acid produced from starch feedstock. Various alternative feedstocks had been used for l-lysine production, but the yield was very low. This study took the first preliminary investigation on l-lysine production from lignocellulose for the replacement of food-crop starch. Corn stover was dry acid pretreated and biodetoxified, then used for enzymatic hydrolysis and l-lysine fermentation by an industrial Corynebacterium glutamicum strain. Various fermentation parameters, nutrient additions, and operation variables were applied and finally 33.8 g/L of l-lysine was obtained. This l-lysine titer is still below that of starch based fermentation, but already 3-5 folds greater than that of other alternative feedstocks based fermentation. A techno-economic analysis was conducted and the minimum selling price of l-lysine (hydrochloride form) was calculated to be $2.445 per kg. The cost reduction by the future improvement could fill the technical and economic gap between the cellulosic and starch based l-lysine production.

Engineering Corynebacterium glutamicum triggers glutamic acid accumulation in biotin-rich corn stover hydrolysate

BACKGROUND

Lignocellulose biomass contains high amount of biotin and resulted in an excessive biotin condition for cellulosic glutamic acid accumulation by Corynebacterium glutamicum. Penicillin or ethambutol triggers cellulosic glutamic acid accumulation, but they are not suitable for practical use due to the fermentation instability and environmental concerns. Efficient glutamic acid production from lignocellulose feedstocks should be achieved without any chemical inductions.

RESULTS

An industrial strain C. glutamicum S9114 was metabolically engineered to achieve efficient glutamic acid accumulation in biotin-excessive corn stover hydrolysate. Among the multiple metabolic engineering efforts, two pathway regulations effectively triggered the glutamic acid accumulation in lignocellulose hydrolysate. The C-terminal truncation of glutamate secretion channel MscCG (ΔC110) led to the successful glutamic acid secretion in corn stover hydrolysate without inductions. Then the α-oxoglutarate dehydrogenase complex (ODHC) activity was attenuated by regulating odhA RBS sequence, and glutamic acid accumulation was further elevated for more than fivefolds. The obtained C. glutamicum XW6 strain reached a record-high titer of 65.2 g/L with the overall yield of 0.63 g/g glucose using corn stover as the starting feedstock without any chemical induction.

CONCLUSIONS

Metabolic engineering method was successfully applied to achieve efficient glutamic acid in biotin-rich lignocellulose hydrolysate for the first time. This study demonstrated the high potential of glutamic acid production from lignocellulose feedstock.

Improved cellulosic ethanol production from corn stover with a low cellulase input using a β-glucosidase-producing yeast following a dry biorefining process

A low-cost and sustainable cellulosic ethanol production is vital for fermentation-based industrial applications. Reducing the expenses of cellulose-deconstruction enzymes is one of the significant challenges to economic cellulose-to-ethanol conversion. Here, we report the improved ethanol production from corn stover after dry biorefining using a natural β-glucosidase-producing strain Clavispora NRRL Y-50464 with a low cellulase dose of 5 mg protein/g glucan under separate enzymatic hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) conditions. Strain Clavispora NRRL Y-50464 exhibited a superior ethanol fermentation performance over Saccharomyces cerevisiae DQ1 under both conditions. It produced an ethanol titer of 38.1 g/L within 96 h at a conversion efficiency of 55.5% with 25% solids loading (w/w) via SSF without addition of extra β-glucosidase supplement. Improved performance of Y-50464 on a bioreactor with a helical stirring apparatus confirmed its advantage over the conventional bioreactors originally designed for liquid fermentations in cellulosic ethanol conversion by SSF. The results of this study suggested that the strain Clavispora NRRL Y-50464 has a potential as a candidate for lower-cost cellulosic ethanol production from lignocellulosic materials.

An Approach of Utilizing Water-Soluble Carbohydrates in Lignocellulose Feedstock for Promotion of Cellulosic l-Lactic Acid Production

Agricultural lignocellulose biomass generally contains certain amounts of water-soluble carbohydrates (WSC) such as glucose, fructose, or sucrose. These sugars are generally degraded in pretreatment at high temperature or discharged with wastewater in a detoxification process. This study proposed an approach of utilizing frequently ignored water-soluble carbohydrates for promotion of cellulosic l-lactic acid production. A simple solid state fermentation was performed during a corn stover storage period to convert the sugars into l-lactic acid and then a dry biorefining technology was applied to convert cellulose and hemicellulose fractions into the same l-lactic acid product. The 5-hydroxymethylfurfural (HMF) formation in pretreatment was significantly reduced and the consequent biodetoxification time was shortened. l-Lactic acid production was increased from 130.2 g/L to 139.0 g/L, and the minimum l-lactic acid selling price was reduced by 5.9%. This study provided an important option of biorefinery processing technology for production of value added biochemicals.

Cascade hydrolysis and fermentation of corn stover for production of high titer gluconic and xylonic acids

Simultaneous saccharification and fermentation (SSF) is an efficient fermentation operation in lignocellulose biorefining. However, SSF may not be applicable when the pH values of hydrolysis and fermentation do not match, or the strong intermediate inhibitors on cellulase activity are generated. This study proposed a cascade hydrolysis and fermentation (CHF) process for cellulosic gluconic acid fermentation to overcome the inhibition of the intermediate glucono-γ-lactone on cellulase activity. The pretreated and detoxified corn stover feedstock was enzymatically hydrolyzed into hydrolysate slurry, then gluconic acid and xylonic acid fermentations were directly conducted by inoculating Gluconobacter oxydans strain without solid/liquid separation. The sugar loss and energy consumption were effectively avoided by moving the solid/liquid separation into the fermentation stage. The experiments and the techno-economic analysis show that the CHF is simple and cost effective fermentation operation when SSF is not applicable.

Rich biotin content in lignocellulose biomass plays the key role in determining cellulosic glutamic acid accumulation by Corynebacterium glutamicum

BACKGROUND

Lignocellulose is one of the most promising alternative feedstocks for glutamic acid production as commodity building block chemical, but the efforts by the dominant industrial fermentation strain Corynebacterium glutamicum failed for accumulating glutamic acid using lignocellulose feedstock.

RESULTS

We identified the existence of surprisingly high biotin concentration in corn stover hydrolysate as the determining factor for the failure of glutamic acid accumulation by Corynebacterium glutamicum. Under excessive biotin content, induction by penicillin resulted in 41.7 ± 0.1 g/L of glutamic acid with the yield of 0.50 g glutamic acid/g glucose. Our further investigation revealed that corn stover contained 353 ± 16 μg of biotin per kg dry solids, approximately one order of magnitude greater than the biotin in corn grain. Most of the biotin remained stable during the biorefining chain and the rich biotin content in corn stover hydrolysate almost completely blocked the glutamic acid accumulation. This rich biotin existence was found to be a common phenomenon in the wide range of lignocellulose biomass and this may be the key reason why the previous studies failed in cellulosic glutamic acid fermentation from lignocellulose biomass. The extended recording of the complete members of all eight vitamin B compounds in lignocellulose biomass further reveals that the major vitamin B members were also under the high concentration levels even after harsh pretreatment.

CONCLUSIONS

The high content of biotin in wide range of lignocellulose biomass feedstocks and the corresponding hydrolysates was discovered and it was found to be the key factor in determining the cellulosic glutamic acid accumulation. The highly reserved biotin and the high content of their other vitamin B compounds in biorefining process might act as the potential nutrients to biorefining fermentations. This study creates a new insight that lignocellulose biorefining not only generates inhibitors, but also keeps nutrients for cellulosic fermentations.

Simultaneous saccharification and aerobic fermentation of high titer cellulosic citric acid by filamentous fungus Aspergillus niger

Simultaneous saccharification and fermentation (SSF) is the most efficient operation in biorefining conversion, but aerobic SSF under high solids loading significantly faces the serious oxygen transfer limitation. This study took the first insight into an aerobic SSF by high oxygen demanding filamentous fungi in highly viscous lignocellulose hydrolysate. The results show that oxygen requirement in the aerobic SSF by Aspergillus niger was well satisfied for production of cellulosic citric acid. The record high citric acid titer of 136.3 g/L and the overall conversion yield of 74.9% of cellulose were obtained by the aerobic SSF. The advantage of SSF to the separate hydrolysis and fermentation (SHF) on citric acid fermentation was compared based on the rigorous Aspen Plus modeling. The techno-economic analysis indicates that the minimum citric acid selling price (MCSP) of $0.603 per kilogram by SSF was highly competitive with the commercial citric acid from starch feedstock.

Engineering Pediococcus acidilactici with xylose assimilation pathway for high titer cellulosic l-lactic acid fermentation

Xylose-assimilating pathways were constructed in the parental Pediococcus acidilactici strain and evolutionarily adapted to yield a highly stable co-fermentation strain for l-lactic acid production. The phosphoketolase pathway (PK) was blocked for reduction of acetic acid generation by disrupting phosphoketolase (pkt) gene. The pentose phosphate pathway (PPP) was reconstructed for xylose assimilation by integrating four heterologous genes encoding transketolase (tkt), transaldolase (tal), xylose isomerase (xylA) and xylulokinase (xylB) into the P. acidilactici chromosome. The xylose-assimilating ability of the constructed strain was significantly improved by long term adaptive evolution. The engineered strain was applied to the simultaneous saccharification and co-fermentation (SSCF) under high solids loading of wheat straw. The l-lactic acid titer, productivity and xylose conversion reached the record high at 130.8±1.6g/L, 1.82±0.0g/L/h, and 94.9±0.0%, respectively. This study provided an important strain and process prototype for production of high titer cellulosic l-lactic acid.

Constructing xylose-assimilating pathways in Pediococcus acidilactici for high titer d-lactic acid fermentation from corn stover feedstock

Xylose-assimilating pathway was constructed in a d-lactic acid producing Pediococcus acidilactici strain and evolutionary adapted to yield a co-fermentation strain P. acidilactici ZY15 with 97.3g/L of d-lactic acid and xylose conversion of 92.6% obtained in the high solids content simultaneous saccharification and co-fermentation (SSCF) of dry dilute acid pretreated and biodetoxified corn stover feedstock. The heterologous genes encoding xylose isomerase (xylA) and xylulokinase (xylB) were screened and integrated into the P. acidilactici chromosome. The metabolic flux to acetic acid in phosphoketolase pathway was re-directed to pentose phosphate pathway by substituting the endogenous phosphoketolase gene (pkt) with the heterologous transketolase (tkt) and transaldolase (tal) genes. The xylose-assimilating ability of the newly constructed P. acidilactici strain was significantly improved by adaptive evolution. This study provided an important strain and process prototype for high titer d-lactic acid production from lignocellulose feedstock with efficient xylose assimilation.