Mechanisms underlying lactic acid tolerance and its influence on lactic acid production in Saccharomyces cerevisiae

Response mechanisms of different Saccharomyces cerevisiae strains to succinic acid

BACKGROUND

The production of succinic acid (SA) from biomass has attracted worldwide interest. Saccharomyces cerevisiae is preferred for SA production due to its strong tolerance to low pH conditions, ease of genetic manipulation, and extensive application in industrial processes. However, when compared with bacterial producers, the SA titers and productivities achieved by engineered S. cerevisiae strains were relatively low. To develop efficient SA-producing strains, it's necessary to clearly understand how S. cerevisiae cells respond to SA.

RESULTS

In this study, we cultivated five S. cerevisiae strains with different genetic backgrounds under different concentrations of SA. Among them, KF7 and NBRC1958 demonstrated high tolerance to SA, whereas NBRC2018 displayed the least tolerance. Therefore, these three strains were chosen to study how S. cerevisiae responds to SA. Under a concentration of 20 g/L SA, only a few differentially expressed genes were observed in three strains. At the higher concentration of 60 g/L SA, the response mechanisms of the three strains diverged notably. For KF7, genes involved in the glyoxylate cycle were significantly downregulated, whereas genes involved in gluconeogenesis, the pentose phosphate pathway, protein folding, and meiosis were significantly upregulated. For NBRC1958, genes related to the biosynthesis of vitamin B6, thiamin, and purine were significantly downregulated, whereas genes related to protein folding, toxin efflux, and cell wall remodeling were significantly upregulated. For NBRC2018, there was a significant upregulation of genes connected to the pentose phosphate pathway, gluconeogenesis, fatty acid utilization, and protein folding, except for the small heat shock protein gene HSP26. Overexpression of HSP26 and HSP42 notably enhanced the cell growth of NBRC1958 both in the presence and absence of SA.

CONCLUSIONS

The inherent activities of small heat shock proteins, the levels of acetyl-CoA and the strains' potential capacity to consume SA all seem to affect the responses and tolerances of S. cerevisiae strains to SA. These factors should be taken into consideration when choosing host strains for SA production. This study provides a theoretical basis and identifies potential host strains for the development of robust and efficient SA-producing strains.

Lactic Acid Production by Lactiplantibacillus plantarum AC 11S-Kinetics and Modeling

Lactic acid is a versatile chemical with wide application in many industries. It can be produced by the fermentation of different sugars by various lactobacilli and investigations on lactic acid production from different substrates and by different strains are still in progress. The present study aimed to study lactic acid production from lactose by Lactiplantibacillus plantarum AC 11S and to choose a mathematical model describing in the best way the experimental data obtained. The influence of initial substrate concentration was investigated, and optimal pH and temperature were determined. An unstructured mathematical model was developed comprising equations for bacterial growth, substrate consumption, and product formation. The model was solved with different terms for specific growth rates considering substrate and/or product inhibition. The best bacterial growth and lactic acid production were achieved at pH = 6.5 and 30 °C. Production of lactic acid was mainly growth-associated, and at initial substrate concentration over 15 g/L, a considerable product inhibition was observed. The parameters of different models were determined and compared. The modified Gompertz equation gave the best fit when solving only the equation for biomass growth at different initial substrate concentrations. Solving the entire set of differential equations for bacterial growth, substrate consumption, and product formation, the best results were obtained when using a variant of the logistic equation for biomass growth. This variant included a term for product inhibition and described in the best way all experimental data. Solving the model for different biomass concentrations showed that an increase in biomass led to a shorter lag phase and the stationary phase was reached faster. The results obtained, optimum conditions and the kinetic model, are good bases for studying pH-controlled fermentation, as well as a continuous process.

Increased CO2 fixation enables high carbon-yield production of 3-hydroxypropionic acid in yeast

CO2 fixation plays a key role to make biobased production cost competitive. Here, we use 3-hydroxypropionic acid (3-HP) to showcase how CO2 fixation enables approaching theoretical-yield production. Using genome-scale metabolic models to calculate the production envelope, we demonstrate that the provision of bicarbonate, formed from CO2, restricts previous attempts for high yield production of 3-HP. We thus develop multiple strategies for bicarbonate uptake, including the identification of Sul1 as a potential bicarbonate transporter, domain swapping of malonyl-CoA reductase, identification of Esbp6 as a potential 3-HP exporter, and deletion of Uga1 to prevent 3-HP degradation. The combined rational engineering increases 3-HP production from 0.14 g/L to 11.25 g/L in shake flask using 20 g/L glucose, approaching the maximum theoretical yield with concurrent biomass formation. The engineered yeast forms the basis for commercialization of bio-acrylic acid, while our CO2 fixation strategies pave the way for CO2 being used as the sole carbon source.

Unveiling the super tolerance of Candida nivariensis to oxidative stress: insights into the involvement of a catalase

Yeast cells involved in fermentation processes face various stressors that disrupt redox homeostasis and cause cellular damage, making the study of oxidative stress mechanisms crucial. In this investigation, we isolated a resilient yeast strain, Candida nivariensis GXAS-CN, capable of thriving in the presence of high concentrations of H2O2. Transcriptomic analysis revealed the up-regulation of multiple antioxidant genes in response to oxidative stress. Deletion of the catalase gene Cncat significantly impacted H2O2-induced oxidative stress. Enzymatic analysis of recombinant CnCat highlighted its highly efficient catalase activity and its essential role in mitigating H2O2. Furthermore, over-expression of CnCat in Saccharomyces cerevisiae improved oxidative resistance by reducing intracellular ROS accumulation. The presence of multiple stress-responsive transcription factor binding sites at the promoters of antioxidative genes indicates their regulation by different transcription factors. These findings demonstrate the potential of utilizing the remarkably tolerant C. nivariensis GXAS-CN or enhancing the resistance of S. cerevisiae to improve the efficiency and cost-effectiveness of industrial fermentation processes.IMPORTANCEEnduring oxidative stress is a crucial trait for fermentation strains. The importance of this research is its capacity to advance industrial fermentation processes. Through an in-depth examination of the mechanisms behind the remarkable H2O2 resistance in Candida nivariensis GXAS-CN and the successful genetic manipulation of this strain, we open the door to harnessing the potential of the catalase CnCat for enhancing the oxidative stress resistance and performance of yeast strains. This pioneering achievement creates avenues for fine-tuning yeast strains for precise industrial applications, ultimately leading to more efficient and cost-effective biotechnological processes.

General mechanisms of weak acid-tolerance and current strategies for the development of tolerant yeasts

Yeast cells are often subjected to various types of weak acid stress in the process of industrial production, food processing, and preservation, resulting in growth inhibition and reduced fermentation performance. Under acidic conditions, weak acids enter the near-neutral yeast cytoplasm and dissociate into protons and anions, leading to cytoplasmic acidification and cell damage. Although some yeast strains have developed the ability to survive weak acids, the complexity and diversity of stresses during industrial production still require the application of appropriate strategies for phenotypes improvement. In this review, we summarized current knowledge concerning weak acid stress response and resistance, which may suggest important targets for further construction of more robust strains. We also highlight current feasible strategies for improving the weak acid resistance of yeasts, such as adaptive laboratory evolution, transcription factors engineering, and cell membrane/wall engineering. Moreover, the challenges and perspectives associated with improving the competitiveness of industrial strains are also discussed. This review provides effective strategies for improving the industrial phenotypes of yeast from multiple dimensions in future studies.

Adaptive laboratory evolution for acetic acid-tolerance matches sourdough challenges with yeast phenotypes

Acetic acid tolerance of Saccharomyces cerevisiae is an important trait in sourdough fermentation processes, where the accumulation of acid by the growth of lactic acid bacteria reduces the yeast metabolic activity. In this work, we have carried out adaptive laboratory evolution (ALE) experiments in two sourdough isolates of S. cerevisiae exposed to acetic acid, or alternatively to acetic acid and myriocin, an inhibitor of sphingolipid biosynthesis that sped-up the evolutionary adaptation. Evolution approaches resulted in acetic tolerance, and surprisingly, increased lactic susceptibility. Four evolved clones, one from each parental strain and evolutionary scheme, were selected on the basis of their potential for CO2 production in sourdough conditions. Among them, two showed phenotypic instability characterized by strong lactic sensitivity after several rounds of growth under unstressed conditions, while two others, displayed increased constitutive acetic tolerance with no loss of growth in lactic medium. Genome sequencing and ploidy level analysis of all strains revealed aneuploidies, which could account for phenotypic heterogeneity. In addition, copy number variations (CNVs), affecting specially to genes involved in ion transport or flocculation, and single nucleotide polymorphisms (SNPs) were identified. Mutations in several genes, ARG82, KEX1, CTK1, SPT20, IRA2, ASG1 or GIS4, were confirmed as involved in acetic and/or lactic tolerance, and new determinants of these phenotypes, MSN5 and PSP2, identified.

Engineering yeast cell factories to produce biodegradable plastics and their monomers: Current status and prospects

Traditional plastic products have caused serious environmental pollution due to difficulty to be degraded in the natural environment. In the recent years, biodegradable plastics are receiving increasing attention due to advantages in natural degradability and environmental friendliness. Biodegradable plastics have potential to be used in food, agriculture, industry, medicine and other fields. However, the high production cost of such plastics is the bottleneck that limits their commercialization and application. Yeasts, including budding yeast and non-conventional yeasts, are widely studied to produce biodegradable plastics and their organic acid monomers. Compared to bacteria, yeast strains are more tolerable to multiple stress conditions including low pH and high temperature, and also have other advantages such as generally regarded as safe, and no phage infection. In addition, synthetic biology and metabolic engineering of yeast have enabled its rapid and efficient engineering for bioproduction using various renewable feedstocks, especially lignocellulosic biomass. This review focuses on the recent progress in biosynthesis technology and strategies of monomeric organic acids for biodegradable polymers, including polylactic acid (PLA), polyhydroxyalkanoate (PHA), polybutylene succinate (PBS), and polybutylene adipate terephthalate (PBAT) using yeast cell factories. Improving the performance of yeast as a cell factory and strategies to improve yeast acid stress tolerance are also discussed. In addition, the critical challenges and future prospects for the production of biodegradable plastic monomer using yeast are also discussed.

Modification of the second PEP4-allele facilitates an industrial Saccharomyces cerevisiae to tolerate tartaric acid stress

The practical significance of constructing robust industrial production strains against organic acid stress lies not only in improving fermentation efficiency but also in reducing manufacturing costs. In a previous study, we constructed an industrial Saccharomyces cerevisiae strain by modifying another PEP4-allele of a mutant that already had one PEP4-allele disrupted. This modification enhanced cellular tolerance to citric acid stress during growth. Unlike citric acid, which S. cerevisiae can consume, tartaric acid is often added to grape must during winemaking to increase total acidity and is not metabolizable. The results of the present study indicate that the modification of the second PEP4-allele improves the cellular tolerance of the strain with one PEP4-allele disrupted against tartaric acid stress during growth and contributes to maintaining intracellular pH homeostasis in cells subjected to tartaric acid stress. Moreover, under tartaric acid stress, a significant improvement in glucose-ethanol conversion performance, conferred by the modification of the second PEP4-allele, was observed. This study not only broadens our understanding of the role of the PEP4-allele in cellular regulation but also provides a prospective approach to reducing the concentration of sulfur dioxide used in winemaking.

Modeling control and transduction of electrochemical gradients in acid-stressed bacteria

Transmembrane electrochemical gradients drive solute uptake and constitute a substantial fraction of the cellular energy pool in bacteria. These gradients act not only as "homeostatic contributors," but also play a dynamic and keystone role in several bacterial functions, including sensing, stress response, and metabolism. At the system level, multiple gradients interact with ion transporters and bacterial behavior in a complex, rapid, and emergent manner; consequently, experiments alone cannot untangle their interdependencies. Electrochemical gradient modeling provides a general framework to understand these interactions and their underlying mechanisms. We quantify the generation, maintenance, and interactions of electrical, proton, and potassium potential gradients under lactic acid-stress and lactic acid fermentation. Further, we elucidate a gradient-mediated mechanism for intracellular pH sensing and stress response. We demonstrate that this gradient model can yield insights on the energetic limitations of membrane transport, and can predict bacterial behavior across changing environments.

Genome engineering of Kluyveromyces marxianus for high D-( -)-lactic acid production under low pH conditions

Saccharomyces cerevisiae is the workhorse of fermentation industry. Upon engineering for D-lactate production by a series of gene deletions, this yeast had deficiencies in cell growth and D-lactate production at high substrate concentrations. Complex nutrients or high cell density were thus required to support growth and D-lactate production with a potential to increase medium and process cost of industrial-scale D-lactate production. As an alternative microbial biocatalyst, a Crabtree-negative and thermotolerant yeast Kluyveromyces marxianus was engineered in this study to produce high titer and yield of D-lactate at a lower pH without growth defects. Only pyruvate decarboxylase 1 (PDC1) gene was replaced by a codon-optimized bacterial D-lactate dehydrogenase (ldhA). Ethanol, glycerol, or acetic acid was not produced by the resulting strain, KMΔpdc1::ldhA. Aeration rate at 1.5 vvm and culture pH 5.0 at 30 °C provided the highest D-lactate titer of 42.97 ± 0.48 g/L from glucose. Yield and productivity of D-lactate, and glucose-consumption rate were 0.85 ± 0.01 g/g, 0.90 ± 0.01 g/(L·h), and 1.06 ± 0.00 g/(L·h), respectively. Surprisingly, D-lactate titer, productivity, and glucose-consumption rate of 52.29 ± 0.68 g/L, 1.38 ± 0.05 g/(L·h), and 1.22 ± 0.00 g/(L·h), respectively, were higher at 42 °C compared to 30 °C. Sugarcane molasses, a low-value carbon, led to the highest D-lactate titer and yield of 66.26 ± 0.81 g/L and 0.91 ± 0.01 g/g, respectively, in a medium without additional nutrients. This study is a pioneer work of engineering K. marxianus to produce D-lactate at the yield approaching theoretical maximum using simple batch process. Our results support the potential of an engineered K. marxianus for D-lactate production on an industrial scale. KEY POINTS: • K. marxianus was engineered by deleting PDC1 and expressing codon-optimized D-ldhA. • The strain allowed high D-lactate titer and yield under pH ranging from 3.5 to 5.0. • The strain produced 66 g/L D-lactate at 30 °C from molasses without any additional nutrients.

Optimal preparation of food waste to increase its utility for sophorolipid production by Starmerella bombicola

Secondary feedstocks, such as food waste (FW), have been used for yeasts (e.g. Starmerella bombicola) to produce sophorolipids (SLs), which are commercially available biosurfactants. However, the quality of FW varies by location and season and may contains chemicals that inhibit SLs production. Therefore, it is crucial to identify such inhibitors and, if possible, remove them, to ensure efficient utilization. In this study, large scale FW was first analysed to determine the concentration of potential inhibitors. Lacticacid, acetic acid and ethanol were identified and found to be inhibitors of the growth of S. bombicola and its SLs production. Various methods were then evaluated for their ability to remove these inhibitors. Finally, a simple and effective strategy for removing inhibitors from FW was developed that complied with the 12 principles of green chemistry and could be adopted by industry for high SLs production.

Enhancement of acid tolerance of Escherichia coli by introduction of molecule chaperone CbpA from extremophile

Molecular chaperone CbpA from extreme acidophile Acidithiobacillus caldus was applied to improve acid tolerance of Escherichia coli via CRISPR/Cas9. Cell growth and viability of plasmid complementary strain indicated the importance of cbpA^Ac for bacteria acid tolerance. With in situ gene replacement by CRISPR/Cas9 system, colony formation unit (CFU) of genome recombinant strain BL21-ΔcbpA/AccbpA showed 7.7 times higher cell viability than deficient strain BL21-ΔcbpA and 2.3 times higher than wild type. Cell morphology observation using Field Emission Scanning Electron Microscopy (FESEM) revealed cell breakage of BL21-ΔcbpA and significant recovery of BL21-ΔcbpA/AccbpA. The intracellular ATP level of all strains gradually decreased along with the increased stress time. Particularly, the value of recombinant strain was 56.0% lower than that of deficient strain after 5 h, indicating that the recombinant strain consumed a lot of energy to resist acid stress. The arginine concentration in BL21-ΔcbpA/AccbpA was double that of BL21-ΔcbpA, while the aspartate and glutamate contents were 14.8% and 6.2% higher, respectively, compared to that of wild type. Moreover, RNA-Seq analysis examined 93 genes down-regulated in BL21-ΔcbpA compared to wild type strain, while 123 genes were up-regulated in BL21-ΔcbpA/AccbpA compared to BL21-ΔcbpA, with an emphasis on energy metabolism, transport, and cell components. Finally, the working model in response to acid stress of cbpA from A. caldus was developed. This study constructed a recombinant strain resistant to acid stress and also provided a reference for enhancing microorganisms' robustness to various conditions.

Microbial production of odd-chain fatty acids

Odd-chain fatty acids (OcFAs) and their derivatives have attracted much attention due to their beneficial physiological effects and their potential to be alternatives to advanced fuels. However, cells naturally produce even-chain fatty acids (EcFAs) with negligible OcFAs. In the process of biosynthesis of fatty acids (FAs), the acetyl-CoA serves as the starter unit for EcFAs, and propionyl-CoA works as the starter unit for OcFAs. The lack of sufficient propionyl-CoA, the precursor, is usually regarded as the main restriction for large-scale bioproduction of OcFAs. In recent years, synthetic biology strategies have been used to modify several microorganisms to produce more propionyl-CoA that would enable an efficient biosynthesis of OcFAs. This review discusses several reported and potential metabolic pathways for propionyl-CoA biosynthesis, followed by advances in engineering several cell factories for OcFAs production. Finally, trends and challenges of synthetic biology driven OcFAs production are discussed.

Recent advances in modified poly (lactic acid) as tissue engineering materials

As an emerging science, tissue engineering and regenerative medicine focus on developing materials to replace, restore or improve organs or tissues and enhancing the cellular capacity to proliferate, migrate and differentiate into different cell types and specific tissues. Renewable resources have been used to develop new materials, resulting in attempts to produce various environmentally friendly biomaterials. Poly (lactic acid) (PLA) is a biopolymer known to be biodegradable and it is produced from the fermentation of carbohydrates. PLA can be combined with other polymers to produce new biomaterials with suitable physicochemical properties for tissue engineering applications. Here, the advances in modified PLA as tissue engineering materials are discussed in light of its drawbacks, such as biological inertness, low cell adhesion, and low degradation rate, and the efforts conducted to address these challenges toward the design of new enhanced alternative biomaterials.

Metabolic engineering of Zymomonas mobilis for co-production of D-lactic acid and ethanol using waste feedstocks of molasses and corncob residue hydrolysate

Lactate is the precursor for polylactide. In this study, a lactate producer of Z. mobilis was constructed by replacing ZMO0038 with LmldhA gene driven by a strong promoter PadhB, replacing ZMO1650 with native pdc gene driven by Ptet, and replacing native pdc with another copy of LmldhA driven by PadhB to divert carbon from ethanol to D-lactate. The resultant strain ZML-pdc-ldh produced 13.8 ± 0.2 g/L lactate and 16.9 ± 0.3 g/L ethanol using 48 g/L glucose. Lactate production of ZML-pdc-ldh was further investigated after fermentation optimization in pH-controlled fermenters. ZML-pdc-ldh produced 24.2 ± 0.6 g/L lactate and 12.9 ± 0.8 g/L ethanol as well as 36.2 ± 1.0 g/L lactate and 40.3 ± 0.3 g/L ethanol, resulting in total carbon conversion rate of 98.3% ± 2.5% and 96.2% ± 0.1% with final product productivity of 1.9 ± 0.0 g/L/h and 2.2 ± 0.0 g/L/h in RMG5 and RMG12, respectively. Moreover, ZML-pdc-ldh produced 32.9 ± 0.1 g/L D-lactate and 27.7 ± 0.2 g/L ethanol as well as 42.8 ± 0.0 g/L D-lactate and 53.1 ± 0.7 g/L ethanol with 97.1% ± 0.0% and 99.1% ± 0.8% carbon conversion rate using 20% molasses or corncob residue hydrolysate, respectively. Our study thus demonstrated that it is effective for lactate production by fermentation condition optimization and metabolic engineering to strengthen heterologous ldh expression while reducing the native ethanol production pathway. The capability of recombinant lactate-producer of Z. mobilis for efficient waste feedstock conversion makes it a promising biorefinery platform for carbon-neutral biochemical production.

Metabolic Engineering and Adaptive Evolution for Efficient Production of l-Lactic Acid in Saccharomyces cerevisiae

l-Lactic acid (LA) is a three-carbon hydroxycarboxylic acid with extensive applications in food, cosmetic, agricultural, pharmaceutical, and bioplastic industries. However, microbial LA production is limited by its intrinsic inefficiency of cellular metabolism. Here, pathway engineering was used to rewire the biosynthetic pathway for LA production in Saccharomyces cerevisiae by screening heterologous l-lactate dehydrogenase, reducing ethanol accumulation, and introducing a bacterial acetyl coenzyme A (acetyl-CoA) synthesis pathway. To improve its intrinsic efficiency of LA export, transporter engineering was conducted by screening the monocarboxylate transporters and then strengthening the capacity of LA export, leading to LA production up to 51.4 g/L. To further enhance its intrinsic efficiency of acid tolerance, adaptive evolution was adopted by cultivating yeast cells with a gradual increase in LA levels during 12 serial subcultures, resulting in a 17.5% increase in LA production to 60.4 g/L. Finally, the engineered strain S.c-NO.2-100 was able to produce 121.5 g/L LA, with a yield of up to 0.81 g/g in a 5-L batch bioreactor. The strategy described here provides a guide for developing efficient cell factories for the production of the other industrially useful organic acids. IMPORTANCE Saccharomyces cerevisiae is one of the most widely engineered cell factories for the production of organic acids. However, microbial production of l-lactic acid is limited by its intrinsic inefficiency of cellular metabolism in S. cerevisiae. Here, the transmission efficiency of the biosynthetic pathway was improved by pathway optimization to increase l-lactic acid production. Then, the synthetic ability for l-lactic acid was further enhanced by adaptive evolution to improve acid tolerance of S. cerevisiae. Based on these strategies, the final engineered S. cerevisiae strain achieved high efficiency of l-lactic acid production. These findings provide new insight into improving the intrinsic efficiency of cellular metabolism and will help to construct superior industrial yeast strains for high-level production of other organic acids.

Antibiofilm Activity of Weissella spp. and Bacillus coagulans Isolated from Equine Skin against Staphylococcus aureus

The aim of this study was to evaluate the antimicrobial and antibiofilm activity of Weissella cibaria, Weissella hellenica and Bacillus coagulans, isolated from equine skin, against biofilm-forming Staphylococcus aureus CCM 4223 and clinical isolate methicillin-resistant S. aureus (MRSA). Non-neutralized cell-free supernatants (nnCFS) of tested skin isolates completely inhibited the growth and biofilm formation of S. aureus strains and caused dispersion of the 24 h preformed biofilm in the range of 21-90%. The majority of the pH-neutralized cell-free supernatants (nCFS) of skin isolates inhibited the biofilm formation of both S. aureus strains in the range of 20-100%. The dispersion activity of B. coagulans nCFS ranged from 17 to 77% and was significantly lower than that of nnCFS, except for B. coagulans 3T27 against S. aureus CCM 4223. Changes in the growth of S. aureus CCM 4223 in the presence of catalase- or trypsin-treated W. hellenica 4/2D23 and W. cibaria 4/8D37 nCFS indicated the role of peroxides and/or bacteriocin in their antimicrobial activities. For the first time, the presence of the fenD gene, associated with biosurfactants production, was detected in B. coagulans. The results of this study showed that selected isolates may have the potential for the prevention and treatment of biofilm-forming S. aureus infections.

Kluyveromyces marxianus as a microbial cell factory for lignocellulosic biomass valorisation

The non-conventional yeast Kluyveromyces marxianus is widely used for several biotechnological applications, mainly due to its thermotolerance, high growth rate, and ability to metabolise a wide range of sugars. These cell traits are strategic for lignocellulosic biomass valorisation and strain diversity prompts the development of robust chassis, either with improved tolerance to lignocellulosic inhibitors or ethanol. This review summarises bioethanol and value-added chemicals production by K. marxianus from different lignocellulosic biomasses. Moreover, metabolic engineering and process optimization strategies developed to expand K. marxianus potential are also compiled, as well as studies reporting cell mechanisms to cope with lignocellulosic-derived inhibitors. The main lignocellulosic-based products are bioethanol, representing 71% of the reports, and xylitol, representing 17% of the reports. K. marxianus also proved to be a good chassis for lactic acid and volatile compounds production from lignocellulosic biomass, although the literature on this matter is still scarce. The increasing advances in genome editing tools and process optimization strategies will widen the K. marxianus-based portfolio products.

l-Lactic Acid Production Using Engineered Saccharomyces cerevisiae with Improved Organic Acid Tolerance

Lactic acid is mainly used to produce bio-based, bio-degradable polylactic acid. For industrial production of lactic acid, engineered Saccharomyces cerevisiae can be used. To avoid cellular toxicity caused by lactic acid accumulation, pH-neutralizing agents are used, leading to increased production costs. In this study, lactic acid-producing S. cerevisiae BK01 was developed with improved lactic acid tolerance through adaptive laboratory evolution (ALE) on 8% lactic acid. The genetic basis of BK01 could not be determined, suggesting complex mechanisms associated with lactic acid tolerance. However, BK01 had distinctive metabolomic traits clearly separated from the parental strain, and lactic acid production was improved by 17% (from 102 g/L to 119 g/L). To the best of our knowledge, this is the highest lactic acid titer produced by engineered S. cerevisiae without the use of pH neutralizers. Moreover, cellulosic lactic acid production by BK01 was demonstrated using acetate-rich buckwheat husk hydrolysates. Particularly, BK01 revealed improved tolerance against acetic acid of the hydrolysates, a major fermentation inhibitor of lignocellulosic biomass. In short, ALE with a high concentration of lactic acid improved lactic acid production as well as acetic acid tolerance of BK01, suggesting a potential for economically viable cellulosic lactic acid production.

Current Progress in Production of Building-Block Organic Acids by Consolidated Bioprocessing of Lignocellulose

Several organic acids have been indicated among the top value chemicals from biomass. Lignocellulose is among the most attractive feedstocks for biorefining processes owing to its high abundance and low cost. However, its highly complex nature and recalcitrance to biodegradation hinder development of cost-competitive fermentation processes. Here, current progress in development of single-pot fermentation (i.e., consolidated bioprocessing, CBP) of lignocellulosic biomass to high value organic acids will be examined, based on the potential of this approach to dramatically reduce process costs. Different strategies for CBP development will be considered such as: (i) design of microbial consortia consisting of (hemi)cellulolytic and valuable-compound producing strains; (ii) engineering of microorganisms that combine biomass-degrading and high-value compound-producing properties in a single strain. The present review will mainly focus on production of organic acids with application as building block chemicals (e.g., adipic, cis,cis-muconic, fumaric, itaconic, lactic, malic, and succinic acid) since polymer synthesis constitutes the largest sector in the chemical industry. Current research advances will be illustrated together with challenges and perspectives for future investigations. In addition, attention will be dedicated to development of acid tolerant microorganisms, an essential feature for improving titer and productivity of fermentative production of acids.