Microbial production of lactic acid

Malolactic fermentation in lingonberry juice and its use as a preservative

Lingonberry is a common wild berry that is often sold as jams and beverages. It naturally contains high amounts of the weak acid preservative benzoic acid making it an interesting ingredient for shelf-life extension. Despite this, their use as a raw ingredient is limited by the inherently intense sour taste. This study aimed to improve the taste of lingonberry juice by subjecting it to malolactic fermentation in order to reduce the sourness, and to investigate the benzoic acid in lingonberries as a natural preservative in juice blends by determining the microbial stability. After initial screening of lactic acid bacteria, a Lactiplantibacillus plantarum strain was used as the starter for subsequent investigations. Upon raising the pH, all malic acid was completely converted to lactic acid after seven days. The fermented juice was mixed with blackcurrant juice in different proportions. Challenge tests of the blends showed Listeria monocytogenes could not grow in any juice samples, while Candida albicans only grew in the pure blackcurrant juice. Aspergillus brasiliensis growth was delayed in all samples containing benzoic acid in a concentration-dependent manner. The sourness and astringency were substantially reduced in the juice with added L. plantarum compared to the unfermented juice.

AFP: Finding pathways accounting for stoichiometry along with atom group tracking in metabolic network

Automatically finding novel pathways plays an important role in the initial designs of metabolic pathways in synthetic biology and metabolic engineering. Although path-finding methods have been successfully applied in identifying valuable synthetic pathways, few efforts have been made in fusing atom group tracking into building stoichiometry model to search metabolic pathways from arbitrary start compound via Mixed Integer Linear Programming (MILP). We propose a novel method called AFP to find metabolic pathways by incorporating atom group tracking into reaction stoichiometry via MILP. AFP tracks the movements of atom groups in the reaction stoichiometry to construct MILP model to search the pathways containing atom groups exchange in the reactions and adapts the MILP model to provide the options of searching pathways from an arbitrary or given compound to the target compound. Combining atom group tracking with reaction stoichiometry to build MILP model for pathfinding may promote the search of well-designed alternative pathways at the stoichiometric modeling level. The experimental comparisons to the known pathways show that our proposed method AFP is more effective to recover the known pathways than other existing methods and is capable of discovering biochemically feasible pathways producing the metabolites of interest.

Production of red beetroot juice by different methods: Kinetics of microbial growth, sugar consumption, and acid production

As a fermentation method, the utilisation of starter culture is a common practice in industrial manufacturing, although spontaneous methods have been employed since ancient times. The objective of this study was to investigate the effect of different production methods on red beetroot juice (RBJ). For this purpose, as a starter culture, the probiotic Lactibasillus paracasei (Lc. paracasei) was inoculated into the RBJ samples after pasteurization. Also, the growth of cells, acid production, and substrate utilisation were monitored throughout the fermentation process of RBJ under two different methods of fermentation. The samples produced by the addition of Lc. paracasei demonstrated a slightly lower decrease in pH values in comparison to the samples obtained by the spontaneous method. The concentration of lactic acid (LA) and acetic acid (AA) at the end of fermentation reveals that Lc. paracasei exhibits a greater capacity for both LA and AA generation compared to the spontaneous method. The ratios of LA and AA molar concentrations of RBJ were determined to be 1.7 and 3.6 for the samples produced by adding Lc. paracasei and the spontaneous method, respectively. The samples produced by adding Lc. paracasei exhibited a greater consumption of sucrose. Both fermentation methods provide LAB counts exceeding 8 log CFU/mL at the end of fermentation. Time demonstrated a significant correlation with LA and AA in the method by adding Lc. paracasei (r = 0.942 and 0.745), respectively (p < 0.01). In both methods, it was demonstrated that while sucrose content decreased during the fermentation period, fructose and glucose content remained constant (p < 0.05).

Characterization of Genomic, Physiological, and Probiotic Features of Lactiplantibacillus plantarum JS21 Strain Isolated from Traditional Fermented Jiangshui

This study aimed to understand the genetic and metabolic traits of a Lactiplantibacillus plantarum JS21 strain and its probiotic abilities through laboratory tests and computer analysis. L. plantarum JS21 was isolated from a traditional fermented food known as "Jiangshui" in Hanzhong city. In this research, the complete genetic makeup of JS21 was determined using Illumina and PacBio technologies. The JS21 genome consisted of a 3.423 Mb circular chromosome and five plasmids. It was found to contain 3023 protein-coding genes, 16 tRNA genes, 64 rRNA operons, 40 non-coding RNA genes, 264 pseudogenes, and six CRISPR array regions. The GC content of the genome was 44.53%. Additionally, the genome harbored three complete prophages. The evolutionary relationship and the genome collinearity of JS21 were compared with other L. plantarum strains. The resistance genes identified in JS21 were inherent. Enzyme genes involved in the Embden-Meyerhof-Parnas (EMP) and phosphoketolase (PK) pathways were detected, indicating potential for facultative heterofermentative pathways. JS21 possessed bacteriocins plnE/plnF genes and genes for polyketide and terpenoid assembly, possibly contributing to its antibacterial properties against Escherichia coli (ATCC 25922), Escherichia coli (K88), Staphylococcus aureus (CMCC 26003), and Listeria monocytogenes (CICC 21635). Furthermore, JS21 carried genes for Na+/H+ antiporters, F0F1 ATPase, and other stress resistance genes, which may account for its ability to withstand simulated conditions of the human gastrointestinal tract in vitro. The high hydrophobicity of its cell surface suggested the potential for intestinal colonization. Overall, L. plantarum JS21 exhibited probiotic traits as evidenced by laboratory experiments and computational analysis, suggesting its suitability as a dietary supplement.

A Comprehensive Review of Bioactive Compounds from Lactic Acid Bacteria: Potential Functions as Functional Food in Dietetics and the Food Industry

Lactic acid bacteria (LAB) are beneficial microbes known for their health-promoting properties. LAB are well known for their ability to produce substantial amounts of bioactive compounds during fermentation. Peptides, exopolysaccharides (EPS), bacteriocins, some amylase, protease, lipase enzymes, and lactic acid are the most important bioactive compounds generated by LAB activity during fermentation. Additionally, the product produced by LAB is dependent on the type of fermentation used. LAB derived from the genera Lactobacillus and Enterococcus are the most popular probiotics at present. Consuming fermented foods has been previously connected to a number of health-promoting benefits such as antibacterial activity and immune system modulation. Furthermore, functional food implementations lead to the application of LAB in therapeutic nutrition such as prebiotic, immunomodulatory, antioxidant, anti-tumor, blood glucose lowering actions. Understanding the characteristics of LAB in diverse sources and its potential as a functional food is crucial for therapeutic applications. This review presents an overview of functional food knowledge regarding interactions between LAB isolated from dairy products (dairy LAB) and fermented foods, as well as the prospect of functioning LAB in human health. Finally, the health advantages of LAB bioactive compounds are emphasized.

Preparation of Sourdoughs Fermented with Isolated Lactic Acid Bacteria and Characterization of Their Antifungal Properties

Traditional sourdough is obtained using a mixture of flour and water stored at room temperature until acidification. Therefore, adding lactic acid bacteria (LAB) can improve the quality and safety of sourdough bread. Faced with this problem, four drying techniques-freeze-drying, spray-drying, low-temperature drying, and drying at low humidity-have been applied. Our goals were to isolate LAB strains with antifungal potential against Aspergillus and Penicillium fungi. The antifungal capacity was evaluated with agar diffusion, co-culture in overlay agar, and a microdilution susceptibility assay. In addition, the antifungal compounds generated in sourdough were analyzed. As a result, dried sourdoughs were prepared with Lactiplantibacillus plantarum TN10, Lactiplantibacillus plantarum TF2, Pediococcus pentosaceus TF8, Pediococcus acidilactici TE4, and Pediococcus pentosaceus TI6. The minimum fungicidal concentrations ranged from 25 g/L versus P. verrucosum and 100 g/L against A. flavus. A total of 27 volatile organic compounds were produced. Moreover, the lactic acid content reached 26 g/kg of dry product, and the phenyllactic concentration was significantly higher than the control. The P. pentosaceus TI6 exhibited a higher antifungal capacity in vitro and demonstrated a higher production of antifungal compounds compared to the other strains; therefore, further studies will evaluate the impact of this sourdough in bread manufacture.

Production of biopolymer precursors beta-alanine and L-lactic acid from CO2 with metabolically versatile Rhodococcus opacus DSM 43205

Hydrogen oxidizing autotrophic bacteria are promising hosts for conversion of CO₂ into chemicals. In this work, we engineered the metabolically versatile lithoautotrophic bacterium R. opacus strain DSM 43205 for synthesis of polymer precursors. Aspartate decarboxylase (panD) or lactate dehydrogenase (ldh) were expressed for beta-alanine or L-lactic acid production, respectively. The heterotrophic cultivations on glucose produced 25 mg L⁻¹ beta-alanine and 742 mg L⁻¹ L-lactic acid, while autotrophic cultivations with CO₂, H₂, and O₂ resulted in the production of 1.8 mg L⁻¹ beta-alanine and 146 mg L⁻¹ L-lactic acid. Beta-alanine was also produced at 345 μg L⁻¹ from CO₂ in electrobioreactors, where H₂ and O₂ were provided by water electrolysis. This work demonstrates that R. opacus DSM 43205 can be engineered to produce chemicals from CO₂ and provides a base for its further metabolic engineering.

A Microphysiometric System Based on LAPS for Real-Time Monitoring of Microbial Metabolism

Macronutrients (carbohydrates, fat and protein) are the cornerstones of daily diet, among which carbohydrates provide energy for the muscles and central nervous system during movement and exercise. The breakdown of carbohydrates starts in the oral cavity, where they are primarily hydrolyzed to glucose and then metabolized to organic acids. The end products may have an impact on the oral microenvironment, so it is necessary to monitor the process of microbial metabolism and to measure the pH change. Although a pH meter has been widely used, it is limited by its sensitivity. We then introduce a light addressable potentiometric sensor (LAPS), which has been used in extracellular acidification detection of living cells with the advantages of being objective, quantitative and highly sensitive. However, it is difficult to use in monitoring bacterial metabolism because bacteria cannot be immobilization on the LAPS chip as easily as living cells. Therefore, a microphysiometric system integrated with Transwell insert and microfluidic LAPS chip was designed and constructed to solve this problem. The decrease in pH caused by glucose fermentation in Lactobacillus rhamnosus was successfully measured by this device. This proves the feasibility of the system for metabolism detection of non-adhere targets such as microorganisms and even 3D cells and organoids.

Optimization of Lactic Acid Production from Pineapple By-Products and an Inexpensive Nitrogen Source Using Lactiplantibacillus plantarum strain 4O8

Lactic acid (LA) is used in food, cosmetic, chemical, and pharmaceutical industries and has recently attracted much attention in the production of biodegradable polymers. The expensive substances including carbon and nitrogen sources involved in its fermentative synthesis and the increasing market demand of LA have prompted scientists to look for inexpensive raw materials from which it can be produced. This research was aimed at determining the optimum conditions of lactic acid (LA) production from pineapple by-products and an inexpensive nitrogen source using Lactiplantibacillus plantarum strain 4O8. After collection and preparation of the carbon source (pineapple by-products) and nitrogen sources (by-products from fish, chicken, and beer brewing industries), they were used for the formulation of 4 different media in terms of nitrogen sources. Then, the proximate compositions of promising nitrogen sources were determined. This was followed by the screening of factors (temperature, carbon source, nitrogen source, MgSO₄, MnSO₄, FeSO₄, KH₂PO₄, and KHPO₄) influencing the production of LA using the definitive plan. Lastly, the optimization process was done using the central composite design. The highest LA productions (14.64 ± 0.05 g/l and 13.4 ± 0.02 g/l) were obtained in production medium supplemented with chicken and fish by-products, respectively, making them the most promising sources of nitrogen. The proximate analysis of these nitrogen sources revealed that their protein contents were 83.00 ± 1.41% DM and 74.00 ± 1.41% DM for chicken by-products and fish by-products, respectively. Concerning the screening of factors, temperature, nitrogen source, and carbon source were the factors that showed a major impact on LA production in the production medium containing chicken by-products as nitrogen source. A pineapple by-product concentration of 141.75 g/l, a nitrogen source volume of 108.99 ml/l, and a temperature of 30.89°C were recorded as the optimum conditions for LA production. The optimization led to a 2.73-fold increase in LA production when compared with the production medium without nitrogen source. According to these results, chicken by-products are a promising and an inexpensive nitrogen source that can be an alternative to yeast extract in lactic acid production.

Lactic acid production by Carnobacterium sp. isolated from a maritime Antarctic lake using eucalyptus enzymatic hydrolysate

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Eucalyptus enzymatic hydrolysate contained mixture of glucose and xylose sugars.

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Carnobacterium sp. produced lactic acid from eucalyptus enzymatic hydrolysate.

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Glucose was consumed faster and completely rather than xylose during fermentation.

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Fed-batch fermentation improved lactic acid production and reached 30 g/L.

Carnobacterium sp., a lactic acid bacterium isolated from a maritime Antarctic lake, was evaluated for lactic acid production from a lignocellulosic hydrolysate. Eucalyptus sawdust, a residue from pulp and paper industries, was subjected to alkaline pretreatment to enhance its enzymatic hydrolysis. Fermentations were performed without and with pH control using eucalyptus enzymatic hydrolysate containing a mixture of glucose and xylose sugars. The sugars were successfully converted into lactic acid in 24 h, resulting in 7.6 g/L of lactic acid and a product yield of 0.50 g/g for pH controlled at 6.5. Fed-batch fermentation performed at a controlled pH of 6.5 improved both the lactic acid production (30 g/L) and the biomass growth (4.2 g/L). l-lactic acid optical purity higher than 95 % was obtained. These results demonstrated the potential usage of Carnobacterium sp in l-lactic acid production from eucalyptus.

Fermentation study of Cassava bagasse starch hydrolyzed's using INIAP 650 and INIAP 651 varieties and a strain of Lactobacillus leichmannii for the lactic acid production

Ecuador is an agricultural country, which has an actual production of starch obtained from cassava. Tuber processing residues do not have an economic impact; for example, the cassava bagasse, only used for plant fertilization and animal feeding. This project aimed to study the influence of the fermentation variables (pH and agitation), on the lactic acid production. Enzymatic hydrolysis of cassava bagasse starch for the varieties INIAP 650 and INIAP 651 was performed using α-amylase and glucoamylase. Then, glucose was fermented by Lactobacillus leichmannii ATCC 7830 strains, varying conditions such as agitation (150 rpm and absence) and pH (4.5, 5.0, and 5.5). Finally, the determination of lactic acid was performed by potentiometric and FTIR analysis. Conversions of cassava bagasse to reduced sugars were 71.66 and 85.05 % for INIAP 650 and INIAP 651 varieties. The best lactic acid concentrations were 27.62 and 33.48 g/L, obtained at pH 5.5 and agitation, for INIAP 650 and INIAP 651 varieties. Qualitative analysis conducted by FTIR spectrophotometry confirmed the presence of lactic acid in the reacted products. Lactic acid production from cassava bagasse starch could contribute to the Manabí and Esmeraldas provinces of Ecuador's economic development.

Metabolism Characteristics of Lactic Acid Bacteria and the Expanding Applications in Food Industry

Lactic acid bacteria are a kind of microorganisms that can ferment carbohydrates to produce lactic acid, and are currently widely used in the fermented food industry. In recent years, with the excellent role of lactic acid bacteria in the food industry and probiotic functions, their microbial metabolic characteristics have also attracted more attention. Lactic acid bacteria can decompose macromolecular substances in food, including degradation of indigestible polysaccharides and transformation of undesirable flavor substances. Meanwhile, they can also produce a variety of products including short-chain fatty acids, amines, bacteriocins, vitamins and exopolysaccharides during metabolism. Based on the above-mentioned metabolic characteristics, lactic acid bacteria have shown a variety of expanded applications in the food industry. On the one hand, they are used to improve the flavor of fermented foods, increase the nutrition of foods, reduce harmful substances, increase shelf life, and so on. On the other hand, they can be used as probiotics to promote health in the body. This article reviews and prospects the important metabolites in the expanded application of lactic acid bacteria from the perspective of bioengineering and biotechnology.

Systematic engineering of Saccharomyces cerevisiae for D-lactic acid production with near theoretical yield

D-lactic acid is a chiral three-carbon organic acid that can improve the thermostability of polylactic acid. Here, we systematically engineered Saccharomyces cerevisiae to produce D-lactic acid from glucose, a renewable carbon source, at near theoretical yield. Specifically, we screened D-lactate dehydrogenase (DLDH) variants from lactic acid bacteria in three different genera and identified the Leuconostoc pseudomesenteroides variant (LpDLDH) as having the highest activity in yeast. We then screened single-gene deletions to minimize the production of the side products ethanol and glycerol as well as prevent the conversion of D-lactic acid back to pyruvate. Based on the results of the DLDH screening and the single-gene deletions, we created a strain called ASc-d789M which overexpresses LpDLDH and contains deletions in glycerol pathway genes GPD1 and GPD2 and lactate dehydrogenase gene DLD1, as well as downregulation of ethanol pathway gene ADH1 using the L-methionine repressible promoter to minimize impact on growth. ASc-d789M produces D-lactic acid at a titer of 17.09 g/L in shake-flasks (yield of 0.89 g/g glucose consumed or 89% of the theoretical yield). Fed-batch fermentation resulted in D-lactic acid titer of 40.03 g/L (yield of 0.81 g/g glucose consumed). Altogether, our work represents progress towards efficient microbial production of D-lactic acid.

Production of 1,3-propanediol and lactic acid from crude glycerol by a microbial consortium from intertidal sludge

OBJECTIVES

To select a microbial consortium from intertidal sludge and evaluate its ability to convert crude glycerol from biodisel to high value-added products such as 1,3-propanediol (1,3-PDO) and lactic acid (LA).

RESULTS

A microbial consortium named CJD-S was selected from intertidal sludge and exhibited excellent performance for the conversion of crude glycerol to 1,3-PDO and LA. The composition of CJD-S was determined to be 85.99% Enterobacteriaceae and 13.75% Enterococcaceae by 16S rRNA gene amplicon high-throughput sequencing. In fed-batch fermentation with crude glycerol under nonsterile conditions, the highest concentrations of 1,3-PDO and LA were 41.47 g/L and 45.86 g/L, respectively.

CONCLUSIONS

The selected microbial consortium, CJD-S, effectively converted crude glycerol to 1,3-PDO and LA under nonsterile conditions and can contribute to the sustainable development of the biodiesel industry.

Limited oxygen conditions as an approach to scale-up and improve D and L-lactic acid production in mineral media and avocado seed hydrolysates with metabolically engineered Escherichia coli

The effectiveness of micro-aeration on lactate (LA) production by metabolically engineered Escherichia coli was evaluated in 1 L bioreactors containing mineral media and glucose (70 g/L). Volumetric oxygen transfer coefficients (k_La) between 12.6 and 28.7 h^-1 increased the specific growth rate (µ) and volumetric productivity (Q_LA) by 300 and 400%, respectively, without a significant decrease in lactate yield (Y_LA), when compared with non-aerated fermentations. A k_La of 12.6 h^-1 was successfully used as a criterion to scale-up the production of L and D-lactate from 1 to 11 and 130 L. Approximately constant Q_LA and Y_LA values were obtained throughout the fermentation scale-up process. Furthermore, a D-lactogenic fermentation was carried out in 1 L bioreactors using avocado seed hydrolysate as a culture medium under the same k_La value, displaying high Q_LA and Y_LA.

Efficient two-step lactic acid production from cassava biomass using thermostable enzyme cocktail and lactic acid bacteria: insights from hydrolysis optimization and proteomics analysis

Lactic acid is an intermediate-volume specialty chemical, used in the production of biodegradable polymers and other chemicals. Although lactic acid production process is well established, however, the cost of production is very high. Therefore, in this study; starchy biomass (cassava) was hydrolyzed with in-house enzyme cocktail prepared from Aspergillus foetidus MTCC508 and Bacillus subtilis RA10. Process optimization using Taguchi experimental design helped to optimize the most effective ratio of fungal and bacterial amylase for effective saccharification of cassava. A higher sugar yield of 379.63 mg/gds was obtained under optimized conditions, using 30 U/gds of bacterial enzyme and 90 U/gds of the fungal enzyme at pH 4 within 48 h of saccharification. Among 11 lactic acid bacteria isolated, Lactobacillus fermentum S1A and Lactobacillus farraginis SS3A produced the highest amount of lactic acid 0.81 g/g and 0.77 g/g, respectively, from the cassava hydrolysate. The study proved the potential renewable source of cassava biomass as a source for fermentable sugars that can be fermented to lactic acid with high yield. In future, this cost-effective and environmental-friendly bioprocess can be upscaled for industrial lactic acid production.

Techno-Economic Analysis of Bio-Based Lactic Acid Production Utilizing Corn Grain as Feedstock

Lactic acid is an important chemical with numerous commercial applications that can be fermentatively produced from biological feedstocks. Producing lactic acid from corn grain could complement the use of already existing infrastructure for corn grain-based ethanol production with a higher value product. The objective of this study was to evaluate the techno-economic feasibility of producing 100,000 metric tons (t) of lactic acid annually from corn grain in a biorefinery. The study estimated the resources (equipment, raw materials, energy, and labor) requirements and costs to produce lactic acid from bacteria, fungi and yeast-based fermentation pathways. Lactic acid production costs were $1181, $1251 and $844, for bacteria, fungi and yeast, respectively. Genetically engineered yeast strains capable of producing lactic acid at low pH support significantly cheaper processes because they do not require simultaneous neutralization and recovery of lactic acid, resulting in lower requirements for chemical, equipment, and utilities. Lactic acid production costs were highly sensitive to sugar-to-lactic-acid conversion rates, grain price, plant size, annual operation hours, and potential use of gypsum. Improvements in process efficiencies and lower equipment and chemical costs would further reduce the cost of lactic acid production from corn grain.

Production of D-Lactic Acid by the Fermentation of Orange Peel Waste Hydrolysate by Lactic Acid Bacteria

Lactic acid is one the most interesting monomer candidates to replace some petroleum- based monomers. The application of conventional poly-lactic acid (PLA) is limited due to insufficient thermal properties. This limitation can be overcome by blending poly-D and poly-L-lactic acid. The main problem is the limited knowledge of D-lactic acid (D-LA) production. Efficient biochemical processes are being developed in order to synthesize D-LA from orange peel waste (OPW). OPW is an interesting renewable raw material for biorefinery processes of biocatalytic, catalytic or thermal nature owing to its low lignin and ash content. Bioprocessing of the pretreated OPW is carried out by enzymatic hydrolysis and fermentation of the released sugars to produce D-LA. Several strains of the species Lactobacillus delbrueckii ssp. bulgaricus have been evaluated for the production of D-LA from OPW hydrolysate using Lactobacillus delbrueckii ssp. delbrueckii CECT 286 as a reference strain since its performance in this kind of substrate have been widely reported in previous studies. Preliminary results show that Lactobacillus delbrueckii ssp. bulgaricus CECT 5037 had the best performance with a yield of 84% w/w for D-LA production and up to 95% (e.e.).

Efficient production of d-lactate from methane in a lactate-tolerant strain of Methylomonas sp. DH-1 generated by adaptive laboratory evolution

BACKGROUND

Methane, a main component of natural gas and biogas, has gained much attention as an abundant and low-cost carbon source. Methanotrophs, which can use methane as a sole carbon and energy source, are promising hosts to produce value-added chemicals from methane, but their metabolic engineering is still challenging. In previous attempts to produce lactic acid (LA) from methane, LA production levels were limited in part due to LA toxicity. We solved this problem by generating an LA-tolerant strain, which also contributes to understanding novel LA tolerance mechanisms.

RESULTS

In this study, we engineered a methanotroph strain Methylomonas sp. DH-1 to produce d-lactic acid (d-LA) from methane. LA toxicity is one of the limiting factors for high-level production of LA. Therefore, we first performed adaptive laboratory evolution of Methylomonas sp. DH-1, generating an LA-tolerant strain JHM80. Genome sequencing of JHM80 revealed the causal gene watR, encoding a LysR-type transcription factor, whose overexpression due to a 2-bp (TT) deletion in the promoter region is partly responsible for the LA tolerance of JHM80. Overexpression of the watR gene in wild-type strain also led to an increase in LA tolerance. When d form-specific lactate dehydrogenase gene from Leuconostoc mesenteroides subsp. mesenteroides ATCC 8293 was introduced into the genome while deleting the glgA gene encoding glycogen synthase, JHM80 produced about 7.5-fold higher level of d-LA from methane than wild type, suggesting that LA tolerance is a critical limiting factor for LA production in this host. d-LA production was further enhanced by optimization of the medium, resulting in a titer of 1.19 g/L and a yield of 0.245 g/g CH4.

CONCLUSIONS

JHM80, an LA-tolerant strain of Methylomonas sp. DH-1, generated by adaptive laboratory evolution was effective in LA production from methane. Characterization of the mutated genes in JHM80 revealed that overexpression of the watR gene, encoding a LysR-type transcription factor, is responsible for LA tolerance. By introducing a heterologous lactate dehydrogenase gene into the genome of JHM80 strain while deleting the glgA gene, high d-LA production titer and yield were achieved from methane.

Biotechnological Production of Organic Acids from Renewable Resources

Biotechnological processes are promising alternatives to petrochemical routes for overcoming the challenges of resource depletion in the future in a sustainable way. The strategies of white biotechnology allow the utilization of inexpensive and renewable resources for the production of a broad range of bio-based compounds. Renewable resources, such as agricultural residues or residues from food production, are produced in large amounts have been shown to be promising carbon and/or nitrogen sources. This chapter focuses on the biotechnological production of lactic acid, acrylic acid, succinic acid, muconic acid, and lactobionic acid from renewable residues, these products being used as monomers for bio-based material and/or as food supplements. These five acids have high economic values and the potential to overcome the "valley of death" between laboratory/pilot scale and commercial/industrial scale. This chapter also provides an overview of the production strategies, including microbial strain development, used to convert renewable resources into value-added products.

Synthesis of Acrylonitrile from Renewable Lactic Acid

Acrylonitrile (ACN) is widely used as monomer in the synthesis of polymers and carbon fibers. Nowadays, its production is based on fossil resources. Herein, an alternative process based on renewable resources is presented. Lactic acid (LA), which can be obtained by fermentation of biomass, was converted to ACN in two steps with an overall selectivity of 57 %. In the first step, a direct amidation of LA in the presence of water was conducted at 230 °C. Zeolites can catalyze the formation of lactamide, and a selectivity of 92 % was reached at 33 % conversion with NH₄ -ZSM-5. In the second step, the dehydration of lactamide to ACN was performed with acetic anhydride, and an ACN selectivity of 62 % was achieved at full conversion.

An integrated process for the production of 1,3-propanediol, lactate and 3-hydroxypropionic acid by an engineered Lactobacillus reuteri

The process economy of food grade 1,3-propanediol (1,3-PD) production by GRAS organisms like Lactobacillus reuteri (L. reuteri), is negatively impacted by the low yield and use of expensive feedstocks. In order to improve the process economy, we have developed a multiproduct process involving the production of three commercially important chemicals, namely, 1,3-PD, lactate and 3-Hydroxypropionic acid (3-HP), by engineered L. reuteri. The maximum 1,3-PD and lactate titer of 41 g/L and 31 g/L, with a volumetric productivity of 1.69 g/L/h and 0.67 g/L/h were achieved, respectively. The maximum 3-HP titer of 5.2 g/L with a volumetric productivity of 1.3 g/L/h, was obtained by biotransformation using cells recovered from the repeated fed-batch process. The volumetric productivity of 1,3-PD obtained in this study is the highest ever reported for this organism. Further cost reduction can be achieved by using waste feedstocks like milk whey, biomass hydrolysate, and crude glycerol.

Production of d-Lactate from Avocado Seed Hydrolysates by Metabolically Engineered Escherichia coli JU15

Agroindustry residues can be used to produce valuable chemicals such as lactic acid, which is a primary chemical platform with many industrial applications. Biotechnological processes are the main approach of lactic acid production; however, culture media has an important impact on their costs. As a result, researchers are exploring various methods of production that use residual or waste biomass as raw materials, most of which are rich in lignocellulose. Nevertheless, starch and micronutrients such as those contained in avocado seeds stand out as promising feedstock for the bioprocess as well. In this study, the lactogenic Escherichia coli strain JU15 was evaluated for producing d-lactate using an avocado seed hydrolysate medium in a controlled stirred-tank bioreactor. The highest lactic acid concentration achieved was 37.8 g L−1 using 120 g L−1 as the content of initial reducing sugars. The results showed that d-lactate can be produced from avocado seed, which hydrolysates to 0.52 g L−1 h−1 using the engineered E. coli JU15. This study may serve as a starting point to further develop bioprocesses for producing metabolites using avocado seed hydrolysates.

Enhanced Production of D-Lactate in Cyanobacteria by Re-Routing Photosynthetic Cyclic and Pseudo-Cyclic Electron Flow

Cyanobacteria are promising chassis strains for the photosynthetic production of platform and specialty chemicals from carbon dioxide. Their efficient light harvesting and metabolic flexibility abilities have allowed a wide range of biomolecules, such as the bioplastic polylactate precursor D-lactate, to be produced, though usually at relatively low yields. In order to increase photosynthetic electron flow towards the production of D-lactate, we have generated several strains of the marine cyanobacterium Synechococcus sp. PCC 7002 (Syn7002) with deletions in genes involved in cyclic or pseudo-cyclic electron flow around photosystem I. Using a variant of the Chlamydomonas reinhardtii D-lactate dehydrogenase (LDH^SRT, engineered to efficiently utilize NADPH in vivo), we have shown that deletion of either of the two flavodiiron flv homologs (involved in pseudo-cyclic electron transport) or the Syn7002 pgr5 homolog (proposed to be a vital part of the cyclic electron transport pathway) is able to increase D-lactate production in Syn7002 strains expressing LDH^SRT and the Escherichia coli LldP (lactate permease), especially at low temperature (25°C) and 0.04% (v/v) CO₂, though at elevated temperatures (38°C) and/or high (1%) CO₂ concentrations, the effect was less obvious. The Δpgr5 background seemed to be particularly beneficial at 25°C and 0.04% (v/v) CO₂, with a nearly 7-fold increase in D-lactate accumulation in comparison to the wild-type background (≈1000 vs ≈150 mg/L) and decreased side effects in comparison to the flv deletion strains. Overall, our results show that manipulation of photosynthetic electron flow is a viable strategy to increase production of platform chemicals in cyanobacteria under ambient conditions.

High-throughput screening of high lactic acid-producing Bacillus coagulans by droplet microfluidic based flow cytometry with fluorescence activated cell sorting

A high-throughput screening system based on droplet microfluidic sorting was developed and employed for screening of high lactic acid-producing Bacillus coagulans. In this system, water-in-oil-in-water (W/O/W) droplets, which were ∼12 pL in volume were used as picoliter-reactors for lactic acid fermentation. A fluorescent sensor was developed and used for monitoring pH which indicated the production of lactic acid. After fermentation, fluorescence activated cell sorting was performed with high sensitivity and speed. Using this microfluidic high-throughput screening system, we found a mutant with a yield of 76 g L⁻¹ lactic acid which was 52% higher than its parent strain with a screening throughput exceeding 10⁶ clones per h.

A high-throughput screening system based on droplet microfluidic sorting was developed and employed for screening of high lactic acid-producing Bacillus coagulans.

High-cell density culture of poly(lactate-co-3-hydroxybutyrate)-producing Escherichia coli by using glucose/xylose-switching fed-batch jar fermentation

Poly(lactate-co-3-hydroxybutyrate) [P(LA-co-3HB)] is produced in engineered Escherichia coli harboring the genes encoding an LA-polymerizing enzyme (LPE) and monomer-supplying enzymes. In this study, high cell-density fed-batch jar fermentation was developed using xylose and/or glucose as the carbon source. Fed-batch fermentation was initially performed with 20 g/L sugar during the batch phase for 24 h, and subsequent sugar feeding from 24 to 86 h. The feeding rate was increased in a stepwise manner. When xylose alone was used for cultivation, the cells produced the polymer at 11.6 g/L, which was higher than the 4.3 g/L obtained using glucose as the sole carbon source. However, in the first 24 h the growth in the glucose culture was greater than in the xylose culture. Based on these results, glucose was used for cell growth (at the initial stage) and xylose was used for polymer production (at the feeding stage). As expected, in the glucose/xylose switching fermentation method, polymer production was significantly enhanced, eventually reaching 26.7 g/L. The enhanced polymer production obtained by using xylose was presumably due to overflow metabolism. In fact, during xylose feeding, acetic acid excretion was greater than that in case of the glucose grown culture, suggesting the channeling of the metabolic flux from acetyl-CoA towards polymer production over into the tricarboxylic acid cycle in the xylose-fed cultures. Therefore, this sequential glucose/xylose feed strategy is potentially useful for production of acetyl-CoA derived compounds in E. coli.

Biogas Biocatalysis: Methanotrophic Bacterial Cultivation, Metabolite Profiling, and Bioconversion to Lactic Acid

Anaerobic digestion (AD) of waste substrates, and renewable biomass and crop residues offers a means to generate energy-rich biogas. However, at present, AD-derived biogas is primarily flared or used for combined heat and power (CHP), in part due to inefficient gas-to-liquid conversion technologies. Methanotrophic bacteria are capable of utilizing methane as a sole carbon and energy source, offering promising potential for biological gas-to-liquid conversion of AD-derived biogas. Here, we report cultivation of three phylogenetically diverse methanotrophic bacteria on biogas streams derived from AD of a series of energy crop residues. Strains maintained comparable central metabolic activity and displayed minimal growth inhibition when cultivated under batch configuration on AD biogas streams relative to pure methane, although metabolite analysis suggested biogas streams increase cellular oxidative stress. In contrast to batch cultivation, growth arrest was observed under continuous cultivation configuration, concurrent with increased biosynthesis and excretion of lactate. We examined the potential for enhanced lactate production via the employ of a pyruvate dehydrogenase mutant strain, ultimately achieving 0.027 g lactate/g DCW/h, the highest reported lactate specific productivity from biogas to date.

Production of D-lactic acid by Lactobacillus delbrueckii ssp. delbrueckii from orange peel waste: techno-economical assessment of nitrogen sources

In this study, the effect of several organic nitrogen sources (namely peptone, meat extract-ME, yeast extract-YE, and corn steep liquor-CSL) on D-lactic acid production by Lactobacillus delbrueckii ssp. delbrueckii has been studied. While lactic acid bacteria (LAB) are well-known for their complex nutritional requirements, organic nitrogen source-related cost can be as high as 38% of total operational costs (OPEX), being its nature and concentration critical factors in the growth and productivity of the selected strain. Corn steep liquor (CSL) has been chosen for its adequacy, on the grounds of the D-lactic acid yield, productivity, and its cost per kilogram of product. Finally, orange peel waste hydrolysate supplemented with 37 g/l CSL has been employed for D-lactic acid production, reaching a final yield of 88% and a productivity of 2.35 g/l h. CSL cost has been estimated at 90.78$/ton of D-lactate.

Effective lactic acid production from waste paper using Streptococcus thermophilus at low enzyme loading assisted by Gleditsia saponin

Waste paper has considerable potential as a raw material for lactic acid (LA) production due to high cellulose content, abundance and low cost. In this study, four kinds of waste papers were used for LA production through simultaneous saccharification and fermentation (SSF) by Streptococcus thermophilus. The SSF of office paper achieved the highest LA concentration (39.71 g/L), while the highest LA yield was observed for magazine (99.56%), followed by office paper (82.85%). High LA concentration is unfavorable to total LA conversion because of product inhibition. However, the addition of Gleditsia saponin (GS) could obtain both high yield and high concentration of LA at a low enzyme loading, indicating that product inhibition could be moderated. A lactic acid yield of 86.30% was obtained from office paper at an enzyme loading of 9 FPU/g-cellulose with GS, which was higher than that of without GS at a higher loading of 18 FPU/g-cellulose.

An optimized fed-batch culture strategy integrated with a one-step fermentation improves L-lactic acid production by Rhizopus oryzae

In previous work, we proposed a novel modified one-step fermentation fed-batch strategy to efficiently generate L-lactic acid (L-LA) using Rhizopus oryzae. In this study, to further enhance efficiency of L-LA production through one-step fermentation in fed-batch cultures, we systematically investigated the initial peptone- and glucose-feeding approaches, including different initial peptone and glucose concentrations and maintained residual glucose levels. Based on the results of this study, culturing R. oryzae with initial peptone and glucose concentrations of 3.0 and 50.0 g/l, respectively, using a fed-batch strategy is an effective approach of producing L-LA through one-step fermentation. Changing the residual glucose had no obvious effect on the generation of L-LA. We determined the maximum LA production and productivity to be 162 g/l and 6.23 g/(l·h), respectively, during the acid production stage. Compared to our previous work, there was almost no change in L-LA production or yield; however, the productivity of L-LA increased by 14.3%.

Future insights in fungal metabolic engineering

Filamentous fungi exhibit versatile abilities, including organic acid fermentation, protein production, and secondary metabolism, amongst others, and thus have applications in the medical and food industries. Previous genomic analyses of several filamentous fungi revealed their further potential as host microorganisms for bioproduction. Recent advancements in molecular genetics, marker recycling, and genome editing could be used to alter transformation and metabolism, based on optimized design carbolated with computer science. In this review, we detail the current applications of filamentous fungi and describe modern molecular genetic tools that could be used to expand the role of these microorganisms in bioproduction. The present review shed light on the possibility of filamentous fungi as host microorganisms in the field of bioproduction in the future.

Potential prebiotic properties of cashew apple (Anacardium occidentale L.) agro-industrial byproduct on Lactobacillus species

BACKGROUND

The prebiotic effects of a cashew apple (Anacardium occidentale L.) agro-industrial byproduct powder (CAP) on different potentially probiotic Lactobacillus strains, namely Lactobacillus acidophilus LA-05, Lactobacillus casei L-26 and Lactobacillus paracasei L-10, were assessed using in vitro experimental models. Accordingly, the growth of the Lactobacillus strains when cultivated in a broth containing CAP (20 or 30 g L^-1 ), glucose (20 g L^-1 ) or fructooligosaccharides (FOS) (20 g L^-1 ) was monitored over 48 h; the prebiotic activity scores of CAP were determined; and the changes in pH values, production of organic acids and consumption of sugars in growth media were verified.

RESULTS

During the 48-h cultivation, similar viable cell counts were observed for the Lactobacillus strains grown in the different media tested. The CAP presented positive prebiotic activity scores toward all the tested Lactobacillus strains, indicating a desirable selective fermentable activity relative to enteric organisms. The cultivation of the Lactobacillus strains in broth containing glucose, FOS or CAP resulted in high viable cell counts, a decreased pH, the production of organic acids and the consumption of sugars over time, revealing intense bacterial metabolic activity.

CONCLUSION

The CAP exerts potential prebiotic effects on different potentially probiotic Lactobacillus strains and should be an added-value ingredient for the food industry. © 2017 Society of Chemical Industry.

Production of fuels and chemicals from xylose by engineered Saccharomyces cerevisiae: a review and perspective

Efficient xylose utilization is one of the most important pre-requisites for developing an economic microbial conversion process of terrestrial lignocellulosic biomass into biofuels and biochemicals. A robust ethanol producing yeast Saccharomyces cerevisiae has been engineered with heterologous xylose assimilation pathways. A two-step oxidoreductase pathway consisting of NAD(P)H-linked xylose reductase and NAD⁺-linked xylitol dehydrogenase, and one-step isomerase pathway using xylose isomerase have been employed to enable xylose assimilation in engineered S. cerevisiae. However, the resulting engineered yeast exhibited inefficient and slow xylose fermentation. In order to improve the yield and productivity of xylose fermentation, expression levels of xylose assimilation pathway enzymes and their kinetic properties have been optimized, and additional optimizations of endogenous or heterologous metabolisms have been achieved. These efforts have led to the development of engineered yeast strains ready for the commercialization of cellulosic bioethanol. Interestingly, xylose metabolism by engineered yeast was preferably respiratory rather than fermentative as in glucose metabolism, suggesting that xylose can serve as a desirable carbon source capable of bypassing metabolic barriers exerted by glucose repression. Accordingly, engineered yeasts showed superior production of valuable metabolites derived from cytosolic acetyl-CoA and pyruvate, such as 1-hexadecanol and lactic acid, when the xylose assimilation pathway and target synthetic pathways were optimized in an adequate manner. While xylose has been regarded as a sugar to be utilized because it is present in cellulosic hydrolysates, potential benefits of using xylose instead of glucose for yeast-based biotechnological processes need to be realized.

Construction of a novel d-lactate producing pathway from dihydroxyacetone phosphate of the Calvin cycle in cyanobacterium, Synechococcus elongatus PCC 7942

Using engineered cyanobacteria to produce various chemicals from carbon dioxide is a promising technology for a sustainable future. Lactate is a valuable commodity that can be used for the biodegradable plastic, polylactic acid. Typically, lactate production using engineered cyanobacteria was via the conversion of pyruvate in glycolysis by lactate dehydrogenase. In cyanobacteria, the metabolic flux in the Calvin cycle is higher than that in glycolysis under photoautotrophic conditions. The construction of a novel lactate producing pathway that uses metabolites from the Calvin cycle could potentially increase lactate productivity in cyanobacteria. In order to develop such a novel lactate production pathway, we engineered a cyanobacterium Synechococcus elongatus PCC 7942 strain that produced lactate directly from carbon dioxide using dihydroxyacetone phosphate (DHAP) via methylglyoxal. We confirmed that wild-type strain of S. elongatus PCC 7942 could produce lactate using exogenous methylglyoxal. A methylglyoxal synthase gene, mgsA, from Escherichia coli was introduced into Synechococcus elongates PCC 7942 for conversion of DHAP to methylglyoxal. This engineered strain produced lactate directly from carbon dioxide. Genes encoding intrinsic putative glyoxalase I, II (Synpcc7942_0638, 1403) and the lactate/H⁺ symporter from E. coli (lldP) were additionally introduced to enhance the production. For higher lactate production, it was important to maintain elevated extracellular pH due to the characteristics of lactate exporting system. In this study, the highest lactate titer of 13.7 mM (1.23 g/l) was achieved during a 24-day incubation with the engineered S. elongatus PCC 7942 strain possessing the novel lactate producing pathway.

Semi-industrial scale (30 m3) fed-batch fermentation for the production of D-lactate by Escherichia coli strain HBUT-D15

D(-)-lactic acid is needed for manufacturing of stereo-complex poly-lactic acid polymer. Large scale D-lactic acid fermentation, however, has yet to be demonstrated. A genetically engineered Escherichia coli strain, HBUT-D, was adaptively evolved in a 15% calcium lactate medium for improved lactate tolerance. The resulting strain, HBUT-D15, was tested at a lab scale (7 L) by fed-batch fermentation with up to 200 g L^-1 of glucose, producing 184-191 g L^-1 of D-lactic acid, with a volumetric productivity of 4.38 g L^-1 h^-1, a yield of 92%, and an optical purity of 99.9%. The HBUT-D15 was then evaluated at a semi-industrial scale (30 m³) via fed-batch fermentation with up to 160 g L^-1 of glucose, producing 146-150 g L^-1 of D-lactic acid, with a volumetric productivity of 3.95-4.29 g L^-1 h^-1, a yield of 91-94%, and an optical purity of 99.8%. These results are comparable to that of current industrial scale L(+)-lactic acid fermentation.

A common bacterial metabolite elicits prion-based bypass of glucose repression

Robust preference for fermentative glucose metabolism has motivated domestication of the budding yeast Saccharomyces cerevisiae. This program can be circumvented by a protein-based genetic element, the [GAR⁺] prion, permitting simultaneous metabolism of glucose and other carbon sources. Diverse bacteria can elicit yeast cells to acquire [GAR⁺], although the molecular details of this interaction remain unknown. Here we identify the common bacterial metabolite lactic acid as a strong [GAR⁺] inducer. Transient exposure to lactic acid caused yeast cells to heritably circumvent glucose repression. This trait had the defining genetic properties of [GAR⁺], and did not require utilization of lactic acid as a carbon source. Lactic acid also induced [GAR⁺]-like epigenetic states in fungi that diverged from S. cerevisiae ~200 million years ago, and in which glucose repression evolved independently. To our knowledge, this is the first study to uncover a bacterial metabolite with the capacity to potently induce a prion.

DOI:http://dx.doi.org/10.7554/eLife.17978.001

We communicate with each other using speech, writing and physical gestures. But how do bacteria, yeast and other single-celled microbes communicate? In 2014, researchers reported a new example of communication between bacteria and yeast in which the bacteria send a chemical message that has a very long-lasting effect on how the yeast grow in certain environments. This in turn also affected the ability of the bacteria to survive in these environments. The identity of the chemical message produced by the bacteria, however, was not known.

Garcia, Dietrich et al. – including one of the researchers from the previous study – used biochemical and genetic approaches to identify the chemical message. The experiments show that the message is a molecule called lactic acid, which is very common in nature and is produced by many bacteria. Garcia, Dietrich et al. found out how much lactic acid is needed to alter the growth of brewer’s yeast, and which genes in yeast are involved in responding to the message from the bacteria. Further experiments suggest that the ability of yeast and bacteria to communicate using lactic acid is likely to have existed for hundreds of millions of years.

The next step following this work will be to identify other chemical messages used by microbes. The human body is packed with billions of bacterial cells, and in some cases yeast can also take up residence. A future challenge will be to find out if bacteria and yeast inside the human body are able to communicate with each other in ways that could affect our health.

DOI:http://dx.doi.org/10.7554/eLife.17978.002

Production of d-lactate from sugarcane bagasse and corn stover hydrolysates using metabolic engineered Escherichia coli strains

In this study, the lactogenic Escherichia coli strain JU15 was used and modified to produce d-lactate (d-LA) from plant hydrolysates with a minimal nutrient addition in pH controlled fermenters. Results showed that strain JU15 produces d-LA with high yield and productivity in laboratory simulated hydrolysate media and actual sugar cane bagasse hemicellulosic hydrolysate. Strain JU15 showed sequential carbon source utilization and acetic acid production. The l-lactic and acetic acid production pathways were deleted in JU15, resulting strain AV03 (JU15 ΔpoxB, ΔackA-pta, ΔmgsA), which showed simultaneous consumption of glucose and xylose and no acetic acid production in the simulated hydrolysate. The d-LA yield from hydrolysate sugars was close to 0.95gD-LA/gsugars in all cases. Our results show that d-LA can be produced from plant hydrolysates in simple batch fermentation processes with a high productivity using engineered E. coli strains at fermenter scales from 0.2 up to 10L.

GSF2 deletion increases lactic acid production by alleviating glucose repression in Saccharomyces cerevisiae

Improving lactic acid (LA) tolerance is important for cost-effective microbial production of LA under acidic fermentation conditions. Previously, we generated LA-tolerant D-LA-producing S. cerevisiae strain JHY5310 by laboratory adaptive evolution of JHY5210. In this study, we performed whole genome sequencing of JHY5310, identifying four loss-of-function mutations in GSF2, SYN8, STM1, and SIF2 genes, which are responsible for the LA tolerance of JHY5310. Among the mutations, a nonsense mutation in GSF2 was identified as the major contributor to the improved LA tolerance and LA production in JHY5310. Deletion of GSF2 in the parental strain JHY5210 significantly improved glucose uptake and D-LA production levels, while derepressing glucose-repressed genes including genes involved in the respiratory pathway. Therefore, more efficient generation of ATP and NAD⁺ via respiration might rescue the growth defects of the LA-producing strain, where ATP depletion through extensive export of lactate and proton is one of major reasons for the impaired growth. Accordingly, alleviation of glucose repression by deleting MIG1 or HXK2 in JHY5210 also improved D-LA production. GSF2 deletion could be applied to various bioprocesses where increasing biomass yield or respiratory flux is desirable.

High-efficiency l-lactic acid production by Rhizopus oryzae using a novel modified one-step fermentation strategy

In this study, lactic acid fermentation by Rhizopus oryzae was investigated using the two different fermentation strategies of one-step fermentation (OSF) and conventional fermentation (CF). Compared to CF, OSF reduced the demurrage of the production process and increased the production of lactic acid. However, the qp was significantly lower than during CF. Based on analysis of μ, qs and qp, a novel modified OSF strategy was proposed. This strategy aimed to achieve a high final concentration of lactic acid, and a high qp by R. oryzae. In this strategy, the maximum lactic acid concentration and productivity of the lactic acid production stage reached 158g/l and 5.45g/(lh), which were 177% and 366% higher, respectively, than the best results from CF. Importantly, the qp and yield did not decrease. This strategy is a convenient and economical method for l-lactic acid fermentation by R. oryzae.