Biotechnological production of citric acid

Metabolomic profiling of oxalate-degrading probiotic Lactobacillus acidophilus and Lactobacillus gasseri

Oxalate, a ubiquitous compound in many plant-based foods, is absorbed through the intestine and precipitates with calcium in the kidneys to form stones. Over 80% of diagnosed kidney stones are found to be calcium oxalate. People who form these stones often experience a high rate of recurrence and treatment options remain limited despite decades of dedicated research. Recently, the intestinal microbiome has become a new focus for novel therapies. Studies have shown that select species of Lactobacillus, the most commonly included genus in modern probiotic supplements, can degrade oxalate in vitro and even decrease urinary oxalate in animal models of Primary Hyperoxaluria. Although the purported health benefits of Lactobacillus probiotics vary significantly between species, there is supporting evidence for their potential use as probiotics for oxalate diseases. Defining the unique metabolic properties of Lactobacillus is essential to define how these bacteria interact with the host intestine and influence overall health. We addressed this need by characterizing and comparing the metabolome and lipidome of the oxalate-degrading Lactobacillus acidophilus and Lactobacillus gasseri using ultra-high-performance liquid chromatography-high resolution mass spectrometry. We report many species-specific differences in the metabolic profiles of these Lactobacillus species and discuss potential probiotic relevance and function resulting from their differential expression. Also described is our validation of the oxalate-degrading ability of Lactobacillus acidophilus and Lactobacillus gasseri, even in the presence of other preferred carbon sources, measuring in vitro 14C-oxalate consumption via liquid scintillation counting.

Effects of Solid Content and Substrate Concentration on Bioleaching of Heavy Metals from Sewage Sludge Using Aspergillus niger

The presence of heavy metals in sewage sludge not only affects the performance of sludge anaerobic digestion process but also restricts the land application of treated sewage sludge. Therefore, a fungi-mediated bioleaching process for simultaneous metal leaching and sludge digestion by Aspergillus niger was developed to treat the sewage sludge containing heavy metals in this study. The effects of two important parameters, sludge solid content and substrate (sucrose) concentration, on the performance of fungal bioleaching were investigated in this study. The results showed that the rate of pH reduction increased with increasing sludge solid contents and sucrose concentrations. In this study, the efficiency of metal removal decreases in the order of Mn > Zn > Ni > Pb. The efficiencies of metal leaching and solid degradation (SS and VSS) were found to be decreased with an increase of sludge solid content and a decrease of sucrose concentration. At 2 days of reaction time, the maximum efficiency of metal solubilization was 95, 56, 21 and 13% for Mn, Zn, Ni and Pb, respectively.

Fermentation of Musa paradisiaca Peels to Produce Citric Acid

Among organic acids, citric acid (CA) features the highest production volume and the greatest economic potential. The steadily increasing demand for CA necessitates the improvement and diversification of the corresponding production techniques via the incorporation of more environmentally friendly and less costly processes such as the bioconversion of agroindustrial by-products. Musa paradisiaca, known as plantain, is a food product of global importance; however, the related by-products are scarcely utilized. Herein, we investigate CA production from M. paradisiaca peels via fermentation with Aspergillus niger. Compositional analysis shows that the above peels contain 623 g·kg−1 total carbohydrates, 374 g·kg−1 starch, and 91 g·kg−1 protein and therefore are rather rich in carbon, with other elements contained in substantial amounts corresponding to K (28 g·kg−1), N (10 g·kg−1), Fe (39 mg·kg−1), Na (71 mg·kg−1), Zn (16 mg·kg−1), and Cu (18 mg·kg−1). Evaluation of solid-substrate fermentation conditions (pH and inoculum loading) reveals that CA production is maximized (29 g·kg−1) at 10% consistency, 30°C, pH 1.4, and inoculum loading = 20 mg, demonstrating that pH is the most important parameter determining fermentation efficiency. As a result, M. paradisiaca peels are concluded to be a suitable substrate for CA biosynthesis via fermentation with A. niger under optimal nutritional conditions.

Determination and study on refractive indices and viscosities of aqueous solutions of citric acid, (citric acid + glycerol), and (citric acid + d-sorbitol) at T = 293.15 K-323.15 K and atmospheric pressure

Citric acid, glycerol, and d-sorbitol are used as important food additives. In this research work, viscosities and refractive indices (the physico-chemical properties) for aqueous solution of citric acid, as well as ternary solutions of (water + d-sorbitol + citric acid) and (water + glycerol + citric acid) were measured in mass fractions of citric acid (0.03-0.21) and at temperatures (T = 293.15, 303.15, 313.15, and 323.15) K and atmospheric pressure. For these solutions, the experimental refractive indices were fitted using a semi-empirical equation which its constant, K_r, was introduced by Koohyar in 2011. This constant can be applied to investigate the power of interactions between solute and solvent molecules in aqueous solutions. Also, the experimental viscosities were fitted by the Jones-Dole and Arrhenius-like equations. Obtained data of this research work can be applied in food industries.

Fungi-assisted phytoextraction of lead: tolerance, plant growth-promoting activities and phytoavailability

Lead (Pb) is known for its low mobility and persistence in soils. The main aim of the present study was to explore potential of different fungal strains to promote phytoextraction of Pb-contaminated soils. Five non-pathogenic fungal strains (Trichoderma harzianum, Penicillium simplicissimum, Aspergillus flavus, Aspergillus niger, and Mucor spp.) were tested for their ability to modify soil properties (pH and organic matter) and to increase Pb phytoavailability at varying concentrations. Lead tolerance of fungal strains followed the decreasing order as A. niger > T. harzianum > A. flavus > Mucor sp. > P. simplicissimum. Lead solubility induced by A. flavus and Mucor spp. was increased by 1.6- and 1.8-fold, respectively, as compared to the control soil (Pb added, without fungi). A. flavus and Mucor spp. lowered the soil pH by - 0.14 and - 0.13 units, in soils spiked with 2000 mg Pb kg^-1. The maximum increase in the percentage of organic matter (OM) recorded was 1.7-fold for A. flavus at 500 mg Pb kg^-1 soil. Plant growth-promoting assays confirmed the beneficial role of these fungal strains. Significantly high production of IAA (249 μg mL^-1) and siderophores (61%) was observed with A. niger, and phosphate solubilization with P. simplicissimum (58 μg mL^-1). Based on the results in Pb-contaminated soils, Pelargonium hortorum L. inoculated with Mucor spp. showed the potential to enhance phytoextraction of Pb by promoting Pb phytoavailability in soil and improving plant biomass production through plant growth-promoting activities.

Evaluation of physical and chemical properties of citric acid industrial waste

This study aimed to evaluate physical and chemical properties and nutritive values of citric acid by-product (CABP) from cassava and to compare its properties with those of cassava root meal (CRM). The physical properties analyzed were color, bulk density, angle of repose, particle size distribution, and ultrastructure morphology. The chemical properties were determined using proximate analysis. Regarding the physical results, the CABP's color was darker, and its bulk density was greater by approximately 64.18% than those of the CRM (p < 0.05). The CABP's angle of repose was significantly lower (p < 0.05) with a freer flow, and the particle size was classified as small with fewer polygonal starch granules but more than the CRM. Regarding the chemical composition results, the CABP contained 0.71% citric acid with pH 4.68 whereas crude protein, ether extract, crude fiber, and gross energy were 6.11%, 2.39%, 18.26%, and 3588.10 kcal/kg, respectively. CABP showed greater and significantly different crude proteins and ether extracts but less gross energy than the CRM (p < 0.05). The results imply that the CABP could be an alternative energy source and used as a CRM substitution in animal feed formulation.

Recovery of lanthanides from hydrocarbon cracking spent catalyst through chemical and biotechnological strategies

The aim of this work is to evaluate the rare earth elements (REEs) recovery from fluid catalytic cracking spent catalyst (FCC-SC) by chemical and biochemical strategies while also examining a route for the valorization of biodiesel-derived glycerin (RG), which is presently unprofitable to refine. Recovery tests for REEs were performed with no pretreatment of the FCC-SC. A chemical leaching investigation was carried out using HCl, HNO₃, NaOH, CaCl₂ and citric acid aqueous solutions (1 mol L^-1, at 30, 50, 60 or 70 ± 1 °C). The leaching tests carried out with 1 mol L^-1 citric acid at 50 °C provided the best recovery of La (27%). Subsequent bioleaching tests were carried out with four strains of Yarrowia lipolytica to evaluate their potential to produce organic acids using RG as the main carbon source. The FCC-SC contains some REEs, predominantly La. Remarkable biorecovery rates for REEs (namely, La (53%), Ce and Nd (both 99%)) were achieved using the Y. lipolytica IM-UFRJ 50678 fermented medium at 50 °C. Thus, here, a sustainable approach to recovering metals from spent cracking catalyst using RG under low-cost and non-energy-intensive processing conditions is reported.

History, Current State, and Emerging Applications of Industrial Biotechnology

The past 150 years have seen remarkable discoveries, rapidly growing biological knowledge, and giant technological leaps providing biotechnological solutions for healthcare, food production, and other societal needs. Genetic engineering, miniaturization, and ever-increasing computing power, in particular, have been key technological drivers for the past few decades. Looking ahead, the eventual transition from fossil resources to biomass and CO₂ demands a shift toward a 'bio-economy' based on novel production processes and engineered organisms.

Overview of Gene Expression Using Filamentous Fungi

Filamentous fungi are lower eukaryotes increasingly used for expression of foreign proteins ranging from industrial enzymes originating from other fungi and bacteria to proteins of mammalian origin, such as antibodies and growth factors. Their strengths include an excellent capacity for protein secretion and their eukaryotic protein processing machinery. Proteins secreted from filamentous fungi are modified in the secretory pathway, with folding, proteolytic processing, and addition of glycans being the main modifications. Unlike from many other expression systems, however, plasmids and host strains for expression of gene products in filamentous fungi are not readily available commercially, and the expression system must thus be stitched together in the laboratory. In this overview, the key elements of fungal expression systems are discussed from a practical point of view and with a view towards the future. The principles and considerations presented here can be applied to a range of filamentous fungi. © 2018 by John Wiley & Sons, Inc.

Proteomic characterization of Aspergillus fumigatus isolated from air and surfaces of the International Space Station

The on-going Microbial Observatory Experiments on the International Space Station (ISS) revealed the presence of various microorganisms that may be affected by the distinct environment of the ISS. The low-nutrient environment combined with enhanced irradiation and microgravity may trigger changes in the molecular suite of microorganisms leading to increased virulence and resistance of microbes. Proteomic characterization of two Aspergillus fumigatus strains, ISSFT-021 and IF1SW-F4, isolated from HEPA filter debris and cupola surface of the ISS, respectively, is presented, along with a comparison to well-studied clinical isolates Af293 and CEA10. In-depth analysis highlights variations in the proteome of both ISS-isolated strains when compared to the clinical strains. Proteins that showed increased abundance in ISS isolates were overall involved in stress responses, and carbohydrate and secondary metabolism. Among the most abundant proteins were Pst2 and ArtA involved in oxidative stress response, PdcA and AcuE responsible for ethanol fermentation and glyoxylate cycle, respectively, TpcA, TpcF, and TpcK that are part of trypacidin biosynthetic pathway, and a toxin Asp-hemolysin. This report provides insight into possible molecular adaptation of filamentous fungi to the unique ISS environment.

Aseptic production of citric and isocitric acid from crude glycerol by genetically modified Yarrowia lipolytica

The unconventional yeast Yarrowia lipolytica is known for its capacity to produce citric or isocitric acid from glycerol. In this study a reduction of production cost was achieved by using cheap crude glycerol and conducting the production at pH 3 to prevent bacterial contamination. In this study a Y. lipolytica strain overexpressing Gut1 and Gut2 was used. For the modified strain, crude glycerol proved to be an excellent substrate for production of citric/isocitric acids in aseptic conditions, as the final concentration of these compounds reached 75.9 ± 1.8 g L^-1 after 7 days of batch production. Interestingly, the concentration of isocitric acid was 42.5 ± 2.4 g L^-1, which is one of the highest concentrations of isocitric acid obtained from a waste substrate. In summary, these data show that organic acids can be efficiently produced by the yeast Y. lipolytica from crude glycerol without any prior purification in aseptic conditions.

Genomic and Genetic Insights Into a Cosmopolitan Fungus, Paecilomyces variotii (Eurotiales)

Species in the genus Paecilomyces, a member of the fungal order Eurotiales, are ubiquitous in nature and impact a variety of human endeavors. Here, the biology of one common species, Paecilomyces variotii, was explored using genomics and functional genetics. Sequencing the genome of two isolates revealed key genome and gene features in this species. A striking feature of the genome was the two-part nature, featuring large stretches of DNA with normal GC content separated by AT-rich regions, a hallmark of many plant-pathogenic fungal genomes. These AT-rich regions appeared to have been mutated by repeat-induced point (RIP) mutations. We developed methods for genetic transformation of P. variotii, including forward and reverse genetics as well as crossing techniques. Using transformation and crossing, RIP activity was identified, demonstrating for the first time that RIP is an active process within the order Eurotiales. A consequence of RIP is likely reflected by a reduction in numbers of genes within gene families, such as in cell wall degradation, and reflected by growth limitations on P. variotii on diverse carbon sources. Furthermore, using these transformation tools we characterized a conserved protein containing a domain of unknown function (DUF1212) and discovered it is involved in pigmentation.

pH selectively regulates citric acid and lipid production in Yarrowia lipolytica W29 during nitrogen-limited growth on glucose

Yarrowia lipolytica has been used to produce both citric acid and lipid-based bioproducts at high titers. In this study, we found that pH differentially affects citric acid and lipid production in Y. lipolytica W29, with citric acid production enhanced at more neutral pH's and lipid production enhanced at more acid pH's. To determine the mechanism governing this pH-dependent switch between citric acid and lipid production, we profiled gene expression at different pH's and found that the relative expression of multiple transporters is increased at neutral pH. These results suggest that this pH-dependent switch is mediated at the level of citric acid transport rather than changes in the expression of the enzymes involved in citric acid and lipid metabolism. In further support of this mechanism, thermodynamic calculations suggest that citric acid secretion is more energetically favorable at neutral pH's, assuming the fully protonated acid is the substrate for secretion. Collectively, these results provide new insights regarding citric acid and lipid production in Y. lipolytica and may offer new strategies for metabolic engineering and process design.

Current strategies and future prospects for enhancing microbial production of citric acid

Aspergillus niger and Yarrowia lipolytica are highly important in citric acid (CA) production. To further minimize the cost of CA bio-production using A. niger and Y. lipolytica, some strategies (e.g., metabolic engineering, efficient mutagenesis, and optimal fermentation strategies) were developed to enhance CA production and low-cost carbon sources were also utilized to decrease CA bio-production cost. In this review, we summarize the recent significant progresses in CA bio-production, including metabolic engineering, efficient mutagenesis and screening methods, optimal fermentation strategies, and use of low-cost carbon sources, and future prospects in this field are also discussed, which could help in the development of CA production industry.

The gold-standard genome of Aspergillus niger NRRL 3 enables a detailed view of the diversity of sugar catabolism in fungi

The fungal kingdom is too large to be discovered exclusively by classical genetics. The access to omics data opens a new opportunity to study the diversity within the fungal kingdom and how adaptation to new environments shapes fungal metabolism. Genomes are the foundation of modern science but their quality is crucial when analysing omics data. In this study, we demonstrate how one gold-standard genome can improve functional prediction across closely related species to be able to identify key enzymes, reactions and pathways with the focus on primary carbon metabolism.

Based on this approach we identified alternative genes encoding various steps of the different sugar catabolic pathways, and as such provided leads for functional studies into this topic. We also revealed significant diversity with respect to genome content, although this did not always correlate to the ability of the species to use the corresponding sugar as a carbon source.

Potential role of the common food additive manufactured citric acid in eliciting significant inflammatory reactions contributing to serious disease states: A series of four case reports

Citric acid naturally exists in fruits and vegetables. However, it is not the naturally occurring citric acid, but the manufactured citric acid (MCA) that is used extensively as a food and beverage additive. Approximately 99% of the world's production of MCA is carried out using the fungus Aspergillus niger since 1919. Aspergilus niger is a known allergen. The FDA placed MCA under the category of GRAS without any research to substantiate this claim. In 2016, 2.3 million tons of MCA were produced, predominantly in China, and approximately 70% is used as a food or beverage additive. There have been no scientific studies performed to evaluate the safety of MCA when ingested in substantial amounts and with chronic exposure. We present four case reports of patients with a history of significant and repetitive inflammatory reactions including respiratory symptoms, joint pain, irritable bowel symptoms, muscular pain and enervation following ingestion of foods, beverages or vitamins containing MCA. We believe that ingestion of the MCA may lead to a harmful inflammatory cascade which manifests differently in different individuals based on their genetic predisposition and susceptibility, and that the use of MCA as an additive in consumable products warrants further studies to document its safety.

A label survey to identify ingredients potentially containing GM organisms to estimate intake exposure in Brazil

OBJECTIVE

To identify ingredients from products and by-products derived from GM crops in packaged food products and to analyse the presence of these ingredients in the foods most commonly consumed by the Brazilian population.

DESIGN

Cross-sectional study.

SETTING

A search of the scientific literature to identify the use of products and by-products derived from GM crops in foods in Brazil and a study of food labels in a supermarket belonging to one of the ten largest supermarket chains in Brazil.

SUBJECTS

To identify the ingredients present in packaged food products and their nomenclatures, the labels of all packaged food products available for sale in a supermarket were analysed. Subsequently, the presence of potential GM ingredients in the foods most commonly consumed by the Brazilian population was analysed.

RESULTS

A total of twenty-eight GM crops' by-products with applications in the food industry (from soyabeans, corn, cotton and a yeast) were identified. Such by-products are presented as food ingredients or additives on food labels with 101 distinct nomenclatures. Most of the variety (63·8 %) and the quantity (64·5 %) of the foods most commonly consumed by Brazilians may contain a least one GM ingredient.

CONCLUSIONS

The presence of at least one potential GM ingredient was observed in more than half of the variety of foods most commonly consumed by the Brazilian population. Such ingredients were identified with distinct nomenclatures and incomplete descriptions, which may make it difficult to identify potential GM foods and confuse consumers when making food choices.

Adsorption and Selective Recovery of Citric Acid with Poly(4-vinylpyridine)

Citric acid (CA) is an important organic acid that is produced on a large scale by fermentation. Current methods to recover CA from the fermentation broth require large amounts of chemicals and produce considerable amounts of waste, while not all CA can be recovered. The use of adsorbents can increase the degree of product recovery and reduce chemical consumption and waste generation. In this work, poly(4-vinylpyridine) (PVP) is evaluated as an adsorbent for CA recovery. It has a high adsorption capacity (>30 wt %) at low pH and a high selectivity for CA at moderate pH in the presence of sulfate anions, two conditions that are frequently encountered during CA recovery. PVP could be efficiently regenerated after adsorption using simple alcohols like methanol and ethanol. Considering selectivity and regeneration, PVP distinctly outperforms more common adsorbents for organic acids, including commercial strongly and weakly basic anion exchangers. The desirable adsorptive features of PVP for CA can be attributed to the low basicity of the pyridine group.

Utilization of biodiesel derived-glycerol for 1,3-PD and citric acid production

Today, biofuels represent a hot topic in the context of petroleum and adjacent products decrease. As biofuels production increase, so does the production of their major byproduct, namely crude glycerol. The efficient usage of raw glycerol will concur to the biodiesel viability. As an inevitable waste of biodiesel manufacturing, glycerol is potentially an attractive substrate for the production of value-added products by fermentation processes, due to its large amounts, low cost and high degree of reduction. One of the most important usages of glycerol is its bioconversion through microbial fermentation to value-added materials like 1,3-propanediol and citric acid. There is a considerable industrial interest in 1,3-propanediol and citric acid production based on microbial fermentations, as it seems to be in competition with traditional technologies utilized for these products. In the present work, yields and concentrations of 1,3-propanediol and citric acid registered for different isolated strains are also described. Microbial bioconversion of glycerol represents a remarkable choice to add value to the biofuel production chain, allowing the biofuel industry to be more competitive. The current review presents certain ways for the bioconversion of crude glycerol into citric acid and 1,3-propanediol with high yields and concentrations achieved by using isolated microorganisms.

Optimization of conditions for decolorization of azo-based textile dyes by multiple fungal species

Wastewater from textile industries contains azo dye residues that negatively affect most environmental systems. The biological treatment of these wastes is the best option due to safety and cost concerns. Here we isolated and identified 19 azo dye-degrading fungi and optimized conditions resulting in enhanced degradation. The fungi belonged to five species of Aspergillus and a single Lichtheimia sp. All fungi were evaluated for their ability to decolorize 20 azo dyes. While the most easily transformable azo dye was direct violet (decolorization ranged from 71.1 to 93.3%), the most resistant to decolorization was fast green azo dye. The greatest degradation potential of azo dyes (direct violet and methyl red) was optimized using the most promising four fungal strains and changing media glucose concentration, nitrogen source, and micronutrients. Biomass, lignin peroxidase, and laccases production were also determined in the optimization studies. The decolorization of both azo dyes by the four fungal strains was greatly enhanced by glucose supplementation. The fungal strains were not able to produce lignin peroxidases in the absence of organic nitrogen source. Both yeast extract and casamino acid supplementation enhanced decolorization of direct violet and methyl red dyes and production of lignin peroxidase by the fungal strains. In contrast, the laccases were absent in the similar medium enriched with the same organic nitrogen sources.

Effect of Different Carbon Sources on Bacterial Nanocellulose Production and Structure Using the Low pH Resistant Strain Komagataeibacter Medellinensis

Bacterial cellulose (BC) is a polymer obtained by fermentation with microorganism of different genera. Recently, new producer species have been discovered, which require identification of the most important variables affecting cellulose production. In this work, the influence of different carbon sources in BC production by a novel low pH-resistant strain Komagataeibacter medellinensis was established. The Hestrin-Schramm culture medium was used as a reference and was compared to other media comprising glucose, fructose, and sucrose, used as carbon sources at three concentrations (1, 2, and 3% w/v). The BC yield and dynamics of carbon consumption were determined at given fermentation times during cellulose production. While the carbon source did not influence the BC structural characteristics, different production levels were determined: glucose > sucrose > fructose. These results highlight considerations to improve BC industrial production and to establish the BC property space for applications in different fields.

Microbial metabolites in nutrition, healthcare and agriculture

Microorganisms are a promising source of an enormous number of natural products, which have made significant contribution to almost each sphere of human, plant and veterinary life. Natural compounds obtained from microorganisms have proved their value in nutrition, agriculture and healthcare. Primary metabolites, such as amino acids, enzymes, vitamins, organic acids and alcohol are used as nutritional supplements as well as in the production of industrial commodities through biotransformation. Whereas, secondary metabolites are organic compounds that are largely obtained by extraction from plants or tissues. They are primarily used in the biopharmaceutical industry due to their capability to reduce infectious diseases in human beings and animals and thus increase the life expectancy. Additionally, microorganisms and their products inevitably play a significant role in sustainable agriculture development.

Enhanced recovery of valuable metals from spent lithium-ion batteries through optimization of organic acids produced by Aspergillus niger

In the present study, spent medium bioleaching method was performed using organic acids produced by Aspergillus niger to dissolve Ni, Co, Mn, Li, Cu and Al from spent lithium-ion batteries (LIBs). Response surface methodology was used to investigate the effects and interactions between the effective factors of sucrose concentration, initial pH, and inoculum size to optimize organic acid production. Maximum citric acid, malic acid, and gluconic acid concentrations of 26,478, 1832.53 and 8433.76ppm, respectively, and a minimum oxalic acid concentration of 305.558ppm were obtained under optimal conditions of 116.90 (gl^-1) sucrose concentration, 3.45% (vv^-1) inoculum size, and a pH value of 5.44. Biogenically-produced organic acids are used for leaching of spent LIBs at different pulp densities. The highest metal recovery of 100% Cu, 100% Li, 77% Mn, and 75% Al occurred at 2% (wv^-1) pulp density; 64% Co and 54% Ni recovery occurred at 1% (wv^-1) pulp density. The bioleaching of metals from spent LIBs can decrease the environmental impact of this waste. The results of this study suggest that the process can be used for large scale industrial purposes.

Adipic acid tolerance screening for potential adipic acid production hosts

BACKGROUND

Biobased processes for the production of adipic acid are of great interest to replace the current environmentally detrimental petrochemical production route. No efficient natural producer of adipic acid has yet been identified, but several approaches for pathway engineering have been established. Research has demonstrated that the microbial production of adipic acid is possible, but the yields and titres achieved so far are inadequate for commercialisation. A plausible explanation may be intolerance to adipic acid. Therefore, in this study, selected microorganisms, including yeasts, filamentous fungi and bacteria, typically used in microbial cell factories were considered to evaluate their tolerance to adipic acid.

RESULTS

Screening of yeasts and bacteria for tolerance to adipic acid was performed in microtitre plates, and in agar plates for A. niger in the presence of adipic acid over a broad range of concentration (0-684 mM). As the different dissociation state(s) of adipic acid may influence cells differently, cultivations were performed with at least two pH values. Yeasts and A. niger were found to tolerate substantially higher concentrations of adipic acid than bacteria, and were less affected by the undissociated form of adipic acid than bacteria. The yeast exhibiting the highest tolerance to adipic acid was Candida viswanathii, showing a reduction in maximum specific growth rate of no more than 10-15% at the highest concentration of adipic acid tested and the tolerance was not dependent on the dissociation state of the adipic acid.

CONCLUSIONS

Tolerance to adipic acid was found to be substantially higher among yeasts and A. niger than bacteria. The explanation of the differences in adipic acid tolerance between the microorganisms investigated are likely related to fundamental differences in their physiology and metabolism. Among the yeasts investigated, C. viswanathii showed the highest tolerance and could be a potential host for a future microbial cell factory for adipic acid.

An accurate description of Aspergillus niger organic acid batch fermentation through dynamic metabolic modelling

BACKGROUND

Aspergillus niger fermentation has provided the chief source of industrial citric acid for over 50 years. Traditional strain development of this organism was achieved through random mutagenesis, but advances in genomics have enabled the development of genome-scale metabolic modelling that can be used to make predictive improvements in fermentation performance. The parent citric acid-producing strain of A. niger, ATCC 1015, has been described previously by a genome-scale metabolic model that encapsulates its response to ambient pH. Here, we report the development of a novel double optimisation modelling approach that generates time-dependent citric acid fermentation using dynamic flux balance analysis.

RESULTS

The output from this model shows a good match with empirical fermentation data. Our studies suggest that citric acid production commences upon a switch to phosphate-limited growth and this is validated by fitting to empirical data, which confirms the diauxic growth behaviour and the role of phosphate storage as polyphosphate.

CONCLUSIONS

The calibrated time-course model reflects observed metabolic events and generates reliable in silico data for industrially relevant fermentative time series, and for the behaviour of engineered strains suggesting that our approach can be used as a powerful tool for predictive metabolic engineering.

Evaluación de factores que afectan la bioacidulación de roca fosfórica bajo condiciones in vitro

El fósforo (P) es un nutriente esencial para el desarrollo de las plantas, desafortunadamente, su disponibilidad en muchos suelos es baja. Consecuentemente, los agricultores aplican altas cantidades de fertilizantes fosfóricos solubles, pero esto es ineficiente y costoso. El uso directo de roca fosfórica (RP) es muy atractivo por su bajo costo; sin embargo, es poco soluble y de baja eficiencia agronómica. Para superar esta limitación, hay un creciente interés en el uso de microorganismos del suelo capaces de disolverla y mejorar su valor como fertilizante. El objetivo de este trabajo fue evaluar el efecto que tienen algunos factores sobre la capacidad del hongo Mortierella sp. para disolver RP bajo condiciones in vitro. Estos factores son: (i) tiempo de incubación, (ii) tipo de RP, (iii) concentración inicial de P soluble y (iv) adición de vitaminas y micronutrientes. Despues del periodo de incubación se midió P en solución y pH. Los resultados indican que producto de la biodisolución de RP la más alta concentración de P en solución se alcanzó al día 5. Por otro lado, la biodisolución de RP fue reducida por la adición de vitaminas y micronutrientes y por el incremento en la concentración inicial de P soluble en el medio. Aunque la disolución microbiana fue más efectiva con la RP de Carolina del Norte, las RP del Huila y Santander presentaron un buen nivel de disolución en un periodo de tiempo corto. La bioacidulación mejorara la efectividad agronómica de la RP para su uso directo o a través de un proceso biotecnológico previo.

Engineering cytosolic acetyl-coenzyme A supply in Saccharomyces cerevisiae: Pathway stoichiometry, free-energy conservation and redox-cofactor balancing

Saccharomyces cerevisiae is an important industrial cell factory and an attractive experimental model for evaluating novel metabolic engineering strategies. Many current and potential products of this yeast require acetyl coenzyme A (acetyl-CoA) as a precursor and pathways towards these products are generally expressed in its cytosol. The native S. cerevisiae pathway for production of cytosolic acetyl-CoA consumes 2 ATP equivalents in the acetyl-CoA synthetase reaction. Catabolism of additional sugar substrate, which may be required to generate this ATP, negatively affects product yields. Here, we review alternative pathways that can be engineered into yeast to optimize supply of cytosolic acetyl-CoA as a precursor for product formation. Particular attention is paid to reaction stoichiometry, free-energy conservation and redox-cofactor balancing of alternative pathways for acetyl-CoA synthesis from glucose. A theoretical analysis of maximally attainable yields on glucose of four compounds (n-butanol, citric acid, palmitic acid and farnesene) showed a strong product dependency of the optimal pathway configuration for acetyl-CoA synthesis. Moreover, this analysis showed that combination of different acetyl-CoA production pathways may be required to achieve optimal product yields. This review underlines that an integral analysis of energy coupling and redox-cofactor balancing in precursor-supply and product-formation pathways is crucial for the design of efficient cell factories.

Effects of heterologous expression of phosphoenolpyruvate carboxykinase and phosphoenolpyruvate carboxylase on organic acid production in Aspergillus carbonarius

Aspergillus carbonarius has a potential as a cell factory for production of various organic acids. In this study, the organic acid profile of A. carbonarius was investigated under different cultivation conditions. Moreover, two heterologous genes, pepck and ppc, which encode phosphoenolpyruvate carboxykinase in Actinobacillus succinogenes and phosphoenolpyruvate carboxylase in Escherichia coli, were inserted individually and in combination in A. carbonarius to enhance the carbon flux toward the reductive TCA branch. Results of transcription analysis and measurement of enzyme activities of phosphoenolpyruvate carboxykinase and phosphoenolpyruvate carboxylase in the corresponding single and double transformants demonstrated that the two heterologous genes were successfully expressed in A. carbonarius. The production of citric acid increased in all the transformants in both glucose- and xylose-based media at pH higher than 3 but did not increase in the pH non-buffered cultivation compared with the wild type.

Metabolic engineering of strains: from industrial-scale to lab-scale chemical production

A plethora of successful metabolic engineering case studies have been published over the past several decades. Here, we highlight a collection of microbially produced chemicals using a historical framework, starting with titers ranging from industrial scale (more than 50 g/L), to medium-scale (5–50 g/L), and lab-scale (0–5 g/L). Although engineered Escherichia coli and Saccharomyces cerevisiae emerge as prominent hosts in the literature as a result of well-developed genetic engineering tools, several novel native-producing strains are gaining attention. This review catalogs the current progress of metabolic engineering towards production of compounds such as acids, alcohols, amino acids, natural organic compounds, and others.

Development and physiological characterization of cellobiose-consuming Yarrowia lipolytica

Yarrowia lipolytica is a promising production host for a wide range of molecules, but limited sugar consumption abilities prevent utilization of an abundant source of renewable feedstocks. In this study we created a Y. lipolytica strain capable of utilizing cellobiose as a sole carbon source by using endogenous promoters to express the cellodextrin transporter cdt-1 and intracellular β-glucosidase gh1-1 from Neurospora crassa. The engineered strain was also capable of simultaneous co-consumption of glucose and cellobiose. Although cellobiose was consumed slower than glucose when engineered strains were cultured with excess nitrogen, culturing with limited nitrogen led to cellobiose consumption rates comparable to those of glucose. Under limited nitrogen conditions, the engineered strain produced citric acid as a major product and we observed greater citric acid yields from cellobiose (0.37 g/g) than glucose (0.28 g/g). Culturing with a sole carbon source of either glucose or cellobiose induced additional differences on cell physiology and metabolism and a link is suggested to evasion of glucose-sensing mechanisms through intracellular creation and consumption of glucose. We ultimately applied this cellobiose-utilization system to produce citric acid from bioconversion of crystalline cellulose through simultaneous saccharification and fermentation (SSF).