Reorganization of a synthetic microbial consortium for one-step vitamin C fermentation

Recent Advances in 2-Keto-l-gulonic Acid Production Using Mixed-Culture Fermentation and Future Prospects

Vitamin C, also known as ascorbic acid, is an essential vitamin that cannot be synthesized by the human body and must be acquired through our diet. At present, the precursor of vitamin C, 2-keto-l-gulonic acid (2-KGA), is typically produced via a two-step fermentation process utilizing three bacterial strains. The second step of this traditional two-step fermentation method involves mixed-culture fermentation employing 2-KGA-producing bacteria (Ketogulonicigenium vulgare) along with associated bacteria. Because K. vulgare has defects in various metabolic pathways, associated bacteria are needed to provide key substances to promote K. vulgare growth and 2-KGA production. Unlike previous reviews where the main focus was the interaction between associated bacteria and K. vulgare, this Review presents the latest scientific research from the perspective of the metabolic pathways associated with 2-KGA production by K. vulgare and the mechanism underlying the interaction between K. vulgare and the associated bacteria. In addition, the dehydrogenases that are responsible for 2-KGA production, the 2-KGA synthesis pathway, strategies for simplifying 2-KGA production via a one-step fermentation route, and, finally, future prospects and research goals in vitamin C production are also presented.

Enhancing the biosynthesis of 2-keto-L-gulonic acid through multi-strategy metabolic engineering in Pseudomonas putida KT2440

2-KGA, a precursor for the synthesis of Vitamin C, is currently produced in China utilizing the "two-step fermentation" technique. Nevertheless, this method exhibits many inherent constraints. This study presents a comprehensive metabolic engineering strategy to establish and optimize a one-step 2-KGA fermentation process from D-sorbitol in Pseudomonas putida KT2440. In general, the endogenous promoters were screened to identify promoter P1 for subsequent heterologous gene expression in KT2440. Following the screening and confirmation of suitable heterologous gene elements such as sldh, sdh, cytc551, pqqAB, and irrE, genetic recombination was performed in KT2440. In comparison to the initial achievement of expressing only sldh and sdh in KT2440, a yield of merely 0.42 g/L was obtained. However, by implementing four metabolic engineering strategies, the recombinant strain KT20 exhibited a significant enhancement in its ability to produce 2-KGA with a remarkable yield of up to 6.5 g/L - representing an impressive 15.48-fold improvement.

Construction of stable microbial consortia for effective biochemical synthesis

Microbial consortia can complete otherwise arduous tasks through the cooperation of multiple microbial species. This concept has been applied to produce commodity chemicals, natural products, and biofuels. However, metabolite incompatibility and growth competition can make the microbial composition unstable, and fluctuating microbial populations reduce the efficiency of chemical production. Thus, controlling the populations and regulating the complex interactions between different strains are challenges in constructing stable microbial consortia. This Review discusses advances in synthetic biology and metabolic engineering to control social interactions within microbial cocultures, including substrate separation, byproduct elimination, crossfeeding, and quorum-sensing circuit design. Additionally, this Review addresses interdisciplinary strategies to improve the stability of microbial consortia and provides design principles for microbial consortia to enhance chemical production.

Genetic and biochemical strategies for regulation of L-ascorbic acid biosynthesis in plants through the L-galactose pathway

Vitamin C (L-ascorbic acid, AsA) is an essential compound with pleiotropic functions in many organisms. Since its isolation in the last century, AsA has attracted the attention of the scientific community, allowing the discovery of the L-galactose pathway, which is the main pathway for AsA biosynthesis in plants. Thus, the aim of this review is to analyze the genetic and biochemical strategies employed by plant cells for regulating AsA biosynthesis through the L-galactose pathway. In this pathway, participates eight enzymes encoded by the genes PMI, PMM, GMP, GME, GGP, GPP, GDH, and GLDH. All these genes and their encoded enzymes have been well characterized, demonstrating their participation in AsA biosynthesis. Also, have described some genetic and biochemical strategies that allow its regulation. The genetic strategy includes regulation at transcriptional and post-transcriptional levels. In the first one, it was demonstrated that the expression levels of the genes correlate directly with AsA content in the tissues/organs of the plants. Also, it was proved that these genes are light-induced because they have light-responsive promoter motifs (e.g., ATC, I-box, GT1 motif, etc.). In addition, were identified some transcription factors that function as activators (e.g., SlICE1, AtERF98, SlHZ24, etc.) or inactivators (e.g., SlL1L4, ABI4, SlNYYA10) regulate the transcription of these genes. In the second one, it was proved that some genes have alternative splicing events and could be a mechanism to control AsA biosynthesis. Also, it was demonstrated that a conserved cis-acting upstream open reading frame (5'-uORF) located in the 5'-untranslated region of the GGP gene induces its post-transcriptional repression. Among the biochemical strategies discovered is the control of the enzyme levels (usually by decreasing their quantities), control of the enzyme catalytic activity (by increasing or decreasing its activity), feedback inhibition of some enzymes (GME and GGP), subcellular compartmentation of AsA, the metabolon assembly of the enzymes, and control of AsA biosynthesis by electron flow. Together, the construction of this basic knowledge has been establishing the foundations for generating genetically improved varieties of fruits and vegetables enriched with AsA, commonly used in animal and human feed.

Design and construction of microbial cell factories based on systems biology

Environmental sustainability is an increasingly important issue in industry. As an environmentally friendly and sustainable way, constructing microbial cell factories to produce all kinds of valuable products has attracted more and more attention. In the process of constructing microbial cell factories, systems biology plays a crucial role. This review summarizes the recent applications of systems biology in the design and construction of microbial cell factories from four perspectives, including functional genes/enzymes discovery, bottleneck pathways identification, strains tolerance improvement and design and construction of synthetic microbial consortia. Systems biology tools can be employed to identify functional genes/enzymes involved in the biosynthetic pathways of products. These discovered genes are introduced into appropriate chassis strains to build engineering microorganisms capable of producing products. Subsequently, systems biology tools are used to identify bottleneck pathways, improve strains tolerance and guide design and construction of synthetic microbial consortia, resulting in increasing the yield of engineered strains and constructing microbial cell factories successfully.

iTRAQ-based proteomics analysis of Bacillus pumilus responses to acid stress and quorum sensing in a vitamin C fermentation system

Microbial consortia play a key role in human health, bioenergy, and food manufacturing due to their strong stability, robustness and versatility. One of the microbial consortia consisting of Ketogulonicigenium vulgare and Bacillus megaterium for the production of the vitamin C precursor, 2-keto-L-gulonic acid (2-KLG), has been widely used for large-scale industrial production. To further investigate the cell-cell communication in microbial consortia, a microbial consortium consisting of Ketogulonicigenium vulgare and Bacillus pumilus was constructed and the differences in protein expression at different fermentation time points (18 h and 40 h) were analyzed by iTRAQ-based proteomics. The results indicated that B. pumilus was subjected to acid shocks in the coculture fermentation system and responded to it. In addition, the quorum sensing system existed in the coculture fermentation system, and B. pumilus could secrete quorum-quenching lactonase (YtnP) to inhibit the signaling pathway of K. vulgare. This study offers valuable guidance for further studies of synthetic microbial consortia.

Microbial production of valuable chemicals by modular co-culture strategy

Nowadays, microbial synthesis has become a common way for producing valuable chemicals. Traditionally, microbial production of valuable chemicals is accomplished by a single strain. For the purpose of increasing the production titer and yield of a recombinant strain, complicated pathways and regulation layers should be fine-tuned, which also brings a heavy metabolic burden to the host. In addition, utilization of various complex and mixed substrates further interferes with the normal growth of the host strain and increases the complexity of strain engineering. As a result, modular co-culture technology, which aims to divide a target complex pathway into separate modules located at different single strains, poses an alternative solution for microbial production. Recently, modular co-culture strategy has been employed for the synthesis of different natural products. Therefore, in this review, various chemicals produced with application of co-cultivation technology are summarized, including co-culture with same species or different species, and regulation of population composition between the co-culture members. In addition, development prospects and challenges of this promising field are also addressed, and possible solution for these issues were also provided.

Microbial Interactions in a Vitamin C Industrial Fermentation System: Novel Insights and Perspectives

In industrial production, the precursor of l-ascorbic acid (L-AA, also referred to as vitamin C), 2-keto-l-gulonic acid (2-KLG), is mainly produced using a classic two-step fermentation process performed by Gluconobacter oxydans, Bacillus megaterium, and Ketogulonicigenium vulgare. In the second step of the two-step fermentation process, the microbial consortium of K. vulgare and B. megaterium is used to achieve 2-KLG production. K. vulgare can transform l-sorbose to 2-KLG, but the yield of 2-KLG is much lower in the monoculture than in the coculture fermentation system. The relationship between the two strains is too diverse to analyze and has been a hot topic in the field of vitamin C fermentation. With the development of omics technology, the relationships between the two strains are well explained; nevertheless, the cell-cell communication is unclear. In this review, based on current omics results, the interactions between the two strains are summarized, and the potential cell-cell communications between the two strains are discussed, which will shed a light on the further understanding of synthetic consortia.

The l-rhamnose-dependent regulator RhaS and its target promoters from Escherichia coli expand the genetic toolkit for regulatable gene expression in the acetic acid bacterium Gluconobacter oxydans

For regulatable target gene expression in the acetic acid bacterium (AAB) Gluconobacter oxydans only recently the first plasmids became available. These systems solely enable AraC- and TetR-dependent induction. In this study we showed that the l-rhamnose-dependent regulator RhaS from Escherichia coli and its target promoters P_rhaBAD, P_rhaT, and P_rhaSR could also be used in G. oxydans for regulatable target gene expression. Interestingly, in contrast to the responsiveness in E. coli, in G. oxydans RhaS increased the expression from P_rhaBAD in the absence of l-rhamnose and repressed P_rhaBAD in the presence of l-rhamnose. Inserting an additional RhaS binding site directly downstream from the −10 region generating promoter variant P_{rhaBAD(+RhaS-BS)} almost doubled the apparent RhaS-dependent promoter strength. Plasmid-based P_rhaBAD and P_{rhaBAD(+RhaS-BS)} activity could be reduced up to 90% by RhaS and l-rhamnose, while a genomic copy of P_{rhaBAD(+RhaS-BS)} appeared fully repressed. The RhaS-dependent repression was largely tunable by l-rhamnose concentrations between 0% and only 0.3% (w/v). The RhaS-P_rhaBAD and the RhaS-P_{rhaBAD(+RhaS-BS)} systems represent the first heterologous repressible expression systems for G. oxydans. In contrast to P_rhaBAD, the E. coli promoter P_rhaT was almost inactive in the absence of RhaS. In the presence of RhaS, the P_rhaT activity in the absence of l-rhamnose was weak, but could be induced up to 10-fold by addition of l-rhamnose, resulting in a moderate expression level. Therefore, the RhaS-P_rhaT system could be suitable for tunable low-level expression of difficult enzymes or membrane proteins in G. oxydans. The insertion of an additional RhaS binding site directly downstream from the E. coli P_rhaT −10 region increased the non-induced expression strength and reversed the regulation by RhaS and l-rhamnose from inducible to repressible. The P_rhaSR promoter appeared to be positively auto-regulated by RhaS and this activation was increased by l-rhamnose. In summary, the interplay of the l-rhamnose-binding RhaS transcriptional regulator from E. coli with its target promoters P_rhaBAD, P_rhaT, P_rhaSR and variants thereof provide new opportunities for regulatable gene expression in G. oxydans and possibly also for simultaneous l-rhamnose-triggered repression and activation of target genes, which is a highly interesting possibility in metabolic engineering approaches requiring redirection of carbon fluxes.

Engineering a tunable bicistronic TetR autoregulation expression system in Gluconobacter oxydans

Acetic acid bacteria are well-known for their ability to incompletely oxidize their carbon sources. Many of the products of these oxidations find industrial uses. Metabolic engineering of acetic acid bacteria would improve production efficiency and yield by allowing controllable gene expression. However, the molecular tools necessary for regulating gene expression have only recently started being explored. To this end the ability of the activation-dependent P_lux system and two constitutive repression P_tet systems were examined for their ability to modulate gene expression in Gluconobacter oxydans. The activation-dependent P_lux system increased gene expression approximately 5-fold regardless of the strength of the constitutive promoter used to express the luxR transcriptional activator. The P_tet system was tunable and had a nearly 20-fold induction when the tetR gene was expressed from the strong constitutive promoters P₀₁₆₉ and P₂₆₄, but only had a 4-fold induction when a weak constitutive promoter (P₄₅₂) was used for tetR expression. However, the P_tet system was somewhat leaky when uninduced. To mitigate this background activity, a bicistronic TetR expression system was constructed. Based on molecular modeling, this system is predicted to have low background activity when not induced with anhydrotetracycline. The bicistronic system was inducible up to >3,000-fold and was highly tunable with almost no background expression when uninduced, making this bicistronic system potentially useful for engineering G. oxydans and possibly other acetic acid bacteria. These expression systems add to the newly growing repertoire of suitable regulatable promoter systems in acetic acid bacteria.

The industrial versatility of Gluconobacter oxydans: current applications and future perspectives

Gluconobacter oxydans is a well-known acetic acid bacterium that has long been applied in the biotechnological industry. Its extraordinary capacity to oxidize a variety of sugars, polyols, and alcohols into acids, aldehydes, and ketones is advantageous for the production of valuable compounds. Relevant G. oxydans industrial applications are in the manufacture of L-ascorbic acid (vitamin C), miglitol, gluconic acid and its derivatives, and dihydroxyacetone. Increasing efforts on improving these processes have been made in the last few years, especially by applying metabolic engineering. Thereby, a series of genes have been targeted to construct powerful recombinant strains to be used in optimized fermentation. Furthermore, low-cost feedstocks, mostly agro-industrial wastes or byproducts, have been investigated, to reduce processing costs and improve the sustainability of G. oxydans bioprocess. Nonetheless, further research is required mainly to make these raw materials feasible at the industrial scale. The current shortage of suitable genetic tools for metabolic engineering modifications in G. oxydans is another challenge to be overcome. This paper aims to give an overview of the most relevant industrial G. oxydans processes and the current strategies developed for their improvement.

Multiscale engineering of microbial cell factories: A step forward towards sustainable natural products industry

Microbial cell factories (bacteria and fungi) are the leading producers of beneficial natural products such as lycopene, carotene, herbal medicine, and biodiesel etc. These microorganisms are considered efficient due to their effective bioprocessing strategy (monoculture- and consortial-based approach) under distinct processing conditions. Meanwhile, the advancement in genetic and process optimization techniques leads to enhanced biosynthesis of natural products that are known functional ingredients with numerous applications in the food, cosmetic and medical industries. Natural consortia and monoculture thrive in nature in a small proportion, such as wastewater, food products, and soils. In similitude to natural consortia, it is possible to engineer artificial microbial consortia and program their behaviours via synthetic biology tools. Therefore, this review summarizes the optimization of genetic and physicochemical parameters of the microbial system for improved production of natural products. Also, this review presents a brief history of natural consortium and describes the functional properties of monocultures. This review focuses on synthetic biology tools that enable new approaches to design synthetic consortia; and highlights the syntropic interactions that determine the performance and stability of synthetic consortia. In particular, the effect of processing conditions and advanced genetic techniques to improve the productibility of both monoculture and consortial based systems have been greatly emphasized. In this context, possible strategies are also discussed to give an insight into microbial engineering for improved production of natural products in the future. In summary, it is concluded that the coupling of genomic modifications with optimum physicochemical factors would be promising for producing a robust microbial cell factory that shall contribute to the increased production of natural products.

Construction of Environmental Synthetic Microbial Consortia: Based on Engineering and Ecological Principles

In synthetic biology, engineering principles are applied to system design. The development of synthetic microbial consortia represents the intersection of synthetic biology and microbiology. Synthetic community systems are constructed by co-cultivating two or more microorganisms under certain environmental conditions, with broad applications in many fields including ecological restoration and ecological theory. Synthetic microbial consortia tend to have high biological processing efficiencies, because the division of labor reduces the metabolic burden of individual members. In this review, we focus on the environmental applications of synthetic microbial consortia. Although there are many strategies for the construction of synthetic microbial consortia, we mainly introduce the most widely used construction principles based on cross-feeding. Additionally, we propose methods for constructing synthetic microbial consortia based on traits and spatial structure from the perspective of ecology to provide a basis for future work.

Methods for the Development of Recombinant Microorganisms for the Production of Natural Products

Metabolic engineering strives to develop microbial strains that are capable of producing a target chemical in a biological organism. There are still many challenges to overcome in order to achieve titers, yields, and productivities necessary for industrial production. The use of recombinant microorganisms to meet these needs is the next step for metabolic engineers. In this chapter, we aim to provide insight on both the applications of metabolic engineering for natural product biosynthesis as well as optimization methods.

Reprogramming microbial populations using a programmed lysis system to improve chemical production

Microbial populations are a promising model for achieving microbial cooperation to produce valuable chemicals. However, regulating the phenotypic structure of microbial populations remains challenging. In this study, a programmed lysis system (PLS) is developed to reprogram microbial cooperation to enhance chemical production. First, a colicin M -based lysis unit is constructed to lyse Escherichia coli. Then, a programmed switch, based on proteases, is designed to regulate the effective lysis unit time. Next, a PLS is constructed for chemical production by combining the lysis unit with a programmed switch. As a result, poly (lactate-co-3-hydroxybutyrate) production is switched from PLH synthesis to PLH release, and the content of free PLH is increased by 283%. Furthermore, butyrate production with E. coli consortia is switched from E. coli BUT003 to E. coli BUT004, thereby increasing butyrate production to 41.61 g/L. These results indicate the applicability of engineered microbial populations for improving the metabolic division of labor to increase the efficiency of microbial cell factories.

Complete biosynthesis of the potential medicine icaritin by engineered Saccharomyces cerevisiae and Escherichia coli

Icaritin is a prenylflavonoid present in the Chinese herbal medicinal plant Epimedium spp. and is under investigation in a phase III clinical trial for advanced hepatocellular carcinoma. Here, we report the biosynthesis of icaritin from glucose by engineered microbial strains. We initially designed an artificial icaritin biosynthetic pathway by identifying a novel prenyltransferase from the Berberidaceae-family species Epimedium sagittatum (EsPT2) that catalyzes the C8 prenylation of kaempferol to yield 8-prenlykaempferol and a novel methyltransferase GmOMT2 from soybean to transfer a methyl to C4'-OH of 8-prenlykaempferol to produce icaritin. We next introduced 11 heterologous genes and modified 12 native yeast genes to construct a yeast strain capable of producing 8-prenylkaempferol with high efficiency. GmOMT2 was sensitive to low pH and lost its activity when expressed in the yeast cytoplasm. By relocating GmOMT2 into mitochondria (higher pH than cytoplasm) of the 8-prenylkaempferol-producing yeast strain or co-culturing the 8-prenylkaempferol-producing yeast with an Escherichia coli strain expressing GmOMT2, we obtained icaritin yields of 7.2 and 19.7 mg/L, respectively. Beyond the characterizing two previously unknown plant enzymes and conducting the first biosynthesis of icaritin from glucose, we describe two strategies of overcoming the widespread issue of incompatible pH conditions encountered in basic and applied bioproduction research. Our findings will facilitate industrial-scale production of icaritin and other prenylflavonoids.

Highly tunable TetR-dependent target gene expression in the acetic acid bacterium Gluconobacter oxydans

Abstract

For the acetic acid bacterium (AAB) Gluconobacter oxydans only recently the first tight system for regulatable target gene expression became available based on the heterologous repressor-activator protein AraC from Escherichia coli and the target promoter P_araBAD. In this study, we tested pure repressor-based TetR- and LacI-dependent target gene expression in G. oxydans by applying the same plasmid backbone and construction principles that we have used successfully for the araC-P_araBAD system. When using a pBBR1MCS-5-based plasmid, the non-induced basal expression of the Tn10-based TetR-dependent expression system was extremely low. This allowed calculated induction ratios of up to more than 3500-fold with the fluorescence reporter protein mNeonGreen (mNG). The induction was highly homogeneous and tunable by varying the anhydrotetracycline concentration from 10 to 200 ng/mL. The already strong reporter gene expression could be doubled by inserting the ribosome binding site AGGAGA into the 3’ region of the P_tet sequence upstream from mNG. Alternative plasmid constructs used as controls revealed a strong influence of transcription terminators and antibiotics resistance gene of the plasmid backbone on the resulting expression performance. In contrast to the TetR-P_tet-system, pBBR1MCS-5-based LacI-dependent expression from P_lacUV5 always exhibited some non-induced basal reporter expression and was therefore tunable only up to 40-fold induction by IPTG. The leakiness of P_lacUV5 when not induced was independent of potential read-through from the lacI promoter. Protein-DNA binding simulations for pH 7, 6, 5, and 4 by computational modeling of LacI, TetR, and AraC with DNA suggested a decreased DNA binding of LacI when pH is below 6, the latter possibly causing the leakiness of LacI-dependent systems hitherto tested in AAB. In summary, the expression performance of the pBBR1MCS-5-based TetR-P_tet system makes this system highly suitable for applications in G. oxydans and possibly in other AAB.

Key Points

• A pBBR1MCS-5-based TetR-P_tet system was tunable up to more than 3500-fold induction.

• A pBBR1MCS-5-based LacI-P_lacUV5 system was leaky and tunable only up to 40-fold.

• Modeling of protein-DNA binding suggested decreased DNA binding of LacI at pH < 6.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11473-x.

Modular optimization in metabolic engineering

There is an increasing demand for bioproducts produced by metabolically engineered microbes, such as pharmaceuticals, biofuels, biochemicals and other high value compounds. In order to meet this demand, modular optimization, the optimizing of subsections instead of the whole system, has been adopted to engineer cells to overproduce products. Research into modularity has focused on traditional approaches such as DNA, RNA, and protein-level modularity of intercellular machinery, by optimizing metabolic pathways for enhanced production. While research into these traditional approaches continues, limitations such as scale-up and time cost hold them back from wider use, while at the same time there is a shift to more novel methods, such as moving from episomal expression to chromosomal integration. Recently, nontraditional approaches such as co-culture systems and cell-free metabolic engineering (CFME) are being investigated for modular optimization. Co-culture modularity looks to optimally divide the metabolic burden between different hosts. CFME seeks to modularly optimize metabolic pathways in vitro, both speeding up the design of such systems and eliminating the issues associated with live hosts. In this review we will examine both traditional and nontraditional approaches for modular optimization, examining recent developments and discussing issues and emerging solutions for future research in metabolic engineering.

Synthetic co-cultures: novel avenues for bio-based processes

In nature, microorganisms live in multi-species communities allowing microbial interactions. These interactions are lost upon establishing a pure culture, increasing the metabolic burden and limiting the metabolic potential of the isolated microbe. In the past years, synthetic microbial co-cultivation, using well-defined consortia of two or more microbes, was increasingly explored for innovative applications in biotechnology. As such, interspecies interactions take place without the complexity of an open mixed culture, minimizing undesired side reactions. Ultimately, synthetic co-cultivation allows to take well-characterized microbes 'off-the-shelf' to create ecosystems with improved process capabilities. This review highlights some of the recent developments on co-cultivation, focusing on waste-to-chemicals conversions. It also addresses fundamental knowledge on microbial interactions deriving from these studies, which is important to further develop our ability to engineer functional co-cultures for bioproduction.

Artificially selecting bacterial communities using propagule strategies

Artificial selection is a promising approach to manipulate microbial communities. Here, we report the outcome of two artificial selection experiments at the microbial community level. Both used "propagule" selection strategies, whereby the best-performing communities are used as the inocula to form a new generation of communities. Both experiments were contrasted to a random selection control. The first experiment used a defined set of strains as the starting inoculum, and the function under selection was the amylolytic activity of the consortia. The second experiment used multiple soil communities as the starting inocula, and the function under selection was the communities' cross-feeding potential. In both experiments, the selected communities reached a higher mean function than the control. In the first experiment, this was caused by a decline in function of the control, rather than an improvement of the selected line. In the second experiment, this response was fueled by the large initial variance in function across communities, and stopped when the top-performing community "fixed" in the metacommunity. Our results are in agreement with basic expectations from breeding theory, pointing to some of the limitations of community-level selection experiments that can inform the design of future studies.

A tunable L-arabinose-inducible expression plasmid for the acetic acid bacterium Gluconobacter oxydans

Abstract

The acetic acid bacterium (AAB) Gluconobacter oxydans incompletely oxidizes a wide variety of carbohydrates and is therefore used industrially for oxidative biotransformations. For G. oxydans, no system was available that allows regulatable plasmid-based expression. We found that the l-arabinose-inducible P_BAD promoter and the transcriptional regulator AraC from Escherichia coli MC4100 performed very well in G. oxydans. The respective pBBR1-based plasmids showed very low basal expression of the reporters β-glucuronidase and mNeonGreen, up to 480-fold induction with 1% l-arabinose, and tunability from 0.1 to 1% l-arabinose. In G. oxydans 621H, l-arabinose was oxidized by the membrane-bound glucose dehydrogenase, which is absent in the multi-deletion strain BP.6. Nevertheless, AraC-P_BAD performed similar in both strains in the exponential phase, indicating that a gene knockout is not required for application of AraC-P_BAD in wild-type G. oxydans strains. However, the oxidation product arabinonic acid strongly contributed to the acidification of the growth medium in 621H cultures during the stationary phase, which resulted in drastically decreased reporter activities in 621H (pH 3.3) but not in BP.6 cultures (pH 4.4). These activities could be strongly increased quickly solely by incubating stationary cells in d-mannitol-free medium adjusted to pH 6, indicating that the reporters were hardly degraded yet rather became inactive. In a pH-controlled bioreactor, these reporter activities remained high in the stationary phase (pH 6). Finally, we created a multiple cloning vector with araC-P_BAD based on pBBR1MCS-5. Together, we demonstrated superior functionality and good tunability of an AraC-P_BAD system in G. oxydans that could possibly also be used in other AAB.

Key points

• We found the AraC-P_BADsystem from E. coli MC4100 was well tunable in G. oxydans.

•  In the absence of AraC orl-arabinose, expression from P_BADwas extremely low.

• This araC-P_BADsystem could also be fully functional in other acetic acid bacteria.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10905-4) contains supplementary material, which is available to authorized users.

Understanding the Mechanisms of Positive Microbial Interactions That Benefit Lactic Acid Bacteria Co-cultures

Microorganisms grow in concert, both in natural communities and in artificial or synthetic co-cultures. Positive interactions between associated microbes are paramount to achieve improved substrate conversion and process performance in biotransformation and fermented food production. The mechanisms underlying such positive interactions have been the focus of numerous studies in recent decades and are now starting to be well characterized. Lactic acid bacteria (LAB) contribute to the final organoleptic, nutritional, and health properties of fermented food products. However, interactions in LAB co-cultures have been little studied, apart from the well-characterized LAB co-culture used for yogurt manufacture. LAB are, however, multifunctional microorganisms that display considerable potential to create positive interactions between them. This review describes why LAB co-cultures are of such interest, particularly in foods, and how their extensive nutritional requirements can be used to favor positive interactions. In that respect, our review highlights the benefits of co-cultures in different areas of application, details the mechanisms underlying positive interactions and aims to show how mechanisms based on nutritional interactions can be exploited to create efficient LAB co-cultures.

Transcriptome Analysis of Gluconobacter oxydans WSH-003 Exposed to Elevated 2-Keto-L-Gulonic Acid Reveals the Responses to Osmotic and Oxidative Stress

Industrial production of 2-keto-L-gulonic acid (2-KLG), the precursor of vitamin C, is mainly achieved by a two-step fermentation process carried out by Gluconobacter oxydans, Bacillus, and Ketogulonicigenium. One of the most promising innovations that could replace this complicated two-step fermentation process is the integration of the essential genes for synthesis of 2-KLG into G. oxydans and use of it as the producer. Therefore, determining the tolerance and response of G. oxydans to 2-KLG is a priority for improving the direct production of 2-KLG in this bacterium. In this study, a global view of the gene expression of G. oxydans WSH-003 in response to 2-KLG challenge was investigated by RNA sequencing. A total of 363 genes of G. oxydans that were differentially expressed in response to 2-KLG were uncovered. The results showed that 2-KLG could lead to oxidative stress, osmotic stress, and DNA damage in G. oxydans.

Distributed flux balance analysis simulations of serial biomass fermentation by two organisms

Intelligent biorefinery design that addresses both the composition of the biomass feedstock as well as fermentation microorganisms could benefit from dedicated tools for computational simulation and computer-assisted optimization. Here we present the BioLego Vn2.0 framework, based on Microsoft Azure Cloud, which supports large-scale simulations of biomass serial fermentation processes by two different organisms. BioLego enables the simultaneous analysis of multiple fermentation scenarios and the comparison of fermentation potential of multiple feedstock compositions. Thanks to the effective use of cloud computing it further allows resource intensive analysis and exploration of media and organism modifications. We use BioLego to obtain biological and validation results, including (1) exploratory search for the optimal utilization of corn biomasses-corn cobs, corn fiber and corn stover-in fermentation biorefineries; (2) analysis of the possible effects of changes in the composition of K. alvarezi biomass on the ethanol production yield in an anaerobic two-step process (S. cerevisiae followed by E. coli); (3) analysis of the impact, on the estimated ethanol production yield, of knocking out single organism reactions either in one or in both organisms in an anaerobic two-step fermentation process of Ulva sp. into ethanol (S. cerevisiae followed by E. coli); and (4) comparison of several experimentally measured ethanol fermentation rates with the predictions of BioLego.

Synthetic microbial consortia for biosynthesis and biodegradation: promises and challenges

Functional differentiation and metabolite exchange enable microbial consortia to perform complex metabolic tasks and efficiently cycle the nutrients. Inspired by the cooperative relationships in environmental microbial consortia, synthetic microbial consortia have great promise for studying the microbial interactions in nature and more importantly for various engineering applications. However, challenges coexist with promises, and the potential of consortium-based technologies is far from being fully harnessed. Thorough understanding of the underlying molecular mechanisms of microbial interactions is greatly needed for the rational design and optimization of defined consortia. These knowledge gaps could be potentially filled with the assistance of the ongoing revolution in systems biology and synthetic biology tools. As current fundamental and technical obstacles down the road being removed, we would expect new avenues with synthetic microbial consortia playing important roles in biological and environmental engineering processes such as bioproduction of desired chemicals and fuels, as well as biodegradation of persistent contaminants.

Recent advances of biofuels and biochemicals production from sustainable resources using co-cultivation systems

Microbial communities are ubiquitous in nature and exhibit several attractive features, such as sophisticated metabolic capabilities and strong environment robustness. Inspired by the advantages of natural microbial consortia, diverse artificial co-cultivation systems have been metabolically constructed for biofuels, chemicals and natural products production. In these co-cultivation systems, especially genetic engineering ones can reduce the metabolic burden caused by the complex of metabolic pathway through labor division, and improve the target product production significantly. This review summarized the most up-to-dated co-cultivation systems used for biofuels, chemicals and nature products production. In addition, major challenges associated with co-cultivation systems are also presented and discussed for meeting further industrial demands.

Advances in the development and application of microbial consortia for metabolic engineering

Recent advances in metabolic engineering enable the production of high-value chemicals via expressing complex biosynthetic pathways in a single microbial host. However, many engineered strains suffer from poor product yields due to redox imbalance and excess metabolic burden, and require compartmentalization of the pathway for optimal function. To address this problem, significant developments have been made towards co-cultivation of more than one engineered microbial strains to distribute metabolic burden between the co-cultivation partners and improve the product yield. In this emerging approach, metabolic pathway modules can be optimized separately in suitable hosts that will then be combined to enable optimal functionality of the complete pathway. This modular approach broadens the possibilities to fine tune sophisticated production platforms and thus achieve the biosynthesis of very complex compounds.

Here, we review the different applications and the overall potential of natural and artificial co-cultivation systems in metabolic engineering in order to improve bioproduction/bioconversion. In addition to the several advantages over monocultures, major challenges and opportunities associated with co-cultivation are also discussed in this review.

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Benefits of using co-cultivation system in metabolic engineering.

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Existence of natural consortia and their application.

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Recent advancement in co-cultivation methodology for bioproductions.

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Challenges in implementing microbial consortia for microbial biosynthesis.

Fermentation Titer Optimization and Impact on Energy and Water Consumption during Downstream Processing

A common focus of fermentation process optimization is the product titer. Different strategies to boost fermentation titer target whole‐cell biocatalyst selection, process control, and medium composition. Working at higher product concentrations reduces the water that needs to be removed in the case of aqueous systems and, therefore, lowers the cost of downstream separation and purification. Different approaches to achieve higher titer in fermentation are examined. Energy and water consumption data collected from different Cargill fermentation plants, i.e., ethanol, lactic acid, and 2‐keto‐L‐gulonic acid, confirm that improvements in fermentation titer play a decisive role in downstream economics and environmental footprint.

Metabolic engineering advances and prospects for amino acid production

Amino acid fermentation is one of the major pillars of industrial biotechnology. The multi-billion USD amino acid market is rising steadily and is diversifying. Metabolic engineering is no longer focused solely on strain development for the bulk amino acids L-glutamate and L-lysine that are produced at the million-ton scale, but targets specialty amino acids. These demands are met by the development and application of new metabolic engineering tools including CRISPR and biosensor technologies as well as production processes by enabling a flexible feedstock concept, co-production and co-cultivation schemes. Metabolic engineering advances are exemplified for specialty proteinogenic amino acids, cyclic amino acids, omega-amino acids, and amino acids functionalized by hydroxylation, halogenation and N-methylation.

Metabolite-based mutualism enhances hydrogen production in a two-species microbial consortium

Sustainable hydrogen production from renewable and low-cost substrates is very important to mitigate environmental and energy-related issues. Microbial consortia are promising for diverse bioenergy and environmental applications, yet microbial interactions are not fully understood. Here, we present comprehensive investigation on how two species in an artificial microbial consortium, consisting of Bacillus cereus A1 and Brevundimonas naejangsanensis B1, mutually cooperate to achieve an overall enhancement in hydrogen production and starch utilization. In this consortium, strains A1 and B1 secrete α-amylase and glucoamylase that are functionally complementary in starch hydrolysis. Moreover, strain A1 converts starch into lactate as a carbon source and electron donor, supporting the cell growth and hydrogen generation of strain B1. In return, strain B1 produces formate as an electron shuttle to strain A1 to enhance hydrogen production. The co-culture re-directs the overall metabolic flux, facilitates the cell growth, and up-regulates the key genes of hydrogen production and starch hydrolysis.

Wang and Tang et al. show that two-species microbial consortium is more efficient in hydrogen production and starch utilization, compared to pure cultures. This work underscores a utility of synthetic microbial consortia, which can be optimized for increasing production of a certain metabolite.

Synthetic Biology Tools to Engineer Microbial Communities for Biotechnology

Microbial consortia have been used in biotechnology processes, including fermentation, waste treatment, and agriculture, for millennia. Today, synthetic biologists are increasingly engineering microbial consortia for diverse applications, including the bioproduction of medicines, biofuels, and biomaterials from inexpensive carbon sources. An improved understanding of natural microbial ecosystems, and the development of new tools to construct synthetic consortia and program their behaviors, will vastly expand the functions that can be performed by communities of interacting microorganisms. Here, we review recent advancements in synthetic biology tools and approaches to engineer synthetic microbial consortia, discuss ongoing and emerging efforts to apply consortia for various biotechnological applications, and suggest future applications.

Overcoming NADPH product inhibition improves D-sorbitol conversion to L-sorbose

Gluconobacter oxydans sorbitol dehydrogenase (GoSLDH) exhibits a higher catalytic efficiency than other l-sorbose producing enzymes. During the reaction catalysed by GoSLDH, NADP⁺ is reduced to NADPH and d-sorbitol is oxidized to l-sorbose. However, GoSLDH activity is inhibited by the NADPH (K_i = 100 μM) formed during the enzymatic reaction. Therefore, Escherichia coli_{gosldh-lrenox} producing both GoSLDH for d-sorbitol oxidation and LreNOX (NAD(P)H oxidase from Lactobacillus reuteri) for NADP⁺ regeneration was generated and used for l-sorbose production. Whole cell biocatalysts with the LreNOX cofactor recycling system showed a high conversion rate (92%) of d-sorbitol to l-sorbose in the presence of low concentration of NADP⁺ (0.5 mM). By alleviating NADPH accumulation during the catalytic reactions, E. coli_{gosldh-lrenox} exhibited 23-fold higher conversion rate of d-sorbitol than E. coli_gosldh. l-Sorbose production by E. coli_{gosldh-lrenox} reached 4.1 g/L after 40 min, which was 20.5-fold higher than that of E. coli_gosldh. We also constructed G. oxydans_gosldh and G. oxydans_{gosldh-lrenox} strains, and they exhibited 1.2- and 2.9-fold higher conversion rates than the wild-type G. oxydans KCTC 1091. The results indicate that overcoming NADPH product inhibition using LreNOX improves chemical production in NADP⁺-dependent enzymatic reactions.

Integrated proteomic and metabolomic analysis of a reconstructed three-species microbial consortium for one-step fermentation of 2-keto-l-gulonic acid, the precursor of vitamin C

Microbial consortia, with the merits of strong stability, robustness, and multi-function, played critical roles in human health, bioenergy, and food manufacture, etc. On the basis of ‘build a consortium to understand it’, a novel microbial consortium consisted of Gluconobacter oxydans, Ketogulonicigenium vulgare and Bacillus endophyticus was reconstructed to produce 2-keto-l-gulonic acid (2-KGA), the precursor of vitamin C. With this synthetic consortium, 73.7 g/L 2-KGA was obtained within 30 h, which is comparable to the conventional industrial method. A combined time-series proteomic and metabolomic analysis of the fermentation process was conducted to further investigate the cell–cell interaction. The results suggested that the existence of B. endophyticus and G. oxydans together promoted the growth of K. vulgare by supplying additional nutrients, and promoted the 2-KGA production by supplying more substrate. Meanwhile, the growth of B. endophyticus and G. oxydans was compromised from the competition of the nutrients by K. vulgare, enabling the efficient production of 2-KGA. This study provides valuable guidance for further study of synthetic microbial consortia.

Global mRNA decay and 23S rRNA fragmentation in Gluconobacter oxydans 621H

Background

Gluconobacter oxydans is a strictly aerobic Gram-negative acetic acid bacterium used industrially for oxidative biotransformations due to its exceptional type of catabolism. It incompletely oxidizes a wide variety of carbohydrates regio- and stereoselectively in the periplasm using membrane-bound dehydrogenases with accumulation of the products in the medium. As a consequence, only a small fraction of the carbon and energy source enters the cell, resulting in a low biomass yield. Additionally, central carbon metabolism is characterized by the absence of a functional glycolysis and absence of a functional tricarboxylic acid (TCA) cycle. Due to these features, G. oxydans is a highly interesting model organism. Here we analyzed global mRNA decay in G. oxydans to describe its characteristic features and to identify short-lived mRNAs representing potential bottlenecks in the metabolism for further growth improvement by metabolic engineering.

Results

Using DNA microarrays we estimated the mRNA half-lives in G. oxydans. Overall, the mRNA half-lives ranged mainly from 3 min to 25 min with a global mean of 5.7 min. The transcripts encoding GroES and GroEL required for proper protein folding ranked at the top among transcripts exhibiting both long half-lives and high abundance. The F-type H⁺-ATP synthase transcripts involved in energy metabolism ranked among the transcripts with the shortest mRNA half-lives. RNAseq analysis revealed low expression levels for genes of the incomplete TCA cycle and also the mRNA half-lives of several of those were short and below the global mean. The mRNA decay analysis also revealed an apparent instability of full-length 23S rRNA. Further analysis of the ribosome-associated rRNA revealed a 23S rRNA fragmentation pattern exhibiting new cleavage regions in 23S rRNAs which were previously not known.

Conclusions

The very short mRNA half-lives of the H⁺-ATP synthase, which is likely responsible for the ATP-proton motive force interconversion in G. oxydans under many or most conditions, is notably in contrast to mRNA decay data from other bacteria. Together with the short mRNA half-lives and low expression of some other central metabolic genes it could limit intended improvements of G. oxydans’ biomass yield by metabolic engineering. Also, further studies are needed to unravel the multistep process of the 23S rRNA fragmentation in G. oxydans.

Electronic supplementary material

The online version of this article (10.1186/s12864-018-5111-1) contains supplementary material, which is available to authorized users.

Synthetic Escherichia coli-Corynebacterium glutamicum consortia for l-lysine production from starch and sucrose

In the biorefinery concept renewable feedstocks are converted to a multitude of value-added compounds irrespective of seasonal or other variations of the complex biomass substrates. Conceptionally, this can be realized by specialized single microbial strains or by co-culturing various strain combinations. In the latter approach strains for substrate conversion and for product formation can be combined. This study addressed the construction of binary microbial consortia based on starch- and sucrose-based production of l-lysine and derived value-added compounds. A commensalism-based synthetic consortium for l-lysine production from sucrose was developed combining an l-lysine auxotrophic, naturally sucrose-negative E. coli strain with a C. glutamicum strain able to produce l-lysine that secretes fructose when grown with sucrose due to deletion of the fructose importer gene ptsF. Mutualistic synthetic consortia with an l-lysine auxotrophic, α-amylase secreting E. coli strain and naturally amylase-negative C. glutamicum strains was implemented for production of valuable fine chemicals from starch.

RNAseq analysis of α-proteobacterium Gluconobacter oxydans 621H

Background

The acetic acid bacterium Gluconobacter oxydans 621H is characterized by its exceptional ability to incompletely oxidize a great variety of carbohydrates in the periplasm. The metabolism of this α-proteobacterium has been characterized to some extent, yet little is known about its transcriptomes and related data. In this study, we applied two different RNAseq approaches. Primary transcriptomes enriched for 5′-ends of transcripts were sequenced to detect transcription start sites, which allow subsequent analysis of promoter motifs, ribosome binding sites, and 5´-UTRs. Whole transcriptomes were sequenced to identify expressed genes and operon structures.

Results

Sequencing of primary transcriptomes of G. oxydans revealed 2449 TSSs, which were classified according to their genomic context followed by identification of promoter and ribosome binding site motifs, analysis of 5´-UTRs including validation of predicted cis-regulatory elements and correction of start codons. 1144 (41%) of all genes were found to be expressed monocistronically, whereas 1634 genes were organized in 571 operons. Together, TSSs and whole transcriptome data were also used to identify novel intergenic (18), intragenic (328), and antisense transcripts (313).

Conclusions

This study provides deep insights into the transcriptional landscapes of G. oxydans. The comprehensive transcriptome data, which we made publicly available, facilitate further analysis of promoters and other regulatory elements. This will support future approaches for rational strain development and targeted gene expression in G. oxydans. The corrections of start codons further improve the high quality genome reference and support future proteome analysis.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4415-x) contains supplementary material, which is available to authorized users.

Microbial consortia: a critical look at microalgae co-cultures for enhanced biomanufacturing

Monocultures have been the preferred production route in the bio-industry, where contamination has been a major bottleneck. In nature, microorganisms usually exist as part of organized communities and consortia, gaining benefits from co-habitation, keeping invaders at bay. There is increasing interest in the use of co-cultures to tackle contamination issues, and simultaneously increase productivity and product diversity. The feasibility of extending the natural phenomenon of co-habitation to the biomanufacturing industry in the form of co-cultures requires careful and systematic consideration of several aspects. This article will critically examine and review current work on microbial co-cultures, with the intent of examining the concept and proposing a design pipeline that can be developed in a biomanufacturing context.

Comparative genomics and metabolomics analyses of the adaptation mechanism in Ketogulonicigenium vulgare-Bacillus thuringiensis consortium

Adaptive evolution by serial subcultivation of co-cultured Bacillus thuringiensis and Ketogulonicigenium vulgare significantly enhanced the productivity of 2-keto-L-gulonic acid in two-step vitamin C production. The adaptation mechanism in K. vulgare-B. thuringiensis consortium was investigated in this study based on comparative genomics and metabolomics studies. It was found that the growth, anti-oxidation, transcription and regulation were significantly enhanced in the adapted consortium. The mutation of the genes, which encode amidohydrolase in adapted K. vulgare (K150) and amino acid permease in adapted B. thuringiensis (B150), resulted in the increase of some amino acids levels in each species, and further enhanced the metabolic exchange and growth ability of the two species. Besides, the mutation of the gene encoding spore germination protein enhanced the metabolic levels of tricarboxylic acid cycle, and decreased the sporulation in B150, which induced its growth. The mutation of the genes, which encode NADPH nitroreductase in K150 and NADPH-dependent FMN reductase in B150, may enhance the ability of anti-oxidation. Overall, the long-term adaptation of K. vulgare and B. thuringiensis influenced the global regulation and made them more inseparable in metabolite exchange. Our work will provide ideas for the molecular design and optimization in microbial consortium.

High precision genome sequencing of engineered Gluconobacter oxydans 621H by combining long nanopore and short accurate Illumina reads

State of the art and novel high-throughput DNA sequencing technologies enable fascinating opportunities and applications in the life sciences including microbial genomics. Short high-quality read data already enable not only microbial genome sequencing, yet can be inadequately to solve problems in genome assemblies and for the analysis of structural variants, especially in engineered microbial cell factories. Single-molecule real-time sequencing technologies generating long reads promise to solve such assembly problems. In our study, we wanted to increase the average read length of long nanopore reads with R9 chemistry and conducted a hybrid approach for the analysis of structural variants to check the genome stability of a recombinant Gluconobacter oxydans 621H strain (IK003.1) engineered for improved growth. Therefore we combined accurate Illumina sequencing technology and low-cost single-molecule nanopore sequencing using the MinION^® device from Oxford Nanopore. In our hybrid approach with a modified library protocol we could increase the average size of nanopore 2D reads to about 18.9kb. Combining the long MinION nanopore reads with the high quality short Illumina reads enabled the assembly of the engineered chromosome into a single contig and comprehensive detection and clarification of 7 structural variants including all three known genetically engineered modifications. We found the genome of IK003.1 was stable over 70 generations of strain handling including 28h of process time in a bioreactor. The long read data revealed a novel 1420 bp transposon-flanked and ORF-containing sequence which was hitherto unknown in the G. oxydans 621H reference. Further analysis and genome sequencing showed that this region is already present in G. oxydans 621H wild-type strains. Our data of G. oxydans 621H wild-type DNA from different resources also revealed in 73 annotated coding sequences about 91 uniform nucleotide differences including InDels. Together, our results contribute to an improved high quality genome reference for G. oxydans 621H which is available via ENA accession PRJEB18739.

Design and construction of synthetic microbial consortia in China

The rapid development of synthetic biology enables the design, construction and optimization of synthetic microbial consortia to achieve specific functions. In China, the "973" project-"Design and Construction of Microbial Consortia" was funded by the National Basic Research Program of China in January 2014. It was proposed to address the fundamental challenges in engineering natural microbial consortia and reconstructing microbial consortia to meet industrial demands. In this review, we will introduce this "973" project, including the significance of microbial consortia, the fundamental scientific issues, the recent research progresses, and some case studies about synthetic microbial consortia in the past two and a half years.

Comparative analysis of L-sorbose dehydrogenase by docking strategy for 2-keto-L-gulonic acid production in Ketogulonicigenium vulgare and Bacillus endophyticus consortium

Improving the yield of 2-keto-L-gulonic acid (2-KGA), the direct precursor of vitamin C, draws more and more attention in industrial production. In this study, we try to increase the 2-KGA productivity by computer-aided selection of genes encoding L-sorbose dehydrogenases (SDH) of Ketogulonicigenium vulgare. First, six SDHs were modeled by docking strategy to predict the binding mode with co-factor PQQ. The binding energy between SSDA1-H/SSDA1-L and PQQ was the highest, followed by SSDA3/SSDA2. The binding energy between SSDA1-P/SSDB and PQQ was the lowest. Then, these genes were overexpressed, respectively, in an industrial strain K. vulgare HKv604. Overexpression of ssda1-l and ssda1-h enhanced the 2-KGA production by 7.89 and 12.56 % in mono-cultured K. vulgare, and by 13.21 and 16.86 % when K. vulgare was co-cultured with Bacillus endophyticus. When the engineered K. vulgare SyBE_Kv000116013 (overexpression of ssda1-p) or SyBE_Kv000116016 (overexpression of ssdb) was co-cultured with B. endophyticus, the 2-KGA production decreased significantly. The docking results were in accordance with the experimental data, which indicated that computer-aided modeling is an efficient strategy for screening more efficient enzymes.

Modular co-culture engineering, a new approach for metabolic engineering

With the development of metabolic engineering, employment of a selected microbial host for accommodation of a designed biosynthetic pathway to produce a target compound has achieved tremendous success in the past several decades. Yet, increasing requirements for sophisticated microbial biosynthesis call for establishment and application of more advanced metabolic engineering methodologies. Recently, important progress has been made towards employing more than one engineered microbial strains to constitute synthetic co-cultures and modularizing the biosynthetic labor between the co-culture members in order to improve bioproduction performance. This emerging approach, referred to as modular co-culture engineering in this review, presents a valuable opportunity for expanding the scope of the broad field of metabolic engineering. We highlight representative research accomplishments using this approach, especially those utilizing metabolic engineering tools for microbial co-culture manipulation. Key benefits and major challenges associated with modular co-culture engineering are also presented and discussed.