Continuous co-production of biomass and bio-oxidized metabolite (sorbose) using Gluconobacter oxydans in a high-oxygen tension bioreactor

New perspectives into Gluconobacter-catalysed biotransformations

Different from other aerobic microorganisms that oxidise carbon sources to water and carbon dioxide, Gluconobacter catalyses the incomplete oxidation of various substrates with regio- and stereoselectivity. This ability, as well as its capacity to release the resulting products into the reaction media, place Gluconobacter as a privileged member of a non-model microorganism class that may boost industrial biotechnology. Knowledge of new technologies applied to Gluconobacter has been piling up in recent years. Advancements in its genetic modification, application of immobilisation tools and careful designs of the transformations, have improved productivities and stabilities of Gluconobacter strains or enabled new bioconversions for the production of valuable marketable chemicals. In this work, the latest advancements applied to Gluconobacter-catalysed biotransformations are summarised with a special focus on recent available tools to improve them. From genetic and metabolic engineering to bioreactor design, the most recent works on the topic are analysed in depth to provide a comprehensive resource not only for scientists and technologists working on/with Gluconobacter, but for the general biotechnologist.

Enhanced production of l-sorbose by systematic engineering of dehydrogenases in Gluconobacter oxydans

l-Sorbose is an essential intermediate for the industrial production of vitamin C (l-ascorbic acid). However, the formation of fructose and some unknown by-products significantly reduces the conversion ratio of D-sorbitol to l-sorbose. This study aimed to identify the key D-sorbitol dehydrogenases in Gluconobacter oxydans WSH-003 by gene knockout. Then, a total of 38 dehydrogenases were knocked out in G. oxydans WSH-003, and 23 dehydrogenase-deficient strains could increase l-sorbose production. G. oxydans-30, wherein a pyrroloquinoline quinone-dependent glucose dehydrogenase was deleted, showed a significant reduction of a by-product with the extension of fermentation time. In addition, the highest conversion ratio of 99.60% was achieved in G. oxydans MD-16, in which 16 different types of dehydrogenases were inactivated consecutively. Finally, the gene vhb encoding hemoglobin was introduced into the strain. The titer of l-sorbose was 298.61 g/L in a 5-L bioreactor. The results showed that the systematic engineering of dehydrogenase could significantly enhance the production of l-sorbose.

Dehydrogenases of acetic acid bacteria

Acetic acid bacteria (AAB) are a group of bacteria that can oxidize many substrates such as alcohols and sugar alcohols and play important roles in industrial biotechnology. A majority of industrial processes that involve AAB are related to their dehydrogenases, including PQQ/FAD-dependent membrane-bound dehydrogenases and NAD(P)⁺-dependent cytoplasmic dehydrogenases. These cofactor-dependent dehydrogenases must effectively regenerate their cofactors in order to function continuously. For PQQ, FAD and NAD(P)⁺ alike, regeneration is directly or indirectly related to the electron transport chain (ETC) of AAB, which plays an important role in energy generation for aerobic cell growth. Furthermore, in changeable natural habitats, ETC components of AAB can be regulated so that the bacteria survive in different environments. Herein, the progressive cascade in an application of AAB, including key dehydrogenases involved in the application, regeneration of dehydrogenase cofactors, ETC coupling with cofactor regeneration and ETC regulation, is systematically reviewed and discussed. As they have great application value, a deep understanding of the mechanisms through which AAB function will not only promote their utilization and development but also provide a reference for engineering of other industrial strains.

Insight in the Recent Application of Polyphenols From Biomass

Biomass polyphenols are bio-active macromolecules with distinct chemical structures in a variety of biomass. In recent years, the study of biomass polyphenols and their application in food and medicine fields has become a research hotspot, which predominantly focuses on the preparation, purification, structural identifications, and measurements of biological activities. Many studies describe methodologies for extraction and application of polyphenols, but comprehensive work to review its physiological activities like drugs and health products are lacking. This paper comprehensively unlocks the bioactivities of antioxidant, antibacterial, antitumor, anticancer, neuroprotection, control of blood sugar, regulation of blood fat, and promotion of gastrointestinal health functions of polyphenols from different biomass sources. This review will serve as an illuminating resource for the global scientific community, especially for those who are actively working to promote the advances of the polyphenols research field.

Rapid Enabling of Gluconobacter oxydans Resistance to High D-Sorbitol Concentration and High Temperature by Microdroplet-Aided Adaptive Evolution

Gluconobacter oxydans is important in the conversion of D-sorbitol into l-sorbose, which is an essential intermediate for industrial-scale production of vitamin C. In a previous study, the strain G. oxydans WSH-004 could directly produce 2-keto-l-gulonic acid (2-KLG). However, its D-sorbitol tolerance was poor compared with that of other common industrial G. oxydans strains, which grew well in the presence of more than 200 g/L of D-sorbitol. This study aimed to use the microbial microdroplet culture (MMC) system for the adaptive evolution of G. oxydans WSH-004 so as to improve its tolerance to high substrate concentration and high temperature. A series of adaptively evolved strains, G. oxydans MMC1-MMC10, were obtained within 90 days. The results showed that the best strain MMC10 grew in a 300 g/L of D-sorbitol medium at 40°C. The comparative genomic analysis revealed that genetic changes related to increased tolerance were mainly in protein translation genes. Compared with the traditional adaptive evolution method, the application of microdroplet-aided adaptive evolution could improve the efficiency in terms of reducing time and simplifying the procedure for strain evolution. This research indicated that the microdroplet-aided adaptive evolution was an effective tool for improving the phenotypes with undemonstrated genotypes in a short time.

Extraction and Application of Natural Rutin From Sophora japonica to Prepare the Novel Fluorescent Sensor for Detection of Copper Ions

Rutin (R), a representative flavonoid found in various biomasses, can be used to prepare different fluorescent sensors for environmental, biological and medical fields. In this work, the natural R in Sophora japonica was extracted and purified to prepare fluorescent-responding sensor systems intended to recognize copper ions with both strong selectivity as well as appropriate sensitivity. Results showed that neat R had no obvious fluorescent emission peak in PBS buffer solution. However, when R and (2-hydroxypropyl)-β-cyclodextrin (CD) were introduced within buffer solution, fluorescent emission intensity was significantly increased due to the resultant R-CD inclusion complex. In addition, the formed R-CD inclusion complex was shown to behave as the aforementioned fluorescent sensor for copper ions through a mechanism of quenched fluorescent emission intensity when R-CD became bound with copper ions. The binding constant value for R-CD with copper ions was 1.33 × 10⁶, allowing for quantification of copper ions between the concentration range of 1.0 × 10^–7–4.2 × 10^–6mol⋅L^–1. Furthermore, the minimum detection limit was found to be 3.5 × 10^–8mol⋅L^–1. This work showed the prepared R-CD inclusion complex was both highly selective and strongly sensitive toward copper ions, indicating that this system could be applied into various fields where copper ions are of concern.

Improvement of pyrroloquinoline quinone-dependent d-sorbitol dehydrogenase activity from Gluconobacter oxydans via expression of Vitreoscilla hemoglobin and regulation of dissolved oxygen tension for the biosynthesis of 6-(N-hydroxyethyl)-amino-6-deoxy-α-l-sorbofuranose

The miglitol intermediate, 6-(N-hydroxyethyl)-amino-6-deoxy-α-l-sorbofuranose (6NSL), is catalyzed from N-2-hydroxyethyl glucamine (NHEG) by resting cells of Gluconobacter oxydans. One of the key factors limiting 6NSL production was the availability of oxygen during both cell cultivation and biotransformation of NHEG to 6NSL. Based on G. oxydans/pBBR1-sldAB-pqqABCDE-tldD (G. oxydans/AB-PQQ), the Vitreoscilla hemoglobin (VHb) was heterologously expressed in G. oxydans to enhance oxygen transfer efficiency and improve 6NSL production. The recombinant G. oxydans/AB-PQQ-VHb displayed higher biomass and NHEG oxidation activity than the control stain. The transcription levels of respiratory chain-related enzyme genes in G. oxydans/AB-PQQ-VHb exhibited up-regulation, indicating that the presence of VHb promoted the respiration. The dissolved oxygen (DO) concentration for cell cultivation was optimized in a 5-L stirred bioreactor. At a DO concentration of 20%, the maximum volumetric oxidation activity of NHEG of G. oxydans/AB-PQQ-VHb in the stirred bioreactor reached 168.3 ± 3.2 U/L. Furthermore, the biotransformation of NHEG to 6NSL using G. oxydans/AB-PQQ-VHb was carried out under different oxygen tensions to investigate the effect of oxygen on 6NSL production. Finally, up to 87.5 ± 5.9 g/L 6NSL was accumulated in the reaction mixture within 16 h when the DO was controlled at 30%.

A techno-practical method for overcoming the biotoxicity and volatility obstacles of butanol and butyric acid during whole-cell catalysis by Gluconobacter oxydans

BACKGROUND

Butyric acid is a platform chemical material, the production of which has been greatly stimulated by the diverse range of downstream applications in many industries. In particular, higher quality butyric acid used in food and medicine, is more dependent on microbiological production methods. Hence, the bio-oxidation of butanol to butyric acid has been identified as a promising method with good potential economic and environmental benefits. However, both butanol and butyric acid are usually intensively toxic to most microorganisms as well as the bio-oxidation pathway. To develop a green, efficient and competitive microbiological method is the primary work to overcome the bottleneck of butyric acid industry.

RESULT

A combined bioprocess was designed with alternative whole-cell catalysis for butyric acid bio-conversion from butanol by Gluconobacter oxydans in a sealed-oxygen supply bioreactor (SOS). In the operation system, the escape of volatile substrates and toxic chemicals to cells can be avoided by the use of a sealed bioreactor, combined with the rejuvenation of cells by supplying energy co-factors. Finally, during a one-batch whole-cell catalysis, the utilization rate of substrate increased from 56.6 to 96.0% by the simple skill. Additionally, the techno-practical bioprocess can realize the purpose of cell-recycling technology through the rejuvenation effect of co-factor. Finally, we obtained 135.3 g/L butyric acid and 216.7 g/L sorbose during a 60-h whole-cell catalysis. This techno-practical technology provides a promising approach to promote the industrial production of butyric acid with more competitiveness.

CONCLUSION

The techno-practical biotechnology has powerfully promoted the process of butyric acid production by microorganisms, especially makes up for the lack of aerobic fermentation in the industry, and surmounts the shortcomings of traditional anaerobic fermentation. At the same time, this technically practical system provides a promising approach for the promotion of the industrial production of butyric acid in a more competitive manner.

Resolving the formidable barrier of oxygen transferring rate (OTR) in ultrahigh-titer bioconversion/biocatalysis by a sealed-oxygen supply biotechnology (SOS)

BACKGROUND

The critical issue in the competitiveness between bioengineering and chemical engineering is the products titer and the volume productivity. The most direct and effective approach usually employs high-density biocatalyst, while the weakened mass transfer and evoked foam problem accompany ultrahigh-density biocatalyst loading and substrate/product titer. In high-density obligate aerobic bioconversion, oxygen as electron acceptor is a speed-limiting step in bioprocesses, but sufficient oxygen supply will lead to the foaming which results in a significant reduction in oxygen utilization and the use of additional defoamers. In this study, we designed a novel sealed-oxygen supply (SOS) biotechnology to resolve the formidable barrier of oxygen transferring rate (OTR), for bio-based fuels and chemical production process.

RESULTS

Based on systemic analysis of whole-cell catalysis in Gluconobacter oxydans, a novel sealed-oxygen supply technology was smartly designed and experimentally performed for biocatalytic oxidation of alcohols, sugars and so on. By a simple operation skill of automatic online supply of oxygen in a sealed stirring tank bioreactor of SOS, OTR barrier and foaming problem was resolved with great ease. We finally obtained ultrahigh-titer products of xylonic acid (XA), 3-hydroxypropionic acid (3-HPA), and erythrulose at 588.4 g/L, 69.4 g/L, and 364.7 g/L, respectively. Moreover, the volume productivity of three chemical products was improved by 150-250% compared with normal biotechnology. This SOS technology provides a promising approach to promote bioengineering competitiveness and advantages over chemical engineering.

CONCLUSION

SOS technology was demonstrated as an economic and universally applicable approach to bio-based fuels and chemicals production by whole-cell catalysis. The novel technology greatly promotes the competitiveness of bioengineering for chemical engineering, and provides a promising platform for the green and environmental use of biofuels.

Efficient biosynthesis of 2-keto-D-gluconic acid by fed-batch culture of metabolically engineered Gluconobacter japonicus

2-keto-d-gluconic acid (2-KGA) is a key precursor for synthesising vitamin C and isovitamin C. However, phage contamination is as constant problem in industrial production of 2-KGA using Pseudomonas fluorescens. Gluconobacter holds promise for producing 2-KGA due to impressive resistance to hypertonicity and acids, and high utilisation of glucose. In this study, the 2-KGA synthesis pathway was regulated to enhance production of 2-KGA and reduce accumulation of the by-products 5-keto-d-gluconic acid (5-KGA) and d-gluconic acid (D-GA) in the 2-KGA producer Gluconobacter japonicus CGMCC 1.49. Knocking out the ga5dh-1 gene from a competitive pathway and overexpressing the ga2dh-A gene from the 2-KGA synthesis pathway via homologous recombination increased the titre of 2-KGA by 63.81% in shake flasks. Additionally, accumulation of 5-KGA was decreased by 63.52% with the resulting G. japonicas-Δga5dh-1-ga2dh-A strain. Using an intermittent fed-batch mode in a 3 L fermenter, 2-KGA reached 235.3 g L⁻¹ with a 91.1% glucose conversion rate. Scaling up in a 15 L fermenter led to stable 2-KGA titre with productivity of 2.99 g L⁻¹ h⁻¹, 11.99% higher than in the 3 L fermenter, and D-GA and 5-KGA by-products were completely converted to 2-KGA.