Diketo-gluconate fermentation by Gluconobacter oxydans

Efficient Production of 2,5-Diketo-D-gluconic Acid by Reducing Browning Levels During Gluconobacter oxydans ATCC 9937 Fermentation

D-Glucose directly generates 2-keto-L-gulonic acid (2-KLG, precursor of vitamin C) through the 2,5-diketo-D-gluconic acid (2,5-DKG) pathway. 2,5-DKG is the main rate-limiting factor of the reaction, and there are few relevant studies on it. In this study, a more accurate quantitative method of 2,5-DKG was developed and used to screen G. oxydans ATCC9937 as the chassis strain for the production of 2,5-DKG. Combining the metabolite profile analysis and knockout and overexpression of production strain, the non-enzymatic browning of 2,5-DKG was identified as the main factor leading to low yield of the target compound. By optimizing the fermentation process, the fermentation time was reduced to 48 h, and 2,5-DKG production peaked at 50.9 g/L, which was 139.02% higher than in the control group. Effectively eliminating browning and reducing the degradation of 2,5-DKG will help increase the conversion of 2,5-DKG to 2-KLG, and finally, establish a one-step D-glucose to 2-KLG fermentation pathway.

Oxidative Fermentation of Acetic Acid Bacteria and Its Products

Acetic acid bacteria (AAB) are a group of Gram-negative, strictly aerobic bacteria, including 19 reported genera until 2021, which are widely found on the surface of flowers and fruits, or in traditionally fermented products. Many AAB strains have the great abilities to incompletely oxidize a large variety of carbohydrates, alcohols and related compounds to the corresponding products mainly including acetic acid, gluconic acid, gulonic acid, galactonic acid, sorbose, dihydroxyacetone and miglitol via the membrane-binding dehydrogenases, which is termed as AAB oxidative fermentation (AOF). Up to now, at least 86 AOF products have been reported in the literatures, but no any monograph or review of them has been published. In this review, at first, we briefly introduce the classification progress of AAB due to the rapid changes of AAB classification in recent years, then systematically describe the enzymes involved in AOF and classify the AOF products. Finally, we summarize the application of molecular biology technologies in AOF researches.

Combinatorial metabolic engineering of industrial Gluconobacter oxydans DSM2343 for boosting 5-keto-D-gluconic acid accumulation

Background

L-(+)-tartaric acid (L-TA) is an important organic acid, which is produced from the cream of tartar or stereospecific hydrolysis of the cis-epoxysuccinate. The former method is limited by the availability of raw material and the latter is dependent on the petrochemical material. Thus, new processes for the economical preparation of L-TA from carbohydrate or renewable resource would be much more attractive. Production of 5-keto-D-gluconate (5-KGA) from glucose by Gluconobacter oxydans is the first step to produce L-TA. The aim of this work is to enhance 5-KGA accumulation using combinatorial metabolic engineering strategies in G. oxydans. The sldAB gene, encoding sorbitol dehydrogenase, was overexpressed in an industrial strain G. oxydans ZJU2 under a carefully selected promoter, P₀₁₆₉. To enhance the efficiency of the oxidation by sldAB, the coenzyme pyrroloquinoline quinone (PQQ) and respiratory chain were engineered. Besides, the role in sldAB overexpression, coenzyme and respiratory chain engineering and their subsequent effects on 5-KGA production were investigated.

Results

An efficient, stable recombinant strain was constructed, whereas the 5-KGA production could be enhanced. By self-overexpressing the sldAB gene in G. oxydans ZJU2 under the constitutive promoter P₀₁₆₉, the resulting strain, G. oxydans ZJU3, produced 122.48 ± 0.41 g/L of 5-KGA. Furthermore, through the coenzyme and respiratory chain engineering, the titer and productivity of 5-KGA reached 144.52 ± 2.94 g/L and 2.26 g/(L · h), respectively, in a 15 L fermenter. It could be further improved the 5-KGA titer by 12.10 % through the fed-batch fermentation under the pH shift and dissolved oxygen tension (DOT) control condition, obtained 162 ± 2.12 g/L with the productivity of 2.53 g/(L · h) within 64 h.

Conclusions

The 5-KGA production could be significantly enhanced with the combinatorial metabolic engineering strategy in Gluconobacter strain, including sldAB overexpression, coenzyme and respiratory chain engineering. Fed-batch fermentation could further enlarge the positive effect and increase the 5-KGA production. All of these demonstrated that the robust recombinant strain can efficiently produce 5-KGA in larger scale to fulfill the industrial production of L-TA from 5-KGA.

Biotechnological applications of acetic acid bacteria

The acetic acid bacteria (AAB) have important roles in food and beverage production, as well as in the bioproduction of industrial chemicals. In recent years, there have been major advances in understanding their taxonomy, molecular biology, and physiology, and in methods for their isolation and identification. AAB are obligate aerobes that oxidize sugars, sugar alcohols, and ethanol with the production of acetic acid as the major end product. This special type of metabolism differentiates them from all other bacteria. Recently, the AAB taxonomy has been strongly rearranged as new techniques using 16S rRNA sequence analysis have been introduced. Currently, the AAB are classified in ten genera in the family Acetobacteriaceae. AAB can not only play a positive role in the production of selected foods and beverages, but they can also spoil other foods and beverages. AAB occur in sugar- and alcohol-enriched environments. The difficulty of cultivation of AAB on semisolid media in the past resulted in poor knowledge of the species present in industrial processes. The first step of acetic acid production is the conversion of ethanol from a carbohydrate carried out by yeasts, and the second step is the oxidation of ethanol to acetic acid carried out by AAB. Vinegar is traditionally the product of acetous fermentation of natural alcoholic substrates. Depending on the substrate, vinegars can be classified as fruit, starch, or spirit substrate vinegars. Although a variety of bacteria can produce acetic acid, mostly members of Acetobacter, Gluconacetobacter, and Gluconobacter are used commercially. Industrial vinegar manufacturing processes fall into three main categories: slow processes, quick processes, and submerged processes. AAB also play an important role in cocoa production, which represents a significant means of income for some countries. Microbial cellulose, produced by AAB, possesses some excellent physical properties and has potential for many applications. Other products of biotransformations by AAB or their enzymes include 2-keto-L-gulonic acid, which is used for the production of vitamin C; D-tagatose, which is used as a bulking agent in food and a noncalorific sweetener; and shikimate, which is a key intermediate for a large number of antibiotics. Recently, for the first time, a pathogenic acetic acid bacterium was described, representing the newest and tenth genus of AAB.

Combination of On-line pH and Oxygen Transfer Rate Measurement in Shake Flasks by Fiber Optical Technique and Respiration Activity MOnitoring System (RAMOS)

Shake flasks are commonly used for process development in biotechnologyindustry. For this purpose a lot of information is required from the growth conditions duringthe fermentation experiments. Therefore, Anderlei et al. developed the RAMOS technology[1, 2], which proviedes on-line oxygen and carbondioxide transfer rates in shake flasks.Besides oxygen consumption, the pH in the medium also plays an important role for thesuccessful cultivation of micro-organisms and for process development. For online pHmeasurement fiber optical methods based on fluorophores are available. Here a combinationof the on-line Oxygen Transfer Rate (OTR) measurements in the RAMOS device with anon-line, fiber optical pH measurement is presented. To demonstrate the application of thecombined measurement techniques, Escherichia coli cultivations were performed and on-line pH measurements were compared with off-line samples. The combination of on-lineOTR and pH measurements gives a lot of information about the cultivation and, therefore, itis a powerful technique for monitoring shake flask experiments as well as for processdevelopment.

The pyrroloquinoline quinone synthesis genes of Gluconobacter oxydans

A Tn5-induced glucose dehydrogenase (GDH) deficient mutant of Gluconobacter oxydans IFO 3293 was characterised. DNA sequencing showed that the insertion site occurred in an open reading frame with homology to the pqqE gene. It was shown that acid production could be restored by addition of the coenzyme pyrroloquinoline quinone (PQQ) to the medium. The pqq cluster of G. oxydans ATCC 9937 was cloned and sequenced. It has five genes pqqA-E. The cluster could complement the Tn5-induced mutation in IFO 3293. Pulsed-field gel electrophoresis suggested that the pqq genes are not closely linked to the ribF gene that produces the riboflavin cofactor for the gluconic acid dehydrogenase.

A mutant of gluconobacter oxydans deficient in gluconic acid dehydrogenase

Gluconobacter oxydans ATCC 9937 was subjected to transposon mutagenesis using Tn5. A non-pigmented mutant was shown to be defective in gluconic acid dehydrogenase and to produce gluconic acid from glucose, whereas the parent strain produced 2, 5-diketogluconic acid. Cloning and sequencing of the region containing the Tn5 insertion showed that the insertion point occurred in an open reading frame homologous (42% amino acid identity) to the ribF genes of Pseudomonas fluorescens and Escherichia coli. The resulting lack of a riboflavin cofactor would explain the loss of enzyme activity.

Characterisation of plasmids from diketogluconic acid producing strains of Gluconobacter oxydans

Gluconobacter oxydans ATCC 9937, which produces 2,5-diketogluconic acid, an intermediate in vitamin C synthesis, has three plasmids of sizes 27.7 kb (pVJ1), 12.3 kb (pVJ2) and 18 kb (pVJ4). A restriction map was constructed of pVJ1. A potential glucose dehydrogenase gene was located on pVJ1 using the polymerase chain reaction with heterologous primers. Two other G. oxydans strains had no detectable plasmid DNA (IFO 12258) and a plasmid (pVJ3) of 9.4 kb (IFO 3293), respectively.

Intergeneric protoplast fusion between Gluconobacter oxydans and Corynebacterium species

Intergeneric protoplast fusion between 2,5-diketo-gluconic acid producing Gluconobacter oxydans (ATCC 9937) and a mutant strain of Corynebacterium species (ATCC 31090), capable of reducing 2,5-diketo-gluconic acid to 2-keto-L-gulonic acid, a penultimate step in vitamin C production) resulted in viable recombinants. Some of the fusion products exhibited the capacity to convert D-glucose to 2-keto-L-gulonic acid, but the conversion rate is low.

Influence of constant and oscillating dissolved oxygen concentrations on keto acid production by Gluconobacter oxydans subsps. melanogenum

Gluconobacter species are known to oxidise glucose via a direct oxidation pathway which is distinct from the pentose phosphate pathway. In the present communication results of an investigation on the influence of different dissolved oxygen concentrations (DO) on the production of 2,5-diketogluconic acid in batch and chemostat cultures are given. DO of 30% relative to air at 1 bar was found as a threshold level for optimum productivity. The positive influence of continuous availability of dissolved oxygen on the process of rapid glucose oxidation was unambiguously shown as the result of induction of membrane bound dehydrogenases involved in direct glucose oxidation. Furthermore data of scale-down experiments in which the organism was cultivated under oscillations of dissolved oxygen, are given. The influences of such oscillations of DO in the region of the established threshold (30% saturation) were found to result in a prolonged lag phase for growth and product formation. The data obtained in this study revealed critical residence times at low DO that could be employed as a criterion for scale up of this aerobic process.