Optimization of the microbial synthesis of dihydroxyacetone in a semi-continuous repeated-fed-batch process by in situ immobilization of Gluconobacter oxydans

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.

Advances on Bacterial and Fungal Biofilms for the Production of Added-Value Compounds

Simple Summary

The production of bio-based materials, including organic acids, antibiotics, enzymes, ethanol, and hydrogen, is generally done by the cultivation of suspended cells rather than using immobilized cells. However, several studies suggest the application of productive biofilms as a reliable alternative for biocatalysis, with many advantages over suspended-growth systems. This review gives an overview of the breakthrough in the application of biofilm platforms for the sustainable production of valuable compounds, with particular insight into the latest advances in the production of recombinant proteins. Productive biofilms are shown to improve production rates and product yields, demonstrating great potential for industrial applications.

Abstract

In recent years, abundant research has been performed on biofilms for the production of compounds with biotechnological and industrial relevance. The use of biofilm platforms has been seen as a compelling approach to producing fine and bulk chemicals such as organic acids, alcohols, and solvents. However, the production of recombinant proteins using this system is still scarce. Biofilm reactors are known to have higher biomass density, operational stability, and potential for long-term operation than suspended cell reactors. In addition, there is an increasing demand to harness industrial and agricultural wastes and biorefinery residues to improve process sustainability and reduce production costs. The synthesis of recombinant proteins and other high-value compounds is mainly achieved using suspended cultures of bacteria, yeasts, and fungi. This review discusses the use of biofilm reactors for the production of recombinant proteins and other added-value compounds using bacteria and fungi.

Repeated production of 6-(N-hydroxyethyl)-amino-6-deoxy-α-L-sorbofuranose by immobilized Gluconobacter oxydans cells with a strategy of in situ exhaustive cell regeneration

The major troubles in 6-(N-hydroxyethyl)-amino-6-deoxy-α-L-sorbofuranose (6NSL) production from N-2-hydroxyethyl glucamine (NHEG) by Gluconobacter oxydans were low cell yield during cell preparation and loss of cells' biocatalytic ability during biotransformation, resulting in high production cost and low 6NSL production. The target of this work was to enhance 6NSL production by reusing cells and improving the cells biocatalytic ability. First, inhibitory effects of substrate and product on 6NSL production, and optimization of cell regeneration condition were investigated, respectively. Then repeated production of 6NSL by immobilized cell using a strategy of in situ exhaustive cell regeneration in a bubble column bioreactor was developed. As a result, the bioprocess underwent nine cycles, the average 6NSL production and conversion rate of NHEG to 6NSL reached 42.6 g L^-1 and 83.1% in each batch was achieved, respectively.

Dihydroxyacetone production from glycerol using Gluconobacter oxydans: Study of medium composition and operational conditions in shaken flasks

The production of dihydroxyacetone from glycerol employing aerobic cultures of Gluconobacter oxydans is studied. Dihydroxyacetone is one of the most important value-added products obtained from glycerol, a by-product of biodiesel production. The effect of organic nitrogen source and initial substrate concentrations has been studied together with the possibility of product inhibition. Afterward, the influence of the main operating conditions (temperature, shaking speed, and initial biomass concentration) on in vivo glycerol dehydrogenase activity has also been considered. The results show no evidence of glycerol inhibition, but an important product inhibition was detected, which has been taken into account in a kinetic model for enzymatic activity description. In terms of operating conditions, pH was found to exert a great impact on glycerol conversion, being necessary to keep it above 4 to ensure complete glycerol conversion. The minimum temperature that maximized enzymatic activity was found to be 30°C. In addition, a surprising decoupling between biomass concentration and dihydroxyacetone production rate was observed when adding increasing nitrogen source concentrations at a fixed shaking speed. Glycerol dehydrogenase activity remains constant despite the increase in biomass concentration, contrary to what would be expected. This fact revealed the existence of a rate limiting factor, identified subsequently as oxygen transfer rate depending on the biomass concentration.

Valorization of Waste Glycerol to Dihydroxyacetone with Biocatalysts Obtained from Gluconobacter oxydans

Waste glycerol is the main by-product generated during biodiesel production, in an amount reaching up to 10% of the produced biofuel. Is there any method which allows changing this waste into industrial valuable compounds? This manuscript describes a method for valorization of crude glycerol via microbial bioconversion. It has been shown that the use of free and immobilized biocatalysts obtained from Gluconobacter oxydans can enable beneficial valorization of crude glycerol to industrially valuable dihydroxyacetone. The highest concentration of this compound, reaching over 20 g·L−1, was obtained after 72 h of biotransformation with free G. oxydans cells, in a medium containing 30 or 50 g·L−1 of waste glycerol. Using a free cell extract resulted in higher concentrations of dihydroxyacetone and a higher valorization efficiency (up to 98%) compared to the reaction with an immobilized cell extract. Increasing waste glycerol concentration to 50 g·L−1 causes neither a faster nor higher increase in product yield and reaction efficiency compared to its initial concentration of 30 g·L−1. The proposed method could be an alternative for utilization of a petrochemical waste into industry applicated chemicals.

Low-cost biotransformation of glycerol to 1,3-dihydroxyacetone through Gluconobacter frateurii in medium with inorganic salts only

Existing dihydroxyacetone (DHA) production practices require the use of yeast extracts, leading to relatively high production costs. This study explores the use of low-cost media comprising glycerol, inorganic salts and Gluconobacter frateurii BCC 36199 in the production of DHA. The medium components are also quantitatively optimized. Regression models describing the linear correlations between the nutrient concentrations and the generated DHA concentration (p), and between the nutrient concentrations and the yield (y_sp ) are developed. Under the optimal conditions according to our regression models, the highest values for p and y_sp are 29·36 g l^-1 and 97·86% g g^-1 respectively. Quantitatively, this study shows positive effects of inorganic salts and adverse effects of excessive amounts of glycerol on DHA production. In particular, the results suggest that low levels of biomass production lead to high levels of DHA production. Consequently, the media containing inorganic nitrogen source from (NH₄ )₂ SO₄ lead to higher yields than organic media containing yeast extract. This study has identified an optimal, low-cost, minimal medium that can effectively enhance DHA production.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study illustrates the advantages of inorganic nutrients supplementation over organic nutrient supplementation for a lower media cost and a higher dihydroxyacetone (DHA) production yield through Gluconobacter frateurii BCC 36199 cultivation. The study found that the use of media that contain only glycerol and inorganic salts enhanced DHA production (DHA-Prod) while keeping the production of bacterial biomass at a sufficient level. Most of the starting material, that is, glycerol, is converted into DHA, which is the target of the production process. The cost of the nitrogen supplement in the DHA-Prod process may be reduced by up to 80% through the use of the inorganic culture medium that has been developed in this study.

Investigations in sonication-induced intensification of crude glycerol fermentation to dihydroxyacetone by free and immobilized Gluconobacter oxydans

This study reports crude glycerol fermentation by G. oxydans for dihydroxyacetone (DHA) production, and intensification of fermentation with sonication. Fermentation was carried out using both free and immobilized cells (on polyurethane foam support) for initial glycerol concentrations of 20, 30 and 50 g/L. Sonication at 20% duty cycle enhanced glycerol consumption by 60-84% with no significant change in cell morphology. Lesser DHA yield in crude glycerol fermentation was attributed to possible formation of inhibitory products. Slight reduction in DHA yield for initial glycerol concentration of 50 g/L was attributed to substrate inhibition. Higher DHA productivity was obtained for immobilized cells. Circular dichroism analysis of intracellular proteins obtained from ultrasound-treated G. oxydans revealed significant reduction in α-helix and β-sheet content. These conformational changes in protein structure could augment activity of intracellular glycerol dehydrogenase, which is manifested in terms of enhanced metabolism of glycerol by G. oxydans.

Process optimization and analysis of product inhibition kinetics of crude glycerol fermentation for 1,3-Dihydroxyacetone production

In present study, statistical optimization of biodiesel-derived crude glycerol fermentation to DHA by immobilized G. oxydans cells over polyurethane foam is reported. Effect of DHA (product) inhibition on crude glycerol fermentation was analyzed using conventional biokinetic models and new model that accounts for both substrate and product inhibition. Optimum values of fermentation parameters were: pH=4.7, temperature=31°C, initial substrate concentration=20g/L. At optimum conditions, DHA yield was 89% (17.83g/L). Effect of product inhibition on fermentation was trivial for DHA concentrations ≤30g/L. At higher concentrations (≥50g/L), kinetics and yield of fermentation showed marked reduction with sharp drop in V_max and K_S values. Inhibition effect was more pronounced for immobilized cells due to restricted transport of fermentation mixture across polyurethane foam. Retention of fermentation mixture in immobilized matrix resulted in higher localized DHA concentration that possibly enhanced inhibition effect.

Repeated biotransformation of glycerol to 1,3-dihydroxyacetone by immobilized cells of Gluconobacter oxydans with glycerol- and urea-feeding strategy in a bubble column bioreactor

Some inorganic nitrogen sources and amino acids instead of yeast extract, which resulted in trouble of product purification, were introduced for 1,3-dihydroxyacetone (DHA) production by biotransformation with Gluconobacter oxydans. The results showed that urea is an optimal nitrogen source. Furthermore, the effects of glycerol- and urea-feeding strategies for DHA production by immobilized cells in a home-made bubble column bioreactor were optimized. Cells immobilization was prepared by cultivation in the bioreactor packed with porous ceramics, and then the broth was removed. Then, repeated biotransformation by continuous-feeding of glycerol and urea was developed. Up to 96.4±4.1g/L of average DHA concentration with 94.8±2.2% of average conversion rate of glycerol to DHA was achieved after 12 cycles of run. Near colorless DHA solution with few impurities was obtained and the production cost could be decreased.

Optimization of 1,3-dihydroxyacetone production from crude glycerol by immobilized Gluconobacter oxydans MTCC 904

This study has addressed the matter of optimization of production of the value added product, dihydroxyacetone, from crude glycerol using immobilized cells of Gluconobacter oxydans. Statistical optimization of the fermentation medium revealed MgSO4·7H2O, (NH4)2SO4 and KH2PO4 as the significant components, in addition to small concentration of yeast extract. As per previous literature, these components augment the activity of glycerol dehydrogenase enzyme in metabolism and provide assimilable nitrogen and sulfur source for cell growth. Yeast extract not only provides essential growth factors, but also accelerates production of alcohol dehydrogenase enzyme due to amino acids present. The DHA yield from crude glycerol (20g/L) with optimized medium is 14.08g/L, which is just 12% lower than the yield for pure glycerol .This study has thus established that proper optimization of fermentation medium reduces the adverse effect of impurities in crude glycerol on fermentation process and DHA yield.

Kinetic analysis of dihydroxyacetone production from crude glycerol by immobilized cells of Gluconobacter oxydans MTCC 904

The present study has investigated kinetic features of bioconversion of biodiesel-derived crude glycerol to dihydroxyacetone with immobilized Gluconobacter oxydans cells using modified Haldane substrate-inhibition model. The results have been compared against free cells and pure glycerol. Relative variations in the kinetic parameters KS, KI, Vmax, n and X reveal that immobilized G. oxydans cells (on PU foam substrate) with crude glycerol as substrate give higher order of inhibition (n) and lower maximum reaction velocities (Vmax). These results are essentially implications of substrate transport restrictions across immobilization matrix, which causes retention of substrate in the matrix and reduction in fractional available substrate (X) for the cells. This causes reduction in both KS (substrate concentration at Vmax/2) and KI (inhibition constant) as compared to free cells. For immobilized cells, substrate concentration (Smax) corresponding to Vmax is practically same for both pure and crude glycerol as substrate.

Improving the production yield and productivity of 1,3-dihydroxyacetone from glycerol fermentation using Gluconobacter oxydans NL71 in a compressed oxygen supply-sealed and stirred tank reactor (COS-SSTR)

In this study, a compressed oxygen gas supply was connected to a sealed aerated stirred tank reactor (COS-SSTR) bio-system, leading to a high-oxygen pressure bioreactor used to improve the bio-transformative performance in the production of 1,3-dihydroxyacetone (DHA) from glycerol using Gluconobacter oxydans NL71. A concentration of 301.2 ± 8.2 g L(-1) DHA was obtained from glycerol after 32 h of fed-batch fermentation in the COS-SSTR system. The volumetric productivity for this process was 9.41 ± 0.23 g L(-1) h(-1), which is presently the highest obtained level of glycerol bioconversion into DHA. These results show that the application of this bioreactor would enable microbial production of DHA from glycerol at the industrial scale.

Simultaneous Bioconversion of Xylose and Glycerol to Xylonic Acid and 1,3-Dihydroxyacetone from the Mixture of Pre-Hydrolysates and Ethanol-Fermented Waste Liquid by Gluconobacter oxydans

Simultaneous bioconversion of xylose and glycerol to xylonic acid and 1,3-dihydroxyacetone (DHA) was realized by using Gluconobacter oxydans (G. oxydans). Currently, the enzymatic hydrolysate to ethanol-fermented waste liquid and the inorganic acid pre-hydrolysate that contain abundant glycerol and xylose were difficult to be utilized or disposed. Based on the method of compressed oxygen supply-sealed and stirred tank reactor system (COS-SSTR), the xylonic acid and 1,3-dihydroxyacetone could be co-produced rapidly with the mixture of the dilute sulfuric acid pre-hydrolysate and ethanol-fermented waste liquid of enzymatic hydrolysate (MPEW) as material. By means of the system, we finally produced 102.3 ± 3.2 g/L xylonic acid and 40.6 ± 1.8 g/L 1,3-dihydroxyacetone at yield of 92.4 ± 2.8 % and 80.6 ± 3.5 % directly and simultaneously from the mixed solution. The central features of this bioprocess application would enable cost-competitive bacterial xylonic acid and 1,3-dihydroxyacetone production from lignocellulosic materials.

Recent progress in the development of solid catalysts for biomass conversion into high value-added chemicals

In recent decades, the substitution of non-renewable fossil resources by renewable biomass as a sustainable feedstock has been extensively investigated for the manufacture of high value-added products such as biofuels, commodity chemicals, and new bio-based materials such as bioplastics. Numerous solid catalyst systems for the effective conversion of biomass feedstocks into value-added chemicals and fuels have been developed. Solid catalysts are classified into four main groups with respect to their structures and substrate activation properties: (a) micro- and mesoporous materials, (b) metal oxides, (c) supported metal catalysts, and (d) sulfonated polymers. This review article focuses on the activation of substrates and/or reagents on the basis of groups (a)-(d), and the corresponding reaction mechanisms. In addition, recent progress in chemocatalytic processes for the production of five industrially important products (5-hydroxymethylfurfural, lactic acid, glyceraldehyde, 1,3-dihydroxyacetone, and furan-2,5-dicarboxylic acid) as bio-based plastic monomers and their intermediates is comprehensively summarized.

Tin-catalyzed conversion of biomass-derived triose sugar and formaldehyde to α-hydroxy-γ-butyrolactone

The direct conversion of biomass-derived 1,3-dihydroxyacetone (DHA) and formaldehyde to α-hydroxy-γ-butyrolactone (HBL) was achieved through the use of tin(iv) chloride and a small amount of water and the yield reached up to 70%. The reaction mechanism was also investigated by incorporating d2-formaldehyde into the reaction mixtures.

Highly active tin(iv) phosphate phase transfer catalysts for the production of lactic acid from triose sugars

Tin phosphate phase transfer catalysts were used efficiently for the conversion of trioses to lactic acid under hydrothermal conditions.

Selective oxidation of glycerol to 1,3-dihydroxyacetone by covalently immobilized glycerol dehydrogenases with higher stability and lower product inhibition

Glycerol dehydrogenase (GlyDH) catalyzes the regioselective oxidation of glycerol to yield 1,3-dihydroxyacetone (DHA); an important building block in chemical industry. Three recombinant GlyDHs from Geobacillus stearothermophilus, from Citrobacter braakii and from Cellulomonas sp. were stabilized by covalent immobilization. The highest activity recoveries (40-50%) of the insoluble preparations were obtained by immobilizing these enzymes in presence of polyethylene glycol (PEG). Noteworthy, these immobilized preparations were more stable and less inhibited by DHA than their soluble counterparts. In particular, GlyDH from G.stearothermophilus immobilized on agarose activated with both amine and glyoxyl groups and crosslinked with dextran aldehyde was 3.7-fold less inhibited by DHA than its soluble form and retained 100% of its initial activity after 18h of incubation at 65°C and pH 7. This is one of the few examples where the same immobilization protocol has minimized enzyme product inhibition and maximized thermal stability.

Whole-cell biocatalysis for selective and productive C-O functional group introduction and modification

During the last decades, biocatalysis became of increasing importance for chemical and pharmaceutical industries. Regarding regio- and stereospecificity, enzymes have shown to be superior compared to traditional chemical synthesis approaches, especially in C-O functional group chemistry. Catalysts established on a process level are diverse and can be classified along a functional continuum starting with single-step biotransformations using isolated enzymes or microbial strains towards fermentative processes with recombinant microorganisms containing artificial synthetic pathways. The complex organization of respective enzymes combined with aspects such as cofactor dependency and low stability in isolated form often favors the use of whole cells over that of isolated enzymes. Based on an inventory of the large spectrum of biocatalytic C-O functional group chemistry, this review focuses on highlighting the potentials, limitations, and solutions offered by the application of self-regenerating microbial cells as biocatalysts. Different cellular functionalities are discussed in the light of their (possible) contribution to catalyst efficiency. The combined achievements in the areas of protein, genetic, metabolic, and reaction engineering enable the development of whole-cell biocatalysts as powerful tools in organic synthesis.

Optimisation of engineered Escherichia coli biofilms for enzymatic biosynthesis of l-halotryptophans

Engineered biofilms comprising a single recombinant species have demonstrated remarkable activity as novel biocatalysts for a range of applications. In this work, we focused on the biotransformation of 5-haloindole into 5-halotryptophan, a pharmaceutical intermediate, using Escherichia coli expressing a recombinant tryptophan synthase enzyme encoded by plasmid pSTB7. To optimise the reaction we compared two E. coli K-12 strains (MC4100 and MG1655) and their ompR234 mutants, which overproduce the adhesin curli (PHL644 and PHL628). The ompR234 mutation increased the quantity of biofilm in both MG1655 and MC4100 backgrounds. In all cases, no conversion of 5-haloindoles was observed using cells without the pSTB7 plasmid. Engineered biofilms of strains PHL628 pSTB7 and PHL644 pSTB7 generated more 5-halotryptophan than their corresponding planktonic cells. Flow cytometry revealed that the vast majority of cells were alive after 24 hour biotransformation reactions, both in planktonic and biofilm forms, suggesting that cell viability was not a major factor in the greater performance of biofilm reactions. Monitoring 5-haloindole depletion, 5-halotryptophan synthesis and the percentage conversion of the biotransformation reaction suggested that there were inherent differences between strains MG1655 and MC4100, and between planktonic and biofilm cells, in terms of tryptophan and indole metabolism and transport. The study has reinforced the need to thoroughly investigate bacterial physiology and make informed strain selections when developing biotransformation reactions.

Biofilms as living catalysts in continuous chemical syntheses

Biofilms are resilient to a wide variety of environmental stresses. This inherited robustness has been exploited mainly for bioremediation. With a better understanding of their physiology, the application of these living catalysts has been extended to the production of bulk and fine chemicals as well as towards biofuels, biohydrogen, and electricity production in microbial fuel cells. Numerous challenges call for novel solutions and concepts of analytics, biofilm reactor design, product recovery, and scale-up strategies. In this review, we highlight recent advancements in spatiotemporal biofilm characterization and new biofilm reactor developments for the production of value-added fine chemicals as well as current challenges and future scenarios.

Use of glycerol for producing 1,3-dihydroxyacetone by Gluconobacter oxydans in an airlift bioreactor

1,3-Dihydroxyacetone can be produced by biotransformation of glycerol with glycerol dehydrogenase from Gluconobacter oxydans cells. Firstly, improvement the activity of glycerol dehydrogenase was carried out by medium optimization. The optimal medium for cell cultivation was composed of 5.6g/l yeast extract, 4.7 g/l glycerol, 42.1g/l mannitol, 0.5 g/l K(2)HPO(4), 0.5 g/l KH(2)PO(4), 0.1g/l MgSO(4)·7H(2)O, and 2.0 g/l CaCO(3) with the initial pH of 4.9. Secondly, an internal loop airlift bioreactor was applied for DHA production from glycerol by resting cells of G. oxydans ZJB09113. Furthermore, the effects of pH, aeration rate and cell content on DHA production and glycerol feeding strategy were investigated. 156.3 ± 7.8 g/l of maximal DHA concentration with 89.8±2.4% of conversion rate of glycerol to DHA was achieved after 72h of biotransformation using 10g/l resting cells at 30°C, pH 5.0 and 1.5vvm of aeration rate.

Catalytic conversion of dihydroxyacetone to lactic acid using metal salts in water

We herein present a study on the application of homogeneous catalysts in the form of metal salts on the conversion of trioses, such as dihydroxyacetone (DHA), and glyceraldehyde (GLY) to lactic acid (LA) in water. A wide range of metal salts (26 in total) were examined. Al(III) salts were identified as the most promising and essentially quantitative LA yields (>90 mol%) were obtained at 140 °C and a reaction time of 90 min. A reaction pathway is proposed and a kinetic model using the power law approach was developed for the conversion of DHA to LA with pyruvaldehyde (PRV) as the intermediate. Good agreement between experimental data and the model was obtained. Model predictions, supported by experiments, indicate that a high yield of LA is favoured in dilute solutions of DHA (0.1 M) at elevated temperatures (180 °C) and reaction times less than 10 min.

Cell immobilization for production of lactic acid biofilms do it naturally

Interest in natural cell immobilization or biofilms for lactic acid fermentation has developed considerably over the last few decades. Many studies report the benefits associated with biofilms as industrial methods for food production and for wastewater treatment, since the formation represents a protective means of microbial growth offering survival advantages to cells in toxic environments. The formation of biofilms is a natural process in which microbial cells adsorb to a support without chemicals or polymers that entrap the cells and is dependent on the reactor environment, microorganism, and characteristics of the support. These unique characteristics enable biofilms to cause chronic infections, disease, food spoilage, and devastating effects as in microbial corrosion. Their distinct resistance to toxicity, high biomass potential, and improved stability over cells in suspension make biofilms a good tool for improving the industrial economics of biological lactic acid production. Lactic acid bacteria and specific filamentous fungi are the main sources of biological lactic acid. Over the past two decades, studies have focused on improving the lactic acid volumetric productivity through reactor design development, new support materials, and improvements in microbial production strains. To illustrate the operational designs applied to the natural immobilization of lactic acid producing microorganisms, this chapter presents the results of a search for optimum parameters and how they are affected by the physical, chemical, and biological variables of the process. We will place particular emphasis upon the relationship between lactic acid productivity attained by various types of reactors, supports, media formulations, and lactic acid producing microorganisms.

Microbial biofilms: a concept for industrial catalysis?

Biofilm reactors have long been commercially used in the treatment of wastewater and off-gas. New opportunities are arising with the rapid expansion of our understanding of biofilm biology over the last few years. Biofilms have great potential as industrial workhorses for the sustainable production of chemicals because of their inherent characteristics of self-immobilization, high resistance to reactants and long-term activity, which all facilitate continuous processing. A variety of biofilm reactor configurations have been explored for productive catalysis and some reactors have been operated continuously for months. Sectors that might particularly benefit from this biofilm approach include synthetic chemistry (ranging from specialty to bulk chemicals), bioenergy, biologics and the food industry.

A microbial biosensor based on bacterial cells immobilized on chitosan matrix

A bio-electrochemical system consisting of Gluconobacter oxydans DSM 2343 cells as a biological material and carbon nanotube (CNT)-free and CNT-modified chitosan as immobilizing matrices has been developed. The measurement was based on the respiratory activity of the cells estimated by the oxygen consumption at -0.7 V (versus the Ag|AgCl reference electrode) due to the metabolic activity in the presence of substrates. The system was calibrated and dependence of signal amplitude on the measuring conditions and cell amount was studied as well as the substrate specificity, pH, temperature and working potential. The biosensors (CNT-modified and unmodified) were demonstrated for the quantification of glucose in the range of 0.05-1.0 mM, at 30 degrees C and pH 7.0 with the 40 s of response time. The linear relationships between sensor response (y; microA/cm(2)) and substrate concentration (x; mM) were defined by the equations of y=1.160x+0.151 (R(2)=0.990) and y=1.261x+0.197 (R(2)=0.982), respectively. All other data were also given as comparison of two systems one with CNT-modified and CNT-free.

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.

Repeated pH-stat fed-batch fermentation for rhamnolipid production with indigenous Pseudomonas aeruginosa S2

Rhamnolipid is one of the most commonly used biosurfactants with the ability to reduce the surface tension of water from 72 to 30 mN/m. An indigenous isolate Pseudomonas aeruginosa S2 possessing excellent ability to produce rhamnolipid was used as a model strain to explore fermentation technology for rhamnolipid production. Using optimal medium and operating conditions (37 degrees C, pH 6.8, and 250 rpm agitation) obtained from batch fermentation, P. aeruginosa S2 was able to produce up to 5.31 g/l of rhamnolipid from glucose-based medium. To further improve the rhamnolipid yield, a pH-stat fed-batch culture was performed by maintaining a constant pH of 6.8 through manipulating glucose feeding. The effect of influent glucose concentration on rhamnolipid yield and productivity was investigated. Using the pH-stat culture, a maximum rhamnolipid concentration (6.06 g/l) and production rate (172.5 ml/h/l) was obtained with 6% glucose in the feed. Moreover, combining pH-stat culture with fill-and-draw operation allowed a stable repeated fed-batch operation for approximately 500 h. A marked increase in rhamnolipid production was achieved, leading to the best rhamnolipid yield of approximately 9.4 g/l during the second repeated run.