In situ product removal (ISPR) in whole cell biotechnology during the last twenty years

The liquid-liquid extractive fermentation of L-lactic acid in a novel semi-partition bioreactor (SPB)

Fermentation technology is commonly used as a mature process to produce numerous products with the help of micro-organisms. However, these organisms are sometimes inhibited by the accumulation of these products or their by-products. One route to circumvent this is via extractive fermentation, which combines the fermentation process with extraction. To facilitate this, novel bioreactor designs are required, such as the semi-partition bioreactor (SPB) which has been recently proposed for in-situ extractive fermentation. The latter combines a fermentation and an extraction unit into a single vessel using a mixer-settler principle. Where the bioproduct is produced in the mixer and removed continuous in the settler. As the SPB functionality is a subject of interest, this study builds on demonstrating different process conditions in the production of a sample bioprocess (lactic acid (LA)) which is susceptible to product inhibition. The results showed a 34.5 g/L LA concentration was obtained in the pH-controlled condition. While LA production can suffer from product inhibition, neutralizing agents can be easily used to curb inhibitory problems, however, the LA fermentation is a simple (and well-studied) example, which can demonstrate an alternative route to avoiding product inhibition (for systems which cannot be rescued using pH control). Hence, to replicate a scenario of product inhibition, two different process conditions were investigated, no pH control with no extraction (non-integrated), and no pH control with integrated extractive fermentation. Key findings showed higher LA concentration in integrated (25.10 g/L) as compared to the non-integrated (14.94 g/L) case with improved yield (0.75 gg^-1 (integrated) versus 0.60 gg^-1 (non-integrated)) and overall productivity (0.35 gL^-1h^-1(integrated) versus 0.20 gL^-1h^-1(non-integrated)) likewise. This is the first demonstration of an SP bioreactor, and shows how the reactor can be applied to improve productivity. Based on these results, the SPB design can be applied to produce any product liable to product inhibition.

Enhancement of triterpene production via in situ extractive fermentation of Sanghuangporus vaninii YC-1

There have been many studies on the activities and polysaccharide production of Sanghuangporus vaninii. However, few studies have looked at triterpene production from S. vaninii using liquid-state fermentation. A method for enhancing the production of triterpenes by in situ extractive fermentation (ISEF) was studied. Eight solvents were investigated as extractants for triterpene production in the ISEF system. The results showed that using vegetable oil as an extractant significantly increased the yield of total triterpenes and biomass of S. vaninii YC-1, reaching 18.98 ± 0.71 and 44.67 ± 2.21 g/L, respectively. In 5 L fermenter experiments, the added vegetable oil improved the dissolved oxygen condition of the fermentation broth and promoted the growth of S. vaninii YC-1. Furthermore, adding vegetable oil increased the expression of fatty acid synthesis-related genes such as FAD2 and SCD, thereby increasing the synthesis of unsaturated fatty acids in the cell membrane of S. vaninii YC-1. Therefore, the cell membrane permeability of S. vaninii YC-1 increased by 19%. Our results indicated that vegetable oil increased the permeability of S. vaninii YC-1 cell membranes to promote the production of total triterpenes. The use of vegetable oil as an extractant was thus effective in increasing the yield of triterpenes in the ISEF system.

Intensification of p-coumaric acid heterologous production using extractive biphasic fermentation

p-coumaric acid (p-CA) can be produced from D-glucose by an engineered S. cerevisiae strain. p-CA has antimicrobial properties and retro-inhibition activity. Moreover, p-CA is a hydrophobic compound, limiting its accumulation in fermentation broth. To overcome these issues all at once, a liquid-liquid extraction in-situ product recovery process using oleyl alcohol as extractant has been implemented in order to continuously extract p-CA from the broth. Media and pH impacts on strain metabolism were assessed, highlighting p-CA decarboxylase endogenous activity. Biphasic fermentations allowed an increase in p-CA respiratory production rates at both pH assessed (13.65 and 9.45 mg L^-1.h^-1 at pH 6 and 4.5, respectively) compared to control ones (10.5 and 7.5 mg L^-1.h^-1 at pH 6 and 4.5, respectively). Biphasic fermentation effects on p-CA decarboxylation were studied showing that continuous removal of p-CA decreased its decarboxylation into 4-vinylphenol at pH 4.5 (57 mg L^-1 in biphasic fermentation vs 173 mg L^-1 in control one).

Ionic liquid-based in situ product removal design exemplified for an acetone-butanol-ethanol fermentation

Selecting an appropriate separation technique is essential for the application of in situ product removal (ISPR) technology in biological processes. In this work, a three-stage systematic design method is proposed as a guide to integrate ionic liquid (IL)-based separation techniques into ISPR. This design method combines the selection of a suitable ISPR processing scheme, the optimal design of an IL-based liquid-liquid extraction (LLE) system followed by process simulation and evaluation. As a proof of concept, results for a conventional acetone-butanol-ethanol fermentation are presented (40,000 ton/year butanol production). In this application, ILs tetradecyl(trihexyl)phosphonium tetracyanoborate ([TDPh][TCB]) and tetraoctylammonium 2-methyl-1-naphthoate ([TOA] [MNaph]) are identified as the optimal solvents from computer-aided IL design (CAILD) method and reported experimental data, respectively. The dynamic simulation results for the fermentation process show that, the productivity of IL-based in situ (fed-batch) process and in situ (batch) process is around 2.7 and 1.8fold that of base case. Additionally, the IL-based in situ (fed-batch) process and in situ (batch) process also have significant energy savings (79.6% and 77.6%) when compared to the base case.

Enhanced production of ε-poly-L-lysine by immobilized Streptomyces ahygroscopicus through repeated-batch or fed-batch fermentation with in situ product removal

ε-Poly-L-lysine (ε-PL) is a naturally-occurring L-lysine homopolymer having a broad-spectrum antimicrobial activity and used widely as a food preservative. In the present study, the combined use of immobilization and in situ product removal (ISPR) was evaluated for the production of ε-PL by Streptomyces ahygroscopicus GIM8. Results showed that ε-PL production in the flask cultures decreased from 0.84 to 0.38-0.56 g/L upon immobilization on loofah sponge with different amounts (0.5-3 g in 50 mL medium in a flask). By applying continuous ISPR to the immobilized flask cultures, ε-PL production as high as 3.51 g/L was obtained compared to 0.51 g/L of the control. A satisfactory titer of 1.84 g/L ε-PL could also be achieved with intermittent ISRP (three cycles of ISPR operation during cultivation). Further investigation showed that low levels of ε-PL retained in the broth appeared to favor its biosynthesis. In the repeated-batch fermentation in a 5 L immobilized bioreactor, with continuous ISPR, the final average ε-PL concentration and productivity were 3.35 g/L and 0.797 g/L/day, respectively, and 3.18 g/L and 0.756 g/L/day for the alternative (intermittent ISPR), in comparison to 1.16 g/L and 0.277 g/L/day with no ISPR usage. In the fed-batch fermentation with immobilized cells, the combined use of intermittent ISPR and extra nutrient feeding increased ε-PL concentration and productivity up to 24.57 g/L and 9.34 g/L/day. The fermentation processes developed could serve as an effective approach for ε-PL production and, moreover, the combination could greatly simplify downstream processing for ε-PL separation and purification.

Design and evaluation of a continuous semipartition bioreactor for in situ liquid-liquid extractive fermentation

Extractive fermentation (or in situ product removal (ISPR)) is an operational method used to combat product inhibition in fermentations. To achieve ISPR, different separation techniques, modes of operation and physical reactor configurations have been proposed. However, the relative paucity of industrial application necessitates continued investigation into reactor systems. This article outlines a bioreactor designed to facilitate in situ product extraction and recovery, through adapting the reaction volume to include a settler and solvent extraction and recycle section. This semipartition bioreactor is proposed as a new mode of operation for continuous liquid-liquid extractive fermentation. The design is demonstrated as a modified bench-top fermentation vessel, initially analysed in terms of fluid dynamic studies, in a model two-liquid phase system. A continuous abiotic simulation of lactic acid (LA) fermentation is then demonstrated. The results show that mixing in the main reaction vessel is unaffected by the inserted settling zone, and that the size of the settling tube effects the maximum volumetric removal rate. In these tests the largest settling tube gave a potential continuous volumetric removal rate of 7.63 ml/min; sufficiently large to allow for continuous product extraction even in a highly productive fermentation. To demonstrate the applicability of the developed reactor, an abiotic simulation of a LA fermentation was performed. LA was added to reactor continuously at a rate of 33ml/h, while continuous in situ extraction removed the LA using 15% trioctylamine in oleyl alcohol. The reactor showed stable LA concentration of 1 g/L, with the balance of the LA successfully extracted and recovered using back extraction. This study demonstrates a potentially useful physical configuration for continuous in situ extraction.

In situ product recovery techniques aiming to obtain biotechnological products: A glance to current knowledge

Biotechnology and bioengineering techniques have been widely used in the production of biofuels, chemicals, pharmaceuticals, and food additives, being considered a "green" form of production because they use renewable and nonpolluting energy sources. On the other hand, in the traditional processes of production, the target product obtained by biotechnological routes must undergo several stages of purification, which makes these processes more expensive. In the past few years, some works have focused on processes that integrate fermentation to the recovery and purification steps necessary to obtain the final product required. This type of process is called in situ product recovery or extractive fermentation. However, there are some differences in the concepts of the techniques used in these bioprocesses. In this way, this review sought to compile relevant content on considerations and procedures that are being used in this field, such as evaporation, liquid-liquid extraction, permeation, and adsorption techniques. Also, the objective of this review was to approach the different configurations in the recent literature of the processes employed and the main bioproducts obtained, which can be used in the food, pharmaceutical, chemical, and/or fuel additives industry. We intended to elucidate concepts of these techniques, considered very recent, but which emerge as a promising alternative for the integration of bioprocesses.

Improving the Yield of Xenocoumacin 1 Enabled by In Situ Product Removal

Xenocoumacin 1 (Xcn1), a major antimicrobial compound produced by Xenorhabdus nematophila CB6, has great potential to be developed into a novel biofungicide. However, its low yield in the producing cells has limited its possible commercial applications. In this study, we explored the effect of in situ product removal (ISPR), a well-established recovery technique, with the use of macroporous resin X-5 on the production of Xcn1 in a fermentation setting. Relative to the routine fermentation process, the yield of Xcn1 was improved from 42.5 to 73.8 μg/mL (1.7-fold) and 12.9 to 60.3 μg/mL (4.7-fold) in three and ten days, respectively. By agar diffusion plate and growth inhibition assays, the antibiotic activity against Bacillus subtilis and Alternaria solani was also found to be improved. Further study revealed that protection of Xcn1 against degradation and decrease in cell self-toxicity as well as upregulation of biosynthesis-related genes of Xcn1 at the transcription level contributed to yield improvement of Xcn1. In addition, resin X-5 significantly altered the metabolite profile of X. nematophila CB6, which could promote the discovery of new antibiotics.

Isolation and Characterization of Ochrobactrum tritici for Penicillin V Potassium Degradation

Substantial concentrations of penicillin V potassium (PVK) have been found in the environment, which may pose potential threats to human health and contribute to the emergence of penicillin-resistant bacterial strains. In this study, antibiotic-degrading bacterial strains for PVK were isolated from sludge and characterized. Ochrobactrum tritici was selected for the biodegradation of PVK with high efficiency. To enhance its PVK degradation ability, a whole-cell biodegradation process was established and optimized using Ochrobactrum tritici. The degradation rate with 0.5 mg/ml PVK reached 100% within 3 h. The potential biodegradation pathway was also investigated. To the best of our knowledge, the present study provides new insights into the biodegradation of PVK using an Ochrobactrum tritici strain, a promising candidate strain for the industrial biodegradation of β-lactam antibiotics.

Substantial concentrations of penicillin V potassium (PVK) have been found in livestock manure, soil, and wastewater effluents, which may pose potential threats to human health and contribute to the emergence of penicillin-resistant bacterial strains. In this study, bacterial strains capable of degrading PVK were isolated from sludge and characterized. Strain X-2 was selected for biodegradation of PVK. Based on morphological observations and 16S rRNA gene sequencing, strain X-2 was identified as an Ochrobactrum tritici strain. To enhance the PVK degradation ability of PVK, a whole-cell biodegradation process of Ochrobactrum tritici X-2 was established and optimized. In the whole-cell biodegradation process, the optimal temperature and pH were 30°C and 7.0, respectively. Under the optimized conditions, the degradation rate using 0.5 mg/ml PVK reached 100% within 3 h. During biodegradation, two major metabolites were detected: penicilloic acid and phenolic acid. The present study provides a novel method for the biodegradation of PVK using Ochrobactrum tritici strains, which represent promising candidates for the industrial biodegradation of PVK.

IMPORTANCE Substantial concentrations of penicillin V potassium (PVK) have been found in the environment, which may pose potential threats to human health and contribute to the emergence of penicillin-resistant bacterial strains. In this study, antibiotic-degrading bacterial strains for PVK were isolated from sludge and characterized. Ochrobactrum tritici was selected for the biodegradation of PVK with high efficiency. To enhance its PVK degradation ability, a whole-cell biodegradation process was established and optimized using Ochrobactrum tritici. The degradation rate with 0.5 mg/ml PVK reached 100% within 3 h. The potential biodegradation pathway was also investigated. To the best of our knowledge, the present study provides new insights into the biodegradation of PVK using an Ochrobactrum tritici strain, a promising candidate strain for the industrial biodegradation of β-lactam antibiotics.

Improved Production and In Situ Recovery of Sesquiterpene (+)-Zizaene from Metabolically-Engineered E. coli

The sesquiterpene (+)-zizaene is the direct precursor of khusimol, the main fragrant compound of the vetiver essential oil from Chrysopogon zizanioides and used in nearly 20% of men's fine perfumery. The biotechnological production of such fragrant sesquiterpenes is a promising alternative towards sustainability; nevertheless, product recovery from fermentation is one of the main constraints. In an effort to improve the (+)-zizaene recovery from a metabolically-engineered Escherichia coli, we developed an integrated bioprocess by coupling fermentation and (+)-zizaene recovery using adsorber extractants. Initially, (+)-zizaene volatilization was confirmed from cultivations with no extractants but application of liquid-liquid phase partitioning cultivation (LLPPC) improved (+)-zizaene recovery nearly 4-fold. Furthermore, solid-liquid phase partitioning cultivation (SLPPC) was evaluated by screening polymeric adsorbers, where Diaion HP20 reached the highest recovery. Bioprocess was scaled up to 2 L bioreactors and in situ recovery configurations integrated to fermentation were evaluated. External recovery configuration was performed with an expanded bed adsorption column and improved (+)-zizaene titers 2.5-fold higher than LLPPC. Moreover, internal recovery configuration (IRC) further enhanced the (+)-zizaene titers 2.2-fold, whereas adsorption velocity was determined as critical parameter for recovery efficiency. Consequently, IRC improved the (+)-zizaene titer 8.4-fold and productivity 3-fold from our last report, achieving a (+)-zizaene titer of 211.13 mg L-1 and productivity of 3.2 mg L-1 h-1. This study provides further knowledge for integration of terpene bioprocesses by in situ product recovery, which could be applied for many terpene studies towards the industrialization of fragrant molecules.

Mating of 2 Laboratory Strains Resulted in Enhanced Production of 2-Phenylethanol by Biotransformation of L-Phenylalanine

2-Phenylethanol (2-PE) is an aromatic alcohol with a rosy scent which is widely used in the food, fragrance, and cosmetic industries. Promising sources of natural 2-PE are microorganisms, especially yeasts, which can produce 2-PE by biosynthesis and biotransformation. Thus, the first challenging goal in the development of biotechnological production of 2-PE is searching for highly productive yeast strains. In the present work, 5 laboratory <i>Saccharomyces cerevisiae</i> strains were tested for the production of 2-PE. Thereafter, 2 of them were hybridized by a mating procedure and, as a result, a new diploid, <i>S. cerevisiae</i> AM1-d, was selected. Within the 72-h batch culture in a medium containing 5 g/L of <smlcap>L</smlcap>-phenylalanine, AM1-d produced 3.83 g/L of 2-PE in a shaking flask. In this way, we managed to select the diploid <i>S. cerevisiae</i> AM1-d strain, showing a 3- and 5-fold increase in 2-PE production in comparison to parental strains. Remarkably, the enhanced production of 2-PE by the hybrid of 2 yeast laboratory strains is demonstrated here for the first time.

Efficient production of the Nylon 12 monomer ω-aminododecanoic acid methyl ester from renewable dodecanoic acid methyl ester with engineered Escherichia coli

The expansion of microbial substrate and product scopes will be an important brick promoting future bioeconomy. In this study, an orthogonal pathway running in parallel to native metabolism and converting renewable dodecanoic acid methyl ester (DAME) via terminal alcohol and aldehyde to 12-aminododecanoic acid methyl ester (ADAME), a building block for the high-performance polymer Nylon 12, was engineered in Escherichia coli and optimized regarding substrate uptake, substrate requirements, host strain choice, flux, and product yield. Efficient DAME uptake was achieved by means of the hydrophobic outer membrane porin AlkL increasing maximum oxygenation and transamination activities 8.3 and 7.6-fold, respectively. An optimized coupling to the pyruvate node via a heterologous alanine dehydrogenase enabled efficient intracellular L-alanine supply, a prerequisite for self-sufficient whole-cell transaminase catalysis. Finally, the introduction of a respiratory chain-linked alcohol dehydrogenase enabled an increase in pathway flux, the minimization of undesired overoxidation to the respective carboxylic acid, and thus the efficient formation of ADAME as main product. The completely synthetic orthogonal pathway presented in this study sets the stage for Nylon 12 production from renewables. Its effective operation achieved via fine tuning the connectivity to native cell functionalities emphasizes the potential of this concept to expand microbial substrate and product scopes.

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.

Overcoming co-product inhibition in the nicotinamide independent asymmetric bioreduction of activated C=C-bonds using flavin-dependent ene-reductases

Eleven flavoproteins from the old yellow enzyme family were found to catalyze the disproportionation (“dismutation”) of conjugated enones. Incomplete conversions, which were attributed to enzyme inhibition by the co-product phenol could be circumvented via in situ co-product removal by scavenging the phenol using the polymeric adsorbent MP-carbonate. The optimized system allowed to reduce an alkene activated by ester groups in a “coupled-substrate” approach via nicotinamide-free hydrogen transfer with >90% conversion and complete stereoselectivity.

In situ magnetic separation of antibody fragments from Escherichia coli in complex media

Background

In situ magnetic separation (ISMS) has emerged as a powerful tool to overcome process constraints such as product degradation or inhibition of target production. In the present work, an integrated ISMS process was established for the production of his-tagged single chain fragment variable (scFv) D1.3 antibodies (“D1.3”) produced by E. coli in complex media. This study investigates the impact of ISMS on the overall product yield as well as its biocompatibility with the bioprocess when metal-chelate and triazine-functionalized magnetic beads were used.

Results

Both particle systems are well suited for separation of D1.3 during cultivation. While the triazine beads did not negatively impact the bioprocess, the application of metal-chelate particles caused leakage of divalent copper ions in the medium. After the ISMS step, elevated copper concentrations above 120 mg/L in the medium negatively influenced D1.3 production. Due to the stable nature of the model protein scFv D1.3 in the biosuspension, the application of ISMS could not increase the overall D1.3 yield as was shown by simulation and experiments.

Conclusions

We could demonstrate that triazine-functionalized beads are a suitable low-cost alternative to selectively adsorb D1.3 fragments, and measured maximum loads of 0.08 g D1.3 per g of beads. Although copper-loaded metal-chelate beads did adsorb his-tagged D1.3 well during cultivation, this particle system must be optimized by minimizing metal leakage from the beads in order to avoid negative inhibitory effects on growth of the microorganisms and target production. Hereby, other types of metal chelate complexes should be tested to demonstrate biocompatibility. Such optimized particle systems can be regarded as ISMS platform technology, especially for the production of antibodies and their fragments with low stability in the medium. The proposed model can be applied to design future ISMS experiments in order to maximize the overall product yield while the amount of particles being used is minimized as well as the number of required ISMS steps.

Use of in situ solid-phase adsorption in microbial natural product fermentation development

It has been half a century since investigators first began experimenting with adding ion exchange resins during the fermentation of microbial natural products. With the development of nonionic polymeric adsorbents in the 1970s, the application of in situ product adsorption in bioprocessing has grown slowly, but steadily. To date, in situ product adsorption strategies have been used in biotransformations, plant cell culture, the production of biofuels, and selected bulk chemicals, such as butanol and lactic acid, as well as in more traditional natural product fermentation within the pharmaceutical industry. Apart from the operational gains in efficiency from the integration of fermentation and primary recovery, the addition of adsorbents during fermentation has repeatedly demonstrated the capacity to significantly increase titers by sequestering the product and preventing or mitigating degradation, feedback inhibition and/or cytotoxic effects. Adoption of in situ product adsorption has been particularly valuable in the early stages of natural product-based drug discovery programs, where quickly and cost-effectively generating multigram quantities of a lead compound can be challenging when using a wild-type strain and fermentation conditions that have not been optimized. While much of the literature involving in situ adsorption describes its application early in the drug development process, this does not imply that the potential for scale-up is limited. To date, commercial-scale processes utilizing in situ product adsorption have reached batch sizes of at least 30,000 l. Here we present examples where in situ product adsorption has been used to improve product titers or alter the ratios among biosynthetically related natural products, examine some of the relevant variables to consider, and discuss the mechanisms by which in situ adsorption may impact the biosynthesis of microbial natural products.

Combined metabolic engineering of precursor and co-factor supply to increase α-santalene production by Saccharomyces cerevisiae

Background

Sesquiterpenes are a class of natural products with a diverse range of attractive industrial proprieties. Due to economic difficulties of sesquiterpene production via extraction from plants or chemical synthesis there is interest in developing alternative and cost efficient bioprocesses. The hydrocarbon α-santalene is a precursor of sesquiterpenes with relevant commercial applications. Here, we construct an efficient Saccharomyces cerevisiae cell factory for α-santalene production.

Results

A multistep metabolic engineering strategy targeted to increase precursor and cofactor supply was employed to manipulate the yeast metabolic network in order to redirect carbon toward the desired product. To do so, genetic modifications were introduced acting to optimize the farnesyl diphosphate branch point, modulate the mevalonate pathway, modify the ammonium assimilation pathway and enhance the activity of a transcriptional activator. The approach employed resulted in an overall α-santalene yield of a 0.0052 Cmmol (Cmmol glucose)^-1 corresponding to a 4-fold improvement over the reference strain. This strategy, combined with a specifically developed continuous fermentation process, led to a final α-santalene productivity of 0.036 Cmmol (g biomass)^-1 h^-1.

Conclusions

The results reported in this work illustrate how the combination of a metabolic engineering strategy with fermentation technology optimization can be used to obtain significant amounts of the high-value sesquiterpene α-santalene. This represents a starting point toward the construction of a yeast “sesquiterpene factory” and for the development of an economically viable bio-based process that has the potential to replace the current production methods.

Microbial production of propionic acid with Propionibacterium freudenreichii using an anion exchanger-based in situ product recovery (ISPR) process with direct and indirect contact of cells

The recovery of an inhibiting product from a bioreactor soon after its formation is an important issue in industrial bioprocess development. In the present study, the potential of the anion exchanger-based in situ product recovery (ISPR) technique for the biocatalytic production of propionic acid was discussed. The focus of the current work was the selection of a suitable configuration of metabolically active cells for application in propionic acid production. Accumulation of propionic acid in fermentation broth caused feedback inhibition of the growth and biotransformation activity of Propionibacterium freudenreichii CICC 10019. Relevant product inhibition kinetics was discussed, and the results showed that keeping the aqueous propionic acid concentration below 10.02 g L⁻¹ was an essential prerequisite for ISPR process. A batch study, in which three ISPR configuration mode designs were compared, was conducted. The comparison indicated that employing an external direct mode had significant advantages over other modes in terms of increased productivity and product yield, with a corresponding decrease in the number of downstream processing steps, as well as in substrate consumption. The fed-batch culture using an external direct mode for the continuous accumulation of propionic acid resulted in a cumulative propionic acid concentration of 62.5 g L⁻¹, with a corresponding product yield of 0.78 g propionic acid/g glucose.

Nicotinamide-independent asymmetric bioreduction of CC-bonds via disproportionation of enones catalyzed by enoate reductases

The asymmetric bioreduction of activated CC-bonds catalyzed by a single flavoprotein was achieved via direct hydrogen transfer from a sacrificial 2-enone or 1,4-dione as hydrogen donor without requirement of a nicotinamide cofactor. Due to its simplicity, this system has clear advantages over conventional FAD-recycling systems.

Characterization of magnetic ion-exchange composites for protein separation from biosuspensions

Downstream processing is a major issue in biotechnological production. A multitude of unit operations with nonsatisfying yield are often used to reach the desired product purity. Direct recovery technologies such as high-gradient magnetic fishing (HGMF) are advantageous because of their ability to separate the desired product in early stages from crude cultivation broths. However, the use of magnetic particles to capture valuable biotechnological products is often linked to the drawback that support particles are expensive and not available in greater quantities. This current work presents new composite magnetic particles that can be used in biotechnology. They are manufactured by a spray drying process. During this process, the nanosized magnetite particles as well as functional ion-exchange nanoparticles are integrated into one particle in which they are linked by a matrix polymer. The production procedure is flexible, scalable, and therefore economical. These particles have good adsorption capacities of up to 85 mg/g adsorbed protein and good binding kinetics. They are resistant to harsh conditions such as short ultrasonic treatment or extreme pHs. In order to test their usefulness in biosuspensions, model proteins were separated using these particles. The anion and cation exchanger particles separated lysozyme (LZ) or BSA from cultivation suspensions. The selectivity of recovery was dependent on other proteins present as is usual for ion-exchange binding mechanisms.

NADH availability limits asymmetric biocatalytic epoxidation in a growing recombinant Escherichia coli strain

Styrene can efficiently be oxidized to (S)-styrene oxide by recombinant Escherichia coli expressing the styrene monooxygenase genes styAB from Pseudomonas sp. strain VLB120. Targeting microbial physiology during whole-cell redox biocatalysis, we investigated the interdependency of styrene epoxidation, growth, and carbon metabolism on the basis of mass balances obtained from continuous two-liquid-phase cultures. Full induction of styAB expression led to growth inhibition, which could be attenuated by reducing expression levels. Operation at subtoxic substrate and product concentrations and variation of the epoxidation rate via the styrene feed concentration allowed a detailed analysis of carbon metabolism and bioconversion kinetics. Fine-tuned styAB expression and increasing specific epoxidation rates resulted in decreasing biomass yields, increasing specific rates for glucose uptake and the tricarboxylic acid (TCA) cycle, and finally saturation of the TCA cycle and acetate formation. Interestingly, the biocatalysis-related NAD(P)H consumption was 3.2 to 3.7 times higher than expected from the epoxidation stoichiometry. Possible reasons include uncoupling of styrene epoxidation and NADH oxidation and increased maintenance requirements during redox biocatalysis. At epoxidation rates of above 21 micromol per min per g cells (dry weight), the absence of limitations by O(2) and styrene and stagnating NAD(P)H regeneration rates indicated that NADH availability limited styrene epoxidation. During glucose-limited growth, oxygenase catalysis might induce regulatory stress responses, which attenuate excessive glucose catabolism and thus limit NADH regeneration. Optimizing metabolic and/or regulatory networks for efficient redox biocatalysis instead of growth (yield) is likely to be the key for maintaining high oxygenase activities in recombinant E. coli.

Biocatalysis for pharmaceutical intermediates: the future is now

Biocatalysis is continuing to gain momentum and is now becoming a key component in the toolbox of the process chemist, with a place alongside chemocatalysis and chromatographic separations. The pharmaceutical industry demands a speed of development that must be on a parallel with conventional chemistry and high optical purity for complex compounds with multiple chiral centres. This review describes how these demands are being addressed to make biocatalysis successful, particularly by the use of micro-scale technology for high-speed catalyst screening and process development alongside discipline integration of biology and engineering with chemistry. Developments in recombinant technology will further expand the repertoire of biocatalysis in the coming years to new chemistries and enable catalyst design to fit the process. Further development of biocatalysis for green chemistry and high productivity processes can also be expected.

Integrated bioprocesses

Integrated bioprocesses have been developed to optimise yield and cost-effectiveness of production of low and high molecular weight molecules. Low molecular weight products are removed from the cultivation medium with in situ extraction, in situ adsorption or crystallisation to avoid product inhibition. One-pot processes are being developed to replace two-stage reactions. Recent developments in the integrated purification of high molecular weight products focus mainly on the integration of solid/liquid separation and initial product recovery such as expanded bed adsorption or extraction in aqueous two-phase systems. Additionally, new approaches for a more efficient processing of inclusion bodies have been developed.