The Lazarus Escherichia coli Effect: Recovery of Productivity on Glycerol/Lactose Mixed Feed in Continuous Biomanufacturing

Continuous cultivation with Escherichia coli has several benefits compared to classical fed-batch cultivation. The economic benefits would be a stable process, which leads to time independent quality of the product, and hence ease the downstream process. However, continuous biomanufacturing with E. coli is known to exhibit a drop of productivity after about 4–5 days of cultivation depending on dilution rate. These cultivations are generally performed on glucose, being the favorite carbon source for E. coli and used in combination with isopropyl β-D-1 thiogalactopyranoside (IPTG) for induction. In recent works, harsh induction with IPTG was changed to softer induction using lactose for T7-based plasmids, with the result of reducing the metabolic stress and tunability of productivity. These mixed feed systems based on glucose and lactose result in high amounts of correctly folded protein. In this study we used different mixed feed systems with glucose/lactose and glycerol/lactose to investigate productivity of E. coli based chemostats. We tested different strains producing three model proteins, with the final aim of a stable long-time protein expression. While glucose fed chemostats showed the well-known drop in productivity after a certain process time, glycerol fed cultivations recovered productivity after about 150 h of induction, which corresponds to around 30 generation times. We want to further highlight that the cellular response upon galactose utilization in E. coli BL21(DE3), might be causing fluctuating productivity, as galactose is referred to be a weak inducer. This “Lazarus” phenomenon has not been described in literature before and may enable a stabilization of continuous cultivation with E. coli using different carbon sources.

Application of Continuous Culture Methods to Recombinant Protein Production in Microorganisms

Depending on the environmental conditions, cells adapt their metabolism and specific growth rate. Rearrangements occur on many different levels such as macromolecular composition, gene and protein expression, morphology and metabolic flux patterns. As the interplay of these processes also determines the output of a recombinant protein producing system, having control over specific growth rate of the culture is advantageous. Continuous culture methods were developed to grow cells in a constant environment and have been used for decades to study basic microbial physiology in a controlled and reproducible manner. Our review summarizes the uses of continuous cultures in cell physiology studies and process development, with a focus on recombinant protein-producing microorganisms.

Parallel steady state studies on a milliliter scale accelerate fed-batch bioprocess design for recombinant protein production with Escherichia coli

In general, fed-batch processes are applied for recombinant protein production with Escherichia coli (E. coli). However, state of the art methods for identifying suitable reaction conditions suffer from severe drawbacks, i.e. direct transfer of process information from parallel batch studies is often defective and sequential fed-batch studies are time-consuming and cost-intensive. In this study, continuously operated stirred-tank reactors on a milliliter scale were applied to identify suitable reaction conditions for fed-batch processes. Isopropyl β-d-1-thiogalactopyranoside (IPTG) induction strategies were varied in parallel-operated stirred-tank bioreactors to study the effects on the continuous production of the recombinant protein photoactivatable mCherry (PAmCherry) with E. coli. Best-performing induction strategies were transferred from the continuous processes on a milliliter scale to liter scale fed-batch processes. Inducing recombinant protein expression by dynamically increasing the IPTG concentration to 100 µM led to an increase in the product concentration of 21% (8.4 g L^-1 ) compared to an implemented high-performance production process with the most frequently applied induction strategy by a single addition of 1000 µM IPGT. Thus, identifying feasible reaction conditions for fed-batch processes in parallel continuous studies on a milliliter scale was shown to be a powerful, novel method to accelerate bioprocess design in a cost-reducing manner. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:1426-1435, 2016.

Metabolic responses to recombinant bioprocesses in Escherichia coli

Escherichia coli has been widely used for the production of recombinant proteins. However, the unbalances between host metabolism and recombinant biosynthesis continue to hamper the efficiency of these recombinant bioprocesses. The additional drainage of biosynthetic precursors toward recombinant processes burdens severely the metabolism of cells that, ultimately, elicits a series of stress responses, reducing biomass growth and recombinant protein production. Several strategies to overcome these metabolic limitations have been implemented; however, in most cases, improvements in recombinant protein expression were achieved at the expense of biomass growth arrest, which significantly hampers the efficiency of recombinant bioprocesses. With the advent of high throughput techniques and modelling approaches that provide a system-level understanding of the cellular systems, it is now expected that new advances in recombinant bioprocesses are achieved. By providing means to deal with these systems, our understanding on the metabolic behaviour of recombinant cells will advance and can be further explored to the design of suitable hosts and more efficient and cost-effective bioprocesses. Here, we review the major metabolic responses associated with recombinant processes and the engineering strategies relevant to overcome these stresses. Moreover, the advantages of applying systems levels engineering strategies to enhance recombinant protein production in E. coli cells are discussed and future perspectives on the advances of mathematical modelling approaches to study these systems are exposed.

Plasmid DNA production for therapeutic applications

Plasmid DNA (pDNA) is the base for promising DNA vaccines and gene therapies against many infectious, acquired, and genetic diseases, including HIV-AIDS, Ebola, Malaria, and different types of cancer, enteric pathogens, and influenza. Compared to conventional vaccines, DNA vaccines have many advantages such as high stability, not being infectious, focusing the immune response to only those antigens desired for immunization and long-term persistence of the vaccine protection. Especially in developing countries, where conventional effective vaccines are often unavailable or too expensive, there is a need for both new and improved vaccines. Therefore the demand of pDNA is expected to rise significantly in the near future. Since the injection of pDNA usually only leads to a weak immune response, several milligrams of DNA vaccine are necessary for immunization protection. Hence, there is a special interest to raise the product yield in order to reduce manufacturing costs. In this chapter, the different stages of plasmid DNA production are reviewed, from the vector design to downstream operation options. In particular, recent advances on cell engineering for improving plasmid DNA production are discussed.

Effects of a recombinant gene expression on ColE1-like plasmid segregation in Escherichia coli

BACKGROUND

Segregation of expression plasmids leads to loss of recombinant DNA from transformed bacterial cells due to the irregular distribution of plasmids between the daughter cells during cell division. Under non-selective conditions this segregational instability results in a heterogeneous population of cells, where the non-productive plasmid-free cells overgrow the plasmid-bearing cells thus decreasing the yield of recombinant protein. Amongst the factors affecting segregational plasmid instability are: the plasmid design, plasmid copy-number, host cell genotype, fermentation conditions etc. This study aims to investigate the influence of transcription and translation on the segregation of recombinant plasmids designed for constitutive gene expression in Escherichia coli LE392 at glucose-limited continuous cultivation. To this end a series of pBR322-based plasmids carrying a synthetic human interferon-gamma (hIFNγ) gene placed under the control of different regulatory elements (promoter and ribosome-binding sites) were used as a model.

RESULTS

Bacterial growth and product formation kinetics of transformed E. coli LE392 cells cultivated continuously were described by a structured kinetic model proposed by Lee et al. (1985). The obtained results demonstrated that both transcription and translation efficiency strongly affected plasmid segregation. The segregation of plasmid having a deleted promoter did not exceed 5% after 190 h of cultivation. The observed high plasmid stability was not related with an increase in the plasmid copy-number. A reverse correlation between the yield of recombinant protein (as modulated by using different ribosome binding sites) and segregational plasmid stability (determined by the above model) was also observed.

CONCLUSIONS

Switching-off transcription of the hIFNγ gene has a stabilising effect on ColE1-like plasmids against segregation, which is not associated with an increase in the plasmid copy-number. The increased constitutive gene expression has a negative effect on segregational plasmid stability. A kinetic model proposed by Lee et al. (1985) was appropriate for description of E. coli cell growth and recombinant product formation in chemostat cultivations.

Expression of beta-lactamase by recombinant Escherichia coli strains containing plasmids of different sizes--effects of pH, phosphate, and dissolved oxygen

The characteristics of growth and synthesis of plasmid-encoded protein were studied for strains of recombinant E. coli JM103 which carried the beta-lactamase gene on plasmids of different sizes. The plasmids used included the vector pUC8 and its recombinant derivatives containing varying-sized inserts of Drosophila DNA (not expressed in E. coli). Luria broth (LB) and a minimal medium (M9) supplemented in some cases with additional inorganic phosphate were used as growth media. There was no evidence of segregational instability in these experiments, where no antibiotic selection pressure was employed. Responses of the recombinant strains to variations in environmental parameters including pH, phosphate concentration in the medium, and aeration rate were examined. While the cell growth rate in LB decreased with pH in the range 7.0-8.0, the bulk beta-lactamase activity was maximized at an intermediate pH. The recombinant cell growth rate decreases with increasing plasmid size in the minimal medium, while such decrease is not significant when a rich medium such as LB is used. There is an intermediate plasmid size in the range studied (2.7-8.7 kb), at which beta-lactamase activity is maximum. While reduction in aeration rate (which determines the dissolved oxygen level) is detrimental for cell growth, it is beneficial for beta-lactamase synthesis. The bulk beta-lactamase activity therefore exhibits a maximum with respect to aeration rate. Cell growth and beta-lactamase production are affected in a similar manner by phosphate concentration in the minimal medium and therefore both are maximized at the same phosphate concentration. This investigation demonstrates clearly how the production of a recombinant plasmid-encoded protein can be maximized by proper manipulation of culture conditions and how it is affected by plasmid size.

Investigation of subpopulation heterogeneity and plasmid stability in recombinant escherichia coli via a simple segregated model

Many microbial and cell cultures exhibit phenomena that can best be described using a segregated modeling approach. Heterogeneties are more marked in recombinant cell cultures because subpopulations, which often exhibit different growth and productivity characteristics, are more easily identified by selective markers. A simple segregated mathematical model that simulates the growth of recombinant Escherichia coli cells is developed. Subpopulations of different growth rate, plasmid replication rate, and plasmid segregation probability are explicitly considered. Results indicate that a third mechanism of plasmid instability, referred to here as a "downward selective pressure," is significant when describing plasmid loss in batch and chemostat cultures. Also, the model agrees well with experimental data from cultures under antibiotic selective pressure. Finally, model simulations of chemostat cultures reveal the importance of initial conditions on culture stability and the possible presence of nonrandom partitioning functions.

Continuous bioconversion of n-octane to octanoic acid by recombinant Escherichia coli (alk(+)) growing in a two-liquid-phase Chemostat

Escherichia coli is able to grow on sugars in the presence of a bulk n-alkane phase. When E. coli is equipped with the alk genes from Pseudomonas oleovorans, the resulting recombinant strain converts n-alkanes into the corresponding alkanoic acids. To study the effects of growth rate and exposure to a bulk apolar phase on the physiology and the productivity of E. coli, we have grown this microorganism in two-liquid-phase continuous cultures containing 5% (v/v) n-octane.In contrast to batch cultures of wild-tape E. coli grown in the presence of n-octane, cells remained viable during the entire continuous culture, which lasted 200 h. Bioconversion of n-octane to n-octanoic acid by a recombinant E. coli (alk(+)) in a two-liquid-phase continuous culture was made possible by optimizing both the recombinant host strain and the conditions of culturing the organism. Continuous production in such two-phase systems has been maintained for the least 125 h without any changes in the product concentration in the fermentation medium. The volumetric productivity was determined as a function of growth rate and showed a maximum at a dilution rate D = 0.32 h(-1), reaching a continuous production rate of 0.5 g octanoate/L . h (4 tons/m(3) . year).

Recombinant protein excretion in Escherichia coli JM103[pUC8]: Effects of plasmid content, ethylenediaminetetraacetate, and phenethyl alcohol on cell membrane permeability

The presence of a high copy number plasmid (pUC8) was found to affect integrity of the cell envelope of Escherichia coli JM103, causing in turn significant release of the plasmid-encoded protein (beta-lactamase). The alterations in cell membrane permeability were evident from the increased susceptibility of recombinant cells to deoxycholic acid and methylene blue, which did not have appreciable effect on plasmid-free cells. The deteriorated cell membrane structure also resulted in a substantial reduction in specific growth rate and mass yield of plasmid-bearing cells. Further enhancement in beta-lactamase excretion was achieved by permeabilizing cell membrane with ethylenediaminetetra-acetate (EDTA) and phenethyl alcohol (PEA). Unlike other commonly used physical and chemical methods for releasing the enzymes accumulated in the cells, application of EDTA and PEA at appropriate concentrations neither led to cell death nor interrupted synthesis of the plasmid-encoded protein. While in situ application of PEA was complicated due to interference with beta-lactamase activity, in situ application of EDTA was found to be an efficient way of releasing the recombinant protein without sacrificing its productivity. The experimental results demonstrate that the presence of EDTA and PEA can substantially reduce the growth rate differential between plasmid-free and plasmid-bearing cells, suggesting possible improvement of plasmid stability by application of these cell membrane-permeabilizing on a periodic basis.

Kinetic studies of recombinant human interferon-gamma expression in continuous cultures of E. coli

A series of continuous cultures was performed to understand the product formation kinetics of recombinant human interferon gamma (rhIFN-gamma) in Escherichia coli at different dilution rates ranging from 0.1 to 0.3 h(-1) in different media. A T7 promoter-based vector was used for expression of IFN-gamma in E. coli BL21 (DE3) cells. The recombinant protein was produced as inclusion bodies, thus allowing a rapid buildup of rhIFN-gamma inside the cell, with the specific product yield (Y(p/X)) reaching a maximum value of 182 mg g(-1) dry cell weight (DCW). In all the media tested, the specific product formation rate (q(p)) was found to be strongly correlated with the specific growth rate (mu), demonstrating the growth-associated nature of product formation. The q(p) values show no significant decline with time postinduction, even though the recombinant protein has been over produced inside the cell. The maximum q(p) level of 75.5 mg g(-1) h(-1) was achieved at the first hour of induction at the dilution rate of 0.3 h(-1). Also, this correlation between q(p) and mu was not critically dependent on media composition, which would made it possible to grow cells in defined media in the growth phase and then push up the specific growth rate just before induction by pulse addition of glucose and yeast extract. This would ensure the twin objectives of high biomass and high specific productivities, leading to high volumetric product concentration.

Development of a highly productive and scalable plasmid DNA production platform

With the applications of DNA vaccines extending from infectious diseases to cancer, achieving the most efficient, reproducible, robust, scalable, and economical production of clinical grade plasmid DNA is paramount to the medical and commercial success of this novel vaccination paradigm. A first generation production process based on the cultivation of Escherichia coli in a chemically defined medium, employing a fed-batch strategy, delivered reasonable volumetric productivities (500-750 mg/L) and proved to perform very well across a wide range of E. coli constructs upon scale-up at industrial scale. However, the presence of monosodium glutamate (MSG) in the formulation of the cultivation and feed solution was found to be a potential cause of process variability. The development of a second generation process, based on a defined cultivation medium and feed solution excluding MSG, was undertaken. Optimization studies, employing a plasmid coding for the HIV gag protein, resulted in cultivation conditions that supported volumetric plasmid titers in excess of 1.2 g/L, while achieving specific yields ranging from 25 to 32 microg plasmid DNA/mg of dry cell weight. When used for the production of clinical supplies, this novel process demonstrated applicability to two other constructs upon scale-up in 2,000-L bioreactors. This second generation process proved to be scalable, robust, and highly productive.

Identification and characterization of IS1 transposition in plasmid amplification mutants of E. coli clones producing DNA vaccines

Merck Research Laboratories has developed a highly productive Escherichia coli fermentation process to produce plasmid DNA for use as vaccines. The process consists of a fed-batch fermentation in a chemically defined medium. Initiation of the feed stream precedes a growth-limited phase in which plasmid DNA is amplified. The fermentation is only maximally productive for a small fraction of E. coli transformants designated as high-producers, while the predominant low-producer population does not amplify plasmid DNA. In experiments undertaken to probe this phenomenon, transposition of the 768-bp E. coli insertion sequence IS1 into an HIV DNA vaccine vector was observed in several low-producer clones. IS1 was found to insert in or near the neomycin resistance gene in nearly a dozen unique sites from within a single population of plasmid molecules. The fraction of IS1-containing plasmids within several clones was determined by quantitative polymerase chain reaction and was found to increase with increasing cultivation time in the chemically defined medium. Because transposition into an antibiotic-resistance gene is unlikely to affect plasmid amplification, the genomes of high- and low-producers of three different HIV DNA vaccine vectors were subsequently profiled by restriction fragment length polymorphism analysis. In all three cases, IS1 insertional mutations were found in the genomes of the predominant low-producers, while the genomes of the high-producers were indistinguishable from untransformed cells. The insertions reside on similarly sized fragments for two of the low-producer clones, and the fragment size is smaller for the third clone. The third clone also produces much less plasmid DNA than a typical low-producer. The results suggest the presence of an IS1 insertional mutation that affects plasmid replication and amplification, possibly in a position-dependent manner.

A memorial review of Jay Bailey's contribution in prokaryotic metabolic engineering

When mentioning prokaryotic metabolic engineering, most people will immediately think of Jay Bailey. Jay's contribution to this fast-growing field is evident and familiar to many. Therefore, instead of a detailed technical review, we attempt in this article to summarize his contribution and dissect reasons for his success in this area from a standpoint of one of his former students (VH) and of a colleague in the field (JCL). This short review is by no means complete and provides only a partial view of Jay's contribution to the metabolic engineering of prokaryotes.

Evolution of a recombinant (gucoamylase-producing) strain of Fusarium venenatum A3/5 in chemostat culture

Fusarium venenatum JeRS 325 is a transformant of strain A3/5 which produces Aspergillus niger glucoamylase (GAM) under the control of a Fusarium oxysporum trypsin-like protease promoter. The evolution of JeRS 325 was studied in glucose-limited chemostat cultures grown on NaNO3 or (NH4)2SO4 as the nitrogen source. Thirteen mutants which were more highly branched and four mutants which were more sparsely branched than the parental strain were isolated from the NaNO3 chemostat. The highly branched mutants detected in this chemostat did not displace the sparsely branched population. The mutants isolated from the NaNO3 chemostat complemented representative strains previously isolated from glucose-limited chemostat cultures of F. venenatum A3/5 grown on (NH4)2SO4, but showed little complementation between themselves. By contrast, a highly branched mutant isolated from the (NH4)2SO4 chemostat culture displaced the sparsely branched mycelial population. None of the mutants isolated from the NaNO3 or (NH4)2SO4 chemostats produced as much GAM as JeRS 325. Southern blot analysis showed that all except one mutant had lost copies of both the glucoamylase and the acetamidase (the selectable marker) genes. However, specific GAM production was not necessarily correlated with the extent of glaA gene loss observed. Further, 10 of the mutants had lost the ability to grow on acetamide as the sole nitrogen source, although they retained copies of the amdS gene. In competition studies, mutants which could not utilize acetamide displaced mutants which could. The presence of foreign DNA in JeRS 325 resulted in a reduced specific growth rate (compared to A3/5), but the presence of the foreign DNA did not prevent the evolution of the strain or the isolation of mutants which had improved growth rates.

Effect of specific production rate of recombinant protein on multimerization of plasmid vector and gene expression level

In fed-batch cultures of recombinant Escherichia coli BL21(DE3)[pT7-G3IL2] at high cell concentration, the post-induction specific growth rate was carefully regulated by controlled medium feed to maximize the synthesis level of recombinant fusion interleukin-2, G3.IL-2. A maximum concentration of G3.IL-2 (11.25 g l(-1)) was achieved in the induced recombinant culture growing at the rate of 0.056 h(-1). A steep decrease in the expression level of G3.IL-2 was observed at the post-induction specific growth rates higher than its optimal value (0.056 h(-1)). In the induced recombinant cultures, plasmid multimerization was observed and highly dependent on specific growth and production rate: a higher post-induction specific growth rate and an increased specific production rate tended to significantly promote it much further. Moreover, plasmid stability was found to decrease rapidly in a faster growing culture.

Quantitative analysis of transcription and translation in gene amplified Chinese hamster ovary cells on the basis of a kinetic model

The elevation of expression levels for secreted glycoproteins by gene amplification in mammalian cells shows a saturation behavior at high levels of gene amplification. At high expression levels a drop in the secretion efficiency for the recombinant protein occurs (Schröder and Friedl, 1997), coinciding with the appearance of misfolded protein in the cell. In this communication we investigated whether additional limitations exist at the levels of transcription and translation. Four Chinese hamster ovary (CHO) cell lines expressing different amounts of human antithrombin III (ATIII) were used as a model system. A tenfold increase in the ATIII cDNA copy number from the lowest to the highest producing cell line coincided with a 38-fold increase in ATIII mRNA levels, and an 80-fold increase in the amount of intracellular ATIII levels. The data was analyzed using a simple kinetic model. The following conclusions were derived: I. The transcriptional activity for the recombinant protein is not saturated. II. Translation itself is not saturated either, but may be downregulated as secretion efficiency drops. III. Two explanations for the previously reported drop in secretion efficiency for the recombinant protein with increasing expression level are possible: A. Protein degradation is an alternative fate for translated ATIII and the fraction of ATIII degraded after translation increases as expression level is increased. B. Translation is downregulated as the secretory apparatus becomes exhausted to maintain cell viability.

Influence of the medium composition and plasmid combination on the growth of recombinant Escherichia coli JM109 and on the production of the fusion protein EcoRI::SPA

Plasmid-free and plasmid-harbouring E. coli JM109 strains were investigated in shaken flasks, stirred tanks in batch and continuous operation. The shaken flask cultivations were performed in M9 minimal medium and in media with various protein supplements. The host hardly grows on M9 minimal medium as opposed to the plasmid-harbouring cells, which grow well on this medium. All of the investigated cells propagate well on protein-containing media. The influence of the combinations of repressor plasmid pRK248cI, the protection plasmid EcoR4 and the production plasmid pMTC48 were determined on the initial specific growth rate of the E. coli JM109 without gene expression, on the yield coefficient of cell growth, acetate concentration and acetate yield coefficient in the yeast extract-containing (HM) medium. The influence of various media on the induction of the gene expression were evaluated. In cultivation media with protein supplement, the growth rate and yield coefficient increased. The variation of the volumetric and specific beta-lactamase activities with the cultivation time were determined in a stirred tank reactor in HM medium. With increasing dilution rate the process performance decreased. Simple relationships exist between the substrate uptake rate and the specific growth rate of the continuous cultivated cells in M9 and HM media. The influence of the dilution rate on the cell mass concentration, colony forming units, acetate formation, yield coefficients of growth and acetate formation, substrate uptake rate, CO2 production rate, ammonium formation rate and beta-lactamase activity in M9 and HM media were determined as well. Carbon balances of the batch and continuous cultivations indicated high carbon recoveries. On account of the higher growth rate of plasmid-harbouring cells than than of the plasmid-free cells, the behaviour of the investigated plasmid-free and plasmid-harbouring E. coli JM109 cells deviates from the published properties of other plasmid-free and plasmid-harbouring E. coli cells.

Kinetics of continuous GM-CSF production by recombinant Saccharomyces cerevisiae in an airlift bioreactor

Continuous production of murine GM-CSF by recombinant Saccharomyces cerevisiae in an airlift bioreactor was studied at three different dilution rates. The reactor was initially fed with a selective medium to increase cell concentration, and then was fed with a rich, nonselective medium for GM-CSF production. Ethanol was used as the main carbon source to provoke GM-CSF expression. In continuous culture, GM-CSF production was maintained for over 150 h, even though the fraction of plasmid-carrying cells continuously dropped to lower than 20%. The stable GM-CSF production during the later phase of the continuous culture was attributed to increased specific cell productivity possibly resulting from an increase in the plasmid copy number in plasmid-carrying cells. This also indicated the possibility of natural selection of high-copy number cells in continuous culture. Plasmid stability was found to be growth rate (dilution rate) dependent; it increased with the dilution rate. Reactor productivity and specific productivity also increased with the dilution rate. A two-parameter kinetic equation was used to model the plasmid stability kinetics. The growth rate ratio between plasmid-carrying and plasmid-free cells increased from 0.996 to 0.998 while the segregational instability or the probability of plasmid loss from each cell division increased from 1.1% to 16% as the dilution rate decreased from 0.11 h-1 to 0.05 h-1. Oscillation of the dilution rate between 0.05 h-1 and 0.11 h-1 stabilized the plasmids and gave a higher productivity than that achieved without oscillation.

Exponentially fed-batch cultures as an alternative to chemostats: the case of penicillin acylase production by recombinant E. coli

Exponentially fed-batch cultures (EFBCs), fed with medium containing a highly concentrated carbon source, are commonly employed for attainment of high cell densities. However, large variations in environmental conditions occur, and quasi-steady-state is usually achieved only for the limiting substrate concentration, restricting the use of such cultures in kinetic characterization studies. In this work we report the production of recombinant penicillin acylase (PA) in EFBC of an E. coli JM101 transformed with the pPA102 plasmid, which includes the PA gene under regulation of the lacZ gene promoter and using isopropyl-beta-thio-galactopyranoside (IPTG) as inducer. The culture was fed with nonconcentrated complete medium, resulting in the attainment of quasi-steady-state conditions not only in substrate concentration, but also in cell concentration, and in the specific rates of growth, product production, and substrate consumption. Similar transient behavior was observed between EFBC and chemostat results. At quasi-steady-state, the dilution rate in the EFBC equaled the growth rate. Specific PA production rate during the fed-batch phase remained relatively constant at each dilution rate and followed typical Luedeking-Piret kinetics, with growth-associated and non-growth-associated constants of 142 U gDCW-1 and 7.2 U gDCW-1 h-1, respectively. Specific glucose consumption rate linearly increased from 0.025 to 0.6 g gDCW-1 h-1 as the dilution rate increased from 0.01 to 0.35 h-1. The maximum specific PA activity increased with decreasing dilution rate, reaching its highest value of 2.0 U mg-1 at a dilution rate of 0.01 h-1, the lowest dilution tested.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of growth rate on stability and gene expression of recombinant plasmids during continuous and high cell density cultivation of Escherichia coli TG1

Continuous and fed-batch cultures of recombinant Escherichia coli TG1 were carried out in order to study plasmid stability and recombinant product formation at different specific growth rates. The aprotinin::beta-galactosidase gene (Ap::lacZ) was placed under the control of two different promoter/repressor systems, the PLac/lacI (pPLac8) and the lambda PL/cIts857 (pPL6) system. The chemically (0.5 mM IPTG) induced gene expression exhibited higher product activity and plasmid stability than the thermally (40 degrees C) induced expression. In fed-batch cultivations with the more stable E. coli TG1(pPLac8) a special feeding strategy allowed bacterial growth with a constant growth rate mu for several hours up to high cell densities. The cloned gene product activity was noticeably effected by the specific growth rate and the cell density at the moment of induction. In particular, the enzyme activities passed a pronounced maximum value in dependence of the set growth rate. The results indicate that fed-batch cultivation strategies are well suited to produce recombinant gene products.

Production of human parathyroid hormone by recombinant Escherichia coli TG1 on synthetic medium

Production of human parathyroid hormone (hPTH) by Escherichia coli TG1:I52cIts was studied. The hPTH is expressed as a fusion protein under control of the bacteriophage lambda pR promoter. The organism grows on glucose/mineral salt medium and the expression of the gene product was investigated under variation of temperature and growth rate prior to and after induction. hPTH formation largely depends on cultivation temperature and is optimal for a temperature shift from 30 to 38 degrees C. Product expression is growth coupled and specific hPTH concentration is independent of growth rate. The results are compared with a previous study on E. coli N4830:pEX-PPTH grown on complex media.

Recombinant protein production in cultures of an Escherichia coli trp- strain

Fermentation conditions were developed in order to achieve simultaneously a high biomass concentration and high-level expression of a hybrid cI-human insulin B peptide gene. In our system, this hybrid gene is under control of the Escherichia coli trp promoter, in a trp- derivative strain of E. coli W3110. The dual role of tryptophan concentration on cellular growth and hybrid gene regulation was studied in 10-1 batch fermentations. In the best batch conditions, a biomass concentration of 12 g dry weight/l can be obtained, and 0.53 g/l of cI-insulin B hybrid protein is produced. Tryptophan in the culture medium is consumed by the growing culture, until a level is reached that causes induction of the hybrid gene. Plasmid loss was detected, as only 62% of the cells retained the recombinant plasmid. In order to increase the hybrid protein production level, a fed-batch culture strategy was developed whereby the specific growth rate of the cells was restrained. Using the same amount of nutrients as in the batch fermentations, it was possible to increase the final biomass concentration to 20 g/l, plasmid-bearing cells in the population to 90% and recombinant hybrid protein to 1.21 g/l.

Determination of plasmid copy number and stability in Clostridium acetobutylicum ATCC 824

The copy number and stability of several plasmid vectors in Clostridium acetobutylicum ATCC 824 were determined. The protocols were modified from the traditional ones to overcome the problems associated with unusual behavior of C. acetobutylicum cells on solid medium. The plasmid copy numbers of pSYL2, pFNK1, pFNK3, and pFNK5 in strain ATCC 824 were 14, 8, 6, and 6, respectively. pSYL2 and pFNK1 were segregationally stable, since the fractions of plasmid-carrying cells after 60 generations of growth without antibiotic (erythromycin) were 73% and 77%, respectively. Vector pFNK1 carrying fermentative genes was found to be rather unstable. The observed instability seemed to be due to the complex host-plasmid interactions by amplified expression of enzymes involved in the tightly regulated primary metabolism of C. acetobutylicum.

High-density Escherichia coli cultivation process for hyperexpression of recombinant porcine growth hormone

Fermentation studies were performed on an Escherichia coli culture that carries a recombinant plasmid composed of an ampicillin-resistant gene, a temperature-regulated pL promoter, and a porcine pituitary cDNA sequence coding for growth hormone. The objective was to achieve high cell density while maintaining the specific expression level of recombinant porcine growth hormone (r-pGH) observed in shake flasks. At a specific expression level of 20% of total cell protein, the cell density of a glucose-limited fed-batch process reached 38 units of OD600 in 14 h, compared to flask cultivation, which resulted in only 1.4 units of OD600 in the same period. The observed critical fermentation conditions for maximal expression included (1) limiting glucose concentration below 1 g l-1 throughout the fed-batch growth and induction phases, (2) keeping postinduction temperature at 42 degrees C for 5-7 h, and (3) maintaining a postinduction growth rate around 0.17-0.21 h-1.

High cell density fermentation of recombinant Escherichia coli with computer-controlled optimal growth rate

In recent years recombinant DNA technology has enabled us to produce various proteins of therapeutic importance with microorganisms. As an appropriate host organism, E. coli plays a dominant role. Yields of E. coli dry cell mass in shaker flask culture range from 1-2 g/L, whereas in fermentors up to 10 g dry cells/L can be achieved. ZIMET and GBF have developed a high cell density fermentation process that produces E. coli (on a glucose/mineral salt medium) up to more than 100 g dry cells/L in a special fed-batch mode. This cultivation strategy prevents oxygen limitation and hence the accumulation of acetate and other metabolic byproducts. The specific growth rate can be adjusted so that product formation reaches its optimum value. An example of the production of alpha1-interferon is presented. The high cell density fermentations were realized in 30- and 450-L Chemap fermentors (ZIMET) and in a three-stage bioreactor scale-up system (72, 300, and 1,500 L) developed in cooperation with GBF and B. Braun Melsungen AG. Multiloop controllers were used to control the process variables.

Cultivation of recombinant E. coli and production of fusion protein in 60-l bubble column and airlift tower loop reactors

E. coli K12 with multicopy plasmid (lambda PR-promoter and temperature-sensitive lambda cI 857 repressor) was cultivated in 60-l bubble column and airlift tower loop reactors. The medium composition, cell concentration, and intracellulary enzyme activity were monitored on-line during batch, fed-batch, and continuous cultivations. The specific growth rates, cell mass yield coefficients, plasmid stabilities, productivities of the amount of active fusion protein (beta-galactosidase activity), concentrations and yields of acetic acid, and volumetric oxygen transfer coefficient were evaluated for different medium compositions and cultivation conditions. The enzyme activity was also monitored during the temperature induction. The results evaluated in the 60-l bubble column and airlift tower loop reactors are compared with those evaluated in a 1-1 stirred-tank reactor.

Construction of vector of Brevibacterium lactofermentum and study of its stability in continuous culture

A 5.7-kb vector plasmid pBK2 was constructed by ligating the kanamycin resistance gene from Escherichia coli plasmid pACYC177 to an endogenous cryptic 4.4-kb plasmid of Brevibacterium lactofermentum ATCC 21086. The vector replicates efficiently and is stably maintained in the host and other coryneforms. However, the copy number varied from 50 to 10 per chromosome-equivalent under different culture conditions. Continuous culture studies showed instability when low dilution rates were used. Co-culture experiments were performed at various dilution rates to measure the growth rate ratio (alpha) of the plasmid-free cells to the plasmid-containing cells. It was observed that at low dilution rates the value of alpha was higher than that at high dilution rates. Thus, the instability of the plasmid can be attributed to the increase in alpha at low dilution rates. Modelling of instability using a random partitioning model of plasmid segregation and experimentally obtained values of alpha showed agreement with experimental data. This demonstrated that active partitioning is not the operative mechanism for plasmid segregation in this case.