Production of ethanol by Clostridium thermosaccharolyticum: I. Effect of cell recycle and environmental parameters

The direct microbial conversion (DMC) process for the production of ethanol from lignocellulosic biomass is limited by low volumetric ethanol production rates due to the low cell densities of Clostridium thermosaccharolyticum which is a key organism for ethanol production in this process. Hence, this study focuses on the use of a continuous- culture cell recycle system to improve the volumetric ethanol productivity and yield of the fermentation of xylose by C. thermosaccharolyticum. Early experiments with the continuous-culture cell recycle system showed a two-fold improvement in volumetric ethanol productivity. However, the ethanol yield at the higher dilution rates suffered because of the large amount of lactate produced. The manipulation of two environmental parameters-iron concentration in the nutrient medium and the N(2) purge rate of the fermentor headspace-allowed a dramatic reduction in the lactate production and a simultaneous improvement in the ethanol titer and yield. Under the improved conditions of increased iron concentration (12.5 mg/L FeSO(4) . 7H(2)O) and decreased N(2) purge rate (0.1 L/min), a continuous culture of C. thermosaccharolyticum operating at a dilution rate of 0.24 h(-1) and 50% cell recycle produced 8.6 g/L ethanol and less than 1 g/L each of acetate and lactate. The volumetric ethanol productivity was 2.2 g/L/h, which is 8 times larger than obtained for a continuous culture operated with no cell recycle and the same specific growth rate.     

Investigation of parameters affecting a fixed bed bioreactor process for recombinant cell lines

A BHK 21 cell line expressing a recombinant antibody was grown in a fixed bed reactor (FBR) system using a porous support made of Siran glass beads. The contribution of five process variables (bead and inoculum sizes; circulation and dilution rates; glutamine concentration of the feed) to the productivity of the process (defined as production rate, effluent product concentration or yield of product on medium supplied) was investigated using a partial factorial experimental design. Individually, none of the variables tested had a significant affect upon productivity. The combination of smaller bead and inoculum sizes, higher circulation and dilution rates, plus higher feed glutamine concentration gave a markedly higher productivity than any other combination of variable levels tested. This combination of variable levels suggested that better results shold be obtained using a fluidised bed reactor system. However, comparison of the productivities of the two systems showed that the FBR gave the better results. This result can be explained in terms of the relationship of Qs_rAb to .Abbreviations C concentration - D dilution rate - FBR fixed bed bioreactor - FIBR fluidised bed bioreactor - Gln glutamine - Qs cell specific rate - Qv volumetric rate - rAb recombinant antibody - X_v viable cell density - specific growth rate     

Monoclonal antibody production in dialyzed continuous suspension culture

Hybridoma cell growth and monoclonal antibody production in dialyzed continuous suspension culture were investigated using a 1.5-L Celligen bioreactor. Medium supplemented with 1.5% fetal bovine serum was fed directly into the reactor at a dilution rate of 0.45 d(-1). Dailysis tubing with a molecular weight cut-off (MWCO) of 1000 was coiled inside the bioreactor. Fresh medium containing no serum or serum substitues passed through the dialysis tubing at flow rates of 2 to 5 L/d. The objective was to remove low molecular weight inhibitors, such as lactic acid and ammonia, by diffusion through the tubing, while continuoulsy replenishing essential nutrients by the same mechanism. Due to the low MWCO of the dialysis tubing high molecular weight components such as growth factors and antibody were not removed by the dialyzing stream. In the batch start-up phase, the monoclonal antibody (MAb) titer was almost 3 times that achieved in typical batch cultures (i.e., 170 to 180 mg/L). During dialyzed continuous operation, a substantial increase (up to 40%) in cell density, monoclonal antibody (MAb) titer, and reactor MAb productivity was observed, as compared with a conventional continuous suspension culture. The cell viability and the specific MAb productivity remained practically constant at different dialysis rates. This finding suggests that the steady state growth and death rate in continuous suspension hybridoma cultures are not direct functions of the nutrient or inhibitor concentrations.     

Investigation of physiological limits and conditions for robust bioprocessing of an extreme halophilic archaeon using external cell retention system

High biological activity and volumetric productivity are considered as prerequisites for efficient bioprocesses, extreme halophilic Archaea have, however, lower growth rates, for which reason halophilic Archaea are so far not used in industrial bioprocesses. To overcome this physiological limit and to achieve increased volumetric productivity, the produced biomass must be retained in a bioreactor, for example equipped with an external cell retention system. In this study, the characterization and parameterization of a bioreactor setup with cell retention was carried out with an extreme halophilic archaeon. Bioprocess quantification was used to demonstrate the process controllability. Focussing on maximizing the volumetric productivity; 10-fold productivity increase was achieved compared to chemostat continuous cultures. Circulation of the broth between the bioreactor and the membrane unit can be however challenging from physiological points-of-view. Hence, operating the system with external cell retention at optimal cross flow rate is physiologically essential: at lower cross flow rates, higher extracellular protein concentrations were measured due to oxygen limitation. In turn, at higher cross flow rates, shear stress reasoned higher concentrations of DNA fragments. This work contributes in a pioneering way to the bioprocess development of extreme halophilic Archaea by optimizing continuous laboratory scale processes regarding robustness and scalability.     

A rational approach to improving productivity in recombinant Pichia pastoris fermentation

A Mut(S) Pichia pastoris strain that had been genetically modified to produce and secrete sea raven antifreeze protein was used as a model system to demonstrate the implementation of a rational, model-based approach to improve process productivity. A set of glycerol/methanol mixed-feed continuous stirred-tank reactor (CSTR) experiments was performed at the 5-L scale to characterize the relationship between the specific growth rate and the cell yield on methanol, the specific methanol consumption rate, the specific recombinant protein formation rate, and the productivity based on secreted protein levels. The range of dilution rates studied was 0. 01 to 0.10 h(-1), and the residual methanol concentration was kept constant at approximately 2 g/L (below the inhibitory level). With the assumption that the cell yield on glycerol was constant, the cell yield on methanol increased from approximately 0.5 to 1.5 over the range studied. A maximum specific methanol consumption rate of 20 mg/g. h was achieved at a dilution rate of 0.06 h(-1). The specific product formation rate and the volumetric productivity based on product continued to increase over the range of dilution rates studied, and the maximum values were 0.06 mg/g. h and 1.7 mg/L. h, respectively. Therefore, no evidence of repression by glycerol was observed over this range, and operating at the highest dilution rate studied maximized productivity. Fed-batch mass balance equations, based on Monod-type kinetics and parameters derived from data collected during the CSTR work, were then used to predict cell growth and recombinant protein production and to develop an exponential feeding strategy using two carbon sources. Two exponential fed-batch fermentations were conducted according to the predicted feeding strategy at specific growth rates of 0.03 h(-1) and 0.07 h(-1) to verify the accuracy of the model. Cell growth was accurately predicted in both fed-batch runs; however, the model underestimated recombinant product concentration. The overall volumetric productivity of both runs was approximately 2.2 mg/L. h, representing a tenfold increase in the productivity compared with a heuristic feeding strategy.     

Continuous production of thienamycin in immobilized cell systems

A novel 2-L bubble column was used to study the continuous, immobilized cell production of thienamycin. Cells of Streptomyces cattleya were immobilized by culturing them in an appropriate growth medium containing 60/80 mesh celite particles. The dilution rate used during the continuous growth phase was 0.2 h(-1). This growth phase was terminated upon the development of heavy cell films (100-500 mum thickness), and the medium was replaced with an appropriate thienamycin production medium. The system was then operated in a batch mode until thienamycin production began. At that time, continuous feeding of the production medium was initiated and the influence of medium composition and dilution rate on CO(2), NH(4), biomass, and thienamycin production investigated. With synthetic production medium, a doubling of the dilution rate from 0.05 to 0.10 h(-1) resulted in a doubling of the thienamycin volumetric productivity. Rates of CO(2) and NH(4) production increased by ca. factors of three and two, respectively. The rate of PO(4) utilization also doubled. When the dilution rate was decreased to 0.05 h(-1), the rates of CO(2) production and PO(4) utilization quickly decreased (i.e., within 3 h). The rates of NH(4) and thienamycin production also decreased but more slowly (i.e., ca. 100 h after the decrease in dilution rate). With complex production medium, the rates of CO(2) production and PO(4) utilization appeared to be a direct function of dilution rate at the dilution rates tested. Thienamycin production in this case was not a function of dilution rate. Comparing the synthetic medium with the complex medium at either dilution rate, the volumetric rate of thienamycin production was higher in the system being fed complex medium. However, the specific activity (units thienamycin/g cell/h) observed with complex medium was lower than that observed with synthetic medium. The higher volumetric productivity observed with complex medium was the result of a high cell loading. The above observations will be discussed in terms of control of thienamycin synthesis and film thickness effects.     

Kinetics of recombinant immunoglobulin production by mammalian cells in continuous culture

A clonal derivative of a transfectant of the SP2/O myeloma cell line producing a chimeric monoclonal antibody was maintained in steady-state, continuous culture at dilution rates ranging from 0.21 to 1.04 day(-1). The steady-state values for nonviable and total cell concentrations increased as the dilution rate decreased, while the viable cell concentration was roughly independent of the dilution rate. At steady state, the specific growth rate increased and the specific death rate decreased as the dilution rate increased. The maximum specific growth rate was 1.15 day(-1). Antibody production was growth associated and the specific rate of antibody production increased linearly as the specific growth rate increased.     

Plasmid stability and kinetics of continuous production of glucoamylase by recombinant <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis> in an airlift bioreactor

Production of glucoamylase by recombinant Saccharomyces cerevisiae C468/pGAC9 (ATCC 20690) in a continuous stirred tank bioreactor was studied at different dilution rates. Plasmid stability was found to be growth (dilution rate) dependent; it increased with the dilution rate. Bioreactor productivity and specific productivity also increased with the dilution rate. A kinetic equation was used to model the plasmid stability kinetics. The growth rate ratio between plasmid-carrying and plasmid-free cells decreased from 1.397 to 1.215, and segregational instability or probability of plasmid loss from each cell division decreased from 0.059 to 0.020 as the dilution rate increased from 0.10 to 0.37 1/h. The specific growth rates increased with dilution rate, while the growth rate difference between plasmid-carrying and plasmid-free cell populations was negligible. This was attributed to the low copy number of the hybrid plasmid pGAC9. Thus, the growth rate had no significant effect on plasmid instability. The proposed kinetics was consistent with experimental results, and the model simulated the experimental data well.     

Continuous ethanol production from Jerusalem artichoke tubers. I. Use of free cells of Kluyveromyces marxianus

The Continuous fermentation of Jerusalem artichoke juice to ethanol by free cells of Kluyveromyces marxianus UCD (FST) 55-82 has been studied in a continuous-stirred-tank bioreactor at 35 degrees C and pH 4.6. A maximum yield of 90% of the theoretical was obtained at a dilution rate of 0.05 h(-1). About 95% of the sugars were utilized at dilution rates lower than 0.15 h(-1). Volumetric ethanol productivity and volumetric biomass productivity reached maximum values of 7 g ETOH/L/h and 0.6 g dry wt/L/h, respectively, at a dilution rate of 0.2 h(-1). The maintenance energy coefficient for K. marxianus culture was found to be 0.46 g sugar/g biomass/h/ Oscillatory behavior was following a change in dilution rate from a previous steady state and from batch to continuous culture. Values of specific ethanol production rate and specific sugar uptake were found to increase almost linearly with the increase of the dilution rate. The maximum specific ethanol production rate and maximum specific sugar uptake rate were found to be 2.6 g ethanol/g/ cell/h and 7.9 sugars/g cell/h, respectively. Washout occurred at a dilution rate of 0.41 h(-1).     

Novel,linked bioreactor system for continuous production of biologics

A novel, alternative intensified cell culture process comprised of a linked bioreactor system is presented. An N-1 perfusion bioreactor maintained cells in a highly proliferative state and provided a continuous inoculum source to a second bioreactor operating as a continuous-flow stirred-tank reactor (CSTR). An initial study evaluated multiple system steady-states by varying N-1 steady-state viable cell densities, N-1 to CSTR working volume ratios, and CSTR dilution rates. After identifying near optimum system steady-state parameters yielding a relatively high volumetric productivity while efficiently consuming media, a subsequent lab-scale experiment demonstrated the startup and long-term operation of the envisioned manufacturing process for 83 days. Additionally, to compensate for the cell-specific productivity loss over time due to cell line instability, the N-1 culture was also replaced with younger generation cells, without disturbing the steady-state of the system. Using the model cell line, the system demonstrated a two-fold volumetric productivity increase over the commercial-ready, optimized fed-batch process.     

Effect of particle loading on GM-CSF production by Saccharomyces cerevisiae in a three-phase fluidized bed bioreactor

Continuous production of a recombinant murine granulocyte-macrophage colony-stimulating factor (MuGM-CSF) by immobilized yeast cells, Saccharomyces cerevisiae strain XV2181 (a/a, Trp1) containing plasmid palphaADH2, in a fluidized bed bioreactor was studied at a 0.03 h(-1) dilution rate and various particle loading rates ranging from 5% to 33% (v/v). Cells were immobilized on porous glass beads fluidized in an air-lift draft tube bioreactor. A selective medium containing glucose was used to start up the reactor. After reaching a stable cell concentration, the reactor feed was switched to a rich, nonselective medium containing ethanol as the carbon source for GM-CSF production. GM-CSF production increased initially and then dropped gradually to a stable level. During the same period, the fraction of plasmid-carrying cells declined continuously to a lower level, depending on the particle loading. The relatively stable GM-CSF production, despite the large decline in the fraction of plasmid-carrying cells, was attributed to cell immobilization. As the particle loading rate increased, the plasmid stability also increased. Also, as the particle loading increased from 5% to 33%, total cell density in the bioreactor increased from 16 to 36 g/L, and reactor volumetric productivity increased from 0.36 to 1.31 mg/L.h. However, the specific productivity of plasmid-carrying cells decreased from 0.55 to 0.07 mg/L.g cell. The decreased specific productivity at higher particle loading rates was attributed to reduced growth efficiency caused by nutrient limitations at higher cell densities. Both the reactor productivity and specific cell productivity increased by two- to threefold or higher when the dilution rate was increased from 0.03 to 0.07 h(-1). (c) 1996 John Wiley & Sons, Inc.     

Production of proteinase A bySaccharomyces cerevesiae in a cell-recycling fermentation system: experiments and computer simulations

Overproduction of proteinase A by recombinantSaccharomyces cerevisiae was investigated by cultivations in a cell-recycling bioreactor. Memebrane filtration was used to separate cells from the broth. Recycling ratios and dilution rates were varied and the effect on enzyme production was studied both experimentally and by computer simulations. Experiments and simulations showed that cell mass and product concentration were enhanced by high ratios of recycling. Additional simulations showed that the proteinase A concentration decreased drastically at high dilution rates and the optimal volumetric productivities were at high dilution rates just below washout and at high ratios of recycling. Cell-recycling fermentation gave much higher volumetric productivities and stable product concentrations in contrast to simple continuous fermentation.     

High production of 1,3-propanediol from industrial glycerol by a newly isolated Clostridium butyricum strain

Batch and continuous cultures of a newly isolated Clostridium butyricum strain were carried out on industrial glycerol, the major by-product of the bio-diesel production process. For both types of cultures, the conversion yield obtained was around 0.55 g of 1,3-propanediol formed per 1 g of glycerol consumed whereas the highest 1,3-propanediol concentration, achieved during the single-stage continuous cultures was 35-48 g l-1. Moreover, the strain presented a strong tolerance at the inhibitory effect of the 1,3-propanediol, even at high concentrations of this substance at the chemostat (e.g. 80 g l-1). 1,3-Propanediol was associated with cell growth whereas acetate and butyrate seemed non growth-associated products. At low and medium dilution rates (until 0.1 h-1), butyrate production was favoured, whereas at higher rates acetate production increased. The maximum 1,3-propanediol volumetric productivity obtained was 5.5 g l-1 h-1. A two-stage continuous fermentation was also carried out. The first stage presented high 1,3-propanediol volumetric productivity, whereas the second stage (with a lower dilution rate) served to further increase the final product concentration. High 1,3-propanediol concentrations were achieved (41-46 g l-1), with a maximum volumetric productivity of 3.4 g l-1 h-1. A cell concentration decrease was reported between the second and the first fermentor.     

Continuous ethanol production from Jerusalem artichoke tubers. II. Use of immobilized cells of Kluyveromyces marxianus

Kluyveromyces marxianus UCD (FST) 55-82 cells were immobilized in Na alginate beads and used in a packed-bed bioreactor system for the continuous production of ethanol from the extract of Jerusalem artichoke tubers. Volumetric ethanol productivities of 104 and 80 g ethanol/ L/h were obtained at 80 and 92% sugar utilization, respectively. The maximum volumetric ethanol productivity of the immobilized cell bioreactor system was found to be 15 times higher than that of an ordinary-stirred-tank (CST) bioreactor using cells of K. marxianus. The immobilized cell bioreactor system was operated continuously at a constant dilution rate of 0.66 h(-1) for 12 days resulting in only an 8% loss of the original immobilized cell activity, which corresponds to an estimated half-life of ca. 72 days. The maximum specific ethanol productivity and maximum specific sugar uptake rate of the immobilized cells were found to be 0.55 g ethanol/g/biomass/h and 1.21 g sugars/g biomass/h, respectively.     

Continuous hybridoma suspension cultures with and without cell retention: kinetics of growth, metabolism and product formation.

A laboratory scale bioreactor was constructed from glass and polycarbonate materials whereby a track-etch membrane (3 microns pore diameter) was integrated into its two-part bottom flange. The reactor performance was evaluated for continuous hybridoma suspension cultures under various conditions of cell retention. A total retention experiment demonstrated that this type of stirred tank reactor cannot be operated at near zero growth rate conditions. Instead, at steady viable cell concentrations of congruent to 3 x 10(6) cells per ml, specific growth and death rates were estimated at 0.60 +/- 0.06 d-1. Specific substrate (glucose, glutamine, O2, amino acids) consumption, by-product (ammonia, alanine, amino acids) and product (antibody) production rates as well as various apparent molar yield coefficients were obtained and are compared to metabolic quotients and yield coefficients previously calculated from standard continuous culture experiments, i.e., without cell retention, at specific growth rates of 0.63 and 1.24 d-1. Furthermore, steady-state data on viable cell and antibody concentrations, spec. mAb productivities, and space-time yields determined before and after a step change (2.5-fold increase) in dilution rate at identical specific growth rates mu are presented.     

Immobilization of the slime mould Dictyostelium discoideum for the continuous production of recombinant human antithrombin III

Recombinant vegetative Dictyostelium discoideum cells were immobilized inside a porous matirx by an inorganic membrane that was permeable to nutrients but not to cells, in order to produce recombinant human antithrombin III. Cells so entrapped could reach up to 15 times higher biomass densities compared with organisms growing freely in suspension. The high cell concentration maintained in the immobilized cell bioreactor caused an increase in specific and volumetric productivity. In continuous operation a maximum volumetric antithrombin productivity of 56 ng h ^–1 ml ^–1 catalyst bulk volume was attained at a dilution rate of 0.016 h ^–1. In addition, the good retention of metabolic activity for several weeks as well as the convenient form of storage and regeneration of the catalytic system were shown. Correspondence to: H. Tiltscher     

Kinetic studies of constitutive human granulocyte-macrophage colony stimulating factor (hGM-CSF) expression in continuous culture of <Emphasis Type="Italic">Pichia pastoris</Emphasis>

Extracellular human granulocyte-macrophage colony stimulating factor (hGM-CSF) expression was studied under the control of the GAP promoter in recombinant Pichia pastoris in a series of continuous culture runs (dilution rates from 0.025 to 0.2 h⁻¹). The inlet feed concentration was also varied and the steady state biomass concentration increased proportionally demonstrating efficient substrate utilization and constancy of the biomass yield coefficient (Y_x/s) for a given dilution rate. The specific product formation rate (q_P) showed a strong correlation with dilution rates demonstrating growth associated product formation of hGM-CSF. The volumetric product concentration achieved at the highest feed concentration (4×) and a dilution rate of 0.2 h⁻¹ was 82 mg l⁻¹ which was 5-fold higher compared to the continuous culture run with 1× feed concentration at the lowest dilution rate thus translating to a 40 fold increase in the volumetric productivity. The specific product yield (Y_P/X) increased slightly from 2 to 2.5 mg g⁻¹, with increasing dilution rates, while it remained fairly invariant, for all feed concentrations demonstrating negligible product degradation or feed back inhibition. The robust nature of this expression system would make it easily amenable to scale up for industrial production.     

Effect of dilution rate on growth, productivity, cell cycle and size, and shear sensitivity of a hybridoma cell in a continuous culture

To study the effects of the growth rate of the hybridoma cell Mn12 on productivity, cell cycle, cell size, and shear sensitivity, six continuous cultures were run at dilution rate of 0.011, 0.021, 0.023, 0.030, 0.042, and 0.058 h(-1). This particular hybridoma cell appeared to be unstable in continuous culture with respect to specific productivity, as a sudden drop occurred after about 30 generations in continuous culture, accompanied by the appearance of two populations with respect to the cytoplasmic lgG content. The specific productivity increased with increasing growth rate. The shear sensitivity of the cell, as measured in a small air-lift loop reactor, increased with increasing growth rate. The mean relative cell size, as determined with a flow cytometer, increased with increasing growth rates. Furthermore, the fraction of cells in the S phase increased, and the fraction of cells in the G1/G0 phase decreased with increasing growth rates. (c) 1993 John Wiley & Sons, Inc.     

Production of Bacillus thuringiensis spores in total cell retention culture and two-stage continuous culture using an internal ceramic filter system

The production of Bacillus thuringiensis spores was investigated in a bioreactor incorporating a ceramic membrane filter to improve spore concentration and volumetric productivity. Two cultivation methods were used in this study: a total cell retention culture (TCRC), and a two-stage continuous culture with partial cell bleeding. In the TCRC, fed by 50 g/L of glucose, a spore concentration of 1.6 x 10(10) CFU/mL was obtained with a spore percentage of greater than 95% and a maximum cell mass of 82.2 g/L. The volumetric productivity was four times higher than that obtained from batch cultivation. In the two-stage continuous culture with partial cell bleeding spore concentration was strongly dependent on the bleed ratio. The spore concentration of 1.8 x 10(9) CFU/mL and the spore percentage of 70% were obtained at the second stage when a bleed ratio of 0.33 and a dilution rate of 0.23 h(-1) were used. (c) 1993 John Wiley & Sons, Inc.     

Continuous cultivation of Hydrogenobacter thermophilus at low partial pressures of H

Growth of the thermophilic H-oxidizing bacterium Hydrogenobacter thermophilus (ITI 553) was achieved in continuous culture using a gas-lift bioreactor. Although the highest equilibrium cell density was obtained at a dilution rate of 0.05 h, the maximum productivity was obtained at 0.1h. At 0.05 h, the maintainable cell density was 7.10 cells/mL at 2-4% oxygen with a gas flow rate of 0.1vvm. The steady-state cell density increased further with increasing gas flow rates. Maximum specific activity of the peroxidase was obtained at 8% oxygen. The total productivity of the enzyme had its maximum at 6-8% oxygen. Increasing gas flow rates had hardly any effect upon peroxidase specific activity, however, productivity was increased due to the increased cell densities obtained.