Step change in the efficiency of centrifugation through cell engineering: co-expression of Staphylococcal nuclease to reduce the viscosity of the bioprocess feedstock

An autonucleolytic suspension HEK293F host cell line for high-titer serum-free AAV5 and AAV9 production with reduced levels of DNA impurity

We sought to engineer mammalian cells to secrete nuclease activity as a step toward removing the need to purchase commercial nucleases as process additions in bioprocessing of AAV5 and AAV9 as gene therapy vectors. Engineering HeLa cells with a serratial nuclease transgene did not bring about nuclease activity in surrounding media whereas engineering serum-free, suspension-adapted HEK293F cells with a staphylococcal nuclease transgene did result in detectable nuclease activity in surrounding media of the resultant stable transfectant cell line, "NuPro-1S." When cultivated in serum-free media, NuPro-1S cells yielded 3.06 × 1010 AAV5 viral genomes (vg)/mL via transient transfection, compared with 3.85 × 109 vg/mL from the parental HEK293F cell line. AAV9 production, followed by purification by ultracentrifugation, yielded 1.8 × 1013 vg/mL from NuPro-1S cells compared with 7.35 × 1012 vg/mL from HEK293F cells. AAV9 from both HEK293F and NuPro-1S showed almost identical ability to transduce cells embedded in a scaffold tissue mimic or cells of mouse neonate brain tissue in vivo. Comparison of agarose gel data indicated that the DNA content of AAV5 and AAV9 process streams from NuPro-1S cells was reduced by approximately 60% compared with HEK293F cells. A similar reduction in HEK293F cells was only achievable with a 50 U/mL Benzonase treatment.

Polyethyleneimine efficiently extracts recombinant cytoplasmatic green fluorescent protein produced in Escherichia coli with high purity

We used a polycationic polymer polyethyleneimine (PEI) to develop a method to extract recombinant proteins produced in the Escherichia coli (E. coli) cytosol. Compared to high pressure homogenization, commonly used to disrupt E. coli cells, our extraction method leads to higher purity of extracts. Upon addition of PEI to the cells, flocculation occurs and the recombinant protein gradually diffuses out of the PEI/cell network. While several aspects such as the E. coli strain, the cell or PEI concentration as well as the protein titer and the pH of the buffer seem to influence the extraction rate, our results show that the PEI molecule (molecular weight and structure) must be chosen appropriately for protein extraction. The method works well with resuspended cells but can also be applied directly to fermentation broths at higher PEI concentration. This extraction approach allows for effective reduction of DNA, endotoxins, and host cell proteins levels by 2 to 4 orders of magnitude, and drastically facilitate the subsequent downstream processing steps such as centrifugation and filtration.

Serum-free lentiviral vector production is compatible with medium-resident nuclease activity arising from adherent HEK293T host cells engineered with a nuclease-encoding transgene

At present lentiviral vector production for cell and gene therapy commonly involves transient plasmid transfection of mammalian cells cultivated in serum-containing media and addition of exogenous nuclease to reduce host cell and plasmid DNA impurities. Switching from serum-containing media to chemically-defined, serum free media, and minimising the number of process additions, are both increasingly regarded as necessary steps for simplifying and potentially automating lentiviral vector bioprocessing in future. Here we adapted human embryonic kidney 293T (HEK293T) cells to grow in serum-free media and also modified these cells with transgenes designed to encode a secreted nuclease activity. Stable transfection of HEK293T cells with transgenes encoding the Staphylococcus aureus nuclease B (NucB) open reading frame with either its native secretion signal peptide, the murine Igκ chain leader sequence or a novel viral transport fusion protein, all resulted in qualitatively detectable nuclease activity in serum-free media. Serum-free transient transfection of human embryonic kidney HEK293T cells stably harbouring the transgene for NucB with its native secretion signal produced active lentivirus in the presence of medium-resident nuclease activity. This lentivirus material was able to transduce the AGF-T immortal T cell line with a green fluorescent protein reporter payload at a level of 2.05 × 10⁵ TU/mL (±3.34 × 10⁴ TU/mL). Sufficient nuclease activity was present in 10 μL of this unconcentrated lentivirus material to degrade 1.5 μg DNA within 2 h at 37 °C, without agitation - conditions compatible with lentivirus production. These observations demonstrate that lentiviral vector production, by transient transfection, is compatible with host cells harbouring a nuclease transgene and evidencing nuclease activity in their surrounding growth media. This work provides a solid basis for future investigations, beyond the scope of this present study, in which commercial and academic groups can apply this approach to therapeutic payloads and potentially omit exogenous nuclease bioprocess additions.

Production of indigo by recombinant bacteria

Indigo is an economically important dye, especially for the textile industry and the dyeing of denim fabrics for jeans and garments. Around 80,000 tonnes of indigo are chemically produced each year with the use of non-renewable petrochemicals and the use and generation of toxic compounds. As many microorganisms and their enzymes are able to synthesise indigo after the expression of specific oxygenases and hydroxylases, microbial fermentation could offer a more sustainable and environmentally friendly manufacturing platform. Although multiple small-scale studies have been performed, several existing research gaps still hinder the effective translation of these biochemical approaches. No article has evaluated the feasibility and relevance of the current understanding and development of indigo biocatalysis for real-life industrial applications. There is no record of either established or practically tested large-scale bioprocess for the biosynthesis of indigo. To address this, upstream and downstream processing considerations were carried out for indigo biosynthesis. 5 classes of potential biocatalysts were identified, and 2 possible bioprocess flowsheets were designed that facilitate generating either a pre-reduced dye solution or a dry powder product. Furthermore, considering the publicly available data on the development of relevant technology and common bioprocess facilities, possible platform and process values were estimated, including titre, DSP yield, potential plant capacities, fermenter size and batch schedule. This allowed us to project the realistic annual output of a potential indigo biosynthesis platform as 540 tonnes. This was interpreted as an industrially relevant quantity, sufficient to provide an annual dye supply to a single industrial-size denim dyeing plant. The conducted sensitivity analysis showed that this anticipated output is most sensitive to changes in the reaction titer, which can bring a 27.8% increase or a 94.4% drop. Thus, although such a biological platform would require careful consideration, fine-tuning and optimization before real-life implementation, the recombinant indigo biosynthesis was found as already attractive for business exploitation for both, luxury segment customers and mass-producers of denim garments.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s40643-023-00626-7.

Recovery of bacterioruberin and proteins using aqueous solutions of surface-active compounds

Haloarchaea microorganisms are little explored marine resources that can be a promising source of valuable compounds with unique characteristics, due to their adaptation to extreme environments. In this work, the extraction of bacterioruberin and proteins from Haloferax mediterranei ATCC 33500 was investigated using aqueous solutions of ionic liquids and surfactants, which were further compared with ethanol. Despite the good performance of ethanol in the extraction of bacterioruberin, the use of aqueous solutions of surface-active compounds allowed the simultaneous release of bacterioruberin and proteins in a multi-product process, with the non-ionic surfactants being identified as the most promising. The optimum operational conditions allowed a maximum extraction yield of 0.37 ± 0.01 mgbacterioruberin gwet biomass -1 and 352 ± 9 mgprotein gwet biomass -1 with an aqueous solution of Tween® 20 (at 182.4 mM) as the extraction solvent. In addition, high purities of bacterioruberin were obtained, after performing a simple induced precipitation using ethanol as an antisolvent to recover the proteins present in the initial extract. Finally, a step for polishing the bacterioruberin was performed, to enable solvent recycling, further closing the process to maximize its circularity.

Mass transfer of proteins in chromatographic media: Comparison of pure and crude feed solutions

Elucidation of intraparticle mass transfer mechanisms in protein chromatography is essential for process design. This study investigates the differences of adsorption and diffusion parameters of basic human fibroblast factor 2 (hFGF2) in a simple (purified) and a complex (clarified homogenate) feed solution on the grafted agarose-based strong cation exchanger Capto S. Microscopic investigations using confocal laser scanning microscopy revealed slower intraparticle diffusion of hFGF2 in the clarified homogenate compared to purified hFGF2. Diffusive adsorption fronts indicated a strong contribution of solid diffusion to the overall mass transfer flux. Protein adsorption methods such as batch uptake and shallow bed as well as breakthrough curve experiments confirmed a 40-fold reduction of the mass transfer flux for hFGF2 in the homogenate compared to pure hFGF2. The slower mass transfer was induced by components of the clarified homogenate. Essentially, the increased dynamic viscosity caused by a higher concentration of dsDNA and membrane lipids in the clarified homogenate contributed to this decrease in mass transfer. Moreover, binding capacity for hFGF2 was much lower in the clarified homogenate and substantially decreased the adsorbed phase driving force for mass transfer.

Selective Release of Recombinant Periplasmic Protein From E. coli Using Continuous Pulsed Electric Field Treatment

To date, high-pressure homogenization is the standard method for cell disintegration before the extraction of cytosolic and periplasmic protein from E. coli. Its main drawback, however, is low selectivity and a resulting high load of host cell impurities. Pulsed electric field (PEF) treatment may be used for selective permeabilization of the outer membrane. PEF is a process which is able to generate pores within cell membranes, the so-called electroporation. It can be readily applied to the culture broth in continuous mode, no additional chemicals are needed, heat generation is relatively low, and it is already implemented at industrial scale in the food sector. Yet, studies about PEF-assisted extraction of recombinant protein from bacteria are scarce. In the present study, continuous electroporation was employed to selectively extract recombinant Protein A from the periplasm of E. coli. For this purpose, a specifically designed flow-through PEF treatment chamber was deployed, operated at 1.5 kg/h, using rectangular pulses of 3 μs at specific energy input levels between 10.3 and 241.9 kJ/kg. Energy input was controlled by variation of the electric field strength (28.4-44.8 kV/cm) and pulse repetition frequency (50-1,000 Hz). The effects of the process parameters on cell viability, product release, and host cell protein (HCP), DNA, as well as endotoxin (ET) loads were investigated. It was found that a maximum product release of 89% was achieved with increasing energy input levels. Cell death also gradually increased, with a maximum inactivation of -0.9 log at 241.9 kJ/kg. The conditions resulting in high release efficiencies while keeping impurities low were electric field strengths ≤ 30 kV/cm and frequencies ≥ 825 Hz. In comparison with high-pressure homogenization, PEF treatment resulted in 40% less HCP load, 96% less DNA load, and 43% less ET load. Therefore, PEF treatment can be an efficient alternative to the cell disintegration processes commonly used in downstream processing.

Improving Fab' fragment retention in an autonucleolytic Escherichia coli strain by swapping periplasmic nuclease translocation signal from OmpA to DsbA

Objectives

To reduce unwanted Fab’ leakage from an autonucleolytic Escherichia coli strain, which co-expresses OmpA-signalled Staphylococcal nuclease and Fab’ fragment in the periplasm, by substituting in Serratial nuclease and the DsbA periplasm translocation signal as alternatives.

Results

We attempted to genetically fuse a nuclease from Serratia marcescens to the OmpA signal peptide but plasmid construction failed, possibly due to toxicity of the resultant nuclease. Combining Serratial nuclease to the DsbA signal peptide was successful. The strain co-expressing this nuclease and periplasmic Fab’ grew in complex media and exhibited nuclease activity detectable by DNAse agar plate but its growth in defined medium was retarded. Fab’ coexpression with Staphylococcal nuclease fused to the DsbA signal peptide resulted in cells exhibiting nuclease activity and growth in defined medium. In cultivation to high cell density in a 5 l bioreactor, DsbA-fused Staphylococcal nuclease co-expression coincided with reduced Fab’ leakage relative to the original autonucleolytic Fab’ strain with OmpA-fused staphylococcal nuclease.

Conclusions

We successfully rescued Fab’ leakage back to acceptable levels and established a basis for future investigation of the linkage between periplasmic nuclease expression and leakage of co-expressed periplasmic Fab’ fragment to the surrounding growth media.

Detecting cell lysis using viscosity monitoring in E. coli fermentation to prevent product loss

Monitoring the physical or chemical properties of cell broths to infer cell status is often challenging due to the complex nature of the broth. Key factors indicative of cell status include cell density, cell viability, product leakage, and DNA release to the fermentation broth. The rapid and accurate prediction of cell status for hosts with intracellular protein products can minimise product loss due to leakage at the onset of cell lysis in fermentation. This article reports the rheological examination of an industrially relevant E. coli fermentation producing antibody fragments (Fab'). Viscosity monitoring showed an increase in viscosity during the exponential phase in relation to the cell density increase, a relatively flat profile in the stationary phase, followed by a rapid increase which correlated well with product loss, DNA release and loss of cell viability. This phenomenon was observed over several fermentations that a 25% increase in broth viscosity (using induction-point viscosity as a reference) indicated 10% product loss. Our results suggest that viscosity can accurately detect cell lysis and product leakage in postinduction cell cultures, and can identify cell lysis earlier than several other common fermentation monitoring techniques. This work demonstrates the utility of rapidly monitoring the physical properties of fermentation broths, and that viscosity monitoring has the potential to be a tool for process development to determine the optimal harvest time and minimise product loss. © 2016 The Authors. Biotechnology Progress published by Wiley Periodicals, Inc. on behalf of American Institute of Chemical Engineers, 32:1069-1076, 2016.

Promoter engineering to optimize recombinant periplasmic Fab' fragment production in Escherichia coli

Fab' fragments have become an established class of biotherapeutic over the last two decades. Likewise, developments in synthetic biology are providing ever more powerful techniques for designing bacterial genes, gene networks and entire genomes that can be used to improve industrial performance of cells used for production of biotherapeutics. We have previously observed significant leakage of an exogenous therapeutic Fab' fragment into the growth medium during high cell density cultivation of an Escherichia coli production strain. In this study we sought to apply a promoter engineering strategy to address the issue of Fab' fragment leakage and its consequent bioprocess challenges. We used site directed mutagenesis to convert the Ptac promoter, present in the plasmid, pTTOD-A33 Fab', to a Ptic promoter which has been shown by others to direct expression at a 35% reduced rate compared to Ptac . We characterized the resultant production trains in which either Ptic or Ptac promoters direct Fab' fragment expression. The Ptic promoter strain showed a 25-30% reduction in Fab' expression relative to the original Ptac strain. Reduced Fab' leakage and increased viability over the course of a fed-batch fermentation were also observed for the Ptic promoter strain. We conclude that cell design steps such as the Ptac to Ptic promoter conversion reported here, can yield significant process benefit and understanding with respect to periplasmic Fab' fragment production. It remains an open question as to whether the influence of transgene expression on periplasmic retention is mediated by global metabolic burden effects or periplasm overcapacity. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:840-847, 2016.

Measuring E. coli and bacteriophage DNA in cell sonicates to evaluate the CAL1 reaction as a synthetic biology standard for qPCR

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We establish the effect of E. coli cellular material on sensitivity of qPCR for detection and quantitation of a lone genomic sequence.

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We demonstrate that LRE qPCR matches performance of the conventional Standard Curve qPCR method with respect to absolute quantitation of a genomic E. coli sequence.

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We characterise the effect of E. coli cellular material on performance of qPCR for detection and quantitation of a bacteriophage DNA sequence.

We measured the impact of the presence of total Escherichia coli (E. coli) cellular material on the performance of the Linear Regression of Efficiency (LRE) method of absolute quantitative PCR (LRE qPCR), which features the putatively universal CAL1 calibration reaction, which we propose as a synthetic biology standard. We firstly used a qPCR reaction in which a sequence present in the lone genomic BirA locus is amplified. Amplification efficiency for this reaction, a key metric for many quantitative qPCR methods, was inhibited by cellular material from bioreactor cultivation to a greater extent than material from shake flask cultivation. We then compared LRE qPCR to the Standard Curve method of absolute qPCR (SC qPCR). LRE qPCR method matched the performance of the SC qPCR when used to measure 417–4.17 × 10⁷ copies of the BirA target sequence present in a shake flask-derived cell sonicates sample, and for 97–9.7 × 10⁵ copies in the equivalent bioreactor-derived sample. A plasmid-encoded T7 bacteriophage sequence was next used to compare the methods. In the presence of cell sonicates from samples of up to OD₆₀₀ = 160, LRE qPCR outperformed SC qPCR in the range of 1.54 × 10⁸–1.54 × 10¹⁰ copies of the T7 target sequence and matched SC qPCR over 1.54 × 10⁴–1.54 × 10⁷ copies. These data suggest the CAL1 standard, combined with the LRE qPCR method, represents an attractive choice as a synthetic biology qPCR standard that performs well even when unpurified industrial samples are used as the source of template material.

IMAC capture of recombinant protein from unclarified mammalian cell feed streams

Fusion-tag affinity chromatography is a key technique in recombinant protein purification. Current methods for protein recovery from mammalian cells are hampered by the need for feed stream clarification. We have developed a method for direct capture using immobilized metal affinity chromatography (IMAC) of hexahistidine (His6) tagged proteins from unclarified mammalian cell feed streams. The process employs radial flow chromatography with 300-500 μm diameter agarose resin beads that allow free passage of cells but capture His-tagged proteins from the feed stream; circumventing expensive and cumbersome centrifugation and/or filtration steps. The method is exemplified by Chinese Hamster Ovary (CHO) cell expression and subsequent recovery of recombinant His-tagged carcinoembryonic antigen (CEA); a heavily glycosylated and clinically relevant protein. Despite operating at a high NaCl concentration necessary for IMAC binding, cells remained over 96% viable after passage through the column with host cell proteases and DNA detected at ∼ 8 U/mL and 2 ng/μL in column flow-through, respectively. Recovery of His-tagged CEA from unclarified feed yielded 71% product recovery. This work provides a basis for direct primary capture of fully glycosylated recombinant proteins from unclarified mammalian cell feed streams.

Enhancing the selective extracellular location of a recombinant E. coli domain antibody by management of fermentation conditions

The preparation of a recombinant protein using Escherichia coli often involves a challenging primary recovery sequence. This is due to the inability to secrete the protein to the extracellular space without a significant degree of cell lysis. This results in the release of nucleic acids, leading to a high viscosity, difficulty to clarify, broth and also to contamination with cell materials such as lipopolysaccharides and host cell proteins. In this paper, we present different fermentation strategies to facilitate the recovery of a V H domain antibody (13.1 kDa) by directing it selectively to the extracellular space and changing the balance between domain antibody to nucleic acid release. The manipulation of the cell growth rate in order to increase the outer cell membrane permeability gave a small ~1.5-fold improvement in released domain antibody to nucleic acid ratio without overall loss of yield. The introduction during fermentation of release agents such as EDTA gave no improvement in the ratio of released domain antibody to nucleic acid and a loss of overall productivity. The use of polyethyleneimine (PEI) during fermentation was with the aim to (a) permeabilise the outer bacterial membrane to release selectively domain antibody and (b) remove selectively by precipitation nucleic acids released during cell lysis. This strategy resulted in up to ~4-fold increase in the ratio of domain antibody to soluble nucleic acid with no reduction in domain antibody overall titre. In addition, a reduction in host cell protein contamination was achieved and there was no increase in endotoxin levels. Similar results were demonstrated with a range of other antibody products prepared in E. coli.

Novel high-throughput deoxyribonuclease 1 assay

Deoxyribonuclease I (DNase I), the most active and abundant apoptotic endonuclease in mammals, is known to mediate toxic, hypoxic, and radiation injuries to the cell. Neither inhibitors of DNase I nor high-throughput methods for screening of high-volume chemical libraries in search of DNase I inhibitors are, however, available. To overcome this problem, we developed a high-throughput DNase I assay. The assay is optimized for a 96-well plate format and based on the increase of fluorescence intensity when fluorophore-labeled oligonucleotide is degraded by the DNase. The assay is highly sensitive to DNase I compared to other endonucleases, reliable (Z' ≥ 0.5), and operationally simple, and it has low operator, intraassay, and interassay variability. The assay was used to screen a chemical library, and several potential DNase I inhibitors were identified. After comparison, 2 hit compounds were selected and shown to protect against cisplatin-induced kidney cell death in vitro. This assay will be suitable for identifying inhibitors of DNase I and, potentially, other endonucleases.

Investigation of the impact of Tat export pathway enhancement on E. coli culture, protein production and early stage recovery

The twin arginine translocation (Tat) pathway occurs naturally in E. coli and has the distinct ability to translocate folded proteins across the inner membrane of the cell. It has the potential to export commercially useful proteins that cannot be exported by the ubiquitous Sec pathway. To better understand the bioprocess potential of the Tat pathway, this article addresses the fermentation and downstream processing performances of E. coli strains with a wild-type Tat system exporting the over-expressed substrate protein FhuD. These were compared to strains cell-engineered to over-express the Tat pathway, since the native export capacity of the Tat pathway is low. This low capacity makes the pathway susceptible to saturation by over-expressed substrate proteins, and can result in compromised cell integrity. However, there is concern in the literature that over-expression of membrane proteins, like those of the Tat pathway, can impact negatively upon membrane integrity itself. Under controlled fermentation conditions E. coli cells with a wild-type Tat pathway showed poor protein accumulation, reaching a periplasmic maximum of only 0.5 mg L⁻¹ of growth medium. Cells over-expressing the Tat pathway showed a 25% improvement in growth rate, avoided pathway saturation, and showed 40-fold higher periplasmic accumulation of FhuD. Moreover, this was achieved whilst conserving the integrity of cells for downstream processing: experimentation comparing the robustness of cells to increasing levels of shear showed no detrimental effect from pathway over-expression. Further experimentation on spheroplasts generated by the lysozyme/osmotic shock method--a scaleable way to release periplasmic protein--showed similar robustness between strains. A scale-down mimic of continuous disk-stack centrifugation predicted clarifications in excess of 90% for both intact cells and spheroplasts. Cells over-expressing the Tat pathway performed comparably to cells with the wild-type system. Overall, engineering E. coli cells to over-express the Tat pathway allowed for greater periplasmic yields of FhuD at the fermentation scale without compromising downstream processing performance.

Growth and productivity impacts of periplasmic nuclease expression in an Escherichia coli Fab' fragment production strain

Host cell engineering is becoming a realistic option in whole bioprocess strategies to maximize product manufacturability. High molecular weight (MW) genomic DNA currently hinders bioprocessing of Escherichia coli by causing viscosity in homogenate feedstocks. We previously showed that co-expressing Staphylococcal nuclease and human Fab' fragment in the periplasm of E. coli enables auto-hydrolysis of genomic DNA upon cell disruption, with a consequent reduction in feedstock viscosity and improvement in clarification performance. Here we report the impact of periplasmic nuclease expression on stability of DNA and Fab' fragment in homogenates, host-strain growth kinetics, cell integrity at harvest and Fab' fragment productivity. Nuclease and Fab' plasmids were shown to exert comparable levels of growth burden on the host W3110 E. coli strain. Nuclease co-expression did not compromise either the growth performance or volumetric yield of the production strain. 0.5 g/L Fab' fragment (75 L scale) and 0.7 g/L (20 L scale) was achieved for both unmodified and cell-engineered production strains. Unexpectedly, nuclease-modified cells achieved maximum Fab' levels 8-10 h earlier than the original, unmodified production strain. Scale-down studies of homogenates showed that nuclease-mediated hydrolysis of high MW DNA progressed to completion within minutes of homogenization, even when homogenates were chilled on ice, with no loss of Fab' product and no need for additional co-factors or buffering.

Dual salt precipitation for the recovery of a recombinant protein from Escherichia coli

When considering worldwide demand for biopharmaceuticals, it becomes necessary to consider alternative process strategies to improve the economics of manufacturing such molecules. To address this issue, the current study investigates precipitation to selectively isolate the product or remove contaminants and thus assist the initial purification of a intracellular protein. The hypothesis tested was that the combination of two or more precipitating agents will alter the solubility profile of the product through synergistic or antagonistic effects. This principle was investigated through several combinations of ammonium sulfate and sodium citrate at different ratios. A synergistic effect mediated by a known electrostatic interaction of citrate ions with Fab' in addition to the typical salting-out effects was observed. On the basis of the results of the solubility studies, a two step primary recovery route was investigated. In the first step termed conditioning, post-homogenization and before clarification, addition of 0.8 M ammonium sulfate extracted 30% additional product. Clarification performance measured using a scale-down disc stack centrifugation mimic determined a four-fold reduction in centrifuge size requirements. Dual salt precipitation in the second step resulted in >98% recovery of Fab' while removing 36% of the contaminant proteins simultaneously.

Evaluation of cell disruption effects on primary recovery of antibody fragments using microscale bioprocessing techniques

Intracellular antibody Fab' fragments periplasmically expressed in Escherichia coli require the release of Fab' from the cells before initial product recovery. This work demonstrates the utility of microscale bioprocessing techniques to evaluate the influence of different cell disruption operations on subsequent solid-liquid separation and product recovery. Initially, the industrial method of Fab' release by thermochemical extraction was established experimentally at the microwell scale and was observed to yield Fab' release consistent with the larger scale process. The influence of two further cell disruption operations, homogenization and sonication, on subsequent Fab' recovery by microfiltration was also examined. The results showed that the heat-extracted cells give better dead-end microfiltration performance in terms of permeate flux and specific cake resistance. In contrast, the cell suspensions prepared by homogenization and sonication showed more efficient product release but with lower product purity and poorer microfiltration performance. Having established the various microscale methods the linked sequence was automated on the deck of a laboratory robotic platform and used to show how different conditions during thermochemical extraction impacted on the optimal performance of the linked unit operations. The results illustrate the power of microscale techniques to evaluate crucial unit operation interactions in a bioprocess sequence using only microliter volumes of feed.

Production and purification of staphylococcal nuclease in Lactococcus lactis using a new expression-secretion system and a pH-regulated mini-reactor

BACKGROUND

Staphylococcal (or micrococcal) nuclease or thermonuclease (SNase or Nuc) is a naturally-secreted nucleic acid degrading enzyme that participates in Staphylococcus aureus spread in the infected host. Purified Nuc protein can be used as an exogenous reagent to clear cellular extracts and improve protein purification. Here, a recombinant form of Nuc was produced and secreted in a Gram-positive host, Lactococcus lactis, and purified from the culture medium.

RESULTS

The gene segment corresponding to the S. aureus nuclease without its signal peptide was cloned in an expression-secretion vector. It was then fused to a lactococcal sequence encoding a signal peptide, and expressed under the control of a lactococcal promoter that is inducible by zinc starvation. An L. lactis subsp cremoris model strain (MG1363) transformed with the resulting plasmid was grown in either of two media (GM17v and CDM) that are free of animal compounds, allowing GMP (Good Manufacturing Practice) production. Induction conditions (concentration of the metal chelator EDTA and timing of addition) in small-scale pH-regulated fermentors were optimized using LacMF (Lactis Multi-Fermentor), a home-made parallel fermentation control system able to monitor 12 reactors simultaneously. Large amounts of recombinant Nuc (rNuc) were produced and secreted in both media, and rNuc was purified from GM17v medium in a single-step procedure.

CONCLUSIONS

In L. lactis, rNuc production and secretion were optimal after induction by 0.5 mM EDTA in small scale (200 mL) GM17v exponential phase cultures (at an OD(600) of 2), leading to a maximal protein yield of 210 mg per L of culture medium. Purified rNuc was highly active, displaying a specific activity of 2000 U/mg.