Extracellular recombinant protein production under continuous culture conditions with Escherichia coli using an alternative plasmid selection mechanism

Pathogen-Specific Bactericidal Method Mediated by Conjugative Delivery of CRISPR-Cas13a Targeting Bacterial Endogenous Transcripts

The emergence of antibiotic-resistant bacteria threatens public health, and the use of broad-spectrum antibiotics often leads to unintended consequences, including disturbing the beneficial gut microbiota and resulting in secondary diseases. Therefore, developing a novel strategy that specifically kills pathogens without affecting the residential microbiota is desirable and urgently needed. Here, we report the development of a precise bactericidal system by taking advantage of CRISPR-Cas13a targeting endogenous transcripts of Salmonella enterica serovar Typhimurium delivered through a conjugative vehicle. In vitro, the CRISPR-Cas13a system exhibited specific killing, growth inhibition, and clearance of S. Typhimurium in mixed microbial flora. In a mouse infection model, the CRISPR-Cas13a system, when delivered by a donor Escherichia coli strain, significantly reduced S. Typhimurium colonization in the intestinal tract. Overall, the results demonstrate the feasibility and efficacy of the designed CRISPR-Cas13a system in selective killing of pathogens and broaden the utility of conjugation-based delivery of bactericidal approaches.

IMPORTANCE Antibiotics with broad-spectrum activities are known to disturb both pathogens and beneficial gut microbiota and cause many undesired side effects, prompting increased interest in developing therapies that specifically eliminate pathogenic bacteria without damaging gut resident flora. To achieve this goal, we developed a strategy utilizing bacterial conjugation to deliver CRISPR-Cas13a programmed to specifically kill S. Typhimurium. This system produced pathogen-specific killing based on CRISPR RNA (crRNAs) targeting endogenous transcripts in pathogens and was shown to be effective in both in vitro and in vivo experiments. Additionally, the system can be readily delivered by conjugation and is adaptable for targeting different pathogens. With further optimization and improvement, the system has the potential to be used for biotherapy and microbial community modification.

Development of an E. coli strain for cell-free ADC manufacturing

Recent advances in cell‐free protein synthesis have enabled the folding and assembly of full‐length antibodies at high titers with extracts from prokaryotic cells. Coupled with the facile engineering of the Escherichia coli translation machinery, E. coli based in vitro protein synthesis reactions have emerged as a leading source of IgG molecules with nonnatural amino acids incorporated at specific locations for producing homogeneous antibody–drug conjugates (ADCs). While this has been demonstrated with extract produced in batch fermentation mode, continuous extract fermentation would facilitate supplying material for large‐scale manufacturing of protein therapeutics. To accomplish this, the IgG‐folding chaperones DsbC and FkpA, and orthogonal tRNA for nonnatural amino acid production were integrated onto the chromosome with high strength constitutive promoters. This enabled co‐expression of all three factors at a consistently high level in the extract strain for the duration of a 5‐day continuous fermentation. Cell‐free protein synthesis reactions with extract produced from cells grown continuously yielded titers of IgG containing nonnatural amino acids above those from extract produced in batch fermentations. In addition, the quality of the synthesized IgGs and the potency of ADC produced with continuously fermented extract were indistinguishable from those produced with the batch extract. These experiments demonstrate that continuous fermentation of E. coli to produce extract for cell‐free protein synthesis is feasible and helps unlock the potential for cell‐free protein synthesis as a platform for biopharmaceutical production.

Efficient strategies to enhance plasmid stability for fermentation of recombinant Escherichia coli harboring tyrosine phenol lyase

OBJECTIVE

To solve the bottleneck of plasmid instability during microbial fermentation of L-DOPA with recombinant Escherichia coli expressing heterologous tyrosine phenol lyase.

RESULTS

The tyrosine phenol lyase from Fusobacterium nucleatum was constitutively expressed in E. coli and a fed-batch fermentation process with temperature down-shift cultivation was performed. Efficient strategies including replacing the original ampicillin resistance gene, as well as inserting cer site that is active for resolving plasmid multimers were applied. As a result, the plasmid stability was increased. The co-use of cer site on plasmid and kanamycin in culture medium resulted in proportion of plasmid containing cells maintained at 100% after fermentation for 35 h. The specific activity of tyrosine phenol lyase reached 1493 U/g dcw, while the volumetric activity increased from 2943 to 14,408 U/L for L-DOPA biosynthesis.

CONCLUSIONS

The established strategies for plasmid stability is not only promoted the applicability of the recombinant cells for L-DOPA production, but also provides important guidance for industrial fermentation with improved microbial productivity.

The Rocky Road From Fed-Batch to Continuous Processing With E. coli

Escherichia coli still serves as a beloved workhorse for the production of many biopharmaceuticals as it fulfills essential criteria, such as having fast doubling times, exhibiting a low risk of contamination, and being easy to upscale. Most industrial processes in E. coli are carried out in fed-batch mode. However, recent trends show that the biotech industry is moving toward time-independent processing, trying to improve the space-time yield, and especially targeting constant quality attributes. In the 1950s, the term "chemostat" was introduced for the first time by Novick and Szilard, who followed up on the previous work performed by Monod. Chemostat processing resulted in a major hype 10 years after its official introduction. However, enthusiasm decreased as experiments suffered from genetic instabilities and physiology issues. Major improvements in strain engineering and the usage of tunable promotor systems facilitated chemostat processes. In addition, critical process parameters have been identified, and the effects they have on diverse quality attributes are understood in much more depth, thereby easing process control. By pooling the knowledge gained throughout the recent years, new applications, such as parallelization, cascade processing, and population controls, are applied nowadays. However, to control the highly heterogeneous cultivation broth to achieve stable productivity throughout long-term cultivations is still tricky. Within this review, we discuss the current state of E. coli fed-batch process understanding and its tech transfer potential within continuous processing. Furthermore, the achievements in the continuous upstream applications of E. coli and the continuous downstream processing of intracellular proteins will be discussed.

Boosting Recombinant Inclusion Body Production-From Classical Fed-Batch Approach to Continuous Cultivation

State of the art microbial recombinant protein production is regularly performed in fed-batch based cultivations. However, these cultivations suffer from highly time-dependent changes in productivity and product quality, leading to high variations in the downstream process. Continuous biomanufacturing offers the possibility of a time independent process, boosting the time-space-yield of the recombinantly produced protein and further reducing costs for production, also as downstream gets more predictive. In the current work, the continuous production of a pharmaceutically relevant protein in form of an inclusion body in E. coli BL21(DE3) was investigated in single vessel cultivations by varying dilution rates using glycerol as carbon source, inducer (lactose or IPTG) and respective inducer concentrations. Attempts to increase low specific productivities observed in single vessel continuous cultivations, led to the establishment of a continuously operated cascade of two stirred tank reactors to spatially separate biomass formation from recombinant protein production. Process performance was substantially improved compared to a single vessel chemostat culture, as specific productivity and space-time yield were boosted using an optimized cascaded process by about a factor of 100. This study shows the potential of a two-stage continuous process as promising alternative to benchmark fed-batch processes achieving constant inclusion body production at a time-independent level.

Two New Plasmid Post-segregational Killing Mechanisms for the Implementation of Synthetic Gene Networks in Escherichia coli

Plasmids are the workhorse of both industrial biotechnology and synthetic biology, but ensuring they remain in bacterial cells is a challenge. Antibiotic selection cannot be used to stabilize plasmids in most real-world applications, and inserting dynamical gene networks into the genome remains challenging. Plasmids have evolved several mechanisms for stability, one of which, post-segregational killing (PSK), ensures that plasmid-free cells do not survive. Here we demonstrate the plasmid-stabilizing capabilities of the axe/txe toxin-antitoxin system and the microcin-V bacteriocin system in the probiotic bacteria Escherichia coli Nissle 1917 and show that they can outperform the commonly used hok/sok. Using plasmid stability assays, automated flow cytometry analysis, mathematical models, and Bayesian statistics we quantified plasmid stability in vitro. Furthermore, we used an in vivo mouse cancer model to demonstrate plasmid stability in a real-world therapeutic setting. These new PSK systems, plus the developed Bayesian methodology, will have wide applicability in clinical and industrial biotechnology.

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.

Use of the SHuffle Strains in Production of Proteins

Escherichia coli continues to be a popular expression host for the production of proteins, yet successful recombinant expression of active proteins to high yields remains a trial and error process. This is mainly due to decoupling of the folding factors of a protein from its native host, when expressed recombinantly in E. coli. Failure to fold could be due to many reasons but is often due to lack of post-translational modifications that are absent in E. coli. One such post-translational modification is the formation of disulfide bonds, a common feature of secreted proteins. The genetically engineered SHuffle cells offer an expression solution to proteins that require disulfide bonds for their folding and activity. The purpose of this protocol unit is to familiarize the researcher with the biology of SHuffle cells and guide the experimental design in order to optimize and increase the chances of successful expression of their desired protein of choice. Example of the expression and purification of a model disulfide-bonded protein DsbC is described in detail. © 2016 by John Wiley & Sons, Inc.