Multiplex Genome Editing by Natural Transformation (MuGENT) for Synthetic Biology in Vibrio natriegens

Vibrio natriegens as a host for rapid biotechnology

Vibrio natriegens is a Gram-negative marine bacterium with an exceptionally fast growth rate and a doubling time of less than 10 min. Its high substrate uptake rates and metabolic prowess make it a promising next-generation workhorse for rapid molecular biology, protein expression, and metabolic engineering.

Intelligent host engineering for metabolic flux optimisation in biotechnology

Optimising the function of a protein of length N amino acids by directed evolution involves navigating a 'search space' of possible sequences of some 20N. Optimising the expression levels of P proteins that materially affect host performance, each of which might also take 20 (logarithmically spaced) values, implies a similar search space of 20P. In this combinatorial sense, then, the problems of directed protein evolution and of host engineering are broadly equivalent. In practice, however, they have different means for avoiding the inevitable difficulties of implementation. The spare capacity exhibited in metabolic networks implies that host engineering may admit substantial increases in flux to targets of interest. Thus, we rehearse the relevant issues for those wishing to understand and exploit those modern genome-wide host engineering tools and thinking that have been designed and developed to optimise fluxes towards desirable products in biotechnological processes, with a focus on microbial systems. The aim throughput is 'making such biology predictable'. Strategies have been aimed at both transcription and translation, especially for regulatory processes that can affect multiple targets. However, because there is a limit on how much protein a cell can produce, increasing kcat in selected targets may be a better strategy than increasing protein expression levels for optimal host engineering.

A dual-function RNA balances carbon uptake and central metabolism in Vibrio cholerae

Bacterial small RNAs (sRNAs) are well known to modulate gene expression by base pairing with trans‐encoded transcripts and are typically non‐coding. However, several sRNAs have been reported to also contain an open reading frame and thus are considered dual‐function RNAs. In this study, we discovered a dual‐function RNA from Vibrio cholerae, called VcdRP, harboring a 29 amino acid small protein (VcdP), as well as a base‐pairing sequence. Using a forward genetic screen, we identified VcdRP as a repressor of cholera toxin production and link this phenotype to the inhibition of carbon transport by the base‐pairing segment of the regulator. By contrast, we demonstrate that the VcdP small protein acts downstream of carbon transport by binding to citrate synthase (GltA), the first enzyme of the citric acid cycle. Interaction of VcdP with GltA results in increased enzyme activity and together VcdR and VcdP reroute carbon metabolism. We further show that transcription of vcdRP is repressed by CRP allowing us to provide a model in which VcdRP employs two different molecular mechanisms to synchronize central metabolism in V. cholerae.

The quorum-sensing systems of Vibrio campbellii DS40M4 and BB120 are genetically and functionally distinct

Vibrio campbellii BB120 (previously classified as Vibrio harveyi) is a fundamental model strain for studying quorum sensing in vibrios. A phylogenetic evaluation of sequenced Vibrio strains in Genbank revealed that BB120 is closely related to the environmental isolate V. campbellii DS40M4. We exploited DS40M4's competence for exogenous DNA uptake to rapidly generate greater than 30 isogenic strains with deletions of genes encoding BB120 quorum-sensing system homologues. Our results show that the quorum-sensing circuit of DS40M4 is distinct from BB120 in three ways: (i) DS40M4 does not produce an acyl homoserine lactone autoinducer but encodes an active orphan LuxN receptor, (ii) the quorum regulatory small RNAs (Qrrs) are not solely regulated by autoinducer signalling through the response regulator LuxO and (iii) the DS40M4 quorum-sensing regulon is much smaller than BB120 (~100 genes vs. ~400 genes, respectively). Using comparative genomics to expand our understanding of quorum-sensing circuit diversity, we observe that conservation of LuxM/LuxN proteins differs widely both between and within Vibrio species. These strains are also phenotypically distinct: DS40M4 exhibits stronger interbacterial cell killing, whereas BB120 forms more robust biofilms and is bioluminescent. These results underscore the need to examine wild isolates for a broader view of bacterial diversity in the marine ecosystem.

A novel global transcriptional perturbation target identified by forward genetics reprograms Vibrio natriegens for improving recombinant protein production

Vibrio natriegens is known to be the fastest-growing free-living bacterium with the potential to be a novel protein expression system other than Escherichia coli. Seven sampled genes of interest (GOIs) encoding biocatalyst enzymes, including Ochrobactrum anthropi-derived ω-transaminase (OATA), were strongly expressed in E. coli but weakly in V. natriegens using the pET expression system. In this study, we fused the C-terminal of OATA with green fluorescent protein (GFP) and obtained V. natriegens mutants that could increase both protein yield and enzyme activity of OATA as well as the other three GOIs by ultraviolet mutagenesis, fluorescence-activated cell sorting (FACS), and OATA colorimetric assay. Furthermore, next-generation sequencing and strain reconstruction revealed that the Y457 variants in the conserved site of endogenous RNA polymerase (RNAP) β' subunit rpoC are responsible for the increase in recombinant protein yield. We speculated that the mutation of rpoC Y457 may reprogram V. natriegens's innate gene transcription, thereby increasing the copy number of pET plasmids and soluble protein yield of certain GOIs. The increase in GOI expression may partly be attributed to the increase in copy number. In conclusion, GOI-GFP fusion combined with FACS is a powerful tool of forward genetics that can be used to obtain a superior expression chassis. If more high-expression-related targets are found for more GOIs, it would make the construction of next-generation protein expression chassis more time-saving.

Effect of Nutritional Determinants and TonB on the Natural Transformation of Riemerella anatipestifer

Riemerella anatipestifer is a gram-negative bacterium that is the first naturally competent bacterium identified in the family Flavobacteriaceae. However, the determinants that influence the natural transformation and the underlying mechanism remain unknown. In this study, we evaluated the effects of various nutritional factors of the GCB medium [glucose, L-glutamine, vitamin B1, Fe (NO₃)₃, NaCl, phosphate, and peptone], on the natural transformation of R. anatipestifer ATCC 11845. Among the assayed nutrients, peptone and phosphate affected the natural transformation of R. anatipestifer ATCC 11845, and the transformation frequency was significantly decreased when phosphate or peptone was removed from the GCB medium. When the iron chelator 2,2′-dipyridyl (Dip) was added, the transformation frequency was decreased by approximately 100-fold and restored gradually when Fe (NO₃)₃ was added, suggesting that the natural transformation of R. anatipestifer ATCC 11845 requires iron. Given the importance of TonB in nutrient transportation, we further identified whether TonB is involved in the natural transformation of R. anatipestifer ATCC 11845. Mutation of tonBA or tonBB, but not tbfA, was shown to inhibit the natural transformation of R. anatipestifer ATCC 11845 in the GCB medium. In parallel, it was shown that the tonBB mutant, but not the tonBA mutant, decreased iron acquisition in the GCB medium. This result suggested that the tonBB mutant affects the natural transformation frequency due to the deficiency of iron utilization.

The Marburg Collection: A Golden Gate DNA Assembly Framework for Synthetic Biology Applications in Vibrio natriegens

Vibrio natriegens is known as the world's fastest growing organism with a doubling time of less than 10 min. This incredible growth speed empowers V. natriegens as a chassis for synthetic and molecular biology, potentially replacing E. coli in many applications. While first genetic parts have been built and tested for V. natriegens, a comprehensive toolkit containing well-characterized and standardized parts did not exist. To close this gap, we created the Marburg Collection-a highly flexible Golden Gate cloning toolbox optimized for the emerging chassis organism V. natriegens, containing 191 genetic parts. The Marburg Collection overcomes the paradigm of plasmid construction-integrating inserts into a backbone-by enabling the de novo assembly of plasmids from basic genetic parts. This allows users to select the plasmid replication origin and resistance part independently, which is highly advantageous when limited knowledge about the behavior of those parts in the target organism is available. Additional design highlights of the Marburg Collection are novel connector parts, which facilitate modular circuit assembly and, optionally, the inversion of individual transcription units to reduce transcriptional crosstalk in multigene constructs. To quantitatively characterize the genetic parts contained in the Marburg Collection in V. natriegens, we developed a reliable microplate reader measurement workflow for reporter experiments and overcame organism-specific challenges. We think the Marburg Collection with its thoroughly characterized parts will provide a valuable resource for the growing V. natriegens community.

The polar flagellar transcriptional regulatory network in Vibrio campbellii deviates from canonical Vibrio species

Swimming motility is a critical virulence factor in pathogenesis for numerous Vibrio species. Vibrio campbellii DS40M4 is a wild isolate that has been recently established as a highly tractable model strain for bacterial genetics studies. We sought to exploit the tractability and relevance of this strain for characterization of flagellar gene regulation in V. campbellii. Using comparative genomics, we identified homologs of V. campbellii flagellar and chemotaxis genes conserved in other members of the Vibrionaceae and determined the transcriptional profile of these loci using differential RNA-seq. We systematically deleted all 63 predicted flagellar and chemotaxis genes in V. campbellii and examined their effects on motility and flagellum production. We specifically focused on the core regulators of the flagellar hierarchy established in other vibrios: RpoN (σ⁵⁴), FlrA, FlrC, and FliA. Our results show that V. campbellii transcription of flagellar and chemotaxis genes is governed by a multi-tiered regulatory hierarchy similar to other motile Vibrio species. However, there are several critical differences in V. campbellii: (i) the σ⁵⁴-dependent regulator FlrA is dispensable for motility, (ii) the flgA, fliEFGHIJ, flrA, and flrBC operons do not require σ⁵⁴ for expression, and (iii) FlrA and FlrC co-regulate class II genes. Our model proposes that the V. campbellii flagellar transcriptional hierarchy has three classes of genes, in contrast to the four-class hierarchy in Vibrio cholerae. Our genetic and phenotypic dissection of the V. campbellii flagellar regulatory network highlights the differences that have evolved in flagellar regulation across the Vibrionaceae. Importance Vibrio campbellii is a Gram-negative bacterium that is free-living and ubiquitous in marine environments and is an important global pathogen of fish and shellfish. Disruption of the flagellar motor significantly decreases host mortality of V. campbellii, suggesting that motility is a key factor in pathogenesis. Using this model organism, we identified >60 genes that encode proteins with predicted structural, mechanical, or regulatory roles in function of the single polar flagellum in V. campbellii. We systematically tested strains containing single deletions of each gene to determine the impact on motility and flagellum production. Our studies have uncovered differences in the regulatory network and function of several genes in V. campbellii as compared to established systems in Vibrio cholerae and Vibrio parahaemolyticus.

Metabolic engineering of Vibrio natriegens

Vibrio natriegens is emerging as a promising host for biotechnology which is basically due to the remarkable intrinsic properties such as the exceptionally high growth and substrate consumption rates. The facultatively anaerobic marine bacterium possesses a versatile metabolism, is able to utilize a variety of substrates as carbon and energy sources and is easy to handle in the lab. These features initiated the rapid development of genetic tools and resulted in extensive engineering of production strains in the past years. Although recent examples illustrate the potential of V. natriegens for biotechnology, a comprehensive understanding of the metabolism and its regulation is still lacking but essential to exploit the full potential of this bacterium. In this review, we summarize the current knowledge on the physiological traits and the genomic organization, provide an overview of the available genetic engineering tools and recent advances in metabolic engineering of V. natriegens. Finally, we discuss the obstacles which have to be overcome in order to establish V. natriegens as industrial production host.

Development and comparison of cell-free protein synthesis systems derived from typical bacterial chassis

Cell-free protein synthesis (CFPS) systems have become an ideal choice for pathway prototyping, protein production, and biosensing, due to their high controllability, tolerance, stability, and ability to produce proteins in a short time. At present, the widely used CFPS systems are mainly based on Escherichia coli strain. Bacillus subtilis, Corynebacterium glutamate, and Vibrio natriegens are potential chassis cells for many biotechnological applications with their respective characteristics. Therefore, to expand the platform of the CFPS systems and options for protein production, four prokaryotes, E. coli, B. subtilis, C. glutamate, and V. natriegens were selected as host organisms to construct the CFPS systems and be compared. Moreover, the process parameters of the CFPS system were optimized, including the codon usage, plasmid synthesis competent cell selection, plasmid concentration, ribosomal binding site (RBS), and CFPS system reagent components. By optimizing and comparing the main influencing factors of different CFPS systems, the systems can be optimized directly for the most influential factors to further improve the protein yield of the systems. In addition, to demonstrate the applicability of the CFPS systems, it was proved that the four CFPS systems all had the potential to produce therapeutic proteins, and they could produce the receptor-binding domain (RBD) protein of SARS-CoV-2 with functional activity. They not only could expand the potential options for in vitro protein production, but also could increase the application range of the system by expanding the cell-free protein synthesis platform.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1186/s40643-021-00413-2.

High-cell-density fed-batch cultivations of Vibrio natriegens

Objectives

With generation times of less than 10 min under optimal conditions, the halophilic Vibrio natriegens is the fastest growing non-pathogenic bacterium isolated so far. The availability of the full genome and genetic engineering tools and its ability to utilize a wide range of carbon sources make V. natriegens an attractive host for biotechnological production processes. However, high-cell-density cultivations, which are desired at industrial-scale have not been described so far.

Results

In this study we report fed-batch cultivations of V. natriegens in deep-well plates and lab-scale bioreactor cultivations at different temperatures in mineral salt medium (MSM). Upon switching from exponential glucose to constant glucose-feeding cell death was induced. Initial NaCl concentrations of 15–18 g L⁻¹ and a temperature reduction from 37 to 30 °C had a positive effect on cell growth. The maximal growth rate in MSM with glucose was 1.36 h⁻¹ with a specific oxygen uptake rate of 22 mmol g_CDW⁻¹ h⁻¹. High biomass yields of up to 55 g L⁻¹ after only 12 h were reached.

Conclusions

The shown fed-batch strategies demonstrate the potential of V. natriegens as a strong producer in industrial biotechnology.

Supplementary Information

The online version of this article (doi:10.1007/s10529-021-03147-5) contains supplementary material, which is available to authorized users.

Natural Transformation in a Classical-Biotype Vibrio cholerae Strain

Vibrio cholerae causes the gastrointestinal illness cholera, which spreads throughout the globe in large pandemics. The current pandemic is caused by O1 El Tor biotype strains, whereas previous pandemics were caused by O1 classical biotype strains. El Tor V. cholerae is noted for its ability to acquire exogenous DNA through chitin-induced natural transformation, which has been exploited for genetic manipulation of El Tor strains in the laboratory. In contrast, the prototypical classical strain O395 lacks this ability, which was suspected to be due to a mutation in the regulatory gene hapR HapR and the regulator TfoX control expression of a third competence regulator, QstR. We found that artificial induction of both TfoX and QstR in the presence of HapR in O395 was required for efficient DNA uptake. However, natural transformation in the classical strain is still orders of magnitude below that of an El Tor strain. O395 expressing HapR could also undergo natural transformation after growth on chitin, which could be increased by artificial induction of TfoX and/or QstR. A plasmid that expresses both TfoX and QstR was created that allowed for consistent DNA uptake in O395 carrying a hapR plasmid. This technique was also used to facilitate cotransformation into O395 of unmarked DNA (ΔlacZ, ΔflaA, ΔflgG) for multiplex genome editing by natural transformation (MuGENT). These results demonstrate that the classical biotype O395 strain is functionally capable of DNA uptake, which allows for the rapid genetic manipulation of its genome.IMPORTANCE Natural transformation (uptake of exogenous DNA) in Vibrio cholerae has contributed to the evolution of these human pathogens. Classical biotype V. cholerae strains were responsible for the first six cholera pandemics but were replaced by El Tor biotype V. cholerae in the current pandemic. This study demonstrates that classical V. cholerae is functionally capable of natural transformation, but inactivation of the transformation regulator HapR and inherent levels of transformation that are lower than those of El Tor V. cholerae suggest that the classical biotype may be less able to utilize natural transformation for horizontal gene transfer.

Engineering Vibrio sp. SP1 for the production of carotenoids directly from brown macroalgae

Macroalgae is regarded as a promising third-generation marine biomass that can be utilized as a sustainable feedstock for bio-industry due to the high sugar level and absence of lignin. Alginate, composed of 1,4-linked D-mannuronate (M) and L-guluronate (G), is one of the major carbohydrates in brown macroalgae. It is difficult to be assimilated by most industrial microorganisms. Therefore, developing engineered microorganisms that can utilize alginate as a feedstock in order to produce natural products from marine biomass is critical. In this study, we isolated, characterized, and sequenced Vibrio sp. SP1 which rapidly grows using alginate as a sole carbon source. We further engineered this strain by introducing genes encoding enzymes under the control of synthetic expression cassettes to produce lycopene and β-carotene which are attractive phytochemicals owing to the antioxidant property. We confirmed that the engineered Vibrio sp. SP1 could successfully produce 2.13 ± 0.37 mg L-1 of lycopene, 2.98 ± 0.43 mg L-1 of β -carotene, respectively, from 10 g L-1 of alginate as a sole carbon source. Furthermore, our engineered strain could directly convert 60 g L-1 of brown macroalgae Sargassum fusiforme into 1.23 ± 0.21 mg L-1 of lycopene without any pretreatment which had been vitally required for the previous productions. As the first demonstrated strain to produce high-value product from Sargassum, Vibrio sp. SP1 is evaluated to be a desirable platform for the brown macroalgae-based biorefinery.

Genetic Code Expansion of Vibrio natriegens

Escherichia coli has been considered as the most used model bacteria in the majority of studies for several decades. However, a new, faster chassis for synthetic biology is emerging in the form of the fast-growing gram-negative bacterium Vibrio natriegens. Different methodologies, well established in E. coli, are currently being adapted for V. natriegens in the hope to enable a much faster platform for general molecular biology studies. Amongst the vast technologies available for E. coli, genetic code expansion, the incorporation of unnatural amino acids into proteins, serves as a robust tool for protein engineering and biorthogonal modifications. Here we designed and adapted the genetic code expansion methodology for V. natriegens and demonstrate an unnatural amino acid incorporation into a protein for the first time in this organism.

Vibrio natriegens as a pET-Compatible Expression Host Complementary to Escherichia coli

Efficient and novel recombinant protein expression systems can further reduce the production cost of enzymes. Vibrio natriegens is the fastest growing free-living bacterium with a doubling time of less than 10 min, which makes it highly attractive as a protein expression host. Here, 196 pET plasmids with different genes of interest (GOIs) were electroporated into the V. natriegens strain VnDX, which carries an integrated T7 RNA polymerase expression cassette. As a result, 65 and 75% of the tested GOIs obtained soluble expression in V. natriegens and Escherichia coli, respectively, 20 GOIs of which showed better expression in the former. Furthermore, we have adapted a consensus "what to try first" protocol for V. natriegens based on Terrific Broth medium. Six sampled GOIs encoding biocatalysts enzymes thus achieved 50-128% higher catalytic efficiency under the optimized expression conditions. Our study demonstrated V. natriegens as a pET-compatible expression host with a spectrum of highly expressed GOIs distinct from E. coli and an easy-to-use consensus protocol, solving the problem that some GOIs cannot be expressed well in E. coli.

A lysate proteome engineering strategy for enhancing cell-free metabolite production

Cell-free systems present a significant opportunity to harness the metabolic potential of diverse organisms. Removing the cellular context provides the ability to produce biological products without the need to maintain cell viability and enables metabolic engineers to explore novel chemical transformation systems. Crude extracts maintain much of a cell’s capabilities. However, only limited tools are available for engineering the contents of the extracts used for cell-free systems. Thus, our ability to take full advantage of the potential of crude extracts for cell-free metabolic engineering is constrained. Here, we employ Multiplex Automated Genomic Engineering (MAGE) to tag proteins for selective depletion from crude extracts so as to specifically direct chemical production. Specific edits to central metabolism are possible without significantly impacting cell growth. Selective removal of pyruvate degrading enzymes resulted in engineered crude lysates that are capable of up to 40-fold increases in pyruvate production when compared to the non-engineered extract. The described approach melds the tools of systems and synthetic biology to showcase the effectiveness of cell-free metabolic engineering for applications like bioprototyping and bioproduction.

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A method of engineering cell-free metabolism in lysates is described.

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Method enables design of cell lysates for enhancing specific metabolic processes.

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Pyruvate consuming enzymes tagged with 6xHis tags have minimal impact on growth.

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Post-lysis pull-down of tagged enzymes enables cell-free pyruvate pooling.

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Lysate engineering strategy permits metabolic states not possible in living cells.

Automating Cloning by Natural Transformation

Affordable and automated cloning platforms are essential to many synthetic biology studies. However, the traditional E. coli-based cloning is a major bottleneck as it requires heat shock or electroporation implemented in the robotic workflows. To overcome this problem, we explored bacterial natural transformation for automatic DNA cloning and engineering. Recombinant plasmids are efficiently generated from Gibson or overlap extension PCR (OE-PCR) products by simply adding the DNA into Acinetobacter baylyi ADP1 cultures. No DNA purification, competence induction, or special equipment is required. Up to 10,000 colonies were obtained per microgram of DNA, while the number of false positive colonies was low. We cloned and engineered 21 biosynthetic gene clusters (BGCs) of various types, with length from 1.5 to 19 kb and GC content from 35% to 72%. One of them, a nucleoside BGC, showed antibacterial activity. Furthermore, the method was easily transferred to a low-cost benchtop robot with consistent cloning efficiency. Thus, this automatic natural transformation (ANT) cloning provides an easy, robust, and affordable platform for high throughput DNA engineering.

Genome Sequence of Vibrio natriegens Phage vB_VnaS-AQKL99

Vibrio natriegens is a naturally occurring marine bacterium that is emerging as a microbiological model system. Here, we describe Aquatic Killer 99 (AQKL99), a novel phage that infects Vibrio natriegens 14048. The genome of the phage is 58,464 bp long, has a GC content of 45.9%, and contains 51 protein-coding genes.

Vibrio natriegens is a naturally occurring marine bacterium that is emerging as a microbiological model system. Here, we describe Aquatic Killer 99 (AQKL99), a novel phage that infects Vibrio natriegens 14048. The genome of the phage is 58,464 bp long, has a GC content of 45.9%, and contains 51 protein-coding genes.

A modular chromosomally integrated toolkit for ectopic gene expression in Vibrio cholerae

The ability to express genes ectopically in bacteria is essential for diverse academic and industrial applications. Two major considerations when utilizing regulated promoter systems for ectopic gene expression are (1) the ability to titrate gene expression by addition of an exogenous inducer and (2) the leakiness of the promoter element in the absence of the inducer. Here, we describe a modular chromosomally integrated platform for ectopic gene expression in Vibrio cholerae. We compare the broadly used promoter elements P_tac and P_BAD to versions that have an additional theophylline-responsive riboswitch (P_tac-riboswitch and P_BAD-riboswitch). These constructs all exhibited unimodal titratable induction of gene expression, however, max induction varied with P_tac > P_BAD > P_BAD-riboswitch > P_tac-riboswitch. We also developed a sensitive reporter system to quantify promoter leakiness and show that leakiness for P_tac > P_tac-riboswitch > P_BAD; while the newly developed P_BAD-riboswitch exhibited no detectable leakiness. We demonstrate the utility of the tightly inducible P_BAD-riboswitch construct using the dynamic activity of type IV competence pili in V. cholerae as a model system. The modular chromosomally integrated toolkit for ectopic gene expression described here should be valuable for the genetic study of V. cholerae and could be adapted for use in other species.

Design and Reconstruction of Regulatory Parts for Fast-frowing Vibrio natriegens Synthetic Biology

The fast-growing Vibrio natriegens is an attractive robust chassis for diverse synthetic biology applications. However, V. natriegens lacks the suitable constitutive regulatory parts for precisely tuning the gene expression and, thus, recapitulating physiologically relevant changes in gene expression levels. In this study, we designed, constructed, and screened the synthetic regulatory parts by varying the promoter region and ribosome binding site element for V. natriegens with different transcriptional or translational strengths, respectively. The fluorescence intensities of the cells with different synthetic regulatory parts could distribute evenly over a wide range of 5 orders of magnitude. The selected synthetic regulatory parts had good stability in both nutrient-rich and minimal media. The precise combinatorial modulation of galP (GalP = galactose permease) and glk (Glk = glucokinase) from Escherichia coli by using three synthetic regulatory parts with different strengths was confirmed in a phosphoenolpyruvate:carbohydrate phosphotransferase system with inactive V. natriegens strain to alter the glucose transport. This work provides the simple, efficient, and standardized constitutive regulatory parts for V. natriegens synthetic biology.

A Hybrid Extracellular Electron Transfer Pathway Enhances the Survival of Vibrio natriegens

Vibrio natriegens is the fastest-growing microorganism discovered to date, making it a useful model for biotechnology and basic research. While it is recognized for its rapid aerobic metabolism, less is known about anaerobic adaptations in V. natriegens or how the organism survives when oxygen is limited. Here, we describe and characterize extracellular electron transfer (EET) in V. natriegens, a metabolism that requires movement of electrons across protective cellular barriers to reach the extracellular space. V. natriegens performs extracellular electron transfer under fermentative conditions with gluconate, glucosamine, and pyruvate. We characterized a pathway in V. natriegens that requires CymA, PdsA, and MtrCAB for Fe(III) citrate and Fe(III) oxide reduction, which represents a hybrid of strategies previously discovered in Shewanella and Aeromonas Expression of these V. natriegens genes functionally complemented Shewanella oneidensis mutants. Phylogenetic analysis of the inner membrane quinol dehydrogenases CymA and NapC in gammaproteobacteria suggests that CymA from Shewanella diverged from Vibrionaceae CymA and NapC. Analysis of sequenced Vibrionaceae revealed that the genetic potential to perform EET is conserved in some members of the Harveyi and Vulnificus clades but is more variable in other clades. We provide evidence that EET enhances anaerobic survival of V. natriegens, which may be the primary physiological function for EET in Vibrionaceae IMPORTANCE Bacteria from the genus Vibrio occupy a variety of marine and brackish niches with fluctuating nutrient and energy sources. When oxygen is limited, fermentation or alternative respiration pathways must be used to conserve energy. In sedimentary environments, insoluble oxide minerals (primarily iron and manganese) are able to serve as electron acceptors for anaerobic respiration by microorganisms capable of extracellular electron transfer, a metabolism that enables the use of these insoluble substrates. Here, we identify the mechanism for extracellular electron transfer in Vibrio natriegens, which uses a combination of strategies previously identified in Shewanella and Aeromonas We show that extracellular electron transfer enhanced survival of V. natriegens under fermentative conditions, which may be a generalized strategy among Vibrio spp. predicted to have this metabolism.

Changes in Vibrio natriegens Growth Under Simulated Microgravity

The growth rate of bacteria increases under simulated microgravity (SMG) with low-shear force. The next-generation microbial chassis Vibrio natriegens (V. natriegens) is a fast-growing Gram-negative, non-pathogenic bacterium with a generation time of less than 10 min. Screening of a V. natriegens strain with faster growth rate was attempted by 2-week continuous long-term culturing under SMG. However, the rapid growth rate of this strain made it difficult to obtain the desired mutant strain with even more rapid growth. Thus, a mutant with slower growth rate emerged. Multi-omics integration analysis was conducted to explore why this mutant grew more slowly, which might inform us about the molecular mechanisms of rapid growth of V. natriegens instead. The transcriptome data revealed that whereas genes related to mechanical signal transduction and flagellin biogenesis were up-regulated, those involved in adaptive responses, anaerobic and nitrogen metabolism, chromosome segregation and cell vitality were down-regulated. Moreover, genome-wide chromosome conformation capture (Hi-C) results of the slower growth mutant and wide type indicated that SMG-induced great changes of genome 3D organization were highly correlated with differentially expressed genes (DEGs). Meanwhile, whole genome re-sequencing found a significant number of structure variations (SVs) were enriched in regions with lower interaction frequency and down-regulated genes in the slower growth mutant compared with wild type (WT), which might represent a prophage region. Additionally, there was also a decreased interaction frequency in regions associated with well-orchestrated chromosomes replication. These results suggested that SMG might regulate local gene expression by sensing stress changes through conformation changes in the genome region of genes involved in flagellin, adaptability and chromosome segregation, thus followed by alteration of some physiological characteristics and affecting the growth rate and metabolic capacity.

Melanin Produced by the Fast-Growing Marine Bacterium Vibrio natriegens through Heterologous Biosynthesis: Characterization and Application

Melanin is a pigment produced by organisms throughout all domains of life. Due to its unique physicochemical properties, biocompatibility, and biostability, there has been an increasing interest in the use of melanin for broad applications. In the vast majority of studies, melanin has been either chemically synthesized or isolated from animals, which has restricted its use to small-scale applications. Using bacteria as biocatalysts is a promising and economical alternative for the large-scale production of biomaterials. In this study, we engineered the marine bacterium Vibrio natriegens, one of the fastest-growing organisms, to synthesize melanin by expressing a heterologous tyrosinase gene and demonstrated that melanin production was much faster than in previously reported heterologous systems. The melanin of V. natriegens was characterized as a polymer derived from dihydroxyindole-2-carboxylic acid (DHICA) and, similarly to synthetic melanin, exhibited several characteristic and useful features. Electron microscopy analysis demonstrated that melanin produced from V. natriegens formed nanoparticles that were assembled as "melanin ghost" structures, and the photoprotective properties of these particles were validated by their protection of cells from UV irradiation. Using a novel electrochemical reverse engineering method, we observed that melanization conferred redox activity to V. natriegens Moreover, melanized bacteria were able to quickly adsorb the organic compound trinitrotoluene (TNT). Overall, the genetic tractability, rapid division time, and ease of culture provide a set of attractive properties that compare favorably to current E. coli production strains and warrant the further development of this chassis as a microbial factory for natural product biosynthesis.IMPORTANCE Melanins are macromolecules that are ubiquitous in nature and impart a large variety of biological functions, including structure, coloration, radiation resistance, free radical scavenging, and thermoregulation. Currently, in the majority of investigations, melanins are either chemically synthesized or extracted from animals, which presents significant challenges for large-scale production. Bacteria have been used as biocatalysts to synthesize a variety of biomaterials due to their fast growth and amenability to genetic engineering using synthetic biology tools. In this study, we engineered the extremely fast-growing bacterium V. natriegens to synthesize melanin nanoparticles by expressing a heterologous tyrosinase gene with inducible promoters. Characterization of the melanin produced from V. natriegens-produced tyrosinase revealed that it exhibited physical and chemical properties similar to those of natural and chemically synthesized melanins, including nanoparticle structure, protection against UV damage, and adsorption of toxic compounds. We anticipate that producing and controlling melanin structures at the nanoscale in this bacterial system with synthetic biology tools will enable the design and rapid production of novel biomaterials for multiple applications.

Diversity in Natural Transformation Frequencies and Regulation across Vibrio Species

Naturally transformable, or competent, bacteria are able to take up DNA from their environment, a key method of horizontal gene transfer for acquisition of new DNA sequences. Our research shows that Vibrio species that inhabit marine environments exhibit a wide diversity in natural transformation capability ranging from nontransformability to high transformation rates in which 10% of cells measurably incorporate new DNA. We show that the role of regulatory systems controlling the expression of competence genes (e.g., quorum sensing) differs throughout both the species and strain levels. We explore natural transformation capabilities of Vibrio campbellii species which have been thus far uncharacterized and find novel regulation of competence. Expression of two key transcription factors, TfoX and QstR, is necessary to stimulate high levels of transformation in Vibrio campbellii and recover low rates of transformation in Vibrio vulnificus.

In Vibrio species, chitin-induced natural transformation enables bacteria to take up DNA from the external environment and integrate it into their genome. Expression of the master competence regulator TfoX bypasses the need for chitin induction and drives expression of the genes required for competence in several Vibrio species. Here, we show that TfoX expression in Vibrio campbellii strains DS40M4 and NBRC 15631 enables high natural transformation frequencies. Conversely, transformation was not achieved in the model quorum-sensing strain V. campbellii BB120 (previously classified as Vibrio harveyi). Surprisingly, we find that quorum sensing is not required for transformation in V. campbellii DS40M4 or Vibrio parahaemolyticus in contrast to the established regulatory pathway in Vibrio cholerae in which quorum sensing is required to activate the competence regulator QstR. Similar to V. cholerae, expression of both QstR and TfoX is necessary for transformation in DS40M4. There is a wide disparity in transformation frequencies among even closely related Vibrio strains, with V. vulnificus having the lowest functional transformation frequency. Ectopic expression of both TfoX and QstR is sufficient to produce a significant increase in transformation frequency in Vibrio vulnificus. To explore differences in competence regulation, we used previously studied V. cholerae competence genes to inform a comparative genomics analysis coupled with transcriptomics. We find that transformation capability cannot necessarily be predicted by the level of gene conservation but rather correlates with competence gene expression following TfoX induction. Thus, we have uncovered notable species- and strain-level variations in the competence gene regulation pathway across the Vibrio genus.

Exploiting the Feedstock Flexibility of the Emergent Synthetic Biology Chassis Vibrio natriegens for Engineered Natural Product Production

A recent goal of synthetic biology has been to identify new chassis that provide benefits lacking in model organisms. Vibrio natriegens is a marine Gram-negative bacterium which is an emergent synthetic biology chassis with inherent benefits: An extremely fast growth rate, genetic tractability, and the ability to grow on a variety of carbon sources ("feedstock flexibility"). Given these inherent benefits, we sought to determine its potential to heterologously produce natural products, and chose beta-carotene and violacein as test cases. For beta-carotene production, we expressed the beta-carotene biosynthetic pathway from the sister marine bacterium Vibrio campbellii, as well as the mevalonate biosynthetic pathway from the Gram-positive bacterium Lactobacillus acidophilus to improve precursor abundance. Violacein was produced by expressing a biosynthetic gene cluster derived from Chromobacterium violaceum. Not only was V. natriegens able to heterologously produce these compounds in rich media, illustrating its promise as a new chassis for small molecule drug production, but it also did so in minimal media using a variety of feedstocks. The ability for V. natriegens to produce natural products with multiple industrially-relevant feedstocks argues for continued investigations into the production of more complex natural products in this chassis.

Synthetic Biology Tools for the Fast-Growing Marine Bacterium Vibrio natriegens

The fast-growing nonmodel marine bacterium Vibrio natriegens has recently garnered attention as a host for molecular biology and biotechnology applications. In order to further its capabilities as a synthetic biology chassis, we have characterized a wide range of genetic parts and tools for use in V. natriegens. These parts include many commonly used resistance markers, promoters, ribosomal binding sites, reporters, terminators, degradation tags, origin of replication sequences, and plasmid backbones. We have characterized the behavior of these parts in different combinations and have compared their functionality in V. natriegens and Escherichia coli. Plasmid stability over time, plasmid copy numbers, and production load on the cells were also evaluated. Additionally, we tested constructs for chemical and optogenetic induction and characterized basic engineered circuit behavior in V. natriegens. The results indicate that, while most parts and constructs work similarly in the two organisms, some deviate significantly. Overall, these results will serve as a primer for anyone interested in engineering V. natriegens and will aid in developing more robust synthetic biology principles and approaches for this nonmodel chassis.

Multiplex genome editing by natural transformation in Vibrio mimicus with potential application in attenuated vaccine development

Vibrio mimicus (V. mimicus) is a significant pathogen in freshwater catfish, though knowledge of virulence determinants and effective vaccine is lacking. Multiplex genome editing by natural transformation (MuGENT) is an easy knockout method, which has successfully used in various bacteria except for V. mimicus. Here, we found V. mimicus strain SCCF01 can uptake exogenous DNA and insert it into genome by natural transformation assay. Subsequently, we exploited this property to make five mutants (△Hem, △TS1, △TS2, △TS1△TS2, and △II), and removed the antibiotic resistance marker by Flp-recombination. Finally, all of the mutants were identified by PCR and RT-PCR. The results showed that combination of natural transformation and FLP-recombination can be applied successfully to generate targeted gene disruptions without the antibiotic resistance marker in V. mimicus. In addition, the five mutants showed mutant could be inherited after several subcultures and a 668-fold decrease in the virulence to yellow catfish (Pelteobagrus fulvidraco). This study provides a convenient method for the genetic manipulation of V. mimicus. It will facilitate the identification and characterization of V. mimicus virulence factors and eventually contribute to a better understanding of V. mimicus pathogenicity and development of attenuated vaccine.

Recent advances in plasmid-based tools for establishing novel microbial chassis

A key challenge for domesticating alternative cultivable microorganisms with biotechnological potential lies in the development of innovative technologies. Within this framework, a myriad of genetic tools has flourished, allowing the design and manipulation of complex synthetic circuits and genomes to become the general rule in many laboratories rather than the exception. More recently, with the development of novel technologies such as DNA automated synthesis/sequencing and powerful computational tools, molecular biology has entered the synthetic biology era. In the beginning, most of these technologies were established in traditional microbial models (known as chassis in the synthetic biology framework) such as Escherichia coli and Saccharomyces cerevisiae, enabling fast advances in the field and the validation of fundamental proofs of concept. However, it soon became clear that these organisms, although extremely useful for prototyping many genetic tools, were not ideal for a wide range of biotechnological tasks due to intrinsic limitations in their molecular/physiological properties. Over the last decade, researchers have been facing the great challenge of shifting from these model systems to non-conventional chassis with endogenous capacities for dealing with specific tasks. The key to address these issues includes the generation of narrow and broad host plasmid-based molecular tools and the development of novel methods for engineering genomes through homologous recombination systems, CRISPR/Cas9 and other alternative methods. Here, we address the most recent advances in plasmid-based tools for the construction of novel cell factories, including a guide for helping with "build-your-own" microbial host.

Generation of a Prophage-Free Variant of the Fast-Growing Bacterium Vibrio natriegens

The fast-growing marine bacterium Vibrio natriegens represents an emerging strain for molecular biology and biotechnology. Genome sequencing and quantitative PCR analysis revealed that the first chromosome of V. natriegens ATCC 14048 contains two prophage regions (VNP1 and VNP2) that are both inducible by the DNA-damaging agent mitomycin C and exhibit spontaneous activation under standard cultivation conditions. Their activation was also confirmed by live cell imaging of an mCherry fusion to the major capsid proteins of VNP1 and VNP2. Transmission electron microscopy visualized the release of phage particles belonging to the Siphoviridae family into the culture supernatant. Freeing V. natriegens from its proviral load, followed by phenotypic characterization, revealed an improved robustness of the prophage-free variant toward DNA-damaging conditions, reduced cell lysis under hypo-osmotic conditions, and an increased pyruvate production compared to wild-type levels. Remarkably, the prophage-free strain outcompeted the wild type in a competitive growth experiment, emphasizing that this strain is a promising platform for future metabolic engineering approaches.IMPORTANCE The fast-growing marine bacterium Vibrio natriegens represents an emerging model host for molecular biology and biotechnology, featuring a reported doubling time of less than 10 minutes. In many bacterial species, viral DNA (prophage elements) may constitute a considerable fraction of the whole genome and may have detrimental effects on the growth and fitness of industrial strains. Genome analysis revealed the presence of two prophage regions in the V. natriegens genome that were shown to undergo spontaneous induction under standard cultivation conditions. In this study, we generated a prophage-free variant of V. natriegens Remarkably, the prophage-free strain exhibited a higher tolerance toward DNA damage and hypo-osmotic stress. Moreover, it was shown to outcompete the wild-type strain in a competitive growth experiment. In conclusion, our study presents the prophage-free variant of V. natriegens as a promising platform strain for future biotechnological applications.

Functional genomics of the rapidly replicating bacterium Vibrio natriegens by CRISPRi

The fast-growing Gram-negative bacterium Vibrio natriegens is an attractive microbial system for molecular biology and biotechnology due to its remarkably short generation time^1,2 and metabolic prowess^3,4. However, efforts to uncover and utilize the mechanisms underlying its rapid growth are hampered by the scarcity of functional genomic data. Here, we develop a pooled genome-wide clustered regularly interspaced short palindromic repeats (CRISPR) interference (CRISPRi) screen to identify a minimal set of genes required for rapid wild-type growth. Targeting 4,565 (99.7%) of predicted protein-coding genes, our screen uncovered core genes comprising putative essential and growth-supporting genes that are enriched for respiratory pathways. We found that 96% of core genes were located on the larger chromosome 1, with growth-neutral duplicates of core genes located primarily on chromosome 2. Our screen also refines metabolic pathway annotations by distinguishing functional biosynthetic enzymes from those predicted on the basis of comparative genomics. Taken together, this work provides a broadly applicable platform for high-throughput functional genomics to accelerate biological studies and engineering of V. natriegens.

Marine Proteobacteria as a source of natural products: advances in molecular tools and strategies

This review covers the recent advances in molecular tools and strategies for studies and use of natural products from marine Proteobacteria.

Chasing bacterial chassis for metabolic engineering: a perspective review from classical to non-traditional microorganisms

The last few years have witnessed an unprecedented increase in the number of novel bacterial species that hold potential to be used for metabolic engineering. Historically, however, only a handful of bacteria have attained the acceptance and widespread use that are needed to fulfil the needs of industrial bioproduction - and only for the synthesis of very few, structurally simple compounds. One of the reasons for this unfortunate circumstance has been the dearth of tools for targeted genome engineering of bacterial chassis, and, nowadays, synthetic biology is significantly helping to bridge such knowledge gap. Against this background, in this review, we discuss the state of the art in the rational design and construction of robust bacterial chassis for metabolic engineering, presenting key examples of bacterial species that have secured a place in industrial bioproduction. The emergence of novel bacterial chassis is also considered at the light of the unique properties of their physiology and metabolism, and the practical applications in which they are expected to outperform other microbial platforms. Emerging opportunities, essential strategies to enable successful development of industrial phenotypes, and major challenges in the field of bacterial chassis development are also discussed, outlining the solutions that contemporary synthetic biology-guided metabolic engineering offers to tackle these issues.

Functionality of Two Origins of Replication in Vibrio cholerae Strains With a Single Chromosome

Chromosomal inheritance in bacteria usually entails bidirectional replication of a single chromosome from a single origin into two copies and subsequent partitioning of one copy each into daughter cells upon cell division. However, the human pathogen Vibrio cholerae and other Vibrionaceae harbor two chromosomes, a large Chr1 and a small Chr2. Chr1 and Chr2 have different origins, an oriC-type origin and a P1 plasmid-type origin, respectively, driving the replication of respective chromosomes. Recently, we described naturally occurring exceptions to the two-chromosome rule of Vibrionaceae: i.e., Chr1 and Chr2 fused single chromosome V. cholerae strains, NSCV1 and NSCV2, in which both origins of replication are present. Using NSCV1 and NSCV2, here we tested whether two types of origins of replication can function simultaneously on the same chromosome or one or the other origin is silenced. We found that in NSCV1, both origins are active whereas in NSCV2 ori2 is silenced despite the fact that it is functional in an isolated context. The ori2 activity appears to be primarily determined by the copy number of the triggering site, crtS, which in turn is determined by its location with respect to ori1 and ori2 on the fused chromosome.

Establishing a Cell-Free Vibrio natriegens Expression System

The fast growing bacterium Vibrio natriegens is an emerging microbial host for biotechnology. Harnessing its productive cellular components may offer a compelling platform for rapid protein production and prototyping of metabolic pathways or genetic circuits. Here, we report the development of a V. natriegens cell-free expression system. We devised a simplified crude extract preparation protocol and achieved >260 μg/mL of superfolder GFP in a small-scale batch reaction after 3 h. Culturing conditions, including growth media and cell density, significantly affect translation kinetics and protein yield of extracts. We observed maximal protein yield at incubation temperatures of 26 or 30 °C, and show improved yield by tuning ions crucial for ribosomal stability. This work establishes an initial V. natriegens cell-free expression system, enables probing of V. natriegens biology, and will serve as a platform to accelerate metabolic engineering and synthetic biology applications.

Establishing a High-Yielding Cell-Free Protein Synthesis Platform Derived from Vibrio natriegens

A new wave of interest in cell-free protein synthesis (CFPS) systems has shown their utility for producing proteins at high titers, establishing genetic regulatory element libraries ( e.g., promoters, ribosome binding sites) in nonmodel organisms, optimizing biosynthetic pathways before implementation in cells, and sensing biomarkers for diagnostic applications. Unfortunately, most previous efforts have focused on a select few model systems, such as Escherichia coli. Broadening the spectrum of organisms used for CFPS promises to better mimic host cell processes in prototyping applications and open up new areas of research. Here, we describe the development and characterization of a facile CFPS platform based on lysates derived from the fast-growing bacterium Vibrio natriegens, which is an emerging host organism for biotechnology. We demonstrate robust preparation of highly active extracts using sonication, without specialized and costly equipment. After optimizing the extract preparation procedure and cell-free reaction conditions, we show synthesis of 1.6 ± 0.05 g/L of superfolder green fluorescent protein in batch mode CFPS, making it competitive with existing E. coli CFPS platforms. To showcase the flexibility of the system, we demonstrate that it can be lyophilized and retain biosynthesis capability, that it is capable of producing antimicrobial peptides, and that our extract preparation procedure can be coupled with the recently described Vmax Express strain in a one-pot system. Finally, to further increase system productivity, we explore a knockout library in which putative negative effectors of CFPS are genetically removed from the source strain. Our V. natriegens-derived CFPS platform is versatile and simple to prepare and use. We expect it will facilitate expansion of CFPS systems into new laboratories and fields for compelling applications in synthetic biology.

Natural Transformation in Vibrio parahaemolyticus: a Rapid Method To Create Genetic Deletions

The Gram-negative bacterium Vibrio parahaemolyticus is an opportunistic human pathogen and the leading cause of seafood-borne acute gastroenteritis worldwide. Recently, this bacterium was implicated as the etiologic agent of a severe shrimp disease with consequent devastating outcomes to shrimp farming. In both cases, acquisition of genetic material via horizontal transfer provided V. parahaemolyticus with new virulence tools to cause disease. Dissecting the molecular mechanisms of V. parahaemolyticus pathogenesis often requires manipulating its genome. Classically, genetic deletions in V. parahaemolyticus are performed using a laborious, lengthy, multistep process. Here, we describe a fast and efficient method to edit this bacterium's genome based on V. parahaemolyticus natural competence. Although this method is similar to one previously described, V. parahaemolyticus requires counterselection for curing of acquired plasmids due to its recalcitrant nature of retaining extrachromosomal DNA. We believe this approach will be of use to the Vibrio community.IMPORTANCE Spreading of vibrios throughout the world correlates with increased global temperatures. As they spread, they find new niches in which to survive, proliferate, and invade. Therefore, genetic manipulation of vibrios is of the utmost importance for studying these species. Here, we have delineated and validated a rapid method to create genetic deletions in Vibrio parahaemolyticus This study provides insightful methodology for studies with other Vibrio species.

Genetic engineering of host organisms for pharmaceutical synthesis

Pharmaceutical production hosts may be derived from almost any organism, from Chinese Hamster Ovary (CHO) cell lines to isolated actinomycetes. Each host can be improved, historically only through adaptive evolution. Recently, the maturation of organism engineering has expanded the available models, methods, and tools for altering host phenotypes. New tools like CRISPR-associated endonucleases promise to enable precise cellular reprogramming and to access previously intractable hosts. In this review, we discuss the most recent advances in engineering several types of pharmaceutical production hosts. These include model organisms, potential platform hosts with advantageous metabolism or physiology, specialized producers capable of unique biosynthesis, and CHO, the most widely used recombinant protein production host. To realize improved engineered hosts, an increasing number of approaches involving DNA sequencing and synthesis, host rewriting technologies, computational methods, and organism engineering strategies must be used. Integrative workflows that enable application of the right combination of methods to the right production host could enable economical production solutions for emerging human health treatments.

High Substrate Uptake Rates Empower Vibrio natriegens as Production Host for Industrial Biotechnology

The productivity of industrial fermentation processes is essentially limited by the biomass-specific substrate consumption rate (qS ) of the applied microbial production system. Since qS depends on the growth rate (μ), we highlight the potential of the fastest-growing nonpathogenic bacterium, Vibrio natriegens, as a novel candidate for future biotechnological processes. V. natriegens grows rapidly in BHIN complex medium with a μ of up to 4.43 h-1 (doubling time of 9.4 min) as well as in minimal medium supplemented with various industrially relevant substrates. Bioreactor cultivations in minimal medium with glucose showed that V. natriegens possesses an exceptionally high qS under aerobic (3.90 ± 0.08 g g-1 h-1) and anaerobic (7.81 ± 0.71 g g-1 h-1) conditions. Fermentations with resting cells of genetically engineered V. natriegens under anaerobic conditions yielded an overall volumetric productivity of 0.56 ± 0.10 g alanine liter-1 min-1 (i.e., 34 g liter-1 h-1). These inherent properties render V. natriegens a promising new microbial platform for future industrial fermentation processes operating with high productivity.IMPORTANCE Low conversion rates are one major challenge to realizing microbial fermentation processes for the production of commodities operating competitively with existing petrochemical approaches. For this reason, we screened for a novel platform organism possessing characteristics superior to those of traditionally employed microbial systems. We identified the fast-growing V. natriegens, which exhibits a versatile metabolism and shows striking growth and conversion rates, as a solid candidate to reach outstanding productivities. Due to these inherent characteristics, V. natriegens can speed up common laboratory routines, is suitable for already existing production procedures, and forms an excellent foundation for engineering next-generation bioprocesses.