A novel arabinose-inducible genetic operation system developed for Clostridium cellulolyticum

Development of inducible promoters for regulating gene expression in Clostridium tyrobutyricum for biobutanol production

Clostridium tyrobutyricum is an anaerobe known for its ability to produce short-chain fatty acids, alcohols, and esters. We aimed to develop inducible promoters for fine-tuning gene expression in C. tyrobutyricum. Synthetic inducible promoters were created by employing an Escherichia coli lac operator to regulate the thiolase promoter (PCathl) from Clostridium acetobutylicum, with the best one (LacI-Pto4s) showing a 5.86-fold dynamic range with isopropyl β- d-thiogalactoside (IPTG) induction. A LT-Pt7 system with a dynamic range of 11.6-fold was then created by combining LacI-Pto4s with a T7 expression system composing of RNA polymerase (T7RNAP) and Pt7lac promoter. Furthermore, two inducible expression systems BgaR-PbgaLA and BgaR-PbgaLB with a dynamic range of ~40-fold were developed by optimizing a lactose-inducible expression system from Clostridium perfringens with modified 5' untranslated region (5' UTR) and ribosome-binding site (RBS). BgaR-PbgaLB was then used to regulate the expressions of a bifunctional aldehyde/alcohol dehydrogenase encoded by adhE2 and butyryl-CoA/acetate Co-A transferase encoded by cat1 in C. tyrobutyricum wild type and Δcat1::adhE2, respectively, demonstrating its efficient inducible gene regulation. The regulated cat1 expression also confirmed that the Cat1-catalyzed reaction was responsible for acetate assimilation in C. tyrobutyricum. The inducible promoters offer new tools for tuning gene expression in C. tyrobutyricum for industrial applications.

Digital PCR: a tool in clostridial mutant selection and detection

The ClosTron mutagenesis system has enabled researchers to efficiently edit the clostridial genome. Since site-specific insertion of the mobile ClosTron insert may cause errors, validation is key. In this paper we describe the use of digital PCR (dPCR) as an alternative tool in selecting clostridial mutant strains. Clostridium perfringens chitinase mutant strains were constructed in which the mobile ClosTron intron was inserted into one of the chitinase genes. On-target insertion of the mobile intron was validated through conventional PCR. In order to confirm the absence of off-target insertions, dPCR was used to determine the amount of the ClosTron intron as well as the amount of a reference gene, located in close proximity to the interrupted gene. Subsequently, mutant strains containing an equivalent amount of both genes were selected as these do not contain additional off-target mobile ClosTron inserts. The outcome of this selection procedure was confirmed through a validated PCR-based approach. In addition to its application in mutant selection, dPCR can be used in other aspects of clostridial research, such as the distinction and easy quantification of different types of strains (wildtype vs. mutant) in complex matrices, such as faecal samples, a process in which other techniques are hampered by bacterial overgrowth (plating) or inhibition by matrix contaminants (qPCR). This research demonstrates that dPCR is indeed a high-throughput method in the selection of clostridial insertion mutants as well as a robust and accurate tool in distinguishing between wildtype and mutant C. perfringens strains, even in a complex matrix such as faeces. KEY POINTS: • Digital PCR as an alternative in ClosTron mutant selection • Digital PCR is an accurate tool in bacterial quantification in a complex matrix • Digital PCR is an alternative tool with great potential to microbiological research.

Development of an efficient ClosTron system for gene disruption in Ruminiclostridium papyrosolvens

Ruminiclostridium papyrosolvens is an anaerobic, mesophilic, and cellulolytic clostridia, promising consolidated bioprocessing (CBP) candidate for producing renewable green chemicals from cellulose, but its metabolic engineering is limited by lack of genetic tools. Here, we firstly employed the endogenous xylan-inducible promoter to control ClosTron system for gene disruption of R. papyrosolvens. The modified ClosTron can be easily transformed into R. papyrosolvens and specifically disrupt targeting genes. Furthermore, a counter selectable system based on uracil phosphoribosyl-transferase (Upp) was successfully established and introduced into the ClosTron system, which resulted in plasmid curing rapidly. Thus, the combination of xylan-inducible ClosTron and upp-based counter selectable system makes the gene disruption more efficient and convenient for successive gene disruption in R. papyrosolvens. KEY POINTS: • Limiting expression of LtrA enhanced the transformation of ClosTron plasmids in R. papyrosolvens. • DNA targeting specificity can be improved by precise management of the expression of LtrA. • Curing of ClosTron plasmids was achieved by introducing the upp-based counter selectable system.

Intestinal Engineered Probiotics as Living Therapeutics: Chassis Selection, Colonization Enhancement, Gene Circuit Design, and Biocontainment

Intestinal probiotics are often used for the in situ treatment of diseases, such as metabolic disorders, tumors, and chronic inflammatory infections. Recently, there has been an increased emphasis on intelligent, customized treatments with a focus on long-term efficacy; however, traditional probiotic therapy has not kept up with this trend. The use of synthetic biology to construct gut-engineered probiotics as live therapeutics is a promising avenue in the treatment of specific diseases, such as phenylketonuria and inflammatory bowel disease. These studies generally involve a series of fundamental design issues: choosing an engineered chassis, improving the colonization ability of engineered probiotics, designing functional gene circuits, and ensuring the safety of engineered probiotics. In this review, we summarize the relevant past research, the progress of current research, and discuss the key issues that restrict the widespread application of intestinal engineered probiotic living therapeutics.

Development of an in vivo methylation system for transformation of Ruminiclostridium cellulolyticum

AIMS

Ruminiclostridium cellulolyticum, an anaerobic cellulolytic bacterium producing an efficient cellulolytic extracellular complex named cellulosome, is a promising host for biofuel production from lignocellulose. This study aims to develop a rapid transformation method for R. cellulolyticum avoiding its restriction system.

METHODS AND RESULTS

The CceI restriction system is a major barrier to introduction of foreign DNA into R. cellulolyticum cells. To improve the transformation efficiency of R. cellulolyticum, the gene encoding CceI methyltransferase (M.CceI) of R. cellulolyticum H10 was functionally expressed in Escherichia coli, resulting in an in vivo methylation system for transformation of R. cellulolyticum. The electrotransformation experiments of R. cellulolyticum H10 with the E. coli-Clostridium shuttle plasmid pMTC6 showed that the transformation efficiency reached up to 2.6 × 10³ ±0.23 × 10³  CFU per μg plasmid DNA. The results demonstrated that the system is able to confer the M.CceI-specific DNA methylation pattern to its resident plasmid, which makes the plasmid resistant to the CceI restriction and efficiently transferred into R. cellulolyticum.

CONCLUSIONS

In this study, we generated an in vivo methylation system of R. cellulolyticum, allowing interspecies DNA transfer and improving transformation efficiency.

SIGNIFICANCE AND IMPACT OF THE STUDY

This research result will greatly facilitate the metabolic engineering of R. cellulolyticum for biofuel production directly from cellulose.

Exploiting heterologous and endogenous CRISPR-Cas systems for genome editing in the probiotic Clostridium butyricum

Clostridium butyricum has been widely used as a probiotic for humans and food animals. However, the mechanisms of beneficial effects of C. butyricum on the host remain poorly understood, largely due to the lack of high-throughput genome engineering tools. Here, we report the exploitation of heterologous Type II CRISPR-Cas9 system and endogenous Type I-B CRISPR-Cas system in probiotic C. butyricum for seamless genome engineering. Although successful genome editing was achieved in C. butyricum when CRISPR-Cas9 system was employed, the expression of toxic cas9 gene result in really poor transformation, spurring us to develop an easy-applicable and high-efficient genome editing tool. Therefore, the endogenous Type I-B CRISPR-Cas machinery located on the megaplasmid of C. butyricum was co-opted for genome editing. In vivo plasmid interference assays identified that ACA and TAA were functional protospacer adjacent motif sequences needed for site-specific CRISPR attacking. Using the customized endogenous CRISPR-Cas system, we successfully deleted spo0A and aldh genes in C. butyricum, yielding an efficiency of up to 100%. Moreover, the conjugation efficiency of endogenous CRISPR-Cas system was dramatically enhanced due to the precluding expression of cas9. Altogether, the two approaches developed herein remarkably expand the existing genetic toolbox available for investigation of C. butyricum.

Genetic sensor-regulators functional in Clostridia

This study addressed the functionality of genetic circuits carrying natural regulatory elements of Clostridium acetobutylicum ATCC 824 in the presence of the respective inducer molecules. Specifically, promoters and their regulators involved in diverse carbon source utilization were characterized using mCherryOpt or beta-galactosidase as a reporter. Consequently, most of the genetic circuits tested in this study were functional in Clostridium acetobutylicum ATCC 824 in the presence of an inducer, leading to the expression of reporter proteins. These genetic sensor-regulators were found to be transferable to another Clostridium species, such as Clostridium beijerinckii NCIMB 8052. The gradual expression of reporter protein was observed as a function of the carbohydrates of interest. A xylose-inducible promoter allows a titratable and robust expression of a reporter protein with stringency and efficacy. This xylose-inducible circuit was seen to enable induction of the expression of reporter proteins in the presence of actual sugar mixtures incorporated in woody hydrolysate wherein glucose and xylose are present as predominant carbon sources.

Screening and application of inducible promoters in Ruminiclostridium papyrosolvens

Ruminiclostridium papyrosolvens is a promising candidate for producing renewable green chemicals from cellulose due to its cellulolytic and ethanologenic capabilities. It is of significance to screen effective, and convenient-to-use inducible promoters that can be used for regulating the gene expression in R. papyrosolvens. We characterized two endogenous inducible promoters and investigated another two exogenous ones on the adaptability in R. papyrosolvens. Both of the endogenous xylan-inducible promoter Pxyl and exogenous lactose-inducible promoter Plac are found of high specificity and stringency. Pxyl has a short time to be induced while Plac has a low concentration of inducer. With these findings, a mazF-based counter selectable system has been constructed for promoting the efficiency of mutant screening via plasmid curing. The inducible gene expression systems provided novel tools for enhancing the capability of genetic manipulation in engineering R. papyrosolvens. SIGNIFICANCE AND IMPACT OF THE STUDY: Four inducible promoters from Clostridia were characterized in R. papyrosolvens. Xylan-inducible promoter Pxyl was found of a short time while lactose-inducible promoter Plac needs a low concentration of inducer to induce. Employing them, we successfully construct a mazF-based counter selectable system, which would be used to increase the mutant screening efficiency via induction of plasmid curing. The inducible gene expression systems provided novel tools for enhancing the capability of genetic manipulation in engineering R. papyrosolvens.

Synthetic Biology Tools for Genome and Transcriptome Engineering of Solventogenic Clostridium

Strains of Clostridium genus are used for production of various value-added products including fuels and chemicals. Development of any commercially viable production process requires a combination of both strain and fermentation process development strategies. The strain development in Clostridium sp. could be achieved by random mutagenesis, and targeted gene alteration methods. However, strain improvement in Clostridium sp. by targeted gene alteration method was challenging due to the lack of efficient tools for genome and transcriptome engineering in this organism. Recently, various synthetic biology tools have been developed to facilitate the strain engineering of solventogenic Clostridium. In this review, we consolidated the recent advancements in toolbox development for genome and transcriptome engineering in solventogenic Clostridium. Here we reviewed the genome-engineering tools employing mobile group II intron, pyrE alleles exchange, and CRISPR/Cas9 with their application for strain development of Clostridium sp. Next, transcriptome engineering tools such as untranslated region (UTR) engineering and synthetic sRNA techniques were also discussed in context of Clostridium strain engineering. Application of any of these discussed techniques will facilitate the metabolic engineering of clostridia for development of improved strains with respect to requisite functional attributes. This might lead to the development of an economically viable butanol production process with improved titer, yield and productivity.

Impact of ammonium sulfite-based sequential pretreatment combinations on two distinct saccharifications of wheat straw

The properties of lignocellulosic substrates obtained from different pretreatments have a big impact on downstream saccharification based on both the fungal cellulase system and the cellulosome-based whole-cell biocatalysis system. However the corresponding effect of these two distinct saccharification strategies has not been comparatively analyzed. In this work, three ammonium sulfite (AS)-based pretreatment combinations (i.e., AS + hydrothermal (HT) pretreatment, AS + xylanase (X) pretreatment, and HT + AS pretreatment) were conducted to treat wheat straw. The obtained pretreated substrates with different properties were saccharified using fungal cellulase or an engineered Clostridium thermocellum strain as the whole-cell biocatalyst, and the ability to release sugar was comparatively evaluated. It was found that for the whole-cell saccharification, the total sugar digestibility of AS + HT/X pretreated wheat straw was 10% higher than that of HT + AS pretreated wheat straw. However, for fungal cellulase-based saccharification, the enzymatic hydrolysis efficiency was less susceptible to the sequence of pretreatment combinations. Hence, the whole-cell biocatalysis system was more sensitive to substrate accessibility compared to the free enzymes. In addition, the characterization and analyses showed that AS + HT/X pretreatment could remove more lignin, generating a more accessible surface with a larger external surface and lower surface lignin coverage, compared to the HT + AS pretreatment. Therefore, the AS + HT/X pretreatment was more compatible with the cellulosome-based whole-cell saccharification.

The impact of substrate properties on wheat straw sugar release from fungal cellulase and whole cell-based CBS was comparatively investigated.

TargeTron Technology Applicable in Solventogenic Clostridia: Revisiting 12 Years' Advances

Clostridium has great potential in industrial application and medical research. But low DNA repair capacity and plasmids transformation efficiency severely delay development and application of genetic tools based on homologous recombination (HR). TargeTron is a gene editing technique dependent on the mobility of group II introns, rather than homologous recombination, which makes it very suitable for gene disruption of Clostridium. The application of TargeTron technology in solventogenic Clostridium is academically reported in 2007 and this tool has been introduced in various clostridia as it is easy to operate, time saving, and reliable. TargeTron has made great progress in solventogenic Clostridium in the aspects of acetone-butanol-ethanol (ABE) fermentation pathway modification, important functional genes identification, and xylose metabolic pathway analysis and reconstruction. In the review, 12 years' advances of TargeTron technology applicable in solventogenic Clostridium, including its principle, technical characteristics, application, and efforts to expand its capabilities, or to avoid potential drawbacks, are revisisted. Some other technologies as putative competitors or collaborators are also discussed. It is believed that TargeTron combined with CRISPR/Cas-assisted gene/base editing and gene-expression regulation system will make a better future for clostridial genetic modification.

Construction of consolidated bio-saccharification biocatalyst and process optimization for highly efficient lignocellulose solubilization

BACKGROUND

The industrial conversion of biomass to high-value biofuels and biochemical is mainly restricted by lignocellulose solubilization. Consolidated bio-saccharification (CBS) is considered a promising process for lignocellulose solubilization depending on whole-cell biocatalysts that simultaneously perform effective cellulase production and hydrolysis. However, it usually takes a long time to reach a high saccharification level using the current CBS biocatalyst and process.

RESULTS

To promote the saccharification efficiency and reduce the cost, a Clostridium thermocellum recombinant strain ∆pyrF::KBm was constructed as a new CBS biocatalyst in this study. The key CBS factors, including the medium, inoculum size and cultivation, and substrate load, were investigated and optimized. The saccharification process was also stimulated by adding free hemicellulases, suggesting the need to further enhance hemicellulase activity of the whole-cell catalyst. Under the optimal conditions, the CBS process was shortened by 50% with pretreated wheat straw as the substrate. The sugar yield reached 0.795 g/g and the saccharification level was 89.3%.

CONCLUSIONS

This work provided a new biocatalyst and an optimized process of CBS and confirmed that CBS is a feasible strategy for cost-efficient solubilization of lignocellulose, which will greatly promote the industrial utilization of lignocellulosic biomass.

Markerless genome editing in Clostridium beijerinckii using the CRISPR-Cpf1 system

CRISPR-Cpf1 is a type V CRISPR system that has recently been exploited for genome engineering purposes. Compared to the well-known Streptococcus pyogenes CRISPR-Cas9 system, the effector protein Cpf1 recognizes T-rich protospacer-adjacent motif (PAM) instead of G-rich PAM (used by CRISPR-Cas9), which could offer a substantial expansion of the existing genetic toolbox for genome editing. In this study, we report the implementation of the Acidaminococcus sp. Cpf1 (AsCpf1) for markerless genome engineering in Clostridium beijerinckii, a prominent species for biosolvent production through the well-known Acetone-Butanol-Ethanol (ABE) pathway. A lactose inducible promoter was used to control the expression of AsCpf1 to decrease its toxicity, while a constitutive small RNA promoter was employed to drive the expression of pre-crRNA. A One-Step-Assembly (OSA) approach was employed to construct the CRISPR-Cpf1-based vector in one single step, which simplified and streamlined the plasmid construction process. Using the customized CRISPR-Cpf1 system, we successfully deleted spo0A and pta genes in C. beijerinckii, with an editing efficiency of up to 100%. Altogether, our results demonstrated the easy programmability and high efficiency of the CRISPR-Cpf1 system for versatile genome engineering purposes. This study provides valuable guidance and essential references for repurposing the CRISPR-Cpf1 system for genome engineering in other microorganisms.

Recent Developments of the Synthetic Biology Toolkit for Clostridium

The Clostridium genus is a large, diverse group consisting of Gram-positive, spore-forming, obligate anaerobic firmicutes. Among this group are historically notorious pathogens as well as several industrially relevant species with the ability to produce chemical commodities, particularly biofuels, from renewable biomass. Additionally, other species are studied for their potential use as therapeutics. Although metabolic engineering and synthetic biology have been instrumental in improving product tolerance, titer, yields, and feed stock consumption capabilities in several organisms, low transformation efficiencies and lack of synthetic biology tools and genetic parts make metabolic engineering within the Clostridium genus difficult. Progress has recently been made to overcome challenges associated with engineering various Clostridium spp. For example, developments in CRISPR tools in multiple species and strains allow greater capability to produce edits with greater precision, faster, and with higher efficiencies. In this mini-review, we will highlight these recent advances and compare them to established methods for genetic engineering in Clostridium. In addition, we discuss the current state and development of Clostridium-based promoters (constitutive and inducible) and reporters. Future progress in this area will enable more rapid development of strain engineering, which would allow for the industrial exploitation of Clostridium for several applications including bioproduction of several commodity products.

Determination of the native features of the exoglucanase Cel48S from Clostridium thermocellum

BACKGROUND

Clostridium thermocellum is considered one of the most efficient natural cellulose degraders because of its cellulosomal system. As the major exoglucanase of cellulosome in C. thermocellum, Cel48S plays key roles and influences the activity and features of cellulosome to a great extent. Thus, it is of great importance to reveal the enzymatic features of Cel48S. However, Cel48S has not been well performed due to difficulties in purifying either recombinant or native Cel48S proteins.

RESULTS

We observed that the soluble fraction of the catalytic domain of Cel48S (Cel48S_CD) obtained by heterologous expression in Escherichia coli and denaturation-refolding treatment contained a large portion of incorrectly folded proteins with low activity. Using a previously developed seamless genome-editing system for C. thermocellum, we achieved direct purification of Cel48S_CD from the culture supernatant of C. thermocellum DSM1313 by inserting a sequence encoding 12 successive histidine residues and a TAA stop codon immediately behind the GH domain of Cel48S. Based on the fully active protein, biochemical and structural analyses were performed to reveal its innate characteristics. The native Cel48S_CD showed high activity of 117.61 ± 2.98 U/mg and apparent substrate preference for crystalline cellulose under the assay conditions. The crystal structure of the native GH48 protein revealed substrate-coupled changes in the residue conformation, indicating induced-fit effects between Cel48S_CD and substrates. Mass spectrum and crystal structural analyses suggested no significant posttranslational modification in the native Cel48S_CD protein.

CONCLUSION

Our results confirmed that the high activity and substrate specificity of Cel48S_CD from C. thermocellum were consistent with its importance in the cellulosome. The structure of the native Cel48S_CD protein revealed evidence of conformational changes during substrate binding. In addition, our study provided a reliable method for in situ purification of cellulosomal and other secretive proteins from C. thermocellum.

Recent Developments of the Synthetic Biology Toolkit for Clostridium

The Clostridium genus is a large, diverse group consisting of Gram-positive, spore-forming, obligate anaerobic firmicutes. Among this group are historically notorious pathogens as well as several industrially relevant species with the ability to produce chemical commodities, particularly biofuels, from renewable biomass. Additionally, other species are studied for their potential use as therapeutics. Although metabolic engineering and synthetic biology have been instrumental in improving product tolerance, titer, yields, and feed stock consumption capabilities in several organisms, low transformation efficiencies and lack of synthetic biology tools and genetic parts make metabolic engineering within the Clostridium genus difficult. Progress has recently been made to overcome challenges associated with engineering various Clostridium spp. For example, developments in CRISPR tools in multiple species and strains allow greater capability to produce edits with greater precision, faster, and with higher efficiencies. In this mini-review, we will highlight these recent advances and compare them to established methods for genetic engineering in Clostridium. In addition, we discuss the current state and development of Clostridium-based promoters (constitutive and inducible) and reporters. Future progress in this area will enable more rapid development of strain engineering, which would allow for the industrial exploitation of Clostridium for several applications including bioproduction of several commodity products.

Efficient whole-cell-catalyzing cellulose saccharification using engineered Clostridium thermocellum

BACKGROUND

Cost-efficient saccharification is one of the main bottlenecks for industrial lignocellulose conversion. Clostridium thermocellum naturally degrades lignocellulose efficiently using the cellulosome, a multiprotein supermolecular complex, and thus can be potentially used as a low-cost catalyst for lignocellulose saccharification. The industrial use of C. thermocellum is restrained due largely to the inhibition of the hydrolysate cellobiose to its cellulosome. Although the supplementation of beta-glucosidase may solve the problem, the production of the enzymes greatly complicates the process and may also increase the cost of saccharification.

RESULTS

To conquer the feedback inhibition and establish an efficient whole-cell catalyst for highly efficient cellulose saccharification, we constructed a recombinant strain of C. thermocellum ∆pyrF::CaBglA which produced a secretory exoglucanase CelS-bearing heterologous BGL using a newly developed seamless genome editing system. Without the extra addition of enzymes, the relative saccharification level of ∆pyrF::CaBglA was stimulated by over twofolds compared to its parent strain ∆pyrF through a two-stage saccharification process with 100 g/L Avicel as the carbon source. The production of reducing sugars and the relative saccharification level were further enhanced to 490 mM and 79.4%, respectively, with increased cell density.

CONCLUSIONS

The high cellulose-degrading ability and sugar productivity suggested that the whole-cell-catalysis strategy for cellulose saccharification is promising, and the C. thermocellum strain ∆pyrF::CaBglA could be potentially used as an efficient whole-cell catalyst for industrial cellulose saccharification.

Dual control system - A novel scaffolding architecture of an inducible regulatory device for the precise regulation of gene expression

Here, we present a novel scaffolding architecture of an inducible regulatory device. This dual control system is completely silent in the off stage and is coupled to the regulation of gene expression at both the transcriptional and translational levels. This system also functions as an AND gate. We demonstrated the effectiveness of the cumate-riboswitch dual control system for the control of pamamycin production in Streptomyces albus. Placing the cre recombinase gene under the control of this system permitted the construction of synthetic devices with non-volatile memory that sense the signal and respond by altering DNA at the chromosomal level, thereby producing changes that are heritable. In addition, we present a library of synthetic inducible promoters based on the previously described cumate switch. With only one inducer and different promoters, we demonstrate that simultaneous modulation of the expression of several genes to different levels in various operons is possible. Because all modules of the AND gates are functional in bacteria other than Streptomyces, we anticipate that these regulatory devices can be used to control gene expression in other Actinobacteria. The features described in this study make these systems promising tools for metabolic engineering and biotechnology in Actinobacteria.

Flavin mononucleotide (FMN)-based fluorescent protein (FbFP) as reporter for promoter screening in Clostridium cellulolyticum

Conventional methods for screening promoters in anaerobic bacteria are generally based on detection of enzymatic reactions and thus usually complicated or strain specific. Therefore a more efficient and universal method will be valuable. Here, using cellulolytic bacteria Clostridium cellulolyticum H10 as a model, we employed an oxygen-independent flavin-based fluorescent protein (FbFP) derived from Pseudomonas putida as a quantitative reporter for the screening of promoter via monitoring fluorescence intensity. The stability and reliability of FbFP fluorescence were proven by the high correlation (R(2)=0.87) between fluorescence intensity and abundance of FbFP. Moreover, two endogenous promoters with exceptional performance were identified and characterized, including a constitutive promoter p3398 and an inducible promoter p1133. Compared to the existing reporter systems widely used in clostridia, this FbFP-based method is more rapid, intuitive and versatile, and the endogenous promoters reported here should enrich the synthetic biology toolbox for this and related organisms.

Low-Carbon Fuel and Chemical Production by Anaerobic Gas Fermentation

World energy demand is expected to increase by up to 40% by 2035. Over this period, the global population is also expected to increase by a billion people. A challenge facing the global community is not only to increase the supply of fuel, but also to minimize fossil carbon emissions to safeguard the environment, at the same time as ensuring that food production and supply is not detrimentally impacted. Gas fermentation is a rapidly maturing technology which allows low carbon fuel and commodity chemical synthesis. Unlike traditional biofuel technologies, gas fermentation avoids the use of sugars, relying instead on gas streams rich in carbon monoxide and/or hydrogen and carbon dioxide as sources of carbon and energy for product synthesis by specialized bacteria collectively known as acetogens. Thus, gas fermentation enables access to a diverse array of novel, large volume, and globally available feedstocks including industrial waste gases and syngas produced, for example, via the gasification of municipal waste and biomass. Through the efforts of academic labs and early stage ventures, process scale-up challenges have been surmounted through the development of specialized bioreactors. Furthermore, tools for the genetic improvement of the acetogenic bacteria have been reported, paving the way for the production of a spectrum of ever-more valuable products via this process. As a result of these developments, interest in gas fermentation among both researchers and legislators has grown significantly in the past 5 years to the point that this approach is now considered amongst the mainstream of emerging technology solutions for near-term low-carbon fuel and chemical synthesis.