Part by Part: Synthetic Biology Parts Used in Solventogenic Clostridia

Advances in synthetic biology toolboxes paving the way for mechanistic understanding and strain engineering of gut commensal Bacteroides spp. and Clostridium spp

The gut microbiota plays a significant role in influencing human immunity, metabolism, development, and behavior by producing a wide range of metabolites. While there is accumulating data on several microbiota-derived small molecules that contribute to host health and disease, our knowledge regarding the molecular mechanisms underlying metabolite-mediated microbe-host interactions remains limited. This is primarily due to the lack of efficient genetic tools for most commensal bacteria, especially those belonging to the dominant phyla Bacteroides spp. and Clostridium spp., which hinders the application of synthetic biology to these gut commensal bacteria. In this review, we provide an overview of recent advances in synthetic biology tools developed for the two dominant genera, as well as their applications in deciphering the mechanisms of microbe-host interactions mediated by microbiota-derived small molecules. We also discuss the potential biomedical applications of engineering commensal bacteria using these toolboxes. Finally, we share our perspective on the future development of synthetic biology tools for a better understanding of small molecule-mediated microbe-host interactions and their engineering for biomedical purposes.

Streamlined assembly of cloning and genome editing vectors for genus Clostridium

Reported herein is a new set of vectors designed to streamline molecular cloning and genome editing by exploiting modern cloning methods. The new vectors build on the existing pMTL8000 vectors that have been a staple of Clostridium research for more than a decade. The introduction of two pairs of type IIS restriction sites flanking an insulated multiple cloning site in both a cloning vector and a CRISPR-Cas9 gene editing vector enables plasmid construction in a "one-pot" reaction, avoiding the more laborious steps of conventional cloning. A synthetic lacZα expression cassette introduced between the cloning sites enables visual detection of background colonies. In addition, distinct selection markers on each vector permit selection of the desired clones according to antibiotic resistance. An example of strain development using the new vectors is demonstrated.

Heterologous Gene Regulation in Clostridia: Rationally Designed Gene Regulation for Industrial and Medical Applications

Several species from the Clostridium genus show promise as industrial solvent producers and cancer therapeutic delivery vehicles. Previous development of shuttle plasmids and genome editing tools has aided the study of these species and enabled their exploitation in industrial and medical applications. Nevertheless, the precise control of gene expression is still hindered by the limited range of characterized promoters. To address this, libraries of promoters (native and synthetic), 5' UTRs, and alternative start codons were constructed. These constructs were tested in Escherichia coli K-12, Clostridium sporogenes NCIMB 10696, and Clostridium butyricum DSM 10702, using β-glucuronidase (gusA) as a gene reporter. Promoter activity was corroborated using a second gene reporter, nitroreductase (nmeNTR) from Neisseria meningitides. A strong correlation was observed between the two reporters. In C. sporogenes and C. butyricum, respectively, changes in GusA activity between the weakest and strongest expressing levels were 129-fold and 78-fold. Similar results were obtained with the nmeNTR. Using the GusA reporter, translation initiation from six alternative (non-AUG) start codons was measured in E. coli, C. sporogenes, and C. butyricum. Clearly, species-specific differences between clostridia and E. coli in translation initiation were observed, and the performance of the start codons was influenced by the upstream 5' UTR sequence. These results highlight a new opportunity for gene control in recombinant clostridia. To demonstrate the value of these results, expression of the sacB gene from Bacillus subtilis was optimized for use as a novel negative selection marker in C. butyricum. In summary, these results indicate improvements in the understanding of heterologous gene regulation in Clostridium species and E. coli cloning strains. This new knowledge can be utilized for rationally designed gene regulation in Clostridium-mediated industrial and medical applications, as well as fundamental research into the biology of Clostridium species.

Establishment of Green- and Red-Fluorescent Reporter Proteins Based on the Fluorescence-Activating and Absorption-Shifting Tag for Use in Acetogenic and Solventogenic Anaerobes

Anaerobic bacteria are promising biocatalysts to produce industrially relevant products from nonfood feedstocks. Several anaerobes are genetically accessible, and various molecular tools for metabolic engineering are available. Still, the use of bright fluorescent reporters, which are commonly used in molecular biological approaches is limited under anaerobic conditions. Therefore, the establishment of different anaerobic fluorescent reporter proteins is of great interest. Here, we present the establishment of the green- and red-fluorescent reporter proteins greenFAST and redFAST for use in different solventogenic and acetogenic bacteria. Green fluorescence of greenFAST was bright in Clostridium saccharoperbutylacetonicum, Clostridium acetobutylicum, Acetobacterium woodii, and Eubacterium limosum, while only C. saccharoperbutylacetonicum showed bright red fluorescence when producing redFAST. We used both reporter proteins in C. saccharoperbutylacetonicum for multicolor approaches. These include the investigation of the co-culture dynamics of metabolically engineered strains. Moreover, we established a tightly regulated inducible two-plasmid system and used greenFAST and redFAST to track the coexistence and interaction of both plasmids under anaerobic conditions in C. saccharoperbutylacetonicum. The establishment of greenFAST and redFAST as fluorescent reporters opens the door for further multicolor approaches to investigate cell dynamics, gene expression, or protein localization under anaerobic conditions.

The challenges of monitoring and manipulating anaerobic microbial communities

Mixed anaerobic microbial communities are a key component in valorization of waste biomass via anaerobic digestion. Similar microbial communities are important as soil and animal microbiomes and have played a critical role in shaping the planet as it is today. Understanding how individual species within communities interact with others and their environment is important for improving performance and potential applications of an inherently green technology. Here, the challenges associated with making measurements critical to assessing the status of anaerobic microbial communities are considered. How these measurements could be incorporated into control philosophies and augment the potential of anaerobic microbial communities to produce different and higher value products from waste materials are discussed. The benefits and pitfalls of current genetic and molecular approaches to measuring and manipulating anaerobic microbial communities and the challenges which should be addressed to realise the potential of this exciting technology are explored.

Monitoring co-cultures of Clostridium carboxidivorans and Clostridium kluyveri by fluorescence in situ hybridization with specific 23S rRNA oligonucleotide probes

The development of co-cultures of clostridial strains which combine different physiological traits represents a promising strategy to achieve the environmentally friendly production of biofuels and chemicals. For the optimization of such co-cultures it is essential to monitor their composition and stability throughout fermentation. FISH is a quick and sensitive method for the specific labeling and quantification of cells within microbial communities. This technique is neither limited by the anaerobic fermenter environment nor by the need of prior genetic modification of strains. In this study, two specific 23S rRNA oligonucleotide probes, ClosKluy and ClosCarb, were designed for the monitoring of C. kluyveri and C. carboxidivorans, respectively. After the optimization of hybridization conditions for both probes, which was achieved at 30% (v/v) formamide, a high specificity was observed with epifluorescence microscopy using cells from different pure reference strains. The discriminating properties of the ClosKluy and ClosCarb probes was verified with samples from heterotrophic co-cultures in anaerobic flasks as well as autotrophic stirred-tank bioreactor co-cultures of C. kluyveri and C. carboxidivorans. Besides being suited to monitor defined co-cultures of these two species, the new specific FISH oligonucleotide probes for C. kluyveri and C. carboxidivorans additionally have potential to be applied in environmental studies.

Anaerobic fluorescent reporters for cell identification, microbial cell biology and high-throughput screening of microbiota and genomic libraries

The lack of real-time reporters in obligate anaerobes has limited studies in gene expression, promoter characterization, library screening, population dynamics, and cell biology in these organisms. While the use of enzymatic, colorimetric, and luminescent reporters has been reported, the need for reliable anaerobic fluorescent proteins is widely acknowledged. Recently, the fluorescent proteins HaloTag, SNAP-tag and FAST have been established as reliable reporters in Clostridium spp., thus suggesting that these reporters can be adopted widely for many obligate anaerobes. With a multitude of labeling options, these anaerobic fluorescent proteins hold a great potential for screening promoters, terminators, and RBS sites, tracking population dynamics in complex multi-species co-cultures, such as microbiomes, screening libraries, and in cell biology studies of protein localization and interactions using high-resolution microscopy.

Development of Strong Anaerobic Fluorescent Reporters for Clostridium acetobutylicum and Clostridium ljungdahlii Using HaloTag and SNAP-tag Proteins

One of the biggest limitations in the study and engineering of anaerobic Clostridium organisms is the lack of strong fluorescent reporters capable of strong and real-time fluorescence. Recently, we developed a strong fluorescent reporter system for Clostridium organisms based on the FAST protein. Here, we report the development of two new strong fluorescent reporter systems for Clostridium organisms based on the HaloTag and SNAP-tag proteins, which produce strong fluorescent signals when covalently bound to fluorogenic ligands. These new fluorescent reporters are orthogonal to the FAST ligands and to each other, allowing for simultaneous labeling and visualization. We used HaloTag and SNAP-tag to label the strictly anaerobic organisms Clostridium acetobutylicum and Clostridium ljungdahlii We have also identified a new strong promoter for protein expression in C. acetobutylicum, based on the phosphotransacetylase gene (pta) from C. ljungdahlii Furthermore, the HaloTag and the SNAP-tag, in combination with the previously described FAST system, were successfully used to measure cell populations in bacterial mixed cultures and showed the simultaneous orthogonal labeling of HaloTag and SNAP-tag together with the FAST protein reporter. Finally, we show the expression of recombinant fusion protein of FAST and the ZapA division protein (from C. acetobutylicum) in C. ljungdahlii. The availability of multiple strong fluorescent reporters is a major addition to the genetic toolkit of Clostridium and other anaerobes that will lead to better understanding of these unique organisms.IMPORTANCE Up to this point, assays and methods involving fluorescent reporter proteins were unavailable or limited in Clostridium organisms and other strict anaerobes. Green fluorescent protein (GFP), mCherry, and flavin-binding proteins (and their derivatives) have been used only in a few clostridia with limited success and yielded low fluorescence compared to aerobic microbial systems. Recently, we reported a new strong fluorescent reporter system based on the FAST protein as a first step in expanding the fluorescence-based reporters for Clostridium and other anaerobic microbial platforms. Additional strong orthogonal fluorescent proteins, with distinct emission spectra are needed to allow for (i) multispecies tracking within the growing field of microbial cocultures and microbiomes, (ii) protein localization and tracking in anaerobes, and (iii) identification and development of natural and synthetic promoters, ribosome-binding sites (RBS), and terminators for optimal protein expression in anaerobes. Here, we present two new strong fluorescent reporter systems based on the HaloTag and SNAP-tag proteins.

Robust microorganisms for biofuel and chemical production from municipal solid waste

Background

Worldwide 3.4 billion tonnes of municipal solid waste (MSW) will be produced annually by 2050, however, current approaches to MSW management predominantly involve unsustainable practices like landfilling and incineration. The organic fraction of MSW (OMSW) typically comprises ~ 50% lignocellulose-rich material but is underexplored as a biomanufacturing feedstock due to its highly inconsistent and heterogeneous composition. This study sought to overcome the limitations associated with studying MSW-derived feedstocks by using OMSW produced from a realistic and reproducible MSW mixture on a commercial autoclave system. The resulting OMSW fibre was enzymatically hydrolysed and used to screen diverse microorganisms of biotechnological interest to identify robust species capable of fermenting this complex feedstock.

Results

The autoclave pre-treated OMSW fibre contained a polysaccharide fraction comprising 38% cellulose and 4% hemicellulose. Enzymatic hydrolysate of OMSW fibre was high in d-glucose (5.5% w/v) and d-xylose (1.8%w/v) but deficient in nitrogen and phosphate. Although relatively low levels of levulinic acid (30 mM) and vanillin (2 mM) were detected and furfural and 5-hydroxymethylfurfural were absent, the hydrolysate contained an abundance of potentially toxic metals (0.6% w/v). Hydrolysate supplemented with 1% yeast extract to alleviate nutrient limitation was used in a substrate-oriented shake-flask screen with eight biotechnologically useful microorganisms (Clostridium saccharoperbutylacetonicum, Escherichia coli, Geobacillus thermoglucosidasius, Pseudomonas putida, Rhodococcus opacus, Saccharomyces cerevisiae, Schizosaccharomyces pombe and Zymomonas mobilis). Each species’ growth and productivity were characterised and three species were identified that robustly and efficiently fermented OMSW fibre hydrolysate without significant substrate inhibition: Z. mobilis, S. cerevisiae and R. opacus, respectively produced product to 69%, 70% and 72% of the maximum theoretical fermentation yield and could theoretically produce 136 kg and 139 kg of ethanol and 91 kg of triacylglycerol (TAG) per tonne of OMSW.

Conclusions

Developing an integrated biorefinery around MSW has the potential to significantly alleviate the environmental burden of current waste management practices. Substrate-oriented screening of a representative and reproducible OMSW-derived fibre identified microorganisms intrinsically suited to growth on OMSW hydrolysates. These species are promising candidates for developing an MSW biorefining platform and provide a foundation for future studies aiming to valorise this underexplored feedstock.

CRISPR-Cas, a highly effective tool for genome editing in Clostridium saccharoperbutylacetonicum N1-4(HMT)

The solventogenic clostridia have long been known for their ability to convert sugars from complex feedstocks into commercially important solvents. Although the acetone-butanol-ethanol process fell out of favour decades ago, renewed interest in sustainability and ‘green’ chemistry has re-established our appetite for reviving technologies such as these, albeit with 21st century improvements. As CRISPR-Cas genome editing tools are being developed and applied to the solventogenic clostridia, their industrial potential is growing. Through integration of new pathways, the beneficial traits and historical track record of clostridial fermentation can be exploited to generate a much wider range of industrially relevant products. Here we show the application of genome editing using the endogenous CRISPR-Cas mechanism of Clostridium saccharoperbutylacetonicum N1-4(HMT), to generate a deletion, SNP and to integrate new DNA into the genome. These technological advancements pave the way for application of clostridial species to the production of an array of products.

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.

Broad range shuttle vector construction and promoter evaluation for the use of Lactobacillus plantarum WCFS1 as a microbial engineering platform

As the field of synthetic biology grows, efforts to deploy complex genetic circuits in nonlaboratory strains of bacteria will continue to be a focus of research laboratories. Members of the Lactobacillus genus are good targets for synthetic biology research as several species are already used in many foods and as probiotics. Additionally, Lactobacilli offer a relatively safe vehicle for microbiological treatment of various health issues considering these commensals are often minor constituents of the gut microbial community and maintain allochthonous behavior. In order to generate a foundation for engineering, we developed a shuttle vector for subcloning in Escherichia coli and used it to characterize the transcriptional and translational activities of a number of promoters native to Lactobacillus plantarum WCFS1. Additionally, we demonstrated the use of this vector system in multiple Lactobacillus species, and provided examples of non-native promoter recognition by both L. plantarum and E. coli strains that might allow a shortcut assessment of circuit outputs. A variety of promoter activities were observed covering a range of protein expression levels peaking at various times throughout growth, and subsequent directed mutations were demonstrated and suggested to further increase the degree of output tuning. We believe these data show the potential for L. plantarum WCFS1 to be used as a nontraditional synthetic biology chassis and provide evidence that our system can be transitioned to other probiotic Lactobacillus species as well.

Direct cell-to-cell exchange of matter in a synthetic Clostridium syntrophy enables CO2 fixation, superior metabolite yields, and an expanded metabolic space

In microbial fermentations at least 33% of the sugar-substrate carbon is lost as CO₂ during pyruvate decarboxylation to acetyl-CoA, with the corresponding electrons lost in the form of H₂. Previous attempts to reduce this carbon and electron loss focused on engineering of a single organism. In nature, most microorganisms live in complex communities where syntrophic interactions result in superior resource utilization. Here, we show that a synthetic syntrophy consisting of the solventogen Clostridium acetobutylicum, which converts simple and complex carbohydrates into a variety of chemicals, and the acetogen C. ljungdahlii which fixes CO₂, achieved carbon recoveries into C2-C4 alcohols almost to the limit of substrate-electron availability, with minimal H₂ and CO₂ release. The syntrophic co-culture produced robust metabolic outcomes over a broad range of starting population ratios of the two organisms. We show that direct cell-to-cell interactions and material exchange among the two microbes enabled unforeseen rearrangements in the metabolism of the individual species that resulted in the production of non-native metabolites, namely isopropanol and 2,3-butanediol. This was accomplished by pathway-specific alterations of gene expression brought about by one organism on the other, and vice versa. While some of these gene-expression alterations can be explained by the exchange of metabolites that induce specific gene expression patterns, others, as demonstrated by co-culture setup in a transwell system, cannot. The latter, for now, would be attributed to complex direct physical interactions among the two organisms, thus providing a glimpse of the potential microbial complexity of simple or multicomponent microbiomes. Such direct material-transfer phenomena have not been documented in the literature. Furthermore, our study shows that syntrophic cultures offer a flexible platform for metabolite production with superior carbon recovery that can also be applied to electron-enhanced fermentations enabling even higher carbon recoveries.

Butyric acid: Applications and recent advances in its bioproduction

Butyric acid is an important C4 organic acid with broad applications. It is currently produced by chemosynthesis from petroleum-based feedstocks. However, the fermentative production of butyric acid from renewable feedstocks has received growing attention because of consumer demand for green products and natural ingredients in foods, pharmaceuticals, animal feed supplements, and cosmetics. In this review, strategies for improving microbial butyric acid production, including strain engineering and novel fermentation process development are discussed and compared regarding product yield, titer, purity and productivity. Future perspectives on strain and process improvements for butyric acid production are also discussed.

Stringency of Synthetic Promoter Sequences in Clostridium Revealed and Circumvented by Tuning Promoter Library Mutation Rates

Collections of characterized promoters of different strengths are key resources for synthetic biology, but are not well established for many important organisms, including industrially relevant Clostridium spp. When generating promoters, reporter constructs are used to measure expression, but classical fluorescent reporter proteins are oxygen-dependent and hence inactive in anaerobic bacteria like Clostridium. We directly compared oxygen-independent reporters of different types in Clostridium acetobutylicum and found that glucuronidase (GusA) from E. coli performed best. Using GusA, a library of synthetic promoters was first generated by a typical approach entailing complete randomization of a constitutive thiolase gene promoter (P_thl) except for the consensus -35 and -10 elements. In each synthetic promoter, the chance of each degenerate position matching P_thl was 25%. Surprisingly, none of the tested synthetic promoters from this library were functional in C. acetobutylicum, even though they functioned as expected in E. coli. Next, instead of complete randomization, we specified lower promoter mutation rates using oligonucleotide primers synthesized using custom mixtures of nucleotides. Using these primers, two promoter libraries were constructed in which the chance of each degenerate position matching P_thl was 79% or 58%, instead of 25% as before. Synthetic promoters from these "stringent" libraries functioned well in C. acetobutylicum, covering a wide range of strengths. The promoters functioned similarly in the distantly related species Clostridium sporogenes, and allowed predictable metabolic engineering of C. acetobutylicum for acetoin production. Besides generating the desired promoters and demonstrating their useful properties, this work indicates an unexpected "stringency" of promoter sequences in Clostridium, not reported previously.