Induced Production, Synthesis, and Immunomodulatory Action of Clostrisulfone, a Diarylsulfone from Clostridium acetobutylicum.

The anaerobe Clostridium acetobutylicum belongs to the most important industrially used bacteria. Whereas genome mining points to a high potential for secondary metabolism in C. acetobutylicum, the functions of most biosynthetic gene clusters are cryptic. We report that the addition of supra-physiological concentrations of cysteine triggered the formation of a novel natural product, clostrisulfone (1). Its structure was fully elucidated by NMR, MS and the chemical synthesis of a reference compound. Clostrisulfone is the first reported natural product with a diphenylsulfone scaffold. A biomimetic synthesis suggests that pentamethylchromanol-derived radicals capture sulfur dioxide to form 1. In a cell-based assay using murine macrophages a biphasic and dose-dependent regulation of the LPS-induced release of nitric oxide was observed in the presence of 1.

Biofilm Polysaccharide Display Platform: A Natural, Renewable, and Biocompatible Material for Improved Lipase Performance.

Most of the microorganisms can form biofilms, which makes biofilms an abundant bioresource to be exploited. Due to the limitations of the application of current immobilization methods for biofilms, we developed an immobilization method called the biofilm polysaccharide display (BPD) strategy while maintaining the native biofilm structure and catalytic microenvironment of Clostridium acetobutylicum B3. Lipase Lip181 showed significant improvements in stability after chemical immobilization. For example, immobilized Lip181 retained 74.23% of its original activity after incubation for 14 days, while free Lip181 was totally deactivated. In addition, immobilized Lip181 maintained high residual activity (pH 5.0-11.0), which showed improved resistance to pH changes. Notably, this method did not decrease but slightly increased the relative activity of Lip181 from 6.39 to 6.78 U/mg. Immobilized Lip181 was used to prepare cinnamyl acetate, and it showed a maximum yield of 85.09%. Overall, this biofilm immobilization method may promote the development of biocatalytic and biofilm materials.

Increased Biobutanol Production by Mediator-Less Electro-Fermentation.

A future bio-economy should not only be based on renewable raw materials but also in the raise of carbon yields of existing production routes. Microbial electrochemical technologies are gaining increased attention for this purpose. In this study, the electro-fermentative production of biobutanol with C. acetobutylicum without the use of exogenous mediators is investigated regarding the medium composition and the reactor design. It is shown that the use of an optimized synthetic culture medium allows higher product concentrations, increased biofilm formation, and higher conductivities compared to a synthetic medium supplemented with yeast extract. Moreover, the optimization of the reactor system results in a doubling of the maximum product concentrations for fermentation products. When a working electrode is polarized at -600 mV vs. Ag/AgCl, a shift from butyrate to acetone and butanol production is induced. This leads to an increased final solvent yield of YABE = 0.202 g g-1 (control 0.103 g g-1 ), which is also reflected in a higher carbon efficiency of 37.6% compared to 23.3% (control) as well as a fourfold decrease in simplified E-factor to 0.43. The results are promising for further development of biobutanol production in bioelectrochemical systems in order to fulfil the principles of Green Chemistry.

The industrial anaerobe Clostridium acetobutylicum uses polyketides to regulate cellular differentiation.

Polyketides are an important class of bioactive small molecules valued not only for their diverse therapeutic applications, but also for their role in controlling interesting biological phenotypes in their producing organisms. While numerous polyketides are known to be derived from aerobic organisms, only a single family of polyketides has been identified from anaerobic organisms. Here we uncover a family of polyketides native to the anaerobic bacterium Clostridium acetobutylicum, an organism well-known for its historical use as an industrial producer of the organic solvents acetone, butanol, and ethanol. Through mutational analysis and chemical complementation assays, we demonstrate that these polyketides act as chemical triggers of sporulation and granulose accumulation in this strain. This study represents a significant addition to the body of work demonstrating the existence and importance of polyketides in anaerobes, and showcases a strategy of manipulating the secondary metabolism of an organism to improve traits relevant for industrial applications.

Immobilization of Clostridium acetobutylicum onto natural textiles and its fermentation properties.

Immobilized fermentation has several advantages over traditional suspended fermentation, including simple and continuous operation, improved fermentation performance and reduced cost. Carrier is the most adjustable element among three elements of immobilized fermentation, including carrier, bacteria and environment. In this study, we characterized carrier roughness and surface properties of four types of natural fibres, including linen, cotton, bamboo fibre and silk, to assess their effects on cell immobilization, fermentation performance and stability. Linen with higher specific surface area and roughness could adsorb more bacteria during immobilized fermentation, thereby improving fermentation performance; thus, linen was selected as a suitable carrier and was applied for acetone-butanol-ethanol (ABE) fermentation. To further improve fermentation performance, we also found that microbes of Clostridium acetobutylicum were negatively charged surfaces during fermentation. Therefore, we then modified linen with polyetherimide (PEI) and steric acid (SA) to increase surface positive charge and improve surface property. During ABE fermentation, the adhesion between modified linen and bacteria was increased, adsorption was increased about twofold compared with that of unmodified linen, and butanol productivity was increased 8.16% and 6.80% with PEI- and SA-modified linen as carriers respectively.

Expression and Characterization of Levansucrase from Clostridium acetobutylicum.

The Clostridium acetobutylicum gene Ca-SacB encoding levansucrase was cloned and expressed in Escherichia coli. Ca-SacB is composed of 1287 bp and encodes 428 amino acid residues, which could convert 150 mmol/L sucrose to levan with the liberation of glucose. The optimum pH and temperature of this enzyme for levan formation were pH 6 and 60 °C, respectively. Levansucrase activity of Ca-SacB was completely abolished by 5 mmol/L Ag+ and Hg2+. The Km and Vmax values for levansucrase were calculated to be 64 mmol/L and 190 µmol/min/mg, respectively. Interestingly, Ca-SacB was found to have high product specificity, and no fructooligosaccharide was identified in the product, indicating that Ca-SacB may be valuable for industrial production of levan. In addition, Ca-SacB is the first characterized levansucrase isolated from an anaerobic bacterium, which should be valuable for exploring new enzyme resources and deepening the understanding of the catalytic mechanisms of levansucrases.

Effect of chemical pretreatments on corn stalk bagasse as immobilizing carrier of Clostridium acetobutylicum in the performance of a fermentation-pervaporation coupled system.

In this study, different pretreatment methods were evaluated for modified the corn stalk bagasse and further used the pretreated bagasse as immobilized carrier in acetone-butanol-ethanol fermentation process. Structural changes of the bagasses pretreated by different methods were analyzed by Fourier transform infrared, crystallinity index and scanning pictures by electron microscope. And the performances of batch fermentation using the corn stalk based carriers were evaluated. Results indicated that the highest ABE concentration of 23.86g/L was achieved using NaOH pretreated carrier in batch fermentation. Immobilized fermentation-pervaporation integration process was further carried out. The integration process showed long-term stability with 225-394g/L of ABE solvents on the permeate side of pervaporation membrane. This novel integration process was found to be an efficient method for biobutanol production.

Impact of the chemicals, essential for the purification process of strict Fe-hydrogenase, on the corrosion of mild steel.

The influence of additional chemical molecules, necessary for the purification process of [Fe]-hydrogenase from Clostridium acetobutylicum, was studied on the anaerobic corrosion of mild steel. At the end of the purification process, the pure [Fe-Fe]-hydrogenase was recovered in a Tris-HCl medium containing three other chemicals at low concentration: DTT, dithionite and desthiobiotin. Firstly, mild steel coupons were exposed in parallel to a 0.1 M pH7 Tris-HCl medium with or without pure hydrogenase. The results showed that hydrogenase and the additional molecules were in competition, and the electrochemical response could not be attributed solely to hydrogenase. Then, solutions with additional chemicals of different compositions were studied electrochemically. DTT polluted the electrochemical signal by increasing the Eoc by 35 mV 24 h after the injection of 300 µL of control solutions with DTT, whereas it drastically decreased the corrosion rate by increasing the charge transfer resistance (Rct 10 times the initial value). Thus, DTT was shown to have a strong antagonistic effect on corrosion and was removed from the purification process. An optimal composition of the medium was selected (0.5 mM dithionite, 7.5 mM desthiobiotin) that simultaneously allowed a high activity of hydrogenase and a lower impact on the electrochemical response for corrosion tests.

Atypical effect of temperature tuning on the insertion of the catalytic iron-sulfur center in a recombinant [FeFe]-hydrogenase.

The expression of recombinant [FeFe]-hydrogenases is an important step for the production of large amount of these enzymes for their exploitation in biotechnology and for the characterization of the protein-metal cofactor interactions. The correct assembly of the organometallic catalytic site, named H-cluster, requires a dedicated set of maturases that must be coexpressed in the microbial hosts or used for in vitro assembly of the active enzymes. In this work, the effect of the post-induction temperature on the recombinant expression of CaHydA [FeFe]-hydrogenase in E. coli is investigated. The results show a peculiar behavior: the enzyme expression is maximum at lower temperatures (20°C), while the specific activity of the purified CaHydA is higher at higher temperature (30°C), as a consequence of improved protein folding and active site incorporation.

Structural analysis of Clostridium acetobutylicum ATCC 824 glycoside hydrolase from CAZy family GH105.

Clostridium acetobutylicum ATCC 824 gene CA_C0359 encodes a putative unsaturated rhamnogalacturonyl hydrolase (URH) with distant amino-acid sequence homology to YteR of Bacillus subtilis strain 168. YteR, like other URHs, has core structural homology to unsaturated glucuronyl hydrolases, but hydrolyzes the unsaturated disaccharide derivative of rhamnogalacturonan I. The crystal structure of the recombinant CA_C0359 protein was solved to 1.6 Šresolution by molecular replacement using the phase information of the previously reported structure of YteR (PDB entry 1nc5) from Bacillus subtilis strain 168. The YteR-like protein is a six-α-hairpin barrel with two ß-sheet strands and a small helix overlaying the end of the hairpins next to the active site. The protein has low primary protein sequence identity to YteR but is structurally similar. The two tertiary structures align with a root-mean-square deviation of 1.4 Šand contain a highly conserved active pocket. There is a conserved aspartic acid residue in both structures, which has been shown to be important for hydration of the C=C bond during the release of unsaturated galacturonic acid by YteR. A surface electrostatic potential comparison of CA_C0359 and proteins from CAZy families GH88 and GH105 reveals the make-up of the active site to be a combination of the unsaturated rhamnogalacturonyl hydrolase and the unsaturated glucuronyl hydrolase from Bacillus subtilis strain 168. Structural and electrostatic comparisons suggests that the protein may have a slightly different substrate specificity from that of YteR.

Improvement of acetone, butanol, and ethanol production from woody biomass using organosolv pretreatment.

A suitable pretreatment is a prerequisite of efficient acetone-butanol-ethanol (ABE) production from wood by Clostridia. In this study, organosolv fractionation, an effective pretreatment with ability to separate lignin as a co-product, was evaluated for ABE production from softwood pine and hardwood elm. ABE production from untreated woods was limited to the yield of 81 g ABE/kg wood and concentration of 5.5 g ABE/L. Thus, the woods were pretreated with aqueous ethanol at elevated temperatures before hydrolysis and fermentation to ABE by Clostridium acetobutylicum. Hydrolysis of pine and elm pretreated at 180 °C for 60 min resulted in the highest sugar concentrations of 16.8 and 23.2 g/L, respectively. The hydrolysate obtained from elm was fermented to ABE with the highest yield of 121.1 g/kg and concentration of 11.6 g/L. The maximum yield of 87.9 g/kg was obtained from pine pretreated for 30 min at 150 °C. Moreover, structural modifications in the woods were investigated and related to the improvements. The woody biomasses are suitable feedstocks for ABE production after the organosolv pretreatment. Effects of the pretreatment conditions on ABE production might be related to the reduced cellulose crystallinity, reduced lignin and hemicellulose content, and lower total phenolic compounds in the hydrolysates.

Effects of H2 and electrochemical reducing power on metabolite production by Clostridium acetobutylicum KCTC1037.

A conventional fermenter (CF), a single-cathode fermenter (SCF), and a double-cathode fermenter (DCF) were employed to evaluate and compare the effects of H2 and electrochemical reducing power on metabolite production by Clostridium acetobutylicum KCTC1037. The source of the external reducing power for CF was H2, for the SCF was electrochemically reduced neutral red-modified graphite felt electrode (NR-GF), and for the DCF was electrochemically reduced combination of NR-GF and platinum plate electrodes (NR-GF/PtP). The metabolites produced from glucose or CO2 by strain KCTC1037 cultivated in the DCF were butyrate, ethanol, and butanol, but ethanol and butanol were not produced from glucose or CO2 by strain KCTC1037 cultivated in the CF and SCF. It is possible that electrochemically reduced NR-GF/PtP is a more effective source of internal and external reducing power than H2 or NR-GF for strain KCTC1037 to produce metabolites from glucose and CO2. This research might prove useful in developing fermentation technology to actualize direct bioalcohol production of fermentation bacteria from CO2.

Atmospheric vs. anaerobic processing of metabolome samples for the metabolite profiling of a strict anaerobic bacterium, Clostridium acetobutylicum.

Well-established metabolome sample preparation is a prerequisite for reliable metabolomic data. For metabolome sampling of a Gram-positive strict anaerobe, Clostridium acetobutylicum, fast filtration and metabolite extraction with acetonitrile/methanol/water (2:2:1, v/v) at -20°C under anaerobic conditions has been commonly used. This anaerobic metabolite processing method is laborious and time-consuming since it is conducted in an anaerobic chamber. Also, there have not been any systematic method evaluation and development of metabolome sample preparation for strict anaerobes and Gram-positive bacteria. In this study, metabolome sampling and extraction methods were rigorously evaluated and optimized for C. acetobutylicum by using gas chromatography/time-of-flight mass spectrometry-based metabolomics, in which a total of 116 metabolites were identified. When comparing the atmospheric (i.e., in air) and anaerobic (i.e., in an anaerobic chamber) processing of metabolome sample preparation, there was no significant difference in the quality and quantity of the metabolomic data. For metabolite extraction, pure methanol at -20°C was a better solvent than acetonitrile/methanol/water (2:2:1, v/v/v) at -20°C that is frequently used for C. acetobutylicum, and metabolite profiles were significantly different depending on extraction solvents. This is the first evaluation of metabolite sample preparation under aerobic processing conditions for an anaerobe. This method could be applied conveniently, efficiently, and reliably to metabolome analysis for strict anaerobes in air.

Proteomic analyses of the phase transition from acidogenesis to solventogenesis using solventogenic and non-solventogenic Clostridium acetobutylicum strains.

The fermentation carried out by the solvent-producing bacterium, Clostridium acetobutylicum, is characterized by two distinct phases: acidogenic and solventogenic phases. Understanding the cellular physiological changes occurring during the phase transition in clostridial fermentation is important for the enhanced production of solvents. To identify protein changes upon entry to stationary phase where solvents are typically produced, we herein analyzed the proteomic profiles of the parental wild type C. acetobutylicum strains, ATCC 824, the non-solventogenic strain, M5 that has lost the solventogenic megaplasmid pSOL1, and the synthetic simplified alcohol forming strain, M5 (pIMP1E1AB) expressing plasmid-based CoA-transferase (CtfAB) and aldehyde/alcohol dehydrogenase (AdhE1). A total of 68 protein spots, corresponding to 56 unique proteins, were unambiguously identified as being differentially present after the phase transitions in the three C. acetobutylicum strains. In addition to changes in proteins known to be involved in solventogenesis (AdhE1 and CtfB), we identified significant alterations in enzymes involved in sugar transport and metabolism, fermentative pathway, heat shock proteins, translation, and amino acid biosynthesis upon entry into the stationary phase. Of these, four increased proteins (AdhE1, CAC0233, CtfB and phosphocarrier protein HPr) and six decreased proteins (butyrate kinase, ferredoxin:pyruvate oxidoreductase, phenylalanyl-tRNA synthetase, adenylosuccinate synthase, pyruvate kinase and valyl-tRNA synthetase) showed similar patterns in the two strains capable of butanol formation. Interestingly, significant changes of several proteins by post-translational modifications were observed in the solventogenic phase. The proteomic data from this study will improve our understanding on how cell physiology is affected through protein levels patterns in clostridia.

Two-codon T-box riboswitch binding two tRNAs.

T-box riboswitches control transcription of downstream genes through the tRNA-binding formation of terminator or antiterminator structures. Previously reported T-boxes were described as single-specificity riboswitches that can bind specific tRNA anticodons through codon-anticodon interactions with the nucleotide triplet of their specifier loop (SL). However, the possibility that T-boxes might exhibit specificity beyond a single tRNA had been overlooked. In Clostridium acetobutylicum, the T-box that regulates the operon for the essential tRNA-dependent transamidation pathway harbors a SL with two potential overlapping codon positions for tRNA(Asn) and tRNA(Glu). To test its specificity, we performed extensive mutagenic, biochemical, and chemical probing analyses. Surprisingly, both tRNAs can efficiently bind the SL in vitro and in vivo. The dual specificity of the T-box is allowed by a single base shift on the SL from one overlapping codon to the next. This feature allows the riboswitch to sense two tRNAs and balance the biosynthesis of two amino acids. Detailed genomic comparisons support our observations and suggest that "flexible" T-box riboswitches are widespread among bacteria, and, moreover, their specificity is dictated by the metabolic interconnection of the pathways under control. Taken together, our results support the notion of a genome-dependent codon ambiguity of the SLs. Furthermore, the existence of two overlapping codons imposes a unique example of tRNA-dependent regulation at the transcriptional level.

A shift in the dominant phenotype governs the pH-induced metabolic switch of Clostridium acetobutylicumin phosphate-limited continuous cultures.

In response to changing extracellular pH levels, phosphate-limited continuous cultures of Clostridium acetobutylicum reversibly switches its metabolism from the dominant formation of acids to the prevalent production of solvents. Previous experimental and theoretical studies have revealed that this pH-induced metabolic switch involves a rearrangement of the intracellular transcriptomic, proteomic and metabolomic composition of the clostridial cells. However, the influence of the population dynamics on the observations reported has so far been neglected. Here, we present a method for linking the pH shift, clostridial growth and the acetone-butanol-ethanol fermentation metabolic network systematically into a model which combines the dynamics of the external pH and optical density with a metabolic model. Furthermore, the recently found antagonistic expression pattern of the aldehyde/alcohol dehydrogenases AdhE1/2 and pH-dependent enzyme activities have been included into this combined model. Our model predictions reveal that the pH-induced metabolic shift under these experimental conditions is governed by a phenotypic switch of predominantly acidogenic subpopulation towards a predominantly solventogenic subpopulation. This model-driven explanation of the pH-induced shift from acidogenesis to solventogenesis by population dynamics casts an entirely new light on the clostridial response to changing pH levels. Moreover, the results presented here underline that pH-dependent growth and pH-dependent specific enzymatic activity play a crucial role in this adaptation. In particular, the behaviour of AdhE1 and AdhE2 seems to be the key factor for the product formation of the two phenotypes, their pH-dependent growth, and thus, the pH-induced metabolic switch in C. acetobutylicum.

Workflow for quantitative proteomic analysis of Clostridium acetobutylicum ATCC 824 using iTRAQ tags.

Clostridium acetobutylicum (Cac) is an anaerobic, endospore-forming, Gram-positive bacterium with tremendous promise for use as a biocatalyst for the production of fuels and solvents. Cac proteomic sample preparation for shotgun analysis typically involves a multitude of reagents for harsh lysis conditions and to maintain protein solubility. We describe a protein extraction and preparation method for Cac that is compatible with proteomic shotgun analysis using isobaric labeling approaches. The method is applied to the analysis of Cac grown under butanol stress and labeled using iTRAQ 4-plex reagents. This method relies on the use of calcium carbonate to facilitate lysis by sonication and a commercially available kit to remove detergents prior to labeling. This workflow resulted in the identification and quantitation of 566 unique proteins using ProteinPilot software with a false discovery rate of 0.01% for peptide matches and 0.70% for protein matches. Ninety-five proteins were found to have statistically higher expression levels in butanol-stressed Cac as compared to non-stressed Cac. Sixty-one proteins were found to have statistically lower expression levels in stressed versus non-stressed cells. This method may be applicable to other Gram-positive organisms.

Yeast extract promotes phase shift of bio-butanol fermentation by Clostridium acetobutylicum ATCC824 using cassava as substrate.

When fermenting on cassava (15-25%, w/v) with Clostridium acetobutylicum ATCC824, a severe delay (18-40 h) was observed in the phase shift from acidogenesis to solventogenesis, compared to the cases of fermenting on corn. By adding yeast extract (2.5 g/L-broth) into cassava meal medium when the delay appeared, the phase shift was triggered and fermentation performances were consequently improved. Total butanol concentrations/butanol productivities, compared to those with cassava substrate alone, increased 15%/80% in traditional fermentation while 86%/79% in extractive fermentation using oleyl alcohol as the extractant, and reached the equivalent levels of those using corn substrate. Analysis of genetic transcriptional levels and measurements of free amino acids in the broth demonstrated that timely and adequate addition of yeast extract could promote phase shift by increasing transcriptional level of ctfAB to 16-fold, and indirectly enhance butanol synthesis through accelerating the accumulation of histidine and aspartic acid families.

Inhibition of [FeFe]-hydrogenases by formaldehyde and wider mechanistic implications for biohydrogen activation.

Formaldehyde-a rapid and reversible inhibitor of hydrogen evolution by [FeFe]-hydrogenases-binds with a strong potential dependence that is almost complementary to that of CO. Whereas exogenous CO binds tightly to the oxidized state known as H(ox) but very weakly to a state two electrons more reduced, formaldehyde interacts most strongly with the latter. Formaldehyde thus intercepts increasingly reduced states of the catalytic cycle, and density functional theory calculations support the proposal that it reacts with the H-cluster directly, most likely targeting an otherwise elusive and highly reactive Fe-hydrido (Fe-H) intermediate.