The names Hungateiclostridium Zhang et al. 2018, Hungateiclostridium thermocellum (Viljoen et al. 1926) Zhang et al. 2018, Hungateiclostridium cellulolyticum (Patel et al. 1980) Zhang et al. 2018, Hungateiclostridium aldrichii (Yang et al. 1990) Zhang et al. 2018, Hungateiclostridium alkalicellulosi (Zhilina et al. 2006) Zhang et al. 2018, Hungateiclostridium clariflavum (Shiratori et al. 2009) Zhang et al. 2018, Hungateiclostridium straminisolvens (Kato et al. 2004) Zhang et al. 2018 and Hungateiclostridium saccincola (Koeck et al. 2016) Zhang et al. 2018 contravene Rule 51b of the International Code of Nomenclature of Prokaryotes and require replacement names in the genus Acetivibrio Patel et al. 1980

Composition and metabolite patterns of caproic acid-producing bacteria during pH-mediated pitmud-Huangshui co-fermentation based on multi-database annotation

The growth and metabolism of caproic acid - producing bacteria (CPBs) in the pit mud (PM) within the Huangshui (HS) are crucial for both the quality of Chinese Nongxiangxing Baijiu (NXXB) fermentation and the aging of the PM. Based on multi-database co-annotation and key affecting factors identification of CPBs, the growth and metabolites pattern of CPBs was studied through the pH-mediated PM-HS co-fermentation. CPBs in PM were mainly from Oscillospiraceae family. The composition and metabolites profiles of CPBs changed with initial pH-adjustment. Lactobacillus (88.61 %-89.41 %) dominated the PM-HS system at an initial pH of 5, with CPBs suppressed. Butyric acid-producing Clostridium (56.18 %-54.53 %, 19.61 %-42.71 %) and CPBs (9.35 %-5.19 %, 65.44 % 50.01 %) co-dominated the PM-HS system with initial pH values of 6 and 7 respectively. This study may help illuminate the role of CPBs in PM aging, facilitating the targeted CPBs-enrichment, unknown CPBs-isolation, and practical regulation of CPBs-bioaugmentation in NXXB ecosystem.

Cellulolytic and hemicellulolytic capacity of Acetivibrio clariflavus

Lignocellulosic biomass represents the largest available renewable source of carbon. It is a recalcitrant mixture of different polysaccharides and lignin. For its complete degradation, a large set of enzymes capable of cleaving its individual components is required. Several anaerobic bacteria produce high-molecular-weight multienzyme complexes called cellulosomes where the enzymes work in synergy for efficient degradation of the complex plant material. One of the anaerobic thermophilic cellulosome-forming bacteria is Acetivibrio clariflavus. Acetivibrio clariflavus was found to be one of the most abundant lignocellulose-solubilizing bacteria in various plant decaying environments. It produces sophisticated cellulosomal complex that is most similar to mesophilic Acetivibrio cellulolyticus cellulosome. In comparison with other anaerobic cellulosome-forming bacteria A. cellulolyticus and Acetivibrio thermocellus, A. clariflavus possesses lower number of cellulolytic enzymes. However, it is significantly better equipped for a degradation of hemicellulose, particularly xylan. Some strains, e.g., 4 - 2a, were also shown to utilize xylose. Efficient saccharification of plant biomass makes A. clariflavus a promising candidate for various biotechnological applications including biofuel production. KEY POINTS: • Acetivibrio clariflavus is anaerobic thermophilic cellulosome-forming bacterium. • Its cellulosomes target mostly cellulose and hemicellulose, in particular xylan. • The strains share most of xylanolytic enzymes but differ in xylose utilization.

Bengtsoniella intestinalis gen. nov., sp. nov., a member of the family Oscillospiraceae, isolated from the hindgut of the marine herbivorous fish Kyphosus sydneyanus

A Gram-stain-negative, non-spore-forming, rod-shaped, obligately anaerobic bacterium, designated strain BP47G, was isolated from the hindgut of a silver drummer (Kyphosus sydneyanus) fish collected from the Hauraki Gulf, New Zealand. Phylogenetic analysis based on the 16S rRNA gene sequence of the isolate indicated that it belonged to the family Oscillospiraceae in the phylum Bacillota. The gene sequence of BP47G was most similar to Oscillibacter valericigenes with 95.23% sequence identity. Isolate BP47G grew on agar medium containing mannitol and fish gut fluid as the sole carbon sources. Clear colonies of ~1 mm diameter grew within a week at 20-28 °C (optimum 28 °C) and pH 7.1-8.5 (optimum 8.5). BP47G tolerated the addition to the medium of up to 1% NaCl. Formate and butyrate were the major fermentation products. The major cellular fatty acids were C12:0, C13:0, iso-C14:0, C16:0 and C16:1 cis 7. Genomic analyses comparing BP47G with its closest relatives indicated low genomic relatedness based on the average nucleotide identity, average amino acid identity, percentage of conserved protein and in silico DNA-DNA hybridization. Supported by the phenotypic and taxonomic characteristics observed in this study, a novel genus and species Bengtsoniella intestinalis gen. nov., sp. nov. is proposed for isolate BP47G (=ICMP 24688=JCM 35770).

Solubilization of sugarcane bagasse by mono and cocultures of thermophilic anaerobes with and without cotreatment

Cotreatment, mechanical disruption of lignocellulosic biomass during microbial fermentation, is a potential alternative to thermochemical pretreatment as a means of increasing the accessibility of lignocellulose to biological attack. Successful implementation of cotreatment requires microbes that can withstand milling, while solubilizing and utilizing carbohydrates from lignocellulose. In this context, cotreatment with thermophilic, lignocellulose-fermenting bacteria has been successfully evaluated for a number of lignocellulosic feedstocks. Here, cotreatment was applied to sugarcane bagasse using monocultures of the cellulose-fermenting Clostridium thermocellum and cocultures with the hemicellulose-fermenting Thermoanaerobacterium thermosaccharolyticum. This resulted in 76 % carbohydrate solubilization (a 1.8-fold increase over non-cotreated controls) on 10 g/L solids loading, having greater effect on the hemicellulose fraction. With cotreatment, fermentation by wild-type cultures at low substrate concentrations increased cumulative product formation by 45 % for the monoculture and 32 % for the coculture. These findings highlight the potential of cotreatment for enhancing deconstruction of sugarcane bagasse using thermophilic bacteria.

The role of AdhE on ethanol tolerance and production in Clostridium thermocellum

Many anaerobic microorganisms use the bifunctional aldehyde and alcohol dehydrogenase enzyme, AdhE, to produce ethanol. One such organism is Clostridium thermocellum, which is of interest for cellulosic biofuel production. In the course of engineering this organism for improved ethanol tolerance and production, we observed that AdhE was a frequent target of mutations. Here, we characterized those mutations to understand their effects on enzymatic activity, as well ethanol tolerance and product formation in the organism. We found that there is a strong correlation between NADH-linked alcohol dehydrogenase (ADH) activity and ethanol tolerance. Mutations that decrease NADH-linked ADH activity increase ethanol tolerance; correspondingly, mutations that increase NADH-linked ADH activity decrease ethanol tolerance. We also found that the magnitude of ADH activity did not play a significant role in determining ethanol titer. Increasing ADH activity had no effect on ethanol titer. Reducing ADH activity had indeterminate effects on ethanol titer, sometimes increasing and sometimes decreasing it. Finally, this study shows that the cofactor specificity of ADH activity was found to be the primary factor affecting ethanol yield. We expect that these results will inform efforts to use AdhE enzymes in metabolic engineering approaches.

Functional contribution of the intestinal microbiome in autism spectrum disorder, attention deficit hyperactivity disorder, and Rett syndrome: a systematic review of pediatric and adult studies

Introduction

Critical phases of neurodevelopment and gut microbiota diversification occur in early life and both processes are impacted by genetic and environmental factors. Recent studies have shown the presence of gut microbiota alterations in neurodevelopmental disorders. Here we performed a systematic review of alterations of the intestinal microbiota composition and function in pediatric and adult patients affected by autism spectrum disorder (ASD), attention-deficit/hyperactivity disorder (ADHD), and Rett syndrome (RETT).

Methods

We searched selected keywords in the online databases of PubMed, Cochrane, and OVID (January 1980 to December 2021) with secondary review of references of eligible articles. Two reviewers independently performed critical appraisals on the included articles using the Critical Appraisal Skills Program for each study design.

Results

Our systematic review identified 18, 7, and 3 original articles describing intestinal microbiota profiles in ASD, ADHD, and RETT, respectively. Decreased Firmicutes and increased Bacteroidetes were observed in the gut microbiota of individuals affected by ASD and ADHD. Proinflammatory cytokines, short-chain fatty acids and neurotransmitter levels were altered in ASD and RETT. Constipation and visceral pain were related to changes in the gut microbiota in patients affected by ASD and RETT. Hyperactivity and impulsivity were negatively correlated with Faecalibacterium (phylum Firmicutes) and positively correlated with Bacteroides sp. (phylum Bacteroidetes) in ADHD subjects. Five studies explored microbiota-or diet-targeted interventions in ASD and ADHD. Probiotic treatments with Lactobacillus sp. and fecal microbiota transplantation from healthy donors reduced constipation and ameliorated ASD symptoms in affected children. Perinatal administration of Lactobacillus sp. prevented the onset of Asperger and ADHD symptoms in adolescence. Micronutrient supplementation improved disease symptomatology in ADHD without causing significant changes in microbiota communities' composition.

Discussion

Several discrepancies were found among the included studies, primarily due to sample size, variations in dietary practices, and a high prevalence of functional gastrointestinal symptoms. Further studies employing longitudinal study designs, larger sample sizes and multi-omics technologies are warranted to identify the functional contribution of the intestinal microbiota in developmental trajectories of the human brain and neurobehavior.

Systematic review registration

https://clinicaltrials.gov/, CRD42020158734.

Novel primers to identify a wider diversity of butyrate-producing bacteria

Butyrate-producing bacteria are a functionally important part of the intestinal tract flora, and the resulting butyric acid is essential for maintaining host intestinal health, regulating the immune system, and influencing energy metabolism. However, butyrate-producing bacteria have not been defined as a coherent phylogenetic group. They are primarily identified using primers for key genes in the butyrate-producing pathway, and their use has been limited to the Bacillota and Bacteroidetes phyla. To overcome this limitation, we developed functional gene primers able to identify butyrate-producing bacteria through the butyrate kinase gene, which encodes the enzyme involved in the final step of the butyrate-producing pathway. Genomes extracted from human and rat feces were used to amplify the target genes through PCR. The obtained sequences were analyzed using BLASTX to construct a developmental tree using the MEGA software. The newly designed butyrate kinase gene primers allowed to recognize a wider diversity of butyrate-producing bacteria than that recognized using currently available primers. Specifically, butyrate-producing bacteria from the Synergistota and Spirochaetota phyla were identified for the first time using these primers. Thus, the developed primers provide a more accurate method for researchers and doctors to identify potential butyrate-producing bacteria and deepen our understanding of butyrate-producing bacterial species.

Amygdalobacter indicium gen. nov., sp. nov., and Amygdalobacter nucleatus sp. nov., gen. nov.: novel bacteria from the family Oscillospiraceae isolated from the female genital tract

Four obligately anaerobic Gram-positive bacteria representing one novel genus and two novel species were isolated from the female genital tract. Both novel species, designated UPII 610-JT and KA00274T, and an additional isolate of each species were characterized utilizing biochemical, genotypic and phylogenetic analyses. All strains were non-motile and non-spore forming, asaccharolytic, non-cellulolytic and indole-negative coccobacilli. Fatty acid methyl ester analysis for UPII 610-JT and KA00274T and additional isolates revealed C16 : 0, C18 : 0, C18:1ω9c and C18:2ω6,9c to be the major fatty acids for both species. UPII 610-JT had a 16S rRNA gene sequence similarity of 99.4 % to an uncultured clone sequence (AY724740) designated as Bacterial Vaginosis Associated Bacterium 2 (BVAB2). KA00274T had a 16S rRNA gene sequence similarity of 96.5 % to UPII 610-JT. Whole genomic DNA mol% G+C content was 42.2 and 39.3 % for UPII 610-JT and KA00274T, respectively. Phylogenetic analyses indicate these isolates represent a novel genus and two novel species within the Oscillospiraceae family. We propose the names Amygdalobacter indicium gen. nov., sp. nov., for UPII 610-JT representing the type strain of this species (=DSM 112989T, =ATCC TSD-274T) and Amygdalobacter nucleatus gen. nov., sp. nov., for KA00274T representing the type strain of this species (=DSM 112988T, =ATCC TSD-275T).

Pyrophosphate as allosteric regulator of ATP-phosphofructokinase in Clostridium thermocellum and other bacteria with ATP- and PPi-phosphofructokinases

The phosphofructokinase (Pfk) reaction represents one of the key regulatory points in glycolysis. While most organisms encode for Pfks that use ATP as phosphoryl donor, some organisms also encode for PP_i-dependent Pfks. Despite this central role, the biochemical characteristics as well as the physiological role of both Pfks is often not known. Clostridium thermocellum is an example of a microorganism that encodes for both Pfks, however, only PP_i-Pfk activity has been detected in cell-free extracts and little is known about the regulation and function of both enzymes. In this study, the ATP- and PP_i-Pfk of C. thermocellum were purified and biochemically characterized. No allosteric regulators were found for PP_i-Pfk amongst common effectors. With fructose-6-P, PP_i, fructose-1,6-bisP, and P_i PP_i-Pfk showed high specificity (K_M < 0.62 mM) and maximum activity (V_max > 156 U mg^-1). In contrast, ATP-Pfk showed much lower affinity (K_0.5 of 9.26 mM) and maximum activity (14.5 U mg^-1) with fructose-6-P. In addition to ATP, also GTP, UTP and ITP could be used as phosphoryl donors. The catalytic efficiency with GTP was 7-fold higher than with ATP, suggesting that GTP is the preferred substrate. The enzyme was activated by NH₄⁺, and pronounced inhibition was observed with GDP, FBP, PEP, and especially with PP_i (K_i of 0.007 mM). Characterization of purified ATP-Pfks originating from eleven different bacteria, encoding for only ATP-Pfk or for both ATP- and PP_i-Pfk, identified that PP_i inhibition of ATP-Pfks could be a common phenomenon for organisms with a PP_i-dependent glycolysis.

A detailed genome-scale metabolic model of Clostridium thermocellum investigates sources of pyrophosphate for driving glycolysis

Lignocellulosic biomass is an abundant and renewable source of carbon for chemical manufacturing, yet it is cumbersome in conventional processes. A promising, and increasingly studied, candidate for lignocellulose bioprocessing is the thermophilic anaerobe Clostridium thermocellum given its potential to produce ethanol, organic acids, and hydrogen gas from lignocellulosic biomass under high substrate loading. Possessing an atypical glycolytic pathway which substitutes GTP or pyrophosphate (PP_i) for ATP in some steps, including in the energy-investment phase, identification, and manipulation of PP_i sources are key to engineering its metabolism. Previous efforts to identify the primary pyrophosphate have been unsuccessful. Here, we explore pyrophosphate metabolism through reconstructing, updating, and analyzing a new genome-scale stoichiometric model for C. thermocellum, iCTH669. Hundreds of changes to the former GEM, iCBI655, including correcting cofactor usages, addressing charge and elemental balance, standardizing biomass composition, and incorporating the latest experimental evidence led to a MEMOTE score improvement to 94%. We found agreement of iCTH669 model predictions across all available fermentation and biomass yield datasets. The feasibility of hundreds of PP_i synthesis routes, newly identified and previously proposed, were assessed through the lens of the iCTH669 model including biomass synthesis, tRNA synthesis, newly identified sources, and previously proposed PP_i-generating cycles. In all cases, the metabolic cost of PP_i synthesis is at best equivalent to investment of one ATP suggesting no direct energetic advantage for the cofactor substitution in C. thermocellum. Even though no unique source of PP_i could be gleaned by the model, by combining with gene expression data two most likely scenarios emerge. First, previously investigated PP_i sources likely account for most PP_i production in wild-type strains. Second, alternate metabolic routes as encoded by iCTH669 can collectively maintain PP_i levels even when previously investigated synthesis cycles are disrupted. Model iCTH669 is available at github.com/maranasgroup/iCTH669.

Riboflavin Supplementation Promotes Butyrate Production in the Absence of Gross Compositional Changes in the Gut Microbiota

Aims: We performed a randomized, placebo-controlled trial, RIBOGUT, to study the effect of 2 weeks supplementation with either 50 or 100 mg/d of riboflavin on (i) Faecalibacterium prausnitzii abundance, (ii) gut microbiota composition, (iii) short-chain fatty acid (SCFA) profiles, and (iv) the satiety and gut hormones. Results: Neither dose of riboflavin, analyzed separately, impacted the abundance of F. prausnitzii, and only minor differences in SCFA concentrations were observed. However, combining the results of the 50 and 100 mg/d groups showed a significant increase in butyrate production. While the gut bacterial diversity was not affected by riboflavin supplementation, the complexity and stability of the bacterial network were enhanced. Oral glucose tolerance tests showed a trend of increased plasma insulin concentration and GLP-1 after 100 mg/d supplementation. Innovation: Dietary supplements, such as vitamins, promote health by either directly targeting host physiology or indirectly via gut microbiota modulation. Here, we show for the first time that riboflavin intervention changes the activity of the microbiota. The butyrate production increased after intervention and although the composition did not change significantly, the network of microbial interactions was enforced. Conclusion: This RIBOGUT study suggests that oral riboflavin supplementation promotes butyrate production in the absence of major shifts in gut microbiota composition. ClinicalTrials.gov Identifier: NCT02929459.

The Roles of Nicotinamide Adenine Dinucleotide Phosphate Reoxidation and Ammonium Assimilation in the Secretion of Amino Acids as Byproducts of Clostridium thermocellum

Clostridium thermocellum is a cellulolytic thermophile that is considered for the consolidated bioprocessing of lignocellulose to ethanol. Improvements in ethanol yield are required for industrial implementation, but the incompletely understood causes of amino acid secretion impede progress. In this study, amino acid secretion was investigated via gene deletions in ammonium-regulated, nicotinamide adenine dinucleotide phosphate (NADPH)-supplying and NADPH-consuming pathways as well as via physiological characterization in cellobiose-limited or ammonium-limited chemostats. First, the contribution of the NADPH-supplying malate shunt was studied with strains using either the NADPH-yielding malate shunt (Δppdk) or a redox-independent conversion of PEP to pyruvate (Δppdk ΔmalE::P_eno-pyk). In the latter, branched-chain amino acids, especially valine, were significantly reduced, whereas the ethanol yield increased from 46 to 60%, suggesting that the secretion of these amino acids balances the NADPH surplus from the malate shunt. The unchanged amino acid secretion in Δppdk falsified a previous hypothesis on an ammonium-regulated PEP-to-pyruvate flux redistribution. The possible involvement of another NADPH-supplier, namely, NADH-dependent reduced ferredoxin:NADP⁺ oxidoreductase (nfnAB), was also excluded. Finally, the deletion of glutamate synthase (gogat) in ammonium assimilation resulted in the upregulation of NADPH-linked glutamate dehydrogenase activity and decreased amino acid yields. Since gogat in C. thermocellum is putatively annotated as ferredoxin-linked, a claim which is supported by the product redistribution observed in this study, this deletion likely replaced ferredoxin with NADPH in ammonium assimilation. Overall, these findings indicate that a need to reoxidize NADPH is driving the observed amino acid secretion, likely at the expense of the NADH needed for ethanol formation. This suggests that metabolic engineering strategies that simplify the redox metabolism and ammonium assimilation can contribute to increased ethanol yields. IMPORTANCE Improving the ethanol yield of C. thermocellum is important for the industrial implementation of this microorganism in consolidated bioprocessing. A central role of NADPH in driving amino acid byproduct formation was demonstrated by eliminating the NADPH-supplying malate shunt and separately by changing the cofactor specificity in ammonium assimilation. With amino acid secretion diverting carbon and electrons away from ethanol, these insights are important for further metabolic engineering to reach industrial requirements on ethanol yield. This study also provides chemostat data that are relevant for training genome-scale metabolic models and for improving the validity of their predictions, especially considering the reduced degree-of-freedom in the redox metabolism of the strains generated here. In addition, this study advances the fundamental understanding on the mechanisms underlying amino acid secretion in cellulolytic Clostridia as well as on the regulation and cofactor specificity in ammonium assimilation. Together, these efforts aid in the development of C. thermocellum for the sustainable consolidated bioprocessing of lignocellulose to ethanol with minimal pretreatment.

Fermentative conversion of unpretreated plant biomass: A thermophilic threshold for indigenous microbial growth

Naturally occurring, microbial contaminants were found in plant biomasses from common bioenergy crops and agricultural wastes. Unexpectedly, indigenous thermophilic microbes were abundant, raising the question of whether they impact thermophilic consolidated bioprocessing fermentations that convert biomass directly into useful bioproducts. Candidate microbial platforms for biomass conversion, Acetivibrio thermocellus (basionym Clostridium thermocellum; Topt 60 °C) and Caldicellulosiruptor bescii (Topt 78 °C), each degraded a wide variety of plant biomasses, but only A. thermocellus was significantly affected by the presence of indigenous microbial populations harbored by the biomass. Indigenous microbial growth was eliminated at ≥75 °C, conditions where C. bescii thrives, but where A. thermocellus cannot survive. Therefore, 75 °C is the thermophilic threshold to avoid sterilizing pre-treatments on the biomass that prevents native microbes from competing with engineered microbes and forming undesirable by-products. Thermophiles that naturally grow at and above 75 °C offer specific advantages as platform microorganisms for biomass conversion into fuels and chemicals.

Ethanol tolerance of Clostridium thermocellum: the role of chaotropicity, temperature and pathway thermodynamics on growth and fermentative capacity

BACKGROUND

Clostridium thermocellum is a promising candidate for consolidated bioprocessing of lignocellulosic biomass to ethanol. The low ethanol tolerance of this microorganism is one of the remaining obstacles to industrial implementation. Ethanol inhibition can be caused by end-product inhibition and/or chaotropic-induced stress resulting in increased membrane fluidization and disruption of macromolecules. The highly reversible glycolysis of C. thermocellum might be especially sensitive to end-product inhibition. The chaotropic effect of ethanol is known to increase with temperature. This study explores the relative contributions of these two aspects to investigate and possibly mitigate ethanol-induced stress in growing and non-growing C. thermocellum cultures.

RESULTS

To separate chaotropic from thermodynamic effects of ethanol toxicity, a non-ethanol producing strain AVM062 (P_{clo1313_2638}::ldh* ∆adhE) was constructed by deleting the bifunctional acetaldehyde/alcohol dehydrogenase gene, adhE, in a lactate-overproducing strain. Exogenously added ethanol lowered the growth rate of both wild-type and the non-ethanol producing mutant. The mutant strain grew quicker than the wild-type at 50 and 55 °C for ethanol concentrations ≥ 10 g L^-1 and was able to reach higher maximum OD₆₀₀ at all ethanol concentrations and temperatures. For the wild-type, the maximum OD₆₀₀ and relative growth rates were higher at 45 and 50 °C, compared to 55 °C, for ethanol concentrations ≥ 15 g L^-1. For the mutant strain, no positive effect on growth was observed at lower temperatures. Growth-arrested cells of the wild-type demonstrated improved fermentative capacity over time in the presence of ethanol concentrations up to 40 g L^-1 at 45 and 50 °C compared to 55 °C.

CONCLUSION

Positive effects of temperature on ethanol tolerance were limited to wild-type C. thermocellum and are likely related to mechanisms involved in the ethanol-formation pathway and redox cofactor balancing. Lowering the cultivation temperature provides an attractive strategy to improve growth and fermentative capacity at high ethanol titres in high-cellulose loading batch cultivations. Finally, non-ethanol producing strains are useful platform strains to study the effects of chaotropicity and thermodynamics related to ethanol toxicity and allow for deeper understanding of growth and/or fermentation cessation under industrially relevant conditions.

Characterization of a novel thermophilic metagenomic GH5 endoglucanase heterologously expressed in Escherichia coli and Saccharomyces cerevisiae

Background

Endoglucanases from thermophilic microorganisms are a valuable resource as they can be used in a wide variety of biotechnological applications including the valorisation of biomass and the production of biofuels. In the present work we analysed the metagenome from the hot spring Muiño da Veiga, located in the northwest of Spain (in the Galicia region), in search for novel thermostable endoglucanases.

Results

Sequence analysis of the metagenome revealed a promising enzyme (Cel776). Predictions on protein structure and conserved amino acid sequences were conducted, as well as expression in heterologous systems with Escherichia coli and Saccharomyces cerevisiae as the host. Cel776Ec was correctly expressed and purified by taking advantage of the His-Tag system, with a yield of 0.346 U/mL in the eluted fraction. Cel776Sc was expressed extracellulary and was easily recovered from the supernatant without the need of further purification, requiring only a concentration step by ultrafiltration, with a significantly higher yield of 531.95 U/mL, revealing a much more suitable system for production of large amounts of the enzyme. Their biochemical characterization revealed biotechnologically interesting enzymes. Both Cel776Ec and Cel776Sc had an optimal temperature of 80 °C and optimal pH of 5. Cel776Ec exhibited high thermostability maintaining its activity for 24 h at 60 °C and maintained its activity longer than Cel776Sc at increasing incubation temperatures. Moreover, its substrate specificity allowed the degradation of both cellulose and xylan. Whereas Cel776Ec was more active in the presence of calcium and magnesium, manganese was found to increase Cel776Sc activity. A stronger inhibitory effect was found for Cel776Ec than Cel776Sc adding detergent SDS to the reaction mix, whereas EDTA only significantly affected Cel776Sc activity.

Conclusions

Our study reports the discovery of a new promising biocatalyst for its application in processes, such as the production of biofuel and the saccharification of plant biomass, due to its bifunctional enzymatic activity as an endoglucanase and as a xylanase, as well as highlights the advantages of a yeast expression system over bacteria.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13068-022-02172-4.

Functional analysis of H+-pumping membrane-bound pyrophosphatase, ADP-glucose synthase, and pyruvate phosphate dikinase as pyrophosphate sources in Clostridium thermocellum

The atypical glycolysis of Clostridium thermocellum is characterized by the use of pyrophosphate (PP_i) as phosphoryl donor for phosphofructokinase (Pfk) and pyruvate phosphate dikinase (Ppdk) reactions. Previously, biosynthetic PP_i was calculated to be stoichiometrically insufficient to drive glycolysis. This study investigates the role of a H⁺-pumping membrane-bound pyrophosphatase, glycogen cycling, a predicted Ppdk-malate shunt cycle and acetate cycling in generating PP_i. Knockout studies and enzyme assays confirmed that clo1313_0823 encodes a membrane-bound pyrophosphatase. Additionally, clo1313_0717-0718 was confirmed to encode ADP-glucose synthase by knockouts, glycogen measurements in C. thermocellum and heterologous expression in E. coli. Unexpectedly, individually-targeted gene deletions of the four putative PP_i sources did not have a significant phenotypic effect. Although combinatorial deletion of all four putative PP_i sources reduced the growth rate by 22% (0.30±0.01 h^-1) and the biomass yield by 38% (0.18±0.00 g_biomass g_substrate^-1), this change was much smaller than what would be expected for stoichiometrically essential PP_i-supplying mechanisms. Growth-arrested cells of the quadruple knockout readily fermented cellobiose indicating that the unknown PP_i-supplying mechanisms are independent of biosynthesis. An alternative hypothesis that ATP-dependent Pfk activity circumvents a need for PP_i altogether, was falsified by enzyme assays, heterologous expression of candidate genes and whole-genome sequencing. As a secondary outcome, enzymatic assays confirmed functional annotation of clo1313_1832 as ATP- and GTP-dependent fructokinase. These results indicate that the four investigated PP_i sources individually and combined play no significant PP_i-supplying role and the true source(s) of PP_i, or alternative phosphorylating mechanisms, that drive glycolysis in C. thermocellum remain(s) elusive. IMPORTANCE Increased understanding of the central metabolism of C. thermocellum is important from a fundamental as well as from a sustainability and industrial perspective. In addition to showing that H⁺-pumping membrane-bound PPase, glycogen cycling, a Ppdk-malate shunt cycle, and acetate cycling are not significant sources of PP_i supply, this study adds functional annotation of four genes and availability of an updated PP_i stoichiometry from biosynthesis to the scientific domain. Together, this aids future metabolic engineering attempts aimed to improve C. thermocellum as a cell factory for sustainable and efficient production of ethanol from lignocellulosic material through consolidated bioprocessing with minimal pretreatment. Getting closer to elucidating the elusive source of PP_i, or alternative phosphorylating mechanisms, for the atypical glycolysis is itself of fundamental importance. Additionally, the findings of this study directly contribute to investigations into trade-offs between thermodynamic driving force versus energy yield of PP_i- and ATP-dependent glycolysis.

Genome-Wide Transcription Factor DNA Binding Sites and Gene Regulatory Networks in Clostridium thermocellum

Clostridium thermocellum is a thermophilic bacterium recognized for its natural ability to effectively deconstruct cellulosic biomass. While there is a large body of studies on the genetic engineering of this bacterium and its physiology to-date, there is limited knowledge in the transcriptional regulation in this organism and thermophilic bacteria in general. The study herein is the first report of a large-scale application of DNA-affinity purification sequencing (DAP-seq) to transcription factors (TFs) from a bacterium. We applied DAP-seq to > 90 TFs in C. thermocellum and detected genome-wide binding sites for 11 of them. We then compiled and aligned DNA binding sequences from these TFs to deduce the primary DNA-binding sequence motifs for each TF. These binding motifs are further validated with electrophoretic mobility shift assay (EMSA) and are used to identify individual TFs’ regulatory targets in C. thermocellum. Our results led to the discovery of novel, uncharacterized TFs as well as homologues of previously studied TFs including RexA-, LexA-, and LacI-type TFs. We then used these data to reconstruct gene regulatory networks for the 11 TFs individually, which resulted in a global network encompassing the TFs with some interconnections. As gene regulation governs and constrains how bacteria behave, our findings shed light on the roles of TFs delineated by their regulons, and potentially provides a means to enable rational, advanced genetic engineering of C. thermocellum and other organisms alike toward a desired phenotype.

A New Subfamily of Glycoside Hydrolase Family 30 with Strict Xylobiohydrolase Function

The Acetivibrio clariflavus (basonym: Clostridium clariflavum) glycoside hydrolase family 30 cellulosomal protein encoded by the Clocl_1795 gene was highly represented during growth on cellulosic substrates. In this report, the recombinantly expressed protein has been characterized and shown to be a non-reducing terminal (NRT)-specific xylobiohydrolase (AcXbh30A). Biochemical function, optimal biophysical parameters, and phylogeny were investigated. The findings indicate that AcXbh30A strictly cleaves xylobiose from the NRT up until an α-1,2-linked glucuronic acid (GA)-decorated xylose if the number of xyloses is even or otherwise a single xylose will remain resulting in a penultimate GA-substituted xylose. Unlike recently reported xylobiohydrolases, AcXbh30A has no other detectable hydrolysis products under our optimized reaction conditions. Sequence analysis indicates that AcXbh30A represents a new GH30 subfamily. This new xylobiohydrolase may be useful for commercial production of industrial quantities of xylobiose.

Adaptability of a caproate-producing bacterium contributes to its dominance in an anaerobic fermentation system

The transformation of diverse feedstocks into medium-chain fatty acids (MCFAs) by mixed cultures is a promising biorefinery route because of the high value of MCFAs. A particular concern is how to maintain the microbial consortia in mixed cultures to achieve stable MCFA production. Chinese strong aroma-type liquor (Baijiu) fermentation system continually produces caproic acid for decades through a spontaneous inoculation of anaerobes from pit mud into fermented grains. Therefore, illuminating the dominant caproate-producing bacterium (CPB) in pit mud and how the CPB sustains in the spontaneous fermentation system will benefit to reveal the microbiological mechanisms of the stable caproate production. Here, we examined pit mud samples across four Chinese strong aroma-type Baijiu producing areas and found that a caproate-producing Caproicibacterium sp. was widely distributed in these distilleries with relative abundance ranging from 1.4% to 35.5% and an average abundance of 11.4%. Through controlling carbon source availability, we achieved different simplified caproate-producing consortia and found that the growth advantage of Caproicibacterium sp. was highly dependent on glucose. Then two strains, named Caproicibacterium sp. LBM19010 and Caproicibacterium sp. JNU-WLY1368, were isolated from pit mud of two regions. The metabolic versatility of this bacterium utilizing starch, maltose, glucose and lactate reflected its adaptability to the fermentation environment where these carbon sources coexist. The simultaneous utilization of glucose and lactate contributed to the balance between cell growth and pH homeostasis. This study reveals that multiple adaptation strategies employed by the predominant CPB promotes its stability and dominance in a saccharide- and lactate-rich anaerobic habitat. IMPORTANCE Chinese strong aroma-type liquor (Baijiu) fermentation environment is a typical medium-chain fatty acid producing system with complex nutrients. Although several studies have revealed the correlation between microbial community composition and abiotic factors, the adaptation mechanisms of dominant species to abiotic environment are still unknown in this special anaerobic habitat. This study identified the predominant CPB in Chinese strong aroma-type Baijiu fermentation system. Metabolic versatility and flexibility of the dominant CPB with a small-size genome indicated that this bacterium can effectively exploit available carbon and nitrogen sources, which could be a key factor to promote its ecological success in a multi-species environment. The understanding of growth and metabolic features of CPB responsible for its dominance in microbial community will not only contribute to the improvement of Chinese strong aroma-type Baijiu production but also expand its potential industrial applications in caproate production.

Thermophilic microbial deconstruction and conversion of natural and transgenic lignocellulose

The potential to convert renewable plant biomasses into fuels and chemicals by microbial processes presents an attractive, less environmentally intense alternative to conventional routes based on fossil fuels. This would best be done with microbes that natively deconstruct lignocellulose and concomitantly form industrially relevant products, but these two physiological and metabolic features are rarely and simultaneously observed in nature. Genetic modification of both plant feedstocks and microbes can be used to increase lignocellulose deconstruction capability and generate industrially relevant products. Separate efforts on plants and microbes are ongoing, but these studies lack a focus on optimal, complementary combinations of these disparate biological systems to obtain a convergent technology. Improving genetic tools for plants have given rise to the generation of low-lignin lines that are more readily solubilized by microorganisms. Most focus on the microbiological front has involved thermophilic bacteria from the genera Caldicellulosiruptor and Clostridium, given their capacity to degrade lignocellulose and to form bio-products through metabolic engineering strategies enabled by ever-improving molecular genetics tools. Bioengineering plant properties to better fit the deconstruction capabilities of candidate consolidated bioprocessing microorganisms has potential to achieve the efficient lignocellulose deconstruction needed for industrial relevance.

Identification and characterization of proteins of unknown function (PUFs) in Clostridium thermocellum DSM 1313 strains as potential genetic engineering targets

BACKGROUND

Mass spectrometry-based proteomics can identify and quantify thousands of proteins from individual microbial species, but a significant percentage of these proteins are unannotated and hence classified as proteins of unknown function (PUFs). Due to the difficulty in extracting meaningful metabolic information, PUFs are often overlooked or discarded during data analysis, even though they might be critically important in functional activities, in particular for metabolic engineering research.

RESULTS

We optimized and employed a pipeline integrating various "guilt-by-association" (GBA) metrics, including differential expression and co-expression analyses of high-throughput mass spectrometry proteome data and phylogenetic coevolution analysis, and sequence homology-based approaches to determine putative functions for PUFs in Clostridium thermocellum. Our various analyses provided putative functional information for over 95% of the PUFs detected by mass spectrometry in a wild-type and/or an engineered strain of C. thermocellum. In particular, we validated a predicted acyltransferase PUF (WP_003519433.1) with functional activity towards 2-phenylethyl alcohol, consistent with our GBA and sequence homology-based predictions.

CONCLUSIONS

This work demonstrates the value of leveraging sequence homology-based annotations with empirical evidence based on the concept of GBA to broadly predict putative functions for PUFs, opening avenues to further interrogation via targeted experiments.

Caproicibacterium amylolyticum gen. nov., sp. nov., a novel member of the family Oscillospiraceae isolated from pit clay used for making Chinese strong aroma-type liquor

An anaerobic, Gram-stain-positive, rod-shaped, motile and spore-forming bacterium, designated strain LBM18003^T, was isolated from pit clay used for making Chinese strong aroma-type liquor. Growth occurred at 20-40 °C (optimum, 30-37 °C), pH 4.5-9.5 (optimum, pH 6.5-7.0) and in the presence of 0.0-1.0 % (w/v) sodium chloride (optimum, 0 %). The predominant fatty acids were C_16:0, C_14:0, C_14:0 DMA and C_16:0 3-OH, and the major polar lipids were diphosphatidylglycerol, phosphatidylglycerol, two unidentified phospholipids and nine unidentified glycolipids. Phylogenetic analysis revealed that strain LBM18003^T is a novel member of the family Oscillospiraceae. The 16S rRNA gene sequence similarities of strain LBM18003^T to its two most closely related species were less than 94.5 % for distinguishing genera, i.e. closely related to Caproiciproducens galactitolivorans JCM 30532^T (94.1 %) and Caproicibacter fermentans DSM 107079^T (93.2 %). The genome size of strain LBM18003^T was 2 996 201 bp and its DNA G+C content was 48.48 mol%. Strain LBM18003^T exhibited 67.8 and 68.1% pairwise-determined whole-genome average nucleotide identity values to Caproiciproducens galactitolivorans JCM 30532^T and Caproicibacter fermentans DSM 107079^T, respectively; and showed 62.2 and 61.0 % the average amino acid identity values to Caproiciproducens galactitolivorans JCM 30532^T and Caproicibacter fermentans DSM 107079^T, respectively; and demonstrated 46.1 and 41.5 % conserved genes to Caproiciproducens galactitolivorans JCM 30532^T and Caproicibacter fermentans DSM 107079^T, respectively. The comparisons of 16S rRNA gene and genome sequences confirmed that strain LBM18003^T represented a novel genus of the family Oscillospiraceae. Based on morphological, physiological, biochemical, chemotaxonomic, genotypic and phylogenetic results, strain LBM18003^T represents a novel species of a novel genus of the family Oscillospiraceae, for which the name Caproicibacterium amylolyticum gen. nov., sp. nov. is proposed. The type strain is LBM18003^T (=GDMCC 1.1626^T=JCM 33783^T).

Laboratory Evolution and Reverse Engineering of Clostridium thermocellum for Growth on Glucose and Fructose

The native ability of Clostridium thermocellum to efficiently solubilize cellulose makes it an interesting platform for sustainable biofuel production through consolidated bioprocessing. Together with other improvements, industrial implementation of C. thermocellum, as well as fundamental studies into its metabolism, would benefit from improved and reproducible consumption of hexose sugars. To investigate growth of C. thermocellum on glucose or fructose, as well as the underlying molecular mechanisms, laboratory evolution was performed in carbon-limited chemostats with increasing concentrations of glucose or fructose and decreasing cellobiose concentrations. Growth on both glucose and fructose was achieved with biomass yields of 0.09 ± 0.00 and 0.18 ± 0.00 gbiomass gsubstrate-1, respectively, compared to 0.15 ± 0.01 gbiomass gsubstrate-1 for wild type on cellobiose. Single-colony isolates had no or short lag times on the monosaccharides, while wild type showed 42 ± 4 h on glucose and >80 h on fructose. With good growth on glucose, fructose, and cellobiose, the fructose isolates were chosen for genome sequence-based reverse metabolic engineering. Deletion of a putative transcriptional regulator (Clo1313_1831), which upregulated fructokinase activity, reduced lag time on fructose to 12 h with a growth rate of 0.11 ± 0.01 h-1 and resulted in immediate growth on glucose at 0.24 ± 0.01 h-1 Additional introduction of a G-to-V mutation at position 148 in cbpA resulted in immediate growth on fructose at 0.32 ± 0.03 h-1 These insights can guide engineering of strains for fundamental studies into transport and the upper glycolysis, as well as maximizing product yields in industrial settings.IMPORTANCEC. thermocellum is an important candidate for sustainable and cost-effective production of bioethanol through consolidated bioprocessing. In addition to unsurpassed cellulose deconstruction, industrial application and fundamental studies would benefit from improvement of glucose and fructose consumption. This study demonstrated that C. thermocellum can be evolved for reproducible constitutive growth on glucose or fructose. Subsequent genome sequencing, gene editing, and physiological characterization identified two underlying mutations with a role in (regulation of) transport or metabolism of the hexose sugars. In light of these findings, such mutations have likely (and unknowingly) also occurred in previous studies with C. thermocellum using hexose-based media with possible broad regulatory consequences. By targeted modification of these genes, industrial and research strains of C. thermocellum can be engineered to (i) reduce glucose accumulation, (ii) study cellodextrin transport systems in vivo, (iii) allow experiments at >120 g liter-1 soluble substrate concentration, or (iv) reduce costs for labeling studies.

Metabolome Analysis of Constituents in Membrane Vesicles for Clostridium thermocellum Growth Stimulation

The cultivation of the cellulolytic bacterium, Clostridium thermocellum, can have cost-effective cellulosic biomass utilizations, such as consolidated bioprocessing, simultaneous biological enzyme production and saccharification. However, these processes require a longer cultivation term of approximately 1 week. We demonstrate that constituents of the C. thermocellum membrane vesicle fraction significantly promoted the growth rate of C. thermocellum. Similarly, cell-free Bacillus subtilis broth was able to increase C. thermocellum growth rate, while several B. subtilis single-gene deletion mutants, e.g., yxeJ, yxeH, ahpC, yxdK, iolF, decreased the growth stimulation ability. Metabolome analysis revealed signal compounds for cell–cell communication in the C. thermocellum membrane vesicle fraction (ethyl 2-decenoate, ethyl 4-decenoate, and 2-dodecenoic acid) and B. subtilis broth (nicotinamide, indole-3-carboxaldehyde, urocanic acid, nopaline, and 6-paradol). These findings suggest that the constituents in membrane vesicles from C. thermocellum and B. subtilis could promote C. thermocellum growth, leading to improved efficiency of cellulosic biomass utilization.

Ruminiclostridium herbifermentans sp. nov., a mesophilic and moderately thermophilic cellulolytic and xylanolytic bacterium isolated from a lab-scale biogas fermenter fed with maize silage

An anaerobic bacterial strain, designated MA18^T, was isolated from a laboratory-scale biogas fermenter fed with maize silage. Cells stained Gram-negative and performed Gram-negative in the KOH test. The peptidoglycan type was found to be A1y-meso-Dpm direct. The major cellular fatty acids were C_14 : 0 iso, C_15 : 0 iso, anteiso and iso DMA as well as a C₁₆ unidentified fatty acid. Oxidase and catalase activities were absent. Cells were slightly curved rods, motile, formed spores and measured approximately 0.35 µm in diameter and 3.0-5.0 µm in length. When cultivated on GS2 agar with cellobiose, round, arched, shiny and slightly yellow-pigmented colonies were formed. The isolate was mesophilic to moderately thermophilic with a growth optimum between 40 and 48 °C. Furthermore, neutral pH values were preferred and up to 1.2 % (w/v) NaCl supplemented to the GS2 medium was tolerated. Producing mainly acetate and ethanol, MA18^T fermented arabinose, cellobiose, crystalline and amorphous cellulose, ribose, and xylan. The genome of MA18^T consists of 4 817 678 bp with a G+C content of 33.16 mol%. In the annotated protein sequences, cellulosomal components were detected. Phylogenetically, MA18^T is most closely related to Ruminiclostridium sufflavum DSM 19573^T (76.88 % average nucleotide identity of the whole genome sequence; 97.23 % 16S rRNA gene sequence similarity) and can be clustered into one clade with other species of the genus Ruminiclostridium, family Oscillospiraceae, class Clostridia. Based on morphological, physiological and genetic characteristics, this strain represents a novel species in the genus Ruminiclostridium. Therefore, the name Ruminiclostridium herbifermentans sp. nov. is proposed. The type strain is MA18^T (=DSM 109966^T=JCM 39124^T).

Cross-feeding and wheat straw extractives enhance growth of Clostridium thermocellum-containing co-cultures for consolidated bioprocessing

Co-cultures consisting of three thermophilic and lignocellulolytic bacteria, namely Clostridium thermocellum, C. stercorarium, and Thermoanaerobacter thermohydrosulfuricus, degrade lignocellulosic material in a synergistic manner. When cultured in a defined minimal medium two of the members appeared to be auxotrophic and unable to grow, but the growth of all species was observed in all co-culture combinations, indicating cross-feeding of unidentified growth factors between the members. Growth factors also appeared to be present in water-soluble extractives obtained from wheat straw, allowing for the growth of the auxotrophic monocultures in the defined minimal medium. Cell enumeration during growth on wheat straw in this medium revealed different growth profiles of the members that varied between the co-cultures. End-product profiles also varied substantially between the cultures, with significantly higher ethanol production in all co-cultures compared to the mono-cultures. Understanding interactions between co-culture members, and the additional nutrients provided by lignocellulosic substrates, will aid us in consolidated bioprocessing design.

Nitrogen-fixing Ability and Nitrogen Fixation-related Genes of Thermophilic Fermentative Bacteria in the Genus Caldicellulosiruptor

Fermentative nitrogen-fixing bacteria have not yet been examined in detail in thermal environments. In the present study, we isolated the thermophilic fermentative bacterium, strain YA01 from a hot spring. This strain grew at temperatures up to 78°C. A phylogenetic analysis based on its 16S rRNA gene sequence indicated that strain YA01 belonged to the genus Caldicellulosiruptor, which are fermentative bacteria in the phylum Firmicutes, with 97.7–98.0% sequence identity to its closest relatives. Strain YA01 clearly exhibited N₂-dependent growth at 70°C. We also confirmed N₂-dependent growth in the relatives of strain YA01, Caldicellulosiruptor hydrothermalis 108 and Caldicellulosiruptor kronotskyensis 2002. The nitrogenase activities of these three strains were examined using the acetylene reduction assay. Similar activities were detected for all tested strains, and were slightly suppressed by the addition of ammonium. A genome analysis revealed that strain YA01, as well as other Caldicellulosiruptor, possessed a gene set for nitrogen fixation, but lacked the nifN gene, which encodes a nitrogenase iron-molybdenum cofactor biosynthesis protein that is commonly detected in nitrogen-fixing bacteria. The amino acid sequences of nitrogenase encoded by nifH, nifD, and nifK shared 92–98% similarity in Caldicellulosiruptor. A phylogenetic tree of concatenated NifHDK sequences showed that NifHDK of Caldicellulosiruptor was in the deepest clade. To the best of our knowledge, this is the first study to demonstrate the nitrogen-fixing ability of fermentative bacteria at 70°C. Caldicellulosiruptor may have retained an ancient nitrogen-fixing enzyme system.

Metabolic Fluxes of Nitrogen and Pyrophosphate in Chemostat Cultures of Clostridium thermocellum and Thermoanaerobacterium saccharolyticum

Clostridium thermocellum and Thermoanaerobacterium saccharolyticum were grown in cellobiose-limited chemostat cultures at a fixed dilution rate. C. thermocellum produced acetate, ethanol, formate, and lactate. Surprisingly, and in contrast to batch cultures, in cellobiose-limited chemostat cultures of T. saccharolyticum, ethanol was the main fermentation product. Enzyme assays confirmed that in C. thermocellum, glycolysis proceeds via pyrophosphate (PP_i)-dependent phosphofructokinase (PFK), pyruvate-phosphate dikinase (PPDK), as well as a malate shunt for the conversion of phosphoenolpyruvate (PEP) to pyruvate. Pyruvate kinase activity was not detectable. In T. saccharolyticum, ATP but not PP_i served as cofactor for the PFK reaction. High activities of both pyruvate kinase and PPDK were present, whereas the activities of a malate shunt enzymes were low in T. saccharolyticum In C. thermocellum, glycolysis via PP_i-PFK and PPDK obeys the equation glucose + 5 NDP + 3 PP_i → 2 pyruvate + 5 NTP + P_i (where NDP is nucleoside diphosphate and NTP is nucleoside triphosphate). Metabolic flux analysis of chemostat data with the wild type and a deletion mutant of the proton-pumping pyrophosphatase showed that a PP_i-generating mechanism must be present that operates according to ATP + P_i → ADP + PP_i Both organisms also produced significant amounts of amino acids in cellobiose-limited cultures. It was anticipated that this phenomenon would be suppressed by growth under nitrogen limitation. Surprisingly, nitrogen-limited chemostat cultivation of wild-type C. thermocellum revealed a bottleneck in pyruvate oxidation, as large amounts of pyruvate and amino acids, mainly valine, were excreted; up to 50% of the nitrogen consumed was excreted again as amino acids.IMPORTANCE This study discusses the fate of pyrophosphate in the metabolism of two thermophilic anaerobes that lack a soluble irreversible pyrophosphatase as present in Escherichia coli but instead use a reversible membrane-bound proton-pumping enzyme. In such organisms, the charging of tRNA with amino acids may become more reversible. This may contribute to the observed excretion of amino acids during sugar fermentation by Clostridium thermocellum and Thermoanaerobacterium saccharolyticum Calculation of the energetic advantage of reversible pyrophosphate-dependent glycolysis, as occurs in Clostridium thermocellum, could not be properly evaluated, as currently available genome-scale models neglect the anabolic generation of pyrophosphate in, for example, polymerization of amino acids to protein. This anabolic pyrophosphate replaces ATP and thus saves energy. Its amount is, however, too small to cover the pyrophosphate requirement of sugar catabolism in glycolysis. Consequently, pyrophosphate for catabolism is generated according to ATP + P_i → ADP + PP_i.

Caproicibacter fermentans gen. nov., sp. nov., a new caproate-producing bacterium and emended description of the genus Caproiciproducens

A strictly anaerobic bacterial strain designated EA1T was isolated from an enrichment culture inoculated with biogas reactor content. Cells of strain EA1T are spore-forming rods (1–3×0.4–0.8 µm) and stain Gram-negative, albeit they possess a Gram-positive type of cell-wall ultrastructure. Growth of strain EA1T was observed at 30 and 37 °C and within a pH range of pH 5–9. The major components recovered in the fatty acid fraction were C14:0, C16:0, C16:0 DMA (dimethyl acetal) and C16:1 ω7c. Strain EA1T fermented several mono- and disaccharides. Metabolic end products from fructose were acetate, butyrate, caproate and lactate. Furthermore, ethanol, CO2 and H2 were identified as products. The genome consists of a chromosome (3.9 Mbp) with 3797 predicted protein-encoding genes and a G+C content of 51.25 mol%. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strain EA1T represents a novel taxon within the family Oscillospiraceae . The most closely related type strains of EA1T, based on 16S rRNA gene sequence identity, are Caproiciproducens galactitolivorans BS-1T (94.9 %), [ Clostridium ] leptum DSM 753T (93.8 %), [ Clostridium ] sporosphaeroides DSM 1294T (91.7 %) and Ruminococcus bromii ATCC 27255T (91.0 %). Further phenotypic characteristics of strain EA1T differentiate it from related, validly described bacterial species. Strain EA1T represents a novel genus and novel species within the family Oscillospiraceae . The proposed name is Caproicibacter fermentans gen. nov., sp. nov. The type strain is EA1T (DSM 107079T=JCM 33110T).

The pentose phosphate pathway of cellulolytic clostridia relies on 6-phosphofructokinase instead of transaldolase

The genomes of most cellulolytic clostridia do not contain genes annotated as transaldolase. Therefore, for assimilating pentose sugars or for generating C5 precursors (such as ribose) during growth on other (non-C5) substrates, they must possess a pathway that connects pentose metabolism with the rest of metabolism. Here we provide evidence that for this connection cellulolytic clostridia rely on the sedoheptulose 1,7-bisphosphate (SBP) pathway, using pyrophosphate-dependent phosphofructokinase (PPi-PFK) instead of transaldolase. In this reversible pathway, PFK converts sedoheptulose 7-phosphate (S7P) to SBP, after which fructose-bisphosphate aldolase cleaves SBP into dihydroxyacetone phosphate and erythrose 4-phosphate. We show that PPi-PFKs of Clostridium thermosuccinogenes and Clostridium thermocellum indeed can convert S7P to SBP, and have similar affinities for S7P and the canonical substrate fructose 6-phosphate (F6P). By contrast, (ATP-dependent) PfkA of Escherichia coli, which does rely on transaldolase, had a very poor affinity for S7P. This indicates that the PPi-PFK of cellulolytic clostridia has evolved the use of S7P. We further show that C. thermosuccinogenes contains a significant SBP pool, an unusual metabolite that is elevated during growth on xylose, demonstrating its relevance for pentose assimilation. Last, we demonstrate that a second PFK of C. thermosuccinogenes that operates with ATP and GTP exhibits unusual kinetics toward F6P, as it appears to have an extremely high degree of cooperative binding, resulting in a virtual on/off switch for substrate concentrations near its K½ value. In summary, our results confirm the existence of an SBP pathway for pentose assimilation in cellulolytic clostridia.