Sporulation of Clostridium acetobutylicum P262 in a Defined Medium

Spore-forming properties and enhanced oxygen tolerance of butyrate-producing Anaerostipes spp

OBJECTIVES

Butyrate producing bacteria are promising candidates for next-generation probiotics. However, they are extremely sensitive to oxygen, which is a significant obstacle to their inclusion in food matrices in a viable form. The present study characterized the spore-forming properties and stress tolerance of human gut butyrate-producing Anaerostipes spp.

METHODS

Spore formation properties in six species of Anaerostipes spp. were studied by in vitro and in silico tests.

RESULTS

Spores were observed from the cells of three species using microscopic analyses, while the remaining three did not form spores under the tested conditions. Spore-forming properties were confirmed by an ethanol treatment. The spores of Anaerostipes caccae were tolerant to oxygen and survived for 15 weeks under atmospheric conditions. Spores tolerated heat stress at 70 °C, but not at 80 °C. An in silico analysis of the conservation of potential sporulation signature genes revealed that the majority of human gut butyrate-producing bacteria were classified as potential spore formers. Comparative genomics revealed that three spore-forming Anaerostipes spp. specifically possessed the spore formation-related genes of bkdR, sodA, and splB, which may be key genes for different sporulation properties in Anaerostipes spp.

CONCLUSIONS

The present study demonstrated the enhanced stress tolerance of butyrate producing Anaerostipes spp. for future probiotic application. Presence of specific gene(s) are possibly keys for sporulation in Anaerostipes spp.

Kinetic model of Clostridium beijerinckii's Acetone-Butanol-Ethanol fermentation considering metabolically diverse cell types

Clostridium beijerinckii population branches into metabolically diverse cell types in batch cultures. Here, we present a new kinetic model of C. beijerinckii's Acetone-Butanol-Ethanol fermentation that considers three cell types: producers of acids (acidogenic), consumer of acids and producers of solvents (solventogenic), and spores cells. The model accurately recapitulates batch culture data. Also, the model estimates cell type-specific kinetic parameters, which can be helpful to improve the operation of the ABE fermentation and give a framework to study acidogenic and solventogenic metabolic pathways. To exemplify the latter, we used a constraint-based model to study how the ABE pathways are used among acidogenic and solventogenic cell types. We found that among both cell types, glycolytic production of ATP and consumption of NAD⁺ varies widely during the fermentation, with their maximum production/consumption rates happening when acidogenic and solventogenic growth rates were at their highest. However, acidogenic cells use the ABE pathway to contribute with an extra 12.5% of the total production of ATP, whereas solventogenic cell types use the ABE pathway to supply more than 75% of the demand for NAD⁺, alternating between the production of lactate and butyrate, being both coupled to the production of NAD⁺.

Sporulation in solventogenic and acetogenic clostridia

The Clostridium genus harbors compelling organisms for biotechnological production processes; while acetogenic clostridia can fix C1-compounds to produce acetate and ethanol, solventogenic clostridia can utilize a wide range of carbon sources to produce commercially valuable carboxylic acids, alcohols, and ketones by fermentation. Despite their potential, the conversion by these bacteria of carbohydrates or C1 compounds to alcohols is not cost-effective enough to result in economically viable processes. Engineering solventogenic clostridia by impairing sporulation is one of the investigated approaches to improve solvent productivity. Sporulation is a cell differentiation process triggered in bacteria in response to exposure to environmental stressors. The generated spores are metabolically inactive but resistant to harsh conditions (UV, chemicals, heat, oxygen). In Firmicutes, sporulation has been mainly studied in bacilli and pathogenic clostridia, and our knowledge of sporulation in solvent-producing or acetogenic clostridia is limited. Still, sporulation is an integral part of the cellular physiology of clostridia; thus, understanding the regulation of sporulation and its connection to solvent production may give clues to improve the performance of solventogenic clostridia. This review aims to provide an overview of the triggers, characteristics, and regulatory mechanism of sporulation in solventogenic clostridia. Those are further compared to the current knowledge on sporulation in the industrially relevant acetogenic clostridia. Finally, the potential applications of spores for process improvement are discussed.Key Points• The regulatory network governing sporulation initiation varies in solventogenic clostridia.• Media composition and cell density are the main triggers of sporulation.• Spores can be used to improve the fermentation process.

Transcriptional analysis of amino acid, metal ion, vitamin and carbohydrate uptake in butanol-producing Clostridium beijerinckii NRRL B-598

In-depth knowledge of cell metabolism and nutrient uptake mechanisms can lead to the development of a tool for improving acetone-butanol-ethanol (ABE) fermentation performance and help to overcome bottlenecks in the process, such as the high cost of substrates and low production rates. Over 300 genes potentially encoding transport of amino acids, metal ions, vitamins and carbohydrates were identified in the genome of the butanol-producing strain Clostridium beijerinckii NRRL B-598, based on similarity searches in protein function databases. Transcriptomic data of the genes were obtained during ABE fermentation by RNA-Seq experiments and covered acidogenesis, solventogenesis and sporulation. The physiological roles of the selected 81 actively expressed transport genes were established on the basis of their expression profiles at particular stages of ABE fermentation. This article describes how genes encoding the uptake of glucose, iron, riboflavin, glutamine, methionine and other nutrients take part in growth, production and stress responses of C. beijerinckii NRRL B-598. These data increase our knowledge of transport mechanisms in solventogenic Clostridium and may be used in the selection of individual genes for further research.

Comparison of expression of key sporulation, solventogenic and acetogenic genes in C. beijerinckii NRRL B-598 and its mutant strain overexpressing spo0A

The production of acetone, butanol and ethanol by fermentation of renewable biomass has potential to become a valuable industrial process. Mechanisms of solvent production and sporulation involve some common regulators in some ABE-producing clostridia, although details of the links between the pathways are not clear. In this study, we compare a wild-type (WT) Clostridium beijerinckii NRRL B-598 with its mutant strain OESpo0A, in which the gene encoding Spo0A, an important regulator of both sporulation and solventogenesis, is overexpressed in terms of solvent and acid production. We also compare morphologies during growth on two different media: TYA broth, where the WT culture sporulates, and RCM, where the WT culture does not. In addition, RT-qPCR-based analysis of expression profiles of spo0A, spoIIE, sigG, spoVD, ald and buk1 genes involved in sporulation or solvent production in these strains, were compared. The OESpo0A mutant did not produce spores and butanol titre was lower compared to the WT, but increased amounts of butyric acid and ethanol were produced. The gene spo0A had high levels of expression in the WT under non-sporulating culture conditions while other selected genes for sporulation factors were downregulated significantly. Similar observations were obtained for OESpo0A where spo0A overexpression and downregulation of other sporulation genes were demonstrated. Higher expression of spo0A led to higher expression of buk1 and ald, which could confirm the role of spo0A in activation of the solventogenic pathway, although solvent production was not affected significantly in the WT and was weakened in the OESpo0A mutant.

Elucidating and reprogramming Escherichia coli metabolisms for obligate anaerobic n-butanol and isobutanol production

Elementary mode (EM) analysis based on the constraint-based metabolic network modeling was applied to elucidate and compare complex fermentative metabolisms of Escherichia coli for obligate anaerobic production of n-butanol and isobutanol. The result shows that the n-butanol fermentative metabolism was NADH-deficient, while the isobutanol fermentative metabolism was NADH redundant. E. coli could grow and produce n-butanol anaerobically as the sole fermentative product but not achieve the maximum theoretical n-butanol yield. In contrast, for the isobutanol fermentative metabolism, E. coli was required to couple with either ethanol- or succinate-producing pathway to recycle NADH. To overcome these "defective" metabolisms, EM analysis was implemented to reprogram the native fermentative metabolism of E. coli for optimized anaerobic production of n-butanol and isobutanol through multiple gene deletion (~8-9 genes), addition (~6-7 genes), up- and downexpression (~6-7 genes), and cofactor engineering (e.g., NADH, NADPH). The designed strains were forced to couple both growth and anaerobic production of n-butanol and isobutanol, which is a useful characteristic to enhance biofuel production and tolerance through metabolic pathway evolution. Even though the n-butanol and isobutanol fermentative metabolisms were quite different, the designed strains could be engineered to have identical metabolic flux distribution in "core" metabolic pathways mainly supporting cell growth and maintenance. Finally, the model prediction in elucidating and reprogramming the native fermentative metabolism of E. coli for obligate anaerobic production of n-butanol and isobutanol was validated with published experimental data.

SpoIIE is necessary for asymmetric division, sporulation, and expression of sigmaF, sigmaE, and sigmaG but does not control solvent production in Clostridium acetobutylicum ATCC 824

In order to better characterize the initial stages of sporulation past Spo0A activation and the associated solventogenesis in the important industrial and model organism Clostridium acetobutylicum, the spoIIE gene was successfully disrupted and its expression was silenced. By silencing spoIIE, sporulation was blocked prior to asymmetric division, and no mature spores or any distinguishable morphogenetic changes developed. Upon plasmid-based complementation of spoIIE, sporulation was restored, although the number of spores formed was below that of the plasmid control strain. To investigate the impact of silencing spoIIE on the regulation of sporulation, transcript levels of sigF, sigE, and sigG were examined by semiquantitative reverse transcription-PCR, and the corresponding σF, σE, and σG protein levels were determined by Western analysis. Expression of sigF was significantly reduced in the inactivation strain, and this resulted in very low σF protein levels. Expression of sigE was barely detected, and no sigG transcript was detected at all; consequently, no σE or σG proteins were detected. These data suggest an autostimulatory role for σF in C. acetobutylicum, in contrast to the model organism for endospore formation, Bacillus subtilis, and confirm that high-level expression of σF is required for expression of σE and σG. Unlike the σF and σE inactivation strains, the SpoIIE inactivation strain did not exhibit inoculum-dependent solvent formation and produced good levels of solvents from both exponential- and stationary-phase inocula. Thus, we concluded that SpoIIE does not control solvent formation.

Meta-analysis and functional validation of nutritional requirements of solventogenic Clostridia growing under butanol stress conditions and coutilization of D-glucose and D-xylose

Recent advances in systems biology, omics, and computational studies allow us to carry out data mining for improving biofuel production bioprocesses. Of particular interest are bioprocesses that center on microbial capabilities to biotransform both the hexose and pentose fractions present in crop residues. This called for a systematic exploration of the components of the media to obtain higher-density cultures and more-productive fermentation operations than are currently found. By using a meta-analysis approach of the transcriptional responses to butanol stress, we identified the nutritional requirements of solvent-tolerant strain Clostridium beijerinckii SA-1 (ATCC 35702). The nutritional requirements identified were later validated using the chemostat pulse-and-shift technique. C. beijerinckii SA-1 was cultivated in a two-stage single-feed-stream continuous production system to test the proposed validated medium formulation, and the coutilization of D-glucose and D-xylose was evaluated by taking advantage of the well-known ability of solventogenic clostridia to utilize a large variety of carbon sources such as mono-, oligo-, and polysaccharides containing pentose and hexose sugars. Our results indicated that C. beijerinckii SA-1 was able to coferment hexose/pentose sugar mixtures in the absence of a glucose repression effect. In addition, our analysis suggests that the solvent and acid resistance mechanisms found in this strain are differentially regulated compared to strain NRRL B-527 and are outlined as the basis of the analysis toward optimizing butanol production.

Inactivation of σF in Clostridium acetobutylicum ATCC 824 blocks sporulation prior to asymmetric division and abolishes σE and σG protein expression but does not block solvent formation

Clostridium acetobutylicum is both a model organism for the understanding of sporulation in solventogenic clostridia and its relationship to solvent formation and an industrial organism for anaerobic acetone-butanol-ethanol (ABE) fermentation. How solvent production is coupled to endospore formation--both stationary-phase events--remains incompletely understood at the molecular level. Specifically, it is unclear how sporulation-specific sigma factors affect solvent formation. Here the sigF gene in C. acetobutylicum was successfully disrupted and silenced. Not only σ(F) but also the sigma factors σ(E) and σ(G) were not detected in the sigF mutant (FKO1), and differentiation was stopped prior to asymmetric division. Since plasmid expression of the spoIIA operon (spoIIAA-spoIIAB-sigF) failed to complement FKO1, the operon was integrated into the FKO1 chromosome to generate strain FKO1-C. In FKO1-C, σ(F) expression was restored along with sporulation and σ(E) and σ(G) protein expression. Quantitative reverse transcription-PCR (RT-PCR) analysis of a select set of genes (csfB, gpr, spoIIP, sigG, lonB, and spoIIR) that could be controlled by σ(F), based on the Bacillus subtilis model, indicated that sigG may be under the control of σ(F), but spoIIR, an important activator of σ(E) in B. subtilis, is not, and neither are the rest of the genes investigated. FKO1 produced solvents at a level similar to that of the parent strain, but solvent levels were dependent on the physiological state of the inoculum. Finally, the complementation strain FKO1-C is the first reported instance of purposeful integration of multiple functional genes into a clostridial chromosome--here, the C. acetobutylicum chromosome--with the aim of altering cell metabolism and differentiation.

Selection of an Asporogenous Strain of Clostridium acetobutylicum in Continuous Culture Under Phosphate Limitation

Based on the observation that cells of Clostridium acetobutylicum unable to store granulose do not initiate sporulation, a staining procedure was developed for the detection of asporogenous mutants. By application of this procedure it was shown that an asporogenous strain of C. acetobutylicum was selected in continuous culture under phosphate limitation.

Characterization, Biosynthesis, and Regulation of Granulose in Clostridium acetobutylicum

Synthesis of granulose was investigated in 15 solvent-producing Clostridium strains. Only one of the strains did not produce granulose. The structure of granulose in Clostridium acetobutylicum P262 consisted of a high-molecular-weight polyglucan containing only (1-->4) linked d-glucopyranose units. Biosynthesis of granulose in C. acetobutylicum P262 was dependent on ADPglucose pyrophosphorylase, and granulose synthase and mutants defective in granulose accumulation lacked either one or both enzyme activities. Granulose-positive revertants exhibited both enzyme activities. ADPglucose pyrophosphorylase and granulose synthase were not subject to allosteric control by metabolites. Granulose accumulation and the biosynthetic enzyme activities were initiated immediately before the pH breakpoint and were detected in cells only at the end of the exponential growth phase. Granulose accumulation did not occur under conditions of nitrogen limitation, excess carbon, or excess energy.

Production and characterization of pure Clostridium spore suspensions

AIMS

A general protocol was derived for optimizing the production of pure, high concentration Clostridium endospore suspensions.

METHODS AND RESULTS

Two sporulation methods were developed that yielded high concentrations of notably pure Clostridium sporogenes, C. hungatei and C. GSA-1 (Greenland ice core isolate) spore suspensions (10 ml of 10(9) spores ml(-1) with >99% purity each). Each method was derived by evaluating combinations of three sporulation conditions, including freeze drying of inocula, heat shock treatment of cultures, and subsequent incubation at suboptimal temperatures that yielded the highest percentage of sporulation. Pure spore suspensions were characterized in terms of dipicolinic acid content, culturability, decimal reduction time (D) value for heat inactivation (100 degrees C) and hydrophobicity.

CONCLUSIONS

While some Clostridium species produce a high percentage of spores with heat shock treatment and suboptimal temperature incubation, other species require the additional step of freeze drying the inocula to achieve a high percentage of sporulation.

SIGNIFICANCE AND IMPACT OF THE STUDY

Pure Clostridium spore suspensions are required for investigating species of medical and environmental importance. Defining the conditions for optimal spore production also provides insight into the underlying mechanisms of Clostridium sporulation.

Development and application of flow-cytometric techniques for analyzing and sorting endospore-forming clostridia

The study of microbial heterogeneity at the single-cell level is a rapidly growing area of research in microbiology and biotechnology due to its significance in pathogenesis, environmental biology, and industrial biotechnologies. However, the tools available for efficiently and precisely probing such heterogeneity are limited for most bacteria. Here we describe the development and application of flow-cytometric (FC) and fluorescence-assisted cell-sorting techniques for the study of endospore-forming bacteria. We show that by combining FC light scattering (LS) with nucleic acid staining, we can discriminate, quantify, and enrich all sporulation-associated morphologies exhibited by the endospore-forming anaerobe Clostridium acetobutylicum. Using FC LS analysis, we quantitatively show that clostridial cultures commonly perform multiple rounds of sporulation and that sporulation is induced earlier by the overexpression of Spo0A, the master regulator of endospore formers. To further demonstrate the power of our approach, we employed FC LS analysis to generate compelling evidence to challenge the long-accepted view in the field that the clostridial cell form is the solvent-forming phenotype.

The transcriptional program underlying the physiology of clostridial sporulation

A detailed microarray analysis of transcription during sporulation of the strict anaerobe and endospore former Clostridium acetobutylicum is presented.

Background

Clostridia are ancient soil organisms of major importance to human and animal health and physiology, cellulose degradation, and the production of biofuels from renewable resources. Elucidation of their sporulation program is critical for understanding important clostridial programs pertaining to their physiology and their industrial or environmental applications.

Results

Using a sensitive DNA-microarray platform and 25 sampling timepoints, we reveal the genome-scale transcriptional basis of the Clostridium acetobutylicum sporulation program carried deep into stationary phase. A significant fraction of the genes displayed temporal expression in six distinct clusters of expression, which were analyzed with assistance from ontological classifications in order to illuminate all known physiological observations and differentiation stages of this industrial organism. The dynamic orchestration of all known sporulation sigma factors was investigated, whereby in addition to their transcriptional profiles, both in terms of intensity and differential expression, their activity was assessed by the average transcriptional patterns of putative canonical genes of their regulon. All sigma factors of unknown function were investigated by combining transcriptional data with predicted promoter binding motifs and antisense-RNA downregulation to provide a preliminary assessment of their roles in sporulation. Downregulation of two of these sigma factors, CAC1766 and CAP0167, affected the developmental process of sporulation and are apparently novel sporulation-related sigma factors.

Conclusion

This is the first detailed roadmap of clostridial sporulation, the most detailed transcriptional study ever reported for a strict anaerobe and endospore former, and the first reported holistic effort to illuminate cellular physiology and differentiation of a lesser known organism.

Transcriptional program of early sporulation and stationary-phase events in Clostridium acetobutylicum

DNA microarray analysis of Clostridium acetobutylicum was used to examine the genomic-scale gene expression changes during the shift from exponential-phase growth and acidogenesis to stationary phase and solventogenesis. Self-organizing maps were used to identify novel expression patterns of functional gene classes, including aromatic and branched-chain amino acid synthesis, ribosomal proteins, cobalt and iron transporters, cobalamin biosynthesis, and lipid biosynthesis. The majority of pSOL1 megaplasmid genes (in addition to the solventogenic genes aad-ctfA-ctfB and adc) had increased expression at the onset of solventogenesis, suggesting that other megaplasmid genes may play a role in stationary-phase phenomena. Analysis of sporulation genes and comparison with published Bacillus subtilis results indicated conserved expression patterns of early sporulation genes, including spo0A, the sigF operon, and putative canonical genes of the sigma(H) and sigma(F) regulons. However, sigE expression could not be detected within 7.5 h of initial spo0A expression, consistent with the observed extended time between the appearance of clostridial forms and endospore formation. The results were compared with microarray comparisons of the wild-type strain and the nonsolventogenic, asporogenous M5 strain, which lacks the pSOL1 megaplasmid. While some results were similar, the expression of primary metabolism genes and heat shock proteins was higher in M5, suggesting a difference in metabolic regulation or a butyrate stress response in M5. The results of this microarray platform and analysis were further validated by comparing gene expression patterns to previously published Northern analyses, reporter assays, and two-dimensional protein electrophoresis data of metabolic genes (including all major solventogenesis genes), sporulation genes, heat shock proteins, and other solventogenesis-induced gene expression.

Spo0A directly controls the switch from acid to solvent production in solvent-forming clostridia

The spo0A genes of Clostridium beijerinckii NCIMB 8052 and Clostridium cellulolyticum ATCC 35319 were isolated and characterized. The C-terminal DNA-binding domains of the predicted products of spo0A from these two organisms, as well as 16 other taxonomically diverse species of Bacillus and Clostridium, show extensive amino acid sequence conservation (56% identity, 65% similarity over 104 residues). A 12-amino-acid motif (SRVERAIRHAIE) that forms the putative DNA recognition helix is particularly highly conserved, suggesting a common DNA target. Insertional inactivation of spo0A in C. beijerinckii blocked the formation of solvents (as well as spores and granulose). Sequences resembling Spo0A-binding motifs (TGNCGAA) are found in the promoter regions of several of the genes whose expression is modulated at the onset of solventogenesis in Clostridium acetobutylicum and C. beijerinckii. These include the upregulated adc gene, encoding acetoacetate decarboxylase (EC 4.1.1. 4), and the downregulated ptb gene, encoding phosphotransbutyrylase (EC 2.3.1.c). In vitro gel retardation experiments using C. acetobutylicum adc and C. beijerinckii ptb promoter fragments and recombinant Bacillus subtilis and C. beijerinckii Spo0A suggested that adc and ptb are directly controlled by Spo0A. The binding affinity was reduced when the 0A boxes were destroyed, and enhanced when they were modified to conform precisely to the consensus sequence. In vivo analysis of wild-type and mutagenized promoters transcriptionally fused to the gusA reporter gene in C. beijerinckii validated this hypothesis. Post-exponential phase expression from the mutagenized adc promoter was substantially reduced, whereas expression from the mutagenized ptb promoter was not shut down at the end of exponential growth.

Sporulation and primary sigma factor homologous genes in Clostridium acetobutylicum

Using a PCR-based approach, we have cloned various sigma factor homologous genes from Clostridium acetobutylicum DSM 792. The nucleotide sequence of the dnaE-sigA operon has been determined and predicts two genes encoding 69- and 43-kDa proteins. The deduced DnaE amino acid sequence has approximately 30% amino acid identity with protein sequences of other primases. The putative sigA gene product shows high homology to primary sigma factors of various bacteria, most significantly to Bacillus subtilis and Staphylococcus aureus. Northern (RNA) blot analysis revealed that both genes from an operon, which is clearly expressed under conditions that allow for cell division. A promoter sequence with significant homology to the sigma H-dependent Bacillus promoters preceded the determined transcriptional start point, 182 bp upstream of the GUG start codon of dnaE. The homologous genes to Bacillus spp. sporulation sigma factors G, E, and K have been cloned and sequenced. Indirect evidence for the existence of sigma F was obtained by identification of a DNA sequence homologous to the respective Bacillus consensus promoter. Southern hybridization analysis indicated the presence of sigma D and sigma H homologous genes in C. acetobutylicum. A new gene group conserved within the eubacteria, but with yet unspecified functions, is described. The data presented here provide strong evidence that at least some of the complex regulation features of sporulation in B. subtilis are conserved in C. acetobutylicum and possibly Clostridium spp.

Transposon mutagenesis of Clostridium acetobutylicum P262: isolation and characterization of solvent deficient and metronidazole resistant mutants

An efficient transposon mutagenesis system using conjugative transposons Tn916 and Tn925::Tn917 was established for Clostridium acetobutylicum P262, an industrial strain which has proved difficult to manipulate genetically. Transposon insertions occurred at several different locations to produce a variety of mutants. An oligosporogenous mutant deficient in acetone and butanol production, and two sporulation-deficient and metronidazole resistant mutants were characterized with respect to differentiation and solvent production. Tn925::Tn917 inserted near a string of adenosine residues and transposon insertion was often multiple.

Preparation of Refractile Spores of Clostridium thermosaccharolyticum Involves a Solventogenic Phase

Conversion of vegetative cells of Clostridium thermosaccharolyticum to refractile endospores was achieved by sequential transfer and dilution at each generation, with a final dilution into a sporulation medium that contained xylan supplemented with excess calcium. The subsequent growth was synchronous and resulted in elongated, solventogenic cells that were then shifted to 35°C to permit further differentiation without cell division. The synchronized cells grown in xylan medium supplemented with Ca gluconate produced total solvents that reached 9.63% (vol/vol). One hundred percent of these elongated solventogenic cells (4.84 × 10 9 cells per ml) entered the sporangial stage and continued to differentiate into refractile spores. Only cells sequentially transferred and diluted at a critical time of the growth cycle are synchronized, induced to elongate (≥fourfold), become highly solventogenic in the presence of excess calcium, and are converted to a homogeneous population of refractile spores.

Absorption edge imaging of bacterial endospores with synchrotron radiation

This article describes a new method of viewing biological specimens by taking advantage of the absorptive characteristics of monochromatic X-rays above and below the absorption edge of a specific element. Bacterial endospores were imaged before and after treatment with an experimental vanadium-containing sporocide using monochromatic synchrotron radiation at the nitrogen absorption edge, and above and below the vanadium LIII absorption edge. This morphological study demonstrates a rapid, easy-to-use method of soft X-ray absorption edge imaging that can be used by the biologist to obtain morphological and elemental information that is not readily accessible using conventional microscopic and analytic techniques.

The acetone-butanol-ethanol fermentation

Renewed interest in the acetone-butanol-ethanol (ABE) fermentation as a route for industrial production of butanol has been evident since the oil crisis of the 1970s. The present review includes an historical recap of the traditional industrial process and culturing practices useful in maintaining viable solvent-producing cultures, and then summarizes new and exciting research on the physiology and genetics of the microorganisms as well as process design. Most of these reports relate to improvements in solvent yield and the overall process, since traditional production is not efficient under present economic conditions. Conclusions are then made on future developments necessary for the establishment of an economically viable industrial process.

Immobilized Clostridium acetobutylicum P262 Mutants for Solvent Production

The production of acetone, butanol, and ethanol by two immobilized, sporulation-deficient (spo) Clostridium acetobutylicum P262 mutants which were held in the solventogenic phase was investigated. The spoA2 mutant, which was an early-sporulation mutant and did not form a forespore septum, produced higher solvent yields than did the spoB mutant which was a late-sporulation mutant and was blocked at a stage after forespore septum formation. The spoA2 mutant was also granulose and capsule negative. In a conventional batch fermentation, the wild-type strain produced 15.44 g of solvents per liter after 50 h at a productivity of 7.41 g of solvents per liter per day. The spoA2 mutant produced 15.42 g of solvents per liter at a productivity of 72.4 g of solvents per liter per day, with a retention time of 2.4 h in a continuous immobilized cell system employing a fluidized bed reactor. This represents a major advance, since the immobilization of wild-type cells showed similar increases in productivity but a ca. fivefold reduction in final product concentrations.