Phosphotransferase Activity in Clostridium acetobutylicum from Acidogenic and Solventogenic Phases of Growth

Clostridium cellulovorans Proteomic Responses to Butanol Stress

Combination of butanol-hyperproducing and hypertolerant phenotypes is essential for developing microbial strains suitable for industrial production of bio-butanol, one of the most promising liquid biofuels. Clostridium cellulovorans is among the microbial strains with the highest potential for direct production of n-butanol from lignocellulosic wastes, a process that would significantly reduce the cost of bio-butanol. However, butanol exhibits higher toxicity compared to ethanol and C. cellulovorans tolerance to this solvent is low. In the present investigation, comparative gel-free proteomics was used to study the response of C. cellulovorans to butanol challenge and understand the tolerance mechanisms activated in this condition. Sequential Window Acquisition of all Theoretical fragment ion spectra Mass Spectrometry (SWATH-MS) analysis allowed identification and quantification of differentially expressed soluble proteins. The study data are available via ProteomeXchange with the identifier PXD024183. The most important response concerned modulation of protein biosynthesis, folding and degradation. Coherent with previous studies on other bacteria, several heat shock proteins (HSPs), involved in protein quality control, were up-regulated such as the chaperones GroES (Cpn10), Hsp90, and DnaJ. Globally, our data indicate that protein biosynthesis is reduced, likely not to overload HSPs. Several additional metabolic adaptations were triggered by butanol exposure such as the up-regulation of V- and F-type ATPases (involved in ATP synthesis/generation of proton motive force), enzymes involved in amino acid (e.g., arginine, lysine, methionine, and branched chain amino acids) biosynthesis and proteins involved in cell envelope re-arrangement (e.g., the products of Clocel_4136, Clocel_4137, Clocel_4144, Clocel_4162 and Clocel_4352, involved in the biosynthesis of saturated fatty acids) and a redistribution of carbon flux through fermentative pathways (acetate and formate yields were increased and decreased, respectively). Based on these experimental findings, several potential gene targets for metabolic engineering strategies aimed at improving butanol tolerance in C. cellulovorans are suggested. This includes overexpression of HSPs (e.g., GroES, Hsp90, DnaJ, ClpC), RNA chaperone Hfq, V- and F-type ATPases and a number of genes whose function in C. cellulovorans is currently unknown.

Mathematical modelling of clostridial acetone-butanol-ethanol fermentation

Clostridial acetone-butanol-ethanol (ABE) fermentation features a remarkable shift in the cellular metabolic activity from acid formation, acidogenesis, to the production of industrial-relevant solvents, solventogensis. In recent decades, mathematical models have been employed to elucidate the complex interlinked regulation and conditions that determine these two distinct metabolic states and govern the transition between them. In this review, we discuss these models with a focus on the mechanisms controlling intra- and extracellular changes between acidogenesis and solventogenesis. In particular, we critically evaluate underlying model assumptions and predictions in the light of current experimental knowledge. Towards this end, we briefly introduce key ideas and assumptions applied in the discussed modelling approaches, but waive a comprehensive mathematical presentation. We distinguish between structural and dynamical models, which will be discussed in their chronological order to illustrate how new biological information facilitates the ‘evolution’ of mathematical models. Mathematical models and their analysis have significantly contributed to our knowledge of ABE fermentation and the underlying regulatory network which spans all levels of biological organization. However, the ties between the different levels of cellular regulation are not well understood. Furthermore, contradictory experimental and theoretical results challenge our current notion of ABE metabolic network structure. Thus, clostridial ABE fermentation still poses theoretical as well as experimental challenges which are best approached in close collaboration between modellers and experimentalists.

Sugar uptake by the solventogenic clostridia

The acetone–butanol–ethanol fermentation of solventogenic clostridia was operated as a successful, worldwide industrial process during the first half of the twentieth century, but went into decline for economic reasons. The recent resurgence in interest in the fermentation has been due principally to the recognised potential of butanol as a biofuel, and development of reliable molecular tools has encouraged realistic prospects of bacterial strains being engineered to optimise fermentation performance. In order to minimise costs, emphasis is being placed on waste feedstock streams containing a range of fermentable carbohydrates. It is therefore important to develop a detailed understanding of the mechanisms of carbohydrate uptake so that effective engineering strategies can be identified. This review surveys present knowledge of sugar uptake and its control in solventogenic clostridia. The major mechanism of sugar uptake is the PEP-dependent phosphotransferase system (PTS), which both transports and phosphorylates its sugar substrates and plays a central role in metabolic regulation. Clostridial genome sequences have indicated the presence of numerous phosphotransferase systems for uptake of hexose sugars, hexose derivatives and disaccharides. On the other hand, uptake of sugars such as pentoses occurs via non-PTS mechanisms. Progress in characterization of clostridial sugar transporters and manipulation of control mechanisms to optimise sugar fermentation is described.

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.

Metabolome remodeling during the acidogenic-solventogenic transition in Clostridium acetobutylicum

The fermentation carried out by the biofuel producer Clostridium acetobutylicum is characterized by two distinct phases. Acidogenesis occurs during exponential growth and involves the rapid production of acids (acetate and butyrate). Solventogenesis initiates as cell growth slows down and involves the production of solvents (butanol, acetone, and ethanol). Using metabolomics, isotope tracers, and quantitative flux modeling, we have mapped the metabolic changes associated with the acidogenic-solventogenic transition. We observed a remarkably ordered series of metabolite concentration changes, involving almost all of the 114 measured metabolites, as the fermentation progresses from acidogenesis to solventogenesis. The intracellular levels of highly abundant amino acids and upper glycolytic intermediates decrease sharply during this transition. NAD(P)H and nucleotide triphosphates levels also decrease during solventogenesis, while low-energy nucleotides accumulate. These changes in metabolite concentrations are accompanied by large changes in intracellular metabolic fluxes. During solventogenesis, carbon flux into amino acids, as well as flux from pyruvate (the last metabolite in glycolysis) into oxaloacetate, decreases by more than 10-fold. This redirects carbon into acetyl coenzyme A, which cascades into solventogenesis. In addition, the electron-consuming reductive tricarboxylic acid (TCA) cycle is shutdown, while the electron-producing oxidative (clockwise) right side of the TCA cycle remains active. Thus, the solventogenic transition involves global remodeling of metabolism to redirect resources (carbon and reducing power) from biomass production into solvent production.

Physiological characteristics of the extreme thermophile Caldicellulosiruptor saccharolyticus: an efficient hydrogen cell factory

Global concerns about climate changes and their association with the use of fossil fuels have accelerated research on biological fuel production. Biological hydrogen production from hemicellulose-containing waste is considered one of the promising avenues. A major economical issue for such a process, however, is the low substrate conversion efficiency. Interestingly, the extreme thermophilic bacterium Caldicellulosiruptor saccharolyticus can produce hydrogen from carbohydrate-rich substrates at yields close to the theoretical maximum of the dark fermentation process (i.e., 4 mol H2/mol hexose). The organism is able to ferment an array of mono-, di- and polysaccharides, and is relatively tolerant to high partial hydrogen pressures, making it a promising candidate for exploitation in a biohydrogen process. The behaviour of this Gram-positive bacterium bears all hallmarks of being adapted to an environment sparse in free sugars, which is further reflected in its low volumetric hydrogen productivity and low osmotolerance. These two properties need to be improved by at least a factor of 10 and 5, respectively, for a cost-effective industrial process. In this review, the physiological characteristics of C. saccharolyticus are analyzed in view of the requirements for an efficient hydrogen cell factory. A special emphasis is put on the tight regulation of hydrogen production in C. saccharolyticus by both redox and energy metabolism. Suggestions for strategies to overcome the current challenges facing the potential use of the organism in hydrogen production are also discussed.

Uncoupler-Resistant Glucose Uptake by the Thermophilic Glycolytic Anaerobe Thermoanaerobacter thermosulfuricus (Clostridium thermohydrosulfuricum)

The transport of glucose across the bacterial cell membrane of Thermoanaerobacter thermosulfuricus (Clostridium thermohydrosulfuricum) Rt8.B1 was governed by a permease which did not catalyze concomitant substrate transport and phosphorylation and thus was not a phosphoenolpyruvate-dependent phosphotransferase. Glucose uptake was carrier mediated, could not be driven by an artificial membrane potential (Deltapsi) in the presence or absence of sodium, and was not sensitive to inhibitors which dissipate the proton motive force (Deltap; tetrachlorosalicylanilide, N,N-dicyclohexylcarboiimide, and 2,4-dinitrophenol), and no uptake of the nonmetabolizable analog 2-deoxyglucose could be demonstrated. The glucokinase apparent K(m) for glucose (0.21 mM) was similar to the K(t) (affinity constant) for glucose uptake (0.15 mM), suggesting that glucokinase controls the rate of glucose uptake. Inhibitors of ATP synthesis (iodoacetate and sodium fluoride) also inhibited glucose uptake, and this effect was due to a reduction in the level of ATP available to glucokinase for glucose phosphorylation. These results indicated that T. thermosulfuricus Rt8.B1 lacks a concentrative uptake system for glucose and that uptake is via facilitated diffusion, followed by ATP-dependent phosphorylation by glucokinase. In T. thermosulfuricus Rt8.B1, glucose is metabolized by the Embden-Meyerhof-Parnas pathway, which yields 2 mol of ATP (G. M. Cook, unpublished data). Since only 1 mol of ATP is used to transport 1 mol of glucose, the energetics of this system are therefore similar to those found in bacteria which possess a phosphotransferase.

Maintenance of DeltapH by a butanol-tolerant mutant of Clostridium beijerinckii

The isolation of Clostridium beijerinckii mutants that are more tolerant of butanol than the wild-type offered the opportunity to investigate whether the membrane activities which are required for maintaining the transmembrane DeltapH (the difference in pH between the cellular interior and exterior) are sensitive targets of butanol toxicity. The DeltapH was measured by the accumulation of [14C]benzoate using late-exponential-phase cells which were suspended in citrate/phosphate buffer at pH 5 (to maximize the DeltapH component of the protonmotive force) and supplemented with glucose and Mg2+. The DeltapH of the butanol-tolerant tolerant mutant, strain BR54, of C. beijerinckii NCIMB 8052 was found to be significantly more tolerant of added butanol than the wild-type. Thus, in potassium citrate/phosphate buffer the mutant cells maintained a DeltapH of 1.4 when butanol was added to a concentration of 1.5 % (w/v), while the wild-type DeltapH was reduced to 0.1. The DeltapH of both strains was completely dissipated with 1.75 % butanol, an effect attributed to a chaotropic effect on the membrane phospholipids. Similar results were obtained in sodium citrate/phosphate buffer. In the absence of added Mg2+, the DeltapH of the mutant decreased in both sodium and potassium citrate/phosphate buffer, but more rapidly in the former. Interestingly, the addition of butanol at low concentrations (0.8 %) prevented this DeltapH dissipation, but only in cells suspended in sodium citrate/phosphate buffer, and not in potassium citrate/phosphate buffer. In wild-type cells the decrease in DeltapH occurred more slowly than in the mutant, and sparing of the DeltapH by 0.8 % butanol was less pronounced. The authors interpret these data to mean that the DeltapH is dissipated in the absence of Mg2+ by a Na+- or K+-linked process, possibly by a Na+/H+ or a K+/H+ antiporter, and that the former is inhibited by butanol. Apparently, butanol can selectively affect a membrane-associated function at concentrations lower than required for the complete dissipation of transmembrane ion gradients. Additionally, since the butanol-tolerant mutant BR54 is deficient in the ability to detoxify methylglyoxal (MG) and contains higher levels of MG than the wild-type, the higher Na+/H+ antiporter activity of the mutant may be due to the greater degree of protein glycation by MG in the mutant cells. The mechanism of butanol tolerance may be an indirect result of the elevated glycation of cell proteins in the mutant strain. Analysis of membrane protein fractions revealed that mutant cells contained significantly lower levels of unmodified arginine residues than those of the wild-type cells, and that unmodified arginine residues of the wild-type were decreased by exposure of the growing cells to added MG.

Metabolism of fructooligosaccharides by Lactobacillus paracasei 1195

Fermentation of fructooligosaccharides (FOS) and other oligosaccharides has been suggested to be an important property for the selection of bacterial strains used as probiotics. However, little information is available on FOS transport and metabolism by lactic acid bacteria and other probiotic bacteria. The objectives of this research were to identify and characterize the FOS transport system of Lactobacillus paracasei 1195. Radiolabeled FOS was synthesized enzymatically from [(3)H]sucrose and purified by column and thin-layer chromatography, yielding three main products: glucose (G) alpha-1,2 linked to two, three, or four fructose (F) units (GF(2), GF(3), and GF(4), respectively). FOS hydrolysis activity was detected only in cell extracts prepared from FOS- or sucrose-grown cells and was absent in cell supernatants, indicating that transport must precede hydrolysis. FOS transport assays revealed that the uptake of GF(2) and GF(3) was rapid, whereas little GF(4) uptake occurred. Competition experiments showed that glucose, fructose, and sucrose reduced FOS uptake but that other mono-, di-, and trisaccharides were less inhibitory. When cells were treated with sodium fluoride, iodoacetic acid, or other metabolic inhibitors, FOS transport rates were reduced by up to 60%; however, ionophores that abolished the proton motive force only slightly decreased FOS transport. In contrast, uptake was inhibited by ortho-vanadate, an inhibitor of ATP-binding cassette transport systems. De-energized cells had low intracellular ATP concentrations and had a reduced capacity to accumulate FOS. These results suggest that FOS transport in L. paracasei 1195 is mediated by an ATP-dependent transport system having specificity for a narrow range of substrates.

Glucose uptake in Clostridium beijerinckii NCIMB 8052 and the solvent-hyperproducing mutant BA101

Glucose uptake and accumulation by Clostridium beijerinckii BA101, a butanol hyperproducing mutant, were examined during various stages of growth. Glucose uptake in C. beijerinckii BA101 was repressed 20% by 2-deoxyglucose and 25% by mannose, while glucose uptake in C. beijerinckii 8052 was repressed 52 and 28% by these sugars, respectively. We confirmed the presence of a phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS) associated with cell extracts of C. beijerinckii BA101 by glucose phosphorylation by PEP. The PTS activity associated with C. beijerinckii BA101 was 50% of that observed for C. beijerinckii 8052. C. beijerinckii BA101 also demonstrated lower PTS activity for fructose and glucitol. Glucose phosphorylation by cell extracts derived from both C. beijerinckii BA101 and 8052 was also dependent on the presence of ATP, a finding consistent with the presence of glucokinase activity in C. beijerinckii extracts. ATP-dependent glucose phosphorylation was predominant during the solventogenic stage, when PEP-dependent glucose phosphorylation was dramatically repressed. A nearly twofold-greater ATP-dependent phosphorylation rate was observed for solventogenic stage C. beijerinckii BA101 than for solventogenic stage C. beijerinckii 8052. These results suggest that C. beijerinckii BA101 is defective in PTS activity and that C. beijerinckii BA101 compensates for this defect with enhanced glucokinase activity, resulting in an ability to transport and utilize glucose during the solventogenic stage.

Physiology of carbohydrate to solvent conversion by clostridia

The solvent-forming clostridia have attracted interest because of their ability to convert a range of carbohydrates to end-products such as acetone, butanol and ethanol. Polymeric substrates such as cellulose, hemicellulose and starch are degraded by extracellular enzymes. The majority of cellulolytic clostridia, typified by Clostridium thermocellum, produce a multi-enzyme cellulase complex in which the organization of components is critical for activity against the crystalline substrate. A variety of enzymes involved in degradation of hemicellulose and starch have been identified in different strains. The products of degradation, and other soluble substrates, are accumulated via membrane-bound transport systems which are generally poorly characterized. It is clear, however, that the phosphoenolpyruvate-dependent phosphotransferase system (PTS) plays a major role in solute uptake in several species. Accumulated substrates are converted by intracellular enzymes to end-products characteristic of the organism, with production of ATP to support growth. The metabolic pathways have been described, but understanding of mechanisms of regulation of metabolism is incomplete. Synthesis of extracellular enzymes and membrane-bound transport systems is commonly subject to catabolite repression in the presence of a readily metabolized source of carbon and energy. While many genes encoding cellulases, xylanases and amylases have been cloned and sequenced, little is known of control of their expression. Although the mechanism of catabolite repression in clostridia is not understood, some recent findings implicate a role for the PTS as in other low G-C Gram-positive bacteria. Emphasis has been placed on describing the mechanisms underlying the switch of C. acetobutylicum fermentations from acidogenic to solventogenic metabolism at the end of the growth phase. Factors involved include a lowered pH and accumulation of undissociated butyric acid, intracellular concentration of ATP and reduced pyridine nucleotides, nutrient limitation, and the interplay between pathways of carbon and electron flow. Genes encoding enzymes of solvent pathways have been cloned and sequenced, and their expression correlated with the pattern of end-product formation in fermentations. There is evidence that the initiation of solvent formation may be subject to control mechanisms similar to other stationary-phase phenomena, including sporulation. The application of recently developed techniques for genetic manipulation of the bacterium is improving understanding of the regulatory circuits, but a complete molecular description of the control of solvent formation remains elusive. Experimental manipulation of the pathways of electron flow in other species has been shown to influence the range and yield of fermentation end-products. Acid-forming clostridia can, under appropriate conditions, be induced to form atypical solvents as products. While the mechanisms of regulation of gene expression are not at all understood, the capacity to adapt in this way further illustrates the metabolic flexibility of clostridial strains.

Glucose uptake by Listeria monocytogenes Scott A and inhibition by pediocin JD

Glucose uptake by Listeria monocytogenes Scott A was inhibited by the bacteriocin pediocin JD and by the protonophore carbonyl cyanide m-chlorophenyhydrazone. Experiments with monensin, nigericin, chlorhexidine diacetate, dinitrophenol, and gramicidin, however, showed that glucose uptake could occur in the absence of a proton motive force. L. monocytogenes cell extracts phosphorylated glucose when phosphoenolpyruvate (PEP) was present in the assay mixture, and whole cells incubated with 2-deoxyglucose accumulated 2-deoxyglucose-6-phosphate, indicating the presence of a PEP-dependent phosphotransferase system in this organism. Glucose phosphorylation also occurred when ATP was present, suggesting that a proton motive force-mediated glucose transport system may also be present. We conclude that L. monocytogenes Scott A accumulates glucose by phosphotransferase and proton motive force-mediated systems, both of which are sensitive to pediocin JD.

Properties of the glucose phosphotransferase system of Clostridium acetobutylicum NCIB 8052

The glucose phosphotransferase system (PTS) of Clostridium acetobutylicum was studied by using cell extracts. The system exhibited a Km for glucose of 34 microM, and glucose phosphorylation was inhibited competitively by mannose and 2-deoxyglucose. The analogs 3-O-methylglucoside and methyl alpha-glucoside did not inhibit glucose phosphorylation significantly. Activity showed no dependence on Mg2+ ions or on pH in the range 6.0 to 8.0. The PTS comprised both soluble and membrane-bound proteins, which interacted functionally with the PTSs of Clostridium pasteurianum, Bacillus subtilis, and Escherichia coli. In addition to a membrane-bound enzyme IIGlc, sugar phosphorylation assays in heterologous systems incorporating extracts of pts mutants of other organisms provided evidence for enzyme I, HPr, and IIIGlc components. The HPr was found in the soluble fraction of C. acetobutylicum extracts, whereas enzyme I, and probably also IIIGlc, was present in both the soluble and membrane fractions, suggesting a membrane location in the intact cell.

Lactose Uptake Driven by Galactose Efflux in Streptococcus thermophilus : Evidence for a Galactose-Lactose Antiporter

Galactose-nonfermenting (Gal - ) Streptococcus thermophilus TS2 releases galactose into the extracellular medium when grown in medium containing excess lactose. Starved and de-energized Gal - cells, however, could be loaded with galactose to levels approximately equal to the extracellular concentration (0 to 50 mM). When loaded cells were separated from the medium and resuspended in fresh broth containing 5 mM lactose, galactose efflux occurred. De-energized, galactose-loaded cells, resuspended in buffer or medium, accumulated [ 14 C]lactose at a greater rate and to significantly higher intracellular concentrations than unloaded cells. Uptake of lactose by loaded cells was inhibited more than that by unloaded cells in the presence of extracellular galactose, indicating that a galactose gradient was involved in the exchange system. When de-energized, galactose-loaded cells were resuspended in carbohydrate-free medium at pH 6.7, a proton motive force (Δp) of 86 to 90 mV was formed, whereas de-energized, nonloaded cells maintained a Δp of about 56 mV. However, uptake of lactose by loaded cells occurred when the proton motive force was abolished by the addition of an uncoupler or in the presence of a proton-translocating ATPase inhibitor. These results support the hypothesis that galactose efflux in Gal - S. thermophilus is electrogenic and that the exchange reaction (lactose uptake and galactose efflux) probably occurs via an antiporter system.

Protein phosphorylation in response to stress in Clostridium acetobutylicum

The possible involvement of protein phosphorylation in the clostridial stress response was investigated by radioactively labeling growing cells of Clostridium acetobutylicum with 32Pi or cell extracts with [gamma-32P]ATP. Several phosphoproteins were identified; these were not affected by the growth stage of the culture. Although the extent of protein phosphorylation was increased by heat stress, the phosphoproteins did not correspond to known stress proteins seen in one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Purified clostridial DnaK, a stress protein, acted as a kinase catalyzing the phosphorylation of a 50-kilodalton protein. The phosphorylation of this protein was enhanced in extracts prepared from heat-stressed cells. Diadenosine-5',5"'-P1,P4-tetraphosphate had no influence on protein phosphorylation.

Metabolism of adenylylated nucleotides in Clostridium acetobutylicum

In response to the stresses imposed by temperature upshift or addition of butanol, Clostridium acetobutylicum cultures accumulated diadenosine-5',5'''-P1,P4-tetraphosphate (Ap4A) and adenosine 5'-P1,P4-tetraphospho-5'-guanosine (Ap4G) to high levels. The two adenylylated nucleotides were also accumulated in batch culture in the absence of imposed stresses when the clostridia switched from the acidogenic phase of growth to the solventogenic phase. Most of the adenylylated nucleotides were extracellular. The intracellular concentrations of these compounds were low throughout batch growth and in cells stressed by added butanol. In contrast to other procaryotes, these clostridia did not possess enzymes to degrade the dinucleotides, as shown with both intact cells and cell-free preparations. Our findings are consistent with the hypothesis that endogenously produced solvents are stressful to the cells, stimulating the synthesis of adenylylated nucleotides. The nucleotides accumulate extracellularly because they cannot be degraded and because the cell membranes are permeabilized by the solvents produced.

The phosphoenolpyruvate:sugar phosphotransferase system in gram-positive bacteria: properties, mechanism, and regulation

This review consists of three major sections. The first and largest section reviews the protein constituents and known properties of the phosphotransferase systems present in well-studied Gram-positive bacteria. These bacteria include species of the following genera: (1) Staphylococcus, (2) Streptococcus, (3) Bacillus, (4) Lactobacillus, (5) Clostridium, (6) Arthrobacter, and (7) Brochothrix. The properties of the different systems are compared. The second major section deals with the regulation of carbohydrate uptake. There are four parts: (1) inhibition by intracellular sugar phosphates in Staphylococcus aureus, (2) PTS-mediated regulation of glycerol uptake in Bacillus subtilis, (3) competition for phospho-HPr in Streptococcus mutans, and (4) the possible involvement of protein kinases in the regulation of sugar uptake via the phosphotransferase system. The third section deals with the phenomenon of inducer expulsion. The first part is concerned with the physiological characterization of the phenomenon; then the consequences of unregulated uptake and expulsion, a futile cycle of energy expenditure, are considered. Finally, the biochemistry of the protein kinase and the protein phosphate phosphatase system, which appears to regulate sugar transport via the phosphotransferase system, is defined. The review, therefore, concentrates on the phosphotransferase system, its functions in carbohydrate transport and phosphorylation, the mechanisms of its regulation, and the mechanism by which it participates in the regulation of other physiological processes in the bacterial cell.

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.

Membrane H + Conductance of Clostridium thermoaceticum and Clostridium acetobutylicum : Evidence for Electrogenic Na + /H + Antiport in Clostridium thermoaceticum

H + conductance in de-energized cells of Clostridium thermoaceticum and Clostridium acetobutylicum was determined from the rate of realkalinization of the medium after an acid pulse. In both organisms, cell membrane proton permeability was increased by fermentation end products and ionophores. In C. thermoaceticum, H + conductance was increased by Na + ions compared with K + as counterions. In these cells, addition of Na + , but not K + , elicited efflux of H + ; H + efflux was stimulated by SCN − and decreased by various ionophores. We concluded that C. thermoaceticum possesses an electrogenic Na + /H + antiporter. In contrast, C. acetobutylicum cells did not have an electrogenic Na + /H + antiporter.

Intracellular Conditions Required for Initiation of Solvent Production by Clostridium acetobutylicum

We investigated the intracellular physiological conditions associated with the induction of butanol-producing enzymes in Clostridium acetobutylicum . During the acidogenic phase of growth, the internal pH decreased in parallel with the decrease in the external pH, but the internal pH did not go below 5.5 throughout batch growth. Butanol was found to dissipate the proton motive force of fermenting C. acetobutylicum cells by decreasing the transmembrane pH gradient, whereas the membrane potential was affected only slightly. In growing cells, the switch from acid to solvent production occurred when the internal undissociated butyric acid concentration reached 13 mM and the total intracellular undissociated acid concentration (acetic plus butyric acids) was at least 40 to 45 mM. Similar values were obtained when cultures were supplemented with 50 mM butyric acid initially or when a phosphate-buffered medium was used instead of an acetate-buffered medium. To measure the induction of the enzymes involved in solvent synthesis, we determined the rates of conversion of butyrate to butanol in growing cells. The rate of butanol formation reached a maximum in the mid-solvent phase, when the butanol concentration was 50 mM. Although more solvent accumulated later, de novo enzyme synthesis decreased and then ceased.