METABOLISM OF PENTOSES BY CLOSTRIDIA II. Clostridium perfringens, Clostridium beijerinckii, andClostridium butylicum

Solvent production from xylose

Xylose is the second most abundant sugar derived from lignocellulose; it is considered less desirable than glucose for fermentation, and strategies that specifically increase xylose utilization in wild type or engineered cells are goals for biofuel production. Issues arise with xylose utilization because of carbohydrate catabolite repression, which is the preferential utilization of glucose relative to xylose in fermentations with both pure and mixed cultures. Taken together the low substrate utilization rates and solvent yields with xylose compared to glucose, many industrial fermentations ignore the xylolytic portion of the reaction in lieu of methods to maintain high glucose. This is shortsighted given the massive potential for xylose generation from a number of sustainable biomass feedstocks, based on utilization of the hemicellulose fraction(s) that enter pretreatment. A number of strategies have been developed in recent years to address xylose utilization and solvent production from xylose in systems with just xylose, or in systems with mixtures of glucose plus xylose, which are more typical of pretreated lignocellulose. The approaches vary in terms of complexity, stability, and ease of introduction to existing fermentation infrastructure (i.e., so-called drop-in fermentation strategies). Some approaches can be considered traditional engineering approaches (e.g., change the reaction conditions), while others are more subtle cellular approaches to eliminate the impacts of catabolite repression. Finally, genetic engineering has been used to increase xylose utilization, although this can be considered a relatively nascent approach compared to manipulations completed to date for glucose utilization.

Exploitation of novel wild type solventogenic strains for butanol production

Butanol has been regarded as an important bulk chemical and advanced biofuel; however, large scaling butanol production by solventogenic Clostridium sp. is still not economically feasible due to the high cost of substrates, low butanol titer and yield caused by the toxicity of butanol and formation of by-products. Renewed interests in biobutanol as biofuel and rapid development in genetic tools have spurred technological advances to strain modifications. Comprehensive reviews regarding these aspects have been reported elsewhere in detail. Meanwhile, more wild type butanol producers with unique properties were also isolated and characterized. However, few reviews addressed these discoveries of novel wild type solventogenic Clostridium sp. strains. Accordingly, this review aims to comprehensively summarize the most recent advances on wild type butanol producers in terms of fermentation patterns, substrate utilization et al. Future perspectives using these native ones as chassis for genetic modification were also discussed.

Calorimetric studies of the growth of anaerobic microbes

This article aims to validate the use of calorimetry to measure the growth of anaerobic microbes. It has been difficult to monitor the growth of strict anaerobes while maintaining optimal growth conditions. Traditionally, optical density and ATP concentration are usually used as measures of the growth of anaerobic microbes. However, to take these measurements it is necessary to extract an aliquot of the culture, which can be difficult while maintaining anaerobic conditions. In this study, calorimetry was used to continuously and nondestructively measure the heat generated by the growth of anaerobic microbes as a function of time. Clostridium acetobutylicum, Clostridium beijerinckii, and Clostridium cellulovorans were used as representative anaerobic microbes. Using a multiplex isothermal calorimeter, we observed that peak time (tp) of C. acetobutylicum heat evolution increased as the inoculation rate decreased. This strong correlation between the inoculation rate and tp showed that it was possible to measure the growth rate of anaerobic microbes by calorimetry. Overall, our results showed that there is a very good correlation between heat evolution and optical density/ATP concentration, validating the use of the method.

Improvement of xylose utilization in Clostridium acetobutylicum via expression of the talA gene encoding transaldolase from Escherichia coli

Clostridium acetobutylicum ATCC 824 was metabolically engineered for improved xylose utilization. The gene talA, which encodes transaldolase from Escherichia coli K-12, was cloned and overexpressed in C. acetobutylicum ATCC 824. Compared with C. acetobutylicum ATCC 824 (824-WT), the transformant bearing the E. coli talA gene (824-TAL) showed improved ability on xylose utilization and solvents production using xylose as the sole carbon source. During the fermentation of xylose and glucose mixtures with three xylose/glucose ratios (approximately 1:2, 1:1 and 2:1), the rate of xylose consumption and final solvents titers of 824-TAL were all higher than those of 824-WT, despite glucose repression on xylose uptake still existing. These results suggest that the insufficiency of transaldolase in the pentose phosphate pathway (PPP) of C. acetobutylicum is one of the bottlenecks for xylose metabolism and therefore, overexpressing the gene encoding transaldolase is able to improve xylose utilization and solvent production.

Enzymes of glucose and pyruvate catabolism in cells, spores, and germinated spores of Clostridium botulinum

Simmons, R. J. (Michigan State University, East Lansing), and R. N. Costilow. Enzymes of glucose and pyruvate catabolism in cells, spores, and germinated spores of Clostridium botulinum. J. Bacteriol. 84:1274-1281. 1962.-An investigation was made of the enzymes of vegetative cells, spores, and germinated spores of Clostridium botulinum 62-A to elucidate a pathway of glucose metabolism. Manometric studies were conducted with intact cells, and various enzymes and enzyme systems were assayed in cell-free and spore-free extracts by use of spectrophotometric and colorimetric procedures. Glucose fermentation was found to be inducible; glucokinase was the controlling enzyme. All other enzymes of the Embden-Meyerhof-Parnas (EMP) pathway were found in both induced and non-induced cells, but they were in relatively low concentrations in the latter. This, plus the fact that no glucose-6-phosphate dehydrogenase was detected, led to the conclusion that glucose is catabolized primarily by the EMP system. A number of glycolytic enzymes were also found in extracts of spores and germinated spores of this organism, but the activities were extremely low as compared with activities in cell extracts. A phosphoroclastic-type reaction was readily demonstrated in both glucose-adapted and non-adapted cells, but not in spores and germinated spores. However, both acetokinase and phosphotransacetylase, as well as coenzyme A transphorase, were detected in spores and germinated-spore extracts, although at very low activity levels as compared with cell extracts. The specific activity of diaphorase in spore extracts was about one-half that of corresponding cell extracts, and the activity of reduced diphosphopyridine nucleotide (DPNH) oxidase was actually higher in the spore extracts. In addition, the DPNH oxidase in spore extracts was considerably more heat-stable than that in extracts of cells or germinated spores.

Regulation and butanol inhibition of D-xylose and D-glucose uptake in Clostridium acetobutylicum

Clostridium acetobutylicum exhibited diauxie growth in the presence of mixtures of glucose and xylose. Both glucose- and xylose-grown cells had a glucose uptake activity. On the other hand, growth on xylose was associated with the induction of a xylose permease activity, which was repressed by glucose in xylose-induced cells. The rate of sugar uptake with increasing sugar concentrations showed saturation kinetics with an apparent Km of 1.25 X 10(-5) M for glucose and 5 X 10(-3) M for xylose. Concomitant with the production of solvents, the activities of the glucose and xylose transport systems decreased. Among the main products of fermentation, butanol was shown to be a potent inhibitor of the growth of the organism and of the rate of sugar uptake as well as of sugar incorporation into cell materials. These inhibitory effects of butanol were more pronounced in xylose-grown cells than in glucose-grown cells. Butanol completely inhibited growth at a concentration of 14 g/liter for cultures growing on glucose and 8 g/liter for cultures growing on xylose. Concentrations of 7 and 10.5 g/liter of butanol caused a 50% inhibition of the xylose and glucose incorporations into cell materials. These inhibitory levels of butanol were found in typical glucose or xylose fermentation.