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PubMed2003

Engineering redox cofactor regeneration for improved pentose fermentation in Saccharomyces cerevisiae.

Pentose fermentation to ethanol with recombinant Saccharomyces cerevisiae is slow and has a low yield. A likely reason for this is that the catabolism of the pentoses D-xylose and L-arabinose through the corresponding fungal pathways creates an imbalance of redox cofactors. The process, although redox neutral, requires NADPH and NAD+, which have to be regenerated in separate processes. NADPH is normally generated through the oxidative part of the pentose phosphate pathway by the action of glucose-6-phosphate dehydrogenase (ZWF1). To facilitate NADPH regeneration, we expressed the recently discovered gene GDP1, which codes for a fungal NADP+-dependent D-glyceraldehyde-3-phosphate dehydrogenase (NADP-GAPDH) (EC 1.2.1.13), in an S. cerevisiae strain with the D-xylose pathway. NADPH regeneration through an NADP-GAPDH is not linked to CO2 production. The resulting strain fermented D-xylose to ethanol with a higher rate and yield than the corresponding strain without GDP1; i.e., the levels of the unwanted side products xylitol and CO2 were lowered. The oxidative part of the pentose phosphate pathway is the main natural path for NADPH regeneration. However, use of this pathway causes wasteful CO2 production and creates a redox imbalance on the path of anaerobic pentose fermentation to ethanol because it does not regenerate NAD+. The deletion of the gene ZWF1 (which codes for glucose-6-phosphate dehydrogenase), in combination with overexpression of GDP1 further stimulated D-xylose fermentation with respect to rate and yield. Through genetic engineering of the redox reactions, the yeast strain was converted from a strain that produced mainly xylitol and CO2 from D-xylose to a strain that produced mainly ethanol under anaerobic conditions.

Anaerobiosis↗
PubMed2003

In situ production of exopolysaccharides during Sourdough fermentation by cereal and intestinal isolates of lactic acid bacteria.

EPS formed by lactobacilli in situ during sourdough fermentation may replace hydrocolloids currently used as texturizing, antistaling, or prebiotic additives in bread production. In this study, a screening of >100 strains of cereal-associated and intestinal lactic acid bacteria was performed for the production of exopolysaccharides (EPS) from sucrose. Fifteen strains produced fructan, and four strains produced glucan. It was remarkable that formation of glucan and fructan was most frequently found in intestinal isolates and strains of the species Lactobacillus reuteri, Lactobacillus pontis, and Lactobacillus frumenti from type II sourdoughs. By the use of PCR primers derived from conserved amino acid sequences of bacterial levansucrase genes, it was shown that 6 of the 15 fructan-producing lactobacilli and none of 20 glucan producers or EPS-negative strains carried a levansucrase gene. In sourdough fermentations, it was determined whether those strains producing EPS in MRS medium modified as described by Stolz et al. (37) and containing 100 g of sucrose liter(-1) as the sole source of carbon also produce the same EPS from sucrose during sourdough fermentation in the presence of 12% sucrose. For all six EPS-producing strains evaluated in sourdough fermentations, in situ production of EPS at levels ranging from 0.5 to 2 g/kg of flour was demonstrated. Production of EPS from sucrose is a metabolic activity that is widespread among sourdough lactic acid bacteria. Thus, the use of these organisms in bread production may allow the replacement of additives.

Amino Acid Sequence↗
PubMed2004

Polydextrose, lactitol, and fructo-oligosaccharide fermentation by colonic bacteria in a three-stage continuous culture system.

In vitro fermentations were carried out by using a model of the human colon to simulate microbial activities of lower gut bacteria. Bacterial populations (and their metabolic products) were evaluated under the effects of various fermentable substrates. Carbohydrates tested were polydextrose, lactitol, and fructo-oligosaccharide (FOS). Bacterial groups of interest were evaluated by fluorescence in situ hybridization as well as by species-specific PCR to determine bifidobacterial species and percent-G+C profiling of the bacterial communities present. Short-chain fatty acids (SCFA) produced during the fermentations were also evaluated. Polydextrose had a stimulatory effect upon colonic bifidobacteria at concentrations of 1 and 2% (using a single and pooled human fecal inoculum, respectively). The bifidogenic effect was sustained throughout all three vessels of the in vitro system (P = 0.01 seen in vessel 3), as corroborated by the bacterial community profile revealed by %G+C analysis. This substrate supported a wide variety of bifidobacteria and was the only substrate where Bifidobacterium infantis was detected. The fermentation of lactitol had a deleterious effect on both bifidobacterial and bacteroides populations (P = 0.01) and decreased total cell numbers. SCFA production was stimulated, however, particularly butyrate (beneficial for host colonocytes). FOS also had a stimulatory effect upon bifidobacterial and lactobacilli populations that used a single inoculum (P = 0.01 for all vessels) as well as a bifidogenic effect in vessels 2 and 3 (P = 0.01) when a pooled inoculum was used. A decrease in bifidobacteria throughout the model was reflected in the percent-G+C profiles.

Bacteria↗
PubMed2005

Inducible gene expression in Lactobacillus reuteri LTH5531 during type II sourdough fermentation.

Lactobacillus reuteri LTH5531 is a dominant member of the microbiota of type II sourdough fermentations. To investigate the genetic background of the ecological performance of LTH5531, in vivo expression technology was used to identify promoters that show elevated levels of expression during growth of this organism in a type II sourdough fermentation. Thirty-eight sourdough-induced fusions were detected, and 29 genes could be identified on the basis of the available sequence information. Four genes encoded stress-related functions (e.g., acid and general stress response), reflecting the harsh conditions prevailing during sourdough fermentation. Further, eight genes were involved in acquisition and synthesis of amino acids and nucleotides, indicating their limited availability in sourdough. The remaining genes were either part of functionally unrelated pathways or encoded hypothetical proteins. The identification of a putative proteinase and a component of the arginine deiminase pathway is of technological interest, as they are potentially involved in the formation of aroma precursors. Our study allowed insight into the transcriptional response of Lactobacillus reuteri to the dough environment, which establishes the molecular basis to investigate bacterial properties that are likely to contribute to the ecological performance of the organism and influence the final outcome of the fermentation.

Bacterial Proteins↗
PubMed2005

Fate and role of ammonium ions during fermentation of citric acid by Aspergillus niger.

Stoichiometric modeling of the early stages of the citric acid fermentation process by Aspergillus niger revealed that ammonium ions combine with a carbon-containing metabolite inside the cell, in a ratio 1:1, to form a nitrogen compound which is then excreted by the mycelium. High-performance liquid chromatography analysis identified glucosamine as the product of the relationship between glucose and ammonium during the early stages of the citric acid fermentation process. Slightly acidic internal pHs, extremely low ammonium ion concentrations inside the cell, and glucosamine synthesis come into direct contradiction with the earlier theory of the ammonium pool inside the cell, regarded as responsible for inhibition of the enzyme phosphofructokinase. At later fermentation stages, when the mycelium is involved in a process of fragmentation and regrowth, the addition of ammonium sulfate leads to a series of events: the formation and secretion of glucosamine in elevated amounts, the short inhibition of citrate synthesis, growth enhancement, the utilization of glucosamine, and finally, the enhancement of citric acid production rates. Obviously, the enzymatic processes underlining the phenomena need to be reexamined. As a by-product of the citric acid fermentation, glucosamine is reported for the first time here. Suitable process manipulations of the system described in this work could lead to successful glucosamine recovery at the point of its highest yield before degradation by the fungus occurs.

Aspergillus niger↗
PubMed2005

Molecular and biochemical analysis of the galactose phenotype of dairy Streptococcus thermophilus strains reveals four different fermentation profiles.

Lactose-limited fermentations of 49 dairy Streptococcus thermophilus strains revealed four distinct fermentation profiles with respect to galactose consumption after lactose depletion. All the strains excreted galactose into the medium during growth on lactose, except for strain IMDOST40, which also displayed extremely high galactokinase (GalK) activity. Among this strain collection eight galactose-positive phenotypes sensu stricto were found and their fermentation characteristics and Leloir enzyme activities were measured. As the gal promoter seems to play an important role in the galactose phenotype, the galR-galK intergenic region was sequenced for all strains yielding eight different nucleotide sequences (NS1 to NS8). The gal promoter played an important role in the Gal-positive phenotype but did not determine it exclusively. Although GalT and GalE activities were detected for all Gal-positive strains, GalK activity could only be detected for two out of eight Gal-positive strains. This finding suggests that the other six S. thermophilus strains metabolize galactose via an alternative route. For each type of fermentation profile obtained, a representative strain was chosen and four complete Leloir gene clusters were sequenced. It turned out that Gal-positive strains contained more amino acid differences within their gal genes than Gal-negative strains. Finally, the biodiversity regarding lactose-galactose utilization among the different S. thermophilus strains used in this study was shown by RAPD-PCR. Five Gal-positive strains that contain nucleotide sequence NS2 in their galR-galK intergenic region were closely related.

Bacterial Proteins↗
PubMed2005

Expression of 17 genes in Clostridium thermocellum ATCC 27405 during fermentation of cellulose or cellobiose in continuous culture.

Clostridium thermocellum is a thermophilic, anaerobic, cellulolytic bacterium that produces ethanol and acetic acid as major fermentation end products. The effect of growth conditions on gene expression in C. thermocellum ATCC 27405 was studied using cells grown in continuous culture under cellobiose or cellulose limitation over a approximately 10-fold range of dilution rates (0.013 to 0.16 h(-1)). Fermentation product distribution displayed similar patterns in cellobiose- or cellulose-grown cultures, including substantial shifts in the proportion of ethanol and acetic acid with changes in growth rate. Expression of 17 genes involved or potentially involved in cellulose degradation, intracellular phosphorylation, catabolite repression, and fermentation end product formation was quantified by real-time PCR, with normalization to two calibrator genes (recA and the 16S rRNA gene) to determine relative expression. Thirteen genes displayed modest (fivefold or less) differences in expression with growth rate or substrate type: sdbA (cellulosomal scaffoldin-dockerin binding protein), cdp (cellodextrin phosphorylase), cbp (cellobiose phosphorylase), hydA (hydrogenase), ldh (lactate dehydrogenase), ack (acetate kinase), one putative type IV alcohol dehydrogenase, two putative cyclic AMP binding proteins, three putative Hpr-like proteins, and a putative Hpr serine kinase. By contrast, four genes displayed >10-fold-reduced levels of expression when grown on cellobiose at dilution rates of >0.05 h(-1): cipA (cellulosomal scaffolding protein), celS (exoglucanase), manA (mannanase), and a second type IV alcohol dehydrogenase. The data suggest that at least some cellulosomal components are transcriptionally regulated but that differences in expression with growth rate or among substrates do not directly account for observed changes in fermentation end product distribution.

Bacterial Proteins↗
PubMed2006

Effect of simultaneous inoculation with yeast and bacteria on fermentation kinetics and key wine parameters of cool-climate chardonnay.

Inoculating grape musts with wine yeast and lactic acid bacteria (LAB) concurrently in order to induce simultaneous alcoholic fermentation (AF) and malolactic fermentation (MLF) can be an efficient alternative to overcome potential inhibition of LAB in wines because of high ethanol concentrations and reduced nutrient content. In this study, the simultaneous inoculation of yeast and LAB into must was compared with a traditional vinification protocol, where MLF was induced after completion of AF. For this, two suitable commercial yeast-bacterium combinations were tested in cool-climate Chardonnay must. The time courses of glucose and fructose, acetaldehyde, several organic acids, and nitrogenous compounds were measured along with the final values of other key wine parameters. Sensory evaluation was done after 12 months of storage. The current study could not confirm a negative impact of simultaneous AF/MLF on fermentation success and kinetics or on final wine parameters. While acetic acid concentrations were slightly increased in wines after simultaneous AF/MLF, the differences were of neither practical nor legal significance. No statistically significant differences were found with regard to the final values of pH or total acidity and the concentrations of ethanol, acetaldehyde, glycerol, citric and lactic acids, and the nitrogen compounds arginine, ammonia, urea, citrulline, and ornithine. Sensory evaluation by a semiexpert panel confirmed the similarity of the wines. However, simultaneous inoculation led to considerable reductions in overall fermentation durations. Furthermore, differences of physiological and microbiological relevance were found. Specifically, we report the vinification of "super-dry" wines devoid of glucose and fructose after simultaneous inoculation of yeast and bacteria.

Climate↗
PubMed1966

Pilot plant glycerol production with a slow-feed osmophilic yeast fermentation.

A slow feed batch fermentation is described for the production of glycerol from sugar. The conversion efficiency was approximately 1 mole of glycerol produced per mole of glucose utilized after the cell growth phase. The glycerol production phase was extended several-fold by periodic glucose addition. The yeast cell count remained constant during this time as limited by phosphate, a deficiency required for an efficient glycerol fermentation. A small amount of phosphate was supplied during the extended fermentation, maintaining an active culture, by the normal autolysis of spent cells. Interfering or inhibitory by-products did not accumulate, and the osmophilic yeasts are tolerant of high glycerol concentrations. These factors combined to allow a particularly efficient fermentation well suited to product enrichment by supplying large quantities of substrate over an extended period.

Carbon Dioxide↗
PubMed1969

Some factors affecting fermentation capacity and net growth of rumen microorganisms.

The fermentation rate of rumen microorganisms obtained from a sheep fed on hay was determined at different hours after feeding. The highest rate of fermentation was obtained immediately after feeding whereas the lowest rate was always before feeding. The effect of adding glucose, sucrose, and starch alone or in combination with urea, sodium citrate, or succinic acid on the maximal fermentation rate was studied. In no case was the maximal fermentation rate greater than that obtained in the presence of concentrate and straw. An investigation on the effect of C/N ratios, dilution, and substrate concentration on the net growth of rumen microorganisms was undertaken. The highest net growth value was obtained with a C/N ratio of 16.5 to 17.5, which is the ratio found for the ration fed to the animal. It was possible to increase net growth approximately threefold by using the proper dilution and fivefold by using the proper feed concentration.

Animal Feed↗
PubMed1961

Theoretical design of continuous antibiotic fermentation units.

A graphic method of predicting antibiotic yields in continuous flow reactors is presented and discussed using the novobiocin fermentation as a model process. Extension to other antibiotic fermentations and steroid bioconversions is emphasized. In the case of the novobiocin fermentation it was concluded that a combination of one growth stage and one or two antibiotic production stages would be the most economic reactor system to conduct such a fermentation on a continuous basis.

Anti-Bacterial Agents↗
PubMed1961

Cacao fermentation. V. Yeasts isolated from cacao beans during the curing process.

Cacao beans must be subjected to fermentation before they are used in making chocolate, and their commercial value is related to a proper procedure. Saccharomyces rosei, Hansenula anomala, Pichia fermentans, Pichia membranaefaciens, and Trichosporon cutaneum were found in fermenting cacao beans. All species isolated during the investigation grew on cacao pulp, but only S. rosei, H. anomala, and P. fermentans exhibited fermenting capacity on the sugars of cacao pulp. Species of the genus Saccharomyces were identified as the agents responsible for the alcoholic phase of the cacao fermentation.

Cacao↗
PubMed1973

Glucose fermentation products in Ruminococcus albus grown in continuous culture with Vibrio succinogenes: changes caused by interspecies transfer of H 2 .

The influence of a H(2)-utilizing organism, Vibrio succinogenes, on the fermentation of limiting amounts of glucose by a carbohydrate-fermenting, H(2)-producing organism, Ruminococcus albus, was studied in continuous cultures. Growth of V. succinogenes depended on the production of H(2) from glucose by R. albus. V. succinogenes used the H(2) produced by R. albus to obtain energy for growth by reducing fumarate in the medium. Fumarate was not metabolized by R. albus alone. The only products detected in continuous cultures of R. albus alone were acetate, ethanol, and H(2). CO(2) was not measured. The only products detected in the mixed cultures were acetate and succinate. No free H(2) was produced. No formate or any other volatile fatty acid, no succinate or other dicarboxylic acids, lactate, alcohols other than ethanol, pyruvate, or other keto-acids, acetoin, or diacetyl were detected in cultures of R. albus alone or in mixed cultures. The moles of product per 100 mol of glucose fermented were approximately 69 for ethanol, 74 for acetate, 237 for H(2) for R. albus alone and 147 for acetate and 384 for succinate for the mixed culture. Each mole of succinate is equivalent to the production of 1 mol of H(2) by R. albus. Thus, in the mixed cultures, ethanol production by R. albus is eliminated with a corresponding increase in acetate and H(2) formation. The mixed-culture pattern is consistent with the hypothesis that nicotinamide adenine dinucleotide (reduced form), formed during glycolysis by R. albus, is reoxidized during ethanol formation when R. albus is grown alone and is reoxidized by conversion to nicotinamide adenine dinucleotide and H(2) when R. albus is grown with V. succinogenes. The ecological significance of this interspecies transfer of H(2) gas and the theoretical basis for its causing changes in fermentation patterns of R. albus are discussed.

Acetates↗
PubMed1973

Glucose utilization and ethanolic fermentation by wild type and extrachromosomal mutants of Neurospora crassa.

Logarithmic growth rates, maximal biomass, specific glucose utilization rates, and ethanol accumulation were measured in aerobic cultures of wild type and extrachromosomal mutants of Neurospora crassa. Maximal biomass and ethanol accumulation of wild type and [mi-1] were proportional to the initial glucose concentration in the range of 2 to 10%. The specific rates of glucose utilization by the mutants were 13- to 20-fold greater than those of wild type in young cultures. The specific rates of glucose utilization by wild type, however, were increased threefold by increasing the ammonium ion concentration in the preculture medium. The suppressor gene f(+) suppressed the excessive glucose utilization and enhanced the growth rate and maximal biomass of [mi-1]. When the mutants were utilizing glucose at excessive rates, ethanol did not appear in the culture medium. Ethanol accumulation was maximum at stationary phase or thereafter, but there was little difference between the maxima of the mutants and wild type. The molar efficiency of the conversion of glucose to ethanol during the entire culture period of wild type and mutants was about 50% and, in the latter stages of fermentation, approached 100%. Replacement of ammonium ion by nitrate in the culture medium suppressed ethanol accumulation by wild type. The relationship of these results to previous observations on respiratory adaptation are discussed. We suggest that the Pasteur effect, the inhibition of fermentation by respiration, may be operative in N. crassa. Factors such as nitrogen source and concentration and oxygen tension, which may serve primarily to regulate the amount and form of respiration would, therefore, indirectly regulate fermentation. The mutants, although transiently deficient in terminal respiratory activity, do not accumulate more ethanol than wild type and, therefore, apparently do not ferment in excess to obtain additional adenosine 5'-triphosphate. We suggest that the excess activity of the alternate form of respiration of the mutants may be related to their excessive rate of glucose utilization by way of the pentose phosphate pathway and the oxidation of excess reduced nicotinamide adenine dinucleotide.

Aerobiosis↗
PubMed1974

Two pathways of glutamate fermentation by anaerobic bacteria.

Two pathways are involved in the fermentation of glutamate to acetate, butyrate, carbon dioxide, and ammonia-the methylaspartate and the hydroxyglutarate pathways which are used by Clostridium tetanomorphum and Peptococcus aerogenes, respectively. Although these pathways give rise to the same products, they are easily distinguished by different labeling patterns of the butyrate when [4-(14)C]glutamate is used as substrate. Schmidt degradation of the radioactive butyrate from C. tetanomorphum yielded equally labeled propionate and carbon dioxide, whereas nearly all the radioactivity of the butyrate from P. aerogenes was recovered in the corresponding propionate. This procedure was used as a test for the pathway of glutamate fermentation by 15 strains (9 species) of anaerobic bacteria. The labeling patterns of the butyrate indicate that glutamate is fermented via the methylaspartate pathway by C. tetani, C. cochlearium, and C. saccarobutyricum, and via the hydroxyglutarate pathway by Acidaminococcus fermentans, C. microsporum, Fusobacterium nucleatum, and F. fusiformis. Enzymes specific for each pathway were assayed in crude extracts of the above organisms. 3-Methylaspartase was found only in clostridia which use the methylaspartate pathway, including Clostridium SB4 and C. sticklandii, which probably degrade glutamate to acetate and carbon dioxide by using a second amino acid as hydrogen acceptor. High levels of 2-hydroxyglutarate dehydrogenase were found exclusively in organisms that use the hydroxyglutarate pathway. The data indicate that only two pathways are involved in the fermentation of glutamate by the bacteria analyzed. The methylaspartate pathway appears to be used only by species of Clostridium, whereas the hydroxyglutarate pathway is used by representatives of several genera.

Acetates↗
PubMed1978

Fermentation of fumarate and L-malate by Clostridium formicoaceticum.

The fermentation of fumarate and L-malate by Clostridium formicoaceticum was investigated. Growing and nongrowing cells degraded fumarate by dismutation to succinate, acetate, and CO2; on the other hand, only small amounts of succinate were detected when the organism was grown on L-malate. This dicarboxylic acid was mainly converted to acetate and CO2. The fermentation balances were modified if bicarbonate or formate were present in the medium. When C. formicoaceticum was grown in the presence of both dicarboxylic acids, fumarate was consumed before L-malate. The latter was mainly converted to acetate, whereas fumarate was fermented to acetate and succinate. Molar growth yields were determined to be 6 g of dry weight per mol of fumarate and 8 g of dry weight per mol of L-malate fermented.

Acetates↗
PubMed1981

Branched-chain amino acid fermentation by a marine spirochete: strategy for starvation survival.

An anaerobic marine spirochete (strain MA-2) fermented glucose and formed ethanol, acetic acid, CO(2), and H(2) as end products. The organism required carbohydrates as growth substrates. Amino acids did not support the growth of strain MA-2. However, when the spirochete was grown in media containing branched-chain amino acids and glucose, significant quantities of 4- and 5-carbon branched-chain volatile fatty acids were formed in addition to products of glucose fermentation. Smaller quantities of branched-chain alcohols were also formed under these conditions. The spirochete converted l-valine, l-isoleucine, and l-leucine to isobutyric, 2-methylbutyric, and isovaleric acids, respectively. CO(2) formation accompanied each of these conversions. Spirochete MA-2 did not require branched-chain amino acids for growth, but these compounds could serve as sole sources of nitrogen for the organism. In addition, the survival of starving cells (no growth substrate available) of spirochete MA-2 was prolonged significantly when l-valine, l-isoleucine, and l-leucine were present in starvation media. Starving cells fermented these amino acids, forming adenosine 5'-triphosphate and branched-chain fatty acids. Our findings indicate that energy derived from amino acid fermentation allows the spirochete to survive periods of growth substrate starvation. Apparently, dissimilation of branched-chain amino acids can provide this bacterium with maintenance energy for cell functions not related to growth. In its natural environment spirochete MA-2 may catabolize branched-chain amino acids as a strategy for survival when growth substrates are not available.

Adenosine Triphosphate↗
PubMed1982

Fermentation and anaerobic respiration by Rhodospirillum rubrum and Rhodopseudomonas capsulata.

Rhodospirillum rubrum and Rhodopseudomonas capsulata were able to grow anaerobically in the dark either by a strict mixed-acid fermentation of sugars or, in the presence of an appropriate electron acceptor, by an energy-linked anaerobic respiration. Both species fermented fructose without the addition of accessory oxidants, but required the initial presence of bicarbonate before fermentative growth could begin. Major products of R. rubrum fermentation were succinate, acetate, propionate, formate, hydrogen, and carbon dioxide; R. capsulata produced major amounts of lactate, acetate, succinate, hydrogen, and carbon dioxide. R. rubrum and R. capsulata were also capable of growing strictly through anaerobic, respiratory mechanisms. Nonfermentable substrates, such as succinate, malate, or acetate, supported growth only in the presence of an electron acceptor such as dimethyl sulfoxide or trimethylamine oxide. Carbon dioxide and dimethyl sulfide were produced during growth of R. rubrum and R. capsulata on succinate plus dimethyl sulfoxide. Molar growth yields from cultures grown anaerobically in the dark on fructose plus dimethyl sulfoxide were 3.8 to 4.6 times higher than values obtained from growth on fructose alone and were 56 to 60% of the values obtained from aerobic, respiratory growth with fructose. Likewise, molar growth yields from anaerobic, respiratory growth conditions with succinate plus dimethyl sulfoxide were 51 to 54% of the values obtained from aerobic, respiratory growth with succinate. The data indicate that dimethyl sulfoxide or trimethylamine oxide as a terminal oxidant is approximately 33 to 41% as efficient as O(2) in conserving energy through electron transport-linked respiration.

Adenosine Triphosphate↗