Parametric studies of ethanol production form xylose and other sugars by recombinant Escherichia coli

Design of a stable ethanologenic bacterial strain without heterologous plasmids and antibiotic resistance genes for efficient ethanol production from concentrated dairy waste

Engineering sustainable bioprocesses that convert abundant waste into fuels is pivotal for efficient production of renewable energy. We previously engineered an Escherichia coli strain for optimized bioethanol production from lactose-rich wastewater like concentrated whey permeate (CWP), a dairy effluent obtained from whey valorization processes. Although attractive fermentation performances were reached, significant improvements are required to eliminate recombinant plasmids, antibiotic resistances and inducible promoters, and increase ethanol tolerance. Here, we report a new strain with chromosomally integrated ethanologenic pathway under the control of a constitutive promoter, without recombinant plasmids and resistance genes. The strain showed extreme stability in 1-month subculturing, with CWP fermentation performances similar to the ethanologenic plasmid-bearing strain. We then investigated conditions enabling efficient ethanol production and sugar consumption by changing inoculum size and CWP concentration, revealing toxicity- and nutritional-related bottlenecks. The joint increase of ethanol tolerance, via adaptive evolution, and supplementation of small ammonium sulphate amounts (0.05% w/v) enabled a fermentation boost with 6.6% v/v ethanol titer, 1.2 g/L/h rate, 82.5% yield, and cell viability increased by three orders of magnitude. Our strain has attractive features for industrial settings and represents a relevant improvement in the existing ethanol production biotechnologies.

Butyric Acid Production by Fermentation: Employing Potential of the Novel Clostridium tyrobutyricum Strain NRRL 67062

In this study, the ability of a novel strain of Clostridium tyrobutyricum NRRL 67062 to produce butyric acid during glucose fermentation was evaluated. The strain was evaluated for substrate and product inhibition in batch experiments using anaerobic tubes. To characterize glucose inhibition, initial glucose concentrations ranging from 60 to 250 g L−1 were used, and it was demonstrated that a glucose concentration of 250 g L−1 exerted strong inhibition on cell growth and fermentation. To evaluate butyric acid inhibition, the culture was challenged with 5–50 g L−1 of butyric acid at an initial pH of 6.5. These experiments were performed without pH control. When challenged with a butyric acid concentration of 50 g L−1, cell growth was slow; however, it produced 8.25 g L−1 of butyric acid. This suggested that the butyric acid tolerance of the culture was 58 g L−1. In a scaled-up batch experiment, which was performed in a 2.5 L fermentor with an initial glucose concentration of 100 g L−1, the pH was controlled at 6.5. In this experiment, the strain produced 57.86 g L−1 of butyric acid and 12.88 g L−1 of acetic acid, thus producing 70.74 g L−1 of total acids with a productivity of 0.69 g·L−1·h−1. A concentration of 70.74 g L−1 of acids equates to a yield of 0.71 g of acid per g consumed glucose. The maximum cell concentration was 3.80 g L−1, which may have been the reason for high productivity in the batch culture. Finally, corn steep liquor (CSL; a commercial nutrient solution) provided greater growth and acid production than the refined medium.

Metabolic engineering of Escherichia coli for the production of butyric acid at high titer and productivity

BACKGROUND

Several anaerobic bacteria produce butyric acid, a commodity chemical with use in chemical, pharmaceutical, food and feed industries, using complex media with acetate as a co-product. Butyrate titer of various recombinant Escherichia coli did not exceed 10 g l^-1 in batch fermentations in any of the media tested.

RESULTS

A recombinant E. coli (strain LW393) that produced butyrate as the major fermentation product was constructed with genes from E. coli, Clostridium acetobutylicum and Treponema denticola. Strain LW393 produced 323 ± 6 mM (28.4 ± 0.4 g l^-1) butyric acid in batch fermentations in mineral salt medium with glucose as C source at a yield of 0.37 ± 0.01 g (g glucose consumed)^-1. Butyrate accounted for 90% of the total products produced by the culture. Supplementing this medium with yeast extract further increased butyric acid titer to 375 ± 4 mM. Average volumetric productivity of butyrate with xylose as C source was 0.89 ± 0.07 g l^-1 h^-1.

CONCLUSIONS

The butyrate titer reported in this study is about 2.5-3-times higher than the values reported for other recombinant E. coli and this is achieved in mineral salt medium with an expectation of lower purification and production cost of butyrate.

Metabolic engineering of Escherichia coli to produce succinate from soybean hydrolysate under anaerobic conditions

It is of great economic interest to produce succinate from low-grade carbon sources, which can enhance the competitiveness of the biological route. In this study, succinate producer Escherichia coli CT550/pHL413KF1 was further engineered to efficiently use the mixed sugars from non-food based soybean hydrolysate to produce succinate under anaerobic conditions. Since many common E. coli strains fail to use galactose anaerobically even if they can use it aerobically, the glucose, and galactose related sugar transporters were deactivated individually and evaluated. The PTS system was found to be important for utilization of mixed sugars, and galactose uptake was activated by deactivating ptsG. In the ptsG- strain, glucose, and galactose were used simultaneously. Glucose was assimilated mainly through the mannose PTS system while galactose was transferred mainly through GalP in a ptsG- strain. A new succinate producing strain, FZ591C which can efficiently produce succinate from the mixed sugars present in soybean hydrolysate was constructed by integration of the high succinate yield producing module and the galactose utilization module into the chromosome of the CT550 ptsG- strain. The succinate yield reached 1.64 mol/mol hexose consumed (95% of maximum theoretical yield) when a mixed sugars feedstock was used as a carbon source. Based on the three monitored sugars, a nominal succinate yield of 1.95 mol/mol was observed as the strain can apparently also use some other minor sugars in the hydrolysate. In this study, we demonstrate that FZ591C can use soybean hydrolysate as an inexpensive carbon source for high yield succinate production under anaerobic conditions, giving it the potential for industrial application.

Fermentation of dihydroxyacetone by engineered Escherichia coli and Klebsiella variicola to products

Methane can be converted to triose dihydroxyacetone (DHA) by chemical processes with formaldehyde as an intermediate. Carbon dioxide, a by-product of various industries including ethanol/butanol biorefineries, can also be converted to formaldehyde and then to DHA. DHA, upon entry into a cell and phosphorylation to DHA-3-phosphate, enters the glycolytic pathway and can be fermented to any one of several products. However, DHA is inhibitory to microbes due to its chemical interaction with cellular components. Fermentation of DHA to d-lactate by Escherichia coli strain TG113 was inefficient, and growth was inhibited by 30 g⋅L^-1 DHA. An ATP-dependent DHA kinase from Klebsiella oxytoca (pDC117d) permitted growth of strain TG113 in a medium with 30 g⋅L^-1 DHA, and in a fed-batch fermentation the d-lactate titer of TG113(pDC117d) was 580 ± 21 mM at a yield of 0.92 g⋅g^-1 DHA fermented. Klebsiella variicola strain LW225, with a higher glucose flux than E. coli, produced 811 ± 26 mM d-lactic acid at an average volumetric productivity of 2.0 g^-1⋅L^-1⋅h^-1 Fermentation of DHA required a balance between transport of the triose and utilization by the microorganism. Using other engineered E. coli strains, we also fermented DHA to succinic acid and ethanol, demonstrating the potential of converting CH₄ and CO₂ to value-added chemicals and fuels by a combination of chemical/biological processes.

Experimental evolution reveals an effective avenue to release catabolite repression via mutations in XylR

Microbial production of fuels and chemicals from lignocellulosic biomass provides promising biorenewable alternatives to the conventional petroleum-based products. However, heterogeneous sugar composition of lignocellulosic biomass hinders efficient microbial conversion due to carbon catabolite repression. The most abundant sugar monomers in lignocellulosic biomass materials are glucose and xylose. Although industrial Escherichia coli strains efficiently use glucose, their ability to use xylose is often repressed in the presence of glucose. Here we independently evolved three E. coli strains from the same ancestor to achieve high efficiency for xylose fermentation. Each evolved strain has a point mutation in a transcriptional activator for xylose catabolic operons, either CRP or XylR, and these mutations are demonstrated to enhance xylose fermentation by allelic replacements. Identified XylR variants (R121C and P363S) have a higher affinity to their DNA binding sites, leading to a xylose catabolic activation independent of catabolite repression control. Upon introducing these amino acid substitutions into the E. coli D-lactate producer TG114, 94% of a glucose-xylose mixture (50 g⋅L-1 each) was used in mineral salt media that led to a 50% increase in product titer after 96 h of fermentation. The two amino acid substitutions in XylR enhance xylose utilization and release glucose-induced repression in different E. coli hosts, including wild type, suggesting its potential wide application in industrial E. coli biocatalysts.

Fermentation of lactose to ethanol in cheese whey permeate and concentrated permeate by engineered Escherichia coli

BACKGROUND

Whey permeate is a lactose-rich effluent remaining after protein extraction from milk-resulting cheese whey, an abundant dairy waste. The lactose to ethanol fermentation can complete whey valorization chain by decreasing dairy waste polluting potential, due to its nutritional load, and producing a biofuel from renewable source at the same time. Wild type and engineered microorganisms have been proposed as fermentation biocatalysts. However, they present different drawbacks (e.g., nutritional supplements requirement, high transcriptional demand of recombinant genes, precise oxygen level, and substrate inhibition) which limit the industrial attractiveness of such conversion process. In this work, we aim to engineer a new bacterial biocatalyst, specific for dairy waste fermentation.

RESULTS

We metabolically engineered eight Escherichia coli strains via a new expression plasmid with the pyruvate-to-ethanol conversion genes, and we carried out the selection of the best strain among the candidates, in terms of growth in permeate, lactose consumption and ethanol formation. We finally showed that the selected engineered microbe (W strain) is able to efficiently ferment permeate and concentrated permeate, without nutritional supplements, in pH-controlled bioreactor. In the conditions tested in this work, the selected biocatalyst could complete the fermentation of permeate and concentrated permeate in about 50 and 85 h on average, producing up to 17 and 40 g/l of ethanol, respectively.

CONCLUSIONS

To our knowledge, this is the first report showing efficient ethanol production from the lactose contained in whey permeate with engineered E. coli. The selected strain is amenable to further metabolic optimization and represents an advance towards efficient biofuel production from industrial waste stream.

Production of d-lactate from sugarcane bagasse and corn stover hydrolysates using metabolic engineered Escherichia coli strains

In this study, the lactogenic Escherichia coli strain JU15 was used and modified to produce d-lactate (d-LA) from plant hydrolysates with a minimal nutrient addition in pH controlled fermenters. Results showed that strain JU15 produces d-LA with high yield and productivity in laboratory simulated hydrolysate media and actual sugar cane bagasse hemicellulosic hydrolysate. Strain JU15 showed sequential carbon source utilization and acetic acid production. The l-lactic and acetic acid production pathways were deleted in JU15, resulting strain AV03 (JU15 ΔpoxB, ΔackA-pta, ΔmgsA), which showed simultaneous consumption of glucose and xylose and no acetic acid production in the simulated hydrolysate. The d-LA yield from hydrolysate sugars was close to 0.95gD-LA/gsugars in all cases. Our results show that d-LA can be produced from plant hydrolysates in simple batch fermentation processes with a high productivity using engineered E. coli strains at fermenter scales from 0.2 up to 10L.

Recent trends in bioethanol production from food processing byproducts

The widespread use of corn starch and sugarcane as sources of sugar for the production of ethanol via fermentation may negatively impact the use of farmland for production of food. Thus, alternative sources of fermentable sugars, particularly from lignocellulosic sources, have been extensively investigated. Another source of fermentable sugars with substantial potential for ethanol production is the waste from the food growing and processing industry. Reviewed here is the use of waste from potato processing, molasses from processing of sugar beets into sugar, whey from cheese production, byproducts of rice and coffee bean processing, and other food processing wastes as sugar sources for fermentation to ethanol. Specific topics discussed include the organisms used for fermentation, strategies, such as co-culturing and cell immobilization, used to improve the fermentation process, and the use of genetic engineering to improve the performance of ethanol producing fermenters.

Fermentation of sweet sorghum derived sugars to butyric acid at high titer and productivity by a moderate thermophile Clostridium thermobutyricum at 50°C

In this study, a moderate thermophile Clostridium thermobutyricum is shown to ferment the sugars in sweet sorghum juice treated with invertase and supplemented with tryptone (10 g L(-1)) and yeast extract (10 g L(-1)) at 50°C to 44 g L(-1) butyrate at a calculated highest volumetric productivity of 1.45 g L(-1)h(-1) (molar butyrate yield of 0.85 based on sugars fermented). This volumetric productivity is among the highest reported for batch fermentations. Sugars from acid and enzyme-treated sweet sorghum bagasse were also fermented to butyrate by this organism with a molar yield of 0.81 (based on the amount of cellulose and hemicellulose). By combining the results from juice and bagasse, the calculated yield of butyric acid is approximately 90 kg per tonne of fresh sweet sorghum stalk. This study demonstrates that C. thermobutyricum can be an effective microbial biocatalyst for production of bio-based butyrate from renewable feedstocks at 50°C.

Genotypic and phenotypic diversity of polyhydroxybutyrate (PHB) producing Pseudomonas putida isolates of Chhattisgarh region and assessment of its phosphate solubilizing ability

A diverse and versatile spectrum of metabolic activities among isolates of fluorescent Pseudomonas putida indicates their adaptability to various niches. These polyhydroxybutyrate producing and phosphate solubilizing isolates showed a high level of functional and genetic versatility among themselves. One of the potential P. putida isolate P132 can contribute as a candidate agent for both biocontrol and PGPR applications. Identified as one of the most efficient PHB producer and phosphate solubilizer, in vitro detection of P132 showed the presence of genes for phenazine, pyrrolnitrin, pyoluteorin and 2,4 diacetylphloroglucinol along with polyhydroxyalkanoate.

Ag43-mediated display of a thermostable β-glucosidase in Escherichia coli and its use for simultaneous saccharification and fermentation at high temperatures

Background

The autotransporter (AT) system can potentially be used in the secretion of saccharolytic enzymes for the production of lignocellulosic biofuels and chemicals using Escherichia coli. Although ATs share similar structural characteristics, their capacity for secreting heterologous proteins widely varies. Additionally, the saccharolytic enzyme selected to be secreted should match the cell growth or cell fermentation conditions of E. coli.

Results

In the search for an AT that suits the physiological performance of the homo-ethanologenic E. coli strain MS04, an expression plasmid based on the AT antigen 43 (Ag43) from E. coli was developed. The β-glucosidase BglC from the thermophile bacterium Thermobifida fusca was displayed on the outer membrane of the E. coli strain MS04 using the Ag43 system (MS04/pAg43BglC). This strain was used to hydrolyze and ferment 40 g/L of cellobiose in mineral media to produce 16.65 g/L of ethanol in 48 h at a yield of 81% of the theoretical maximum. Knowing that BglC shows its highest activity at 50°C and retains more than 70% of its activity at pH 6, therefore E. coli MS04/pAg43BglC was used to ferment crystalline cellulose (Avicel) in a simultaneous saccharification and fermentation (SSF) process using a commercial cocktail of cellulases (endo and exo) at pH 6 and at a relatively high temperature for E. coli (45°C). As much as 22 g/L of ethanol was produced in 48 h.

Conclusions

The Ag43-BglC system can be used in E. coli strains without commercial β-glucosidases, reducing the quantities of commercial enzymes needed for the SSF process. Furthermore, the present work shows that E. coli cells are able to ferment sugars at 45°C during the SSF process using 40 g/L of Avicel, reducing the gap between the working conditions of the commercial saccharolytic enzymes and ethanologenic E. coli.

Investigation of ptsG gene in response to xylose utilization in Corynebacterium glutamicum

Corynebacterium glutamicum strains NC-2 were able to grow on xylose as sole carbon sources in our previous work. Nevertheless, it exhibited the major shortcoming that the xylose consumption was repressed in the presence of glucose. So far, regarding C. glutamicum, there are a number of reports on ptsG gene, the glucose-specific transporter, involved in glucose metabolism. Recently, we found ptsG had influence on xylose utilization and investigated the ptsG gene in response to xylose utilization in C. glutamicum with the aim to improve xylose consumption and simultaneously utilized glucose and xylose. The ptsG-deficient mutant could grow on xylose, while exhibiting noticeably reduced growth on xylose as sole carbon source. A mutant deficient in ptsH, a general PTS gene, exhibited a similar phenomenon. When complementing ptsG gene, the mutant ΔptsG-ptsG restored the ability to grow on xylose similarly to NC-2. These indicate that ptsG gene is not only essential for metabolism on glucose but also important in xylose utilization. A ptsG-overexpressing recombinant strain could not accelerate glucose or xylose metabolism. When strains were aerobically cultured in a sugar mixture of glucose and xylose, glucose and xylose could not be utilized simultaneously. Interestingly, the ΔptsG strain could co-utilize glucose and xylose under oxygen-deprived conditions, though the consumption rate of glucose and xylose dramatically declined. It was the first report of ptsG gene in response to xylose utilization in C. glutamicum.

Saccharification and fermentation of Sugar Cane bagasse by Klebsiella oxytoca P2 containing chromosomally integrated genes encoding the Zymomonas mobilis ethanol pathway

Pretreatment of sugar cane bagasse is essential for a simultaneous saccharification and fermentation (SSF) process which uses recombinant Klebsiella oxytoca strain P2 and Genencor Spezyme CE. Strain P2 has been genetically engineered to express Zymomonas mobilis genes encoding the ethanol pathway and retains the native ability to transport and metabolize cellobiose (minimizing the need for extracellular cellobiase). In SSF studies with this organism, both the rate of ethanol production and ethanol yield were limited by saccharification at 10 and 20 filter papaer units (FPU) g(-1) acid-treated bagasse. Dilute slurries of biomass were converted to ethanol more efficiently (over 72% of theoretical yield) in simple batch fermentations than slurries containing high solids albeit with the production of lower levels of ethanol. With high solids (i.e., 160 g acid-treated bagasse L(-1)), a combination of 20 FPU cellulase g(-1) bagasse, preincubation under saccharification conditions, and additional grinding (to reduce particle size) were required to produce ca. 40 g ethanol L(-1). Alternatively, almost 40 g ethanol L(-1) was produced with 10 FPU cellulase g(-1) bagasse by incorporating a second saccharification step (no further enzyme addition) followed by a second inoculation and short fermentation. In this way, a theoretical ethanol yield of over 70% was achieved with the production of 20 g ethanol 800 FPU(-1) of commercial cellulase. (c) 1994 John Wiley & Sons, Inc.

Production of medium-chain-length polyhydroxyalkanoates by sequential feeding of xylose and octanoic acid in engineered Pseudomonas putida KT2440

Background

Pseudomonas putida KT2440 is able to synthesize large amounts of medium-chain-length polyhydroxyalkanoates (mcl-PHAs). To reduce the substrate cost, which represents nearly 50% of the total PHA production cost, xylose, a hemicellulose derivate, was tested as the growth carbon source in an engineered P. putida KT2440 strain.

Results

The genes encoding xylose isomerase (XylA) and xylulokinase (XylB) from Escherichia coli W3110 were introduced into P. putida KT2440. The recombinant KT2440 exhibited a XylA activity of 1.47 U and a XylB activity of 0.97 U when grown on a defined medium supplemented with xylose. The cells reached a maximum specific growth rate of 0.24 h^-1 and a final cell dry weight (CDW) of 2.5 g L^-1 with a maximal yield of 0.5 g CDW g^-1 xylose. Since no mcl-PHA was accumulated from xylose, mcl-PHA production can be controlled by the addition of fatty acids leading to tailor-made PHA compositions. Sequential feeding strategy was applied using xylose as the growth substrate and octanoic acid as the precursor for mcl-PHA production. In this way, up to 20% w w^-1 of mcl-PHA was obtained. A yield of 0.37 g mcl-PHA per g octanoic acid was achieved under the employed conditions.

Conclusions

Sequential feeding of relatively cheap carbohydrates and expensive fatty acids is a practical way to achieve more cost-effective mcl-PHA production. This study is the first reported attempt to produce mcl-PHA by using xylose as the growth substrate. Further process optimizations to achieve higher cell density and higher productivity of mcl-PHA should be investigated. These scientific exercises will undoubtedly contribute to the economic feasibility of mcl-PHA production from renewable feedstock.

Cell surface display of a β-glucosidase employing the type V secretion system on ethanologenic Escherichia coli for the fermentation of cellobiose to ethanol

We used the autodisplay system AIDA-I, which belongs to the type V secretion system (TVSS), to display the β-glucosidase BglC from Thermobifida fusca on the outer membrane of the ethanologenic Escherichia coli strain MS04 (MG1655 ∆pflB, ∆adhE, ∆frdA, ∆xylFGH, ∆ldhA, PpflB::pdc (Zm)-adhB (Zm)). MS04 that was transformed with the plasmid pAIDABglCRHis showed cellobiase activity (171 U/g(CDW)) and fermented 40 g/l cellobiose in mineral medium in 60 h with an ethanol yield of 81 % of the theoretical maximum. Whole-cell protease treatment, SDS-PAGE, and Western-blot analysis demonstrated that BglC was attached to the external surface of the outer membrane of MS04. When attached to the cells, BglC showed 93.3 % relative activity in the presence of 40 g/l ethanol and retained 100 % of its activity following 2 days of incubation at 37 °C with the same ethanol concentration. This study shows the potential of the TVSS (AIDA-I) and BglC as tools for the production of lignocellulosic bio-commodities.

Ethanol production from wheat straw by recombinant Escherichia coli strain FBR5 at high solid loading

Ethanol production by a recombinant bacterium from wheat straw (WS) at high solid loading by separate hydrolysis and fermentation (SHF) and simultaneous saccharification and fermentation (SSF) was studied. The yield of total sugars from dilute acid pretreated WS (150 g/L) after enzymatic saccharification was 86.3±1.5 g/L. The pretreated WS was bio-abated by growing a fungal strain aerobically in the liquid portion for 16 h. The recombinant Escherichia coli strain FBR5 produced 41.1±1.1 gethanol/L from non-abated WS hydrolyzate (total sugars, 86.6±0.3 g/L) in 168 h at pH7.0 and 35 °C. The bacterium produced 41.8±0.0 g ethanol/L in 120 h from the bioabated WS by SHF. It produced 41.6±0.7 g ethanol/L in 120 h from bioabated WS by fed-batch SSF. This is the first report of the production of above 4% ethanol from a lignocellulosic hydrolyzate by the recombinant bacterium.

Continuous ethanol production from wheat straw hydrolysate by recombinant ethanologenic Escherichia coli strain FBR5

Continuous production of ethanol from alkaline peroxide pretreated and enzymatically saccharified wheat straw hydrolysate by ethanologenic recombinant Escherichia coli strain FBR5 was investigated under various conditions at controlled pH 6.5 and 35 °C. The strain FBR5 was chosen because of its ability to ferment both hexose and pentose sugars under semi-anaerobic conditions without using antibiotics. The average ethanol produced from the available sugars (21.9-47.8 g/L) ranged from 8.8 to 17.3 g/L (0.28-0.45 g/g available sugars, 0.31-0.48 g/g sugar consumed) with ethanol productivity of 0.27-0.78 g l(-1)h(-1) in a set of 14 continuous culture (CC) runs (16-105 days). During these CC runs, no loss of ethanol productivity was observed. This is the first report on the continuous production of ethanol by the recombinant bacterium from a lignocellulosic hydrolysate.

Adaptive evolution of Escherichia coli inactivated in the phosphotransferase system operon improves co-utilization of xylose and glucose under anaerobic conditions

Modification of the phosphoenolpyruvate/sugar phosphotransferase system (PTS) has shown improvement in sugar coassimilation in Escherichia coli production strains. However, in preliminary experiments under anaerobic conditions, E. coli strains with an inactive PTS and carrying pLOI1594, which encodes pyruvate decarboxylase and alcohol dehydrogenase from Zymomonas mobilis, were unable to grow. These PTS⁻ strains were previously evolved under aerobic conditions to grow rapidly in glucose (PTS⁻ Glucose+ phenotype). Thus, in this work, applying a continuous culture strategy under anaerobic conditions, we generate a new set of evolved PTS⁻ Glucose+ mutants, VH30N1 to VH30N6. Contrary to aerobically evolved mutants, strains VH30N2 and VH30N4 carrying pLOI1594 grew in anaerobiosis; also, their growth capacity was restored in a 100%, showing specific growth rates (μ ~ 0.12 h⁻¹) similar to the PTS+ parental strain (μ = 0.11 h⁻¹). In cultures of VH30N2/pLOI1594 and VH30N4/pLOI1594 using a glucose-xylose mixture, xylose was totally consumed and consumption of sugars occurred in a simultaneous manner indicating that catabolic repression is alleviated in these strains. Also, the efficient sugar coassimilation by the evolved strains caused an increment in the ethanol yields.

Comparing the fermentation performance of Escherichia coli KO11, Saccharomyces cerevisiae 424A(LNH-ST) and Zymomonas mobilis AX101 for cellulosic ethanol production

BACKGROUND

Fermentations using Escherichia coli KO11, Saccharomyces cerevisiae 424A(LNH-ST), and Zymomonas mobilis AX101 are compared side-by-side on corn steep liquor (CSL) media and the water extract and enzymatic hydrolysate from ammonia fiber expansion (AFEX)-pretreated corn stover.

RESULTS

The three ethanologens are able produce ethanol from a CSL-supplemented co-fermentation at a metabolic yield, final concentration and rate greater than 0.42 g/g consumed sugars, 40 g/L and 0.7 g/L/h (0-48 h), respectively. Xylose-only fermentation of the tested ethanologenic bacteria are five to eight times faster than 424A(LNH-ST) in the CSL fermentation.All tested strains grow and co-ferment sugars at 15% w/v solids loading equivalent of ammonia fiber explosion (AFEX)-pretreated corn stover water extract. However, both KO11 and 424A(LNH-ST) exhibit higher growth robustness than AX101. In 18% w/w solids loading lignocellulosic hydrolysate from AFEX pretreatment, complete glucose fermentations can be achieved at a rate greater than 0.77 g/L/h. In contrast to results from fermentation in CSL, S. cerevisiae 424A(LNH-ST) consumed xylose at the greatest extent and rate in the hydrolysate compared to the bacteria tested.

CONCLUSIONS

Our results confirm that glucose fermentations among the tested strains are effective even at high solids loading (18% by weight). However, xylose consumption in the lignocellulosic hydrolysate is the major bottleneck affecting overall yield, titer or rate of the process. In comparison, Saccharomyces cerevisiae 424A(LNH-ST) is the most relevant strains for industrial production for its ability to ferment both glucose and xylose from undetoxified and unsupplemented hydrolysate from AFEX-pretreated corn stover at high yield.

Bioethanol fermentation by recombinant E. coli FBR5 and its robust mutant FBHW using hot-water wood extract hydrolyzate as substrate

Hemicellulose is a potential by-product currently under-utilized in the papermaking industry. It is a hetero-carbohydrate polymer. For hardwood hemicelluloses, D-xylose is the major component upon depolymerization. At SUNY-ESF, wood extracts were obtained by extracting sugar maple wood chips with hot water at an elevated temperature. The wood extracts were then concentrated and acid hydrolyzed. Ethanologenic bacteria, E. coli FBR5, had a good performance in pure xylose medium for ethanol production. However, FBR5 was strongly inhibited in dilute sulfuric acid hydrolyzate of hot-water wood extract. FBR5 was challenged by hot-water wood extract hydrolyzate in this study. After repeated strain adaptation, an improved strain: E. coli FBHW was obtained. Fermentation experiments indicated that FBHW was resistant to the toxicity of hydrolyzate in the fermentation media of concentrated hydrolyzate, and xylose was completely utilized by the strain to produce ethanol. FBHW was grown in the concentrated hydrolyzate without any detoxification treatment and has yielded 36.8g/L ethanol.

ATP limitation in a pyruvate formate lyase mutant of Escherichia coli MG1655 increases glycolytic flux to D-lactate

A derivative strain of Escherichia coli MG1655 for D-lactate production was constructed by deleting the pflB, adhE and frdA genes; this strain was designated "CL3." Results show that the CL3 strain grew 44% slower than its parental strain under nonaerated (fermentative) conditions due to the inactivation of the main acetyl-CoA production pathway. In contrast to E. coli B and W3110 pflB derivatives, we found that the MG1655 pflB derivative is able to grow in mineral media with glucose as the sole carbon source under fermentative conditions. The glycolytic flux was 2.8-fold higher in CL3 when compared to the wild-type strain, and lactate yield on glucose was 95%. Although a low cell mass formed under fermentative conditions with this strain (1.2 g/L), the volumetric productivity of CL3 was 1.31 g/L h. In comparison with the parental strain, CL3 has a 22% lower ATP/ADP ratio. In contrast to wild-type E. coli, the ATP yield from glucose to lactate is 2 ATP/glucose, so CL3 has to improve its glycolytic flux in order to fulfill its ATP needs in order to grow. The aceF deletion in strains MG1655 and CL3 indicates that the pyruvate dehydrogenase (PDH) complex is functional under glucose-fermentative conditions. These results suggest that the pyruvate to acetyl-CoA flux in CL3 is dependent on PDH activity and that the decrease in the ATP/ADP ratio causes an increase in the flux of glucose to lactate.

Deletion of methylglyoxal synthase gene (mgsA) increased sugar co-metabolism in ethanol-producing Escherichia coli

The use of lignocellulose as a source of sugars for bioproducts requires the development of biocatalysts that maximize product yields by fermenting mixtures of hexose and pentose sugars to completion. In this study, we implicate mgsA encoding methylglyoxal synthase (and methylglyoxal) in the modulation of sugar metabolism. Deletion of this gene (strain LY168) resulted in the co-metabolism of glucose and xylose, and accelerated the metabolism of a 5-sugar mixture (mannose, glucose, arabinose, xylose and galactose) to ethanol.

Homolactic fermentation from glucose and cellobiose using Bacillus subtilis

Backgroung

Biodegradable plastics can be made from polylactate, which is a polymer made from lactic acid. This compound can be produced from renewable resources as substrates using microorganisms. Bacillus subtilis is a Gram-positive bacterium recognized as a GRAS microorganism (generally regarded as safe) by the FDA. B. subtilis produces and secretes different kind of enzymes, such as proteases, cellulases, xylanases and amylases to utilize carbon sources more complex than the monosaccharides present in the environment. Thus, B. subtilis could be potentially used to hydrolyze carbohydrate polymers contained in lignocellulosic biomass to produce chemical commodities. Enzymatic hydrolysis of the cellulosic fraction of agroindustrial wastes produces cellobiose and a lower amount of glucose. Under aerobic conditions, B. subtilis grows using cellobiose as substrate.

Results

In this study, we proved that under non-aerated conditions, B. subtilis ferments cellobiose to produce L-lactate with 82% of the theoretical yield, and with a specific rate of L-lactate production similar to that one obtained fermenting glucose. Under fermentative conditions in a complex media supplemented with glucose, B. subtilis produces L-lactate and a low amount of 2,3-butanediol. To increase the L-lactate production of this organism, we generated the B subtilis CH1 alsS^- strain that lacks the ability to synthesize 2,3-butanediol. Inactivation of this pathway, that competed for pyruvate availability, let a 15% increase in L-lactate yield from glucose compared with the parental strain. CH1 alsS^- fermented 5 and 10% of glucose to completion in mineral medium supplemented with yeast extract in four and nine days, respectively. CH1 alsS^- produced 105 g/L of L-lactate in this last medium supplemented with 10% of glucose. The L-lactate yield was up to 95% using mineral media, and the optical purity of L-lactate was of 99.5% since B. subtilis has only one gene (lctE) that exclusively encodes a L-lactate deshydrogenase.

Conclusion

This study shows that by taking advantage of the cellobiose utilization capability and osmotic stress high resistance of B. subtilis, a robust process for L-lactate production can be developed.

Anaerobic respiration in engineered Escherichia coli with an internal electron acceptor to produce fuel ethanol

Environmental concerns and unease with U.S. dependence on foreign oil have renewed interest in converting biomass into fuel ethanol. The volume of plant matter available makes lignocellulose conversion to ethanol desirable, although no one isolated organism has been shown to break bonds in lignocellulose and efficiently metabolize resulting sugars into one product. This work reviews directed engineering coupled with metabolic evolution resulting in microbial biocatalysts that produce up to 45 g L(-1) ethanol in 48 hours in a simple mineral salts medium and that convert various compounds of lignocellulosic materials to ethanol. Mutations contributing to ethanologenesis are discussed along with adding enzymatic capabilities to existing biocatalysts in order to decrease the commercial enzymes required to reduce plant matter into fermentable sugars.

Microbial conversion of sugars from plant biomass to lactic acid or ethanol

Concerns for our environment and unease with our dependence on foreign oil have renewed interest in converting plant biomass into fuels and 'green' chemicals. The volume of plant matter available makes lignocellulose conversion desirable, although no single isolated organism has been shown to depolymerize lignocellulose and efficiently metabolize the resulting sugars into a specific product. This work reviews selected chemicals and fuels that can be produced from microbial fermentation of plant-derived cell-wall sugars and directed engineering for improvement of microbial biocatalysts. Lactic acid and ethanol production are highlighted, with a focus on engineered Escherichia coli.

Metabolic regulation analysis of an ethanologenic Escherichia coli strain based on RT-PCR and enzymatic activities

BACKGROUND

A metabolic regulation study was performed, based upon measurements of enzymatic activities, fermentation performance, and RT-PCR analysis of pathways related to central carbon metabolism, in an ethanologenic Escherichia coli strain (CCE14) derived from lineage C. In comparison with previous engineered strains, this E coli derivative has a higher ethanol production rate in mineral medium, as a result of the elevated heterologous expression of the chromosomally integrated genes encoding PDCZm and ADHZm (pyruvate decarboxylase and alcohol dehydrogenase from Zymomonas mobilis). It is suggested that this behavior might be due to lineage differences between E. coli W and C.

RESULTS

This study demonstrated that the glycolytic flux is controlled, in this case, by reactions outside glycolysis, i.e., the fermentative pathways. Changes in ethanol production rate in this ethanologenic strain result in low organic acid production rates, and high glycolytic and ethanologenic fluxes, that correlate with enhanced transcription and enzymatic activity levels of PDCZm and ADHZm. Furthermore, a higher ethanol yield (90% of the theoretical) in glucose-mineral media was obtained with CCE14 in comparison with previous engineered E. coli strains, such as KO11, that produces a 70% yield under the same conditions.

CONCLUSION

Results suggest that a higher ethanol formation rate, caused by ahigher PDCZm and ADHZm activities induces a metabolic state that cells compensate through enhanced glucose transport, ATP synthesis, and NAD-NADH+H turnover rates. These results show that glycolytic enzymatic activities, present in E. coli W and C under fermentative conditions, are sufficient to contend with increases in glucose consumption and product formation rates.

Specific ethanol production rate in ethanologenic Escherichia coli strain KO11 Is limited by pyruvate decarboxylase

Modification of ethanol productivity and yield, using mineral medium supplemented with glucose or xylose as carbon sources, was studied in ethanologenic Escherichia coli KO11 by increasing the activity of five key carbon metabolism enzymes. KO11 efficiently converted glucose or xylose to ethanol with a yield close to 100% of the theoretical maximum when growing in rich medium. However, when KO11 ferments glucose or xylose in mineral medium, the ethanol yields decreased to only 70 and 60%, respectively. An increase in GALP(Ec) (permease of galactose-glucose-xylose) or PGK(Ec) (phosphoglycerate kinase) activities did not change xylose or glucose and ethanol flux. However, when PDC(Zm) (pyruvate decarboxylase from Zymomonas mobilis) activity was increased 7-fold, the yields of ethanol from glucose or xylose were increased to 85 and 75%, respectively, and organic acid formation rates were reduced. Furthermore, as a response to a reduction in acetate and ATP yield, and a limited PDC(Zm) activity, an increase in PFK(Ec) (phosphofructokinase) or PYK(Bs) (pyruvate kinase from Bacillus stearothermophilus) activity drastically reduced glucose or xylose consumption and ethanol formation flux. This experimental metabolic control analysis showed that ethanol flux in KO11 is negatively controlled by phosphofructokinase and pyruvate kinase, and positively influenced by the PDC(Zm) activity level.

Metabolic engineering of Bacillus subtilis for ethanol production: lactate dehydrogenase plays a key role in fermentative metabolism

Wild-type Bacillus subtilis ferments 20 g/liter glucose in 48 h, producing lactate and butanediol, but not ethanol or acetate. To construct an ethanologenic B. subtilis strain, homologous recombination was used to disrupt the native lactate dehydrogenase (LDH) gene (ldh) by chromosomal insertion of the Zymomonas mobilis pyruvate decarboxylase gene (pdc) and alcohol dehydrogenase II gene (adhB) under the control of the ldh native promoter. The values of the intracellular PDC and ADHII enzymatic activities of the engineered B. subtilis BS35 strain were similar to those found in an ethanologenic Escherichia coli strain. BS35 produced ethanol and butanediol; however, the cell growth and glucose consumption rates were reduced by 70 and 65%, respectively, in comparison to those in the progenitor strain. To eliminate butanediol production, the acetolactate synthase gene (alsS) was inactivated. In the BS36 strain (BS35 delta alsS), ethanol production was enhanced, with a high yield (89% of the theoretical); however, the cell growth and glucose consumption rates remained low. Interestingly, kinetic characterization of LDH from B. subtilis showed that it is able to oxidize NADH and NADPH. The expression of the transhydrogenase encoded by udhA from E. coli allowed a partial recovery of the cell growth rate and an early onset of ethanol production. Beyond pyruvate-to-lactate conversion and NADH oxidation, an additional key physiological role of LDH for glucose consumption under fermentative conditions is suggested. Long-term cultivation showed that 8.9 g/liter of ethanol can be obtained using strain BS37 (BS35 delta alsS udhA+). As far as we know, this is the highest ethanol titer and yield reported with a B. subtilis strain.

Hydrolysis of grapefruit peel waste with cellulase and pectinase enzymes

Approximately 1 million metric tons of grapefruit were processed in the 2003/04 season resulting in 500,000 metric tons of peel waste. Grapefruit peel waste is usually dried, pelletized, and sold as a low-value cattle feed. This study tested different loadings of commercial cellulase and pectinase enzymes and pH levels to hydrolyze grapefruit peel waste to produce sugars. Pectinase and cellulase loadings of 0, 1, 2, 5, and 10mgprotein/g peel dry matter were tested at 45 degrees C. Hydrolyses were supplemented with 2.1mg beta-glucosidase protein/g peel dry matter. Five mg pectinase/g peel dry matter and 2mgcellulase/g peel dry matter were the lowest loadings to yield the most glucose. Optimum pH was 4.8. Cellulose, pectin, and hemicellulose in grapefruit peel waste can be hydrolyzed by pectinase and cellulase enzymes to monomer sugars, which can then be used by microorganisms to produce ethanol and other fermentation products.

Effect of lignocellulose-derived inhibitors on growth of and ethanol production by growth-arrested Corynebacterium glutamicum R

In cellulosic ethanol production, pretreatment of a biomass to facilitate enzymatic hydrolysis inevitably yields fermentation inhibitors such as organic acids, furans, and phenols. With representative inhibitors included in the medium at various concentrations, individually or in various combinations, ethanol production by Corynebacterium glutamicum R under growth-arrested conditions was investigated. In the presence of various inhibitors, the 62 to 100% ethanol productivity retained by the C. glutamicum R-dependent method far exceeded that retained by previously reported methods.

Development of ethanologenic bacteria

The utilization of lignocellulosic biomass as a petroleum alternative faces many challenges. This work reviews recent progress in the engineering of Escherichia coli and Klebsiella oxytoca to produce ethanol from biomass with minimal nutritional supplementation. A combination of directed engineering and metabolic evolution has resulted in microbial biocatalysts that produce up to 45 g L(-1) ethanol in 48 h in a simple mineral salts medium, and convert various lignocellulosic materials to ethanol. Mutations contributing to ethanologenesis are discussed. The ethanologenic biocatalyst design approach was applied to other commodity chemicals, including optically pure D: (-)- and L: (+)-lactic acid, succinate and L: -alanine with similar success. This review also describes recent progress in growth medium development, the reduction of hemicellulose hydrolysate toxicity and reduction of the demand for fungal cellulases.

Performance and stability of ethanologenic Escherichia coli strain FBR5 during continuous culture on xylose and glucose

Escherichia coli FBR5 containing recombinant genes for ethanol production on plasmids that are also required for anaerobic growth was cultivated continuously on 50 g/l xylose or glucose in the absence of antibiotics and without the use of special measures to limit the entry of oxygen into the fermenter. Under chemostat conditions, stable ethanol yields of ca. 80-85% of the theoretical were obtained on both sugars over 26 days at dilution rates of 0.045/h (xylose) and 0.075/h (glucose), with average plasmid retention rates of 96% (xylose) and 97% (glucose). In a continuous fluidized bed fermenter, with the cells immobilized on porous glass beads, the extent of plasmid retention by the free cells fell rapidly, while that of the immobilized cells remained constant. This was shown to be due to diffusion of oxygen through the tubing used to recirculate the medium and free cells. A change to oxygen-impermeable tubing led to a stable high rate of plasmid retention (more than 96% of both the free and immobilized cells) with ethanol yields of ca. 80% on a 50 g/l xylose feed. The maximum permissible level of oxygen availability consistent with high plasmid retention by the strain appears to be of the order of 0.1 mmol per hour per gram dry biomass, based on measurements of the rate of oxygen penetration into the fermenters. Revertant colonies lacking the ethanologenic plasmid were easily detectable by their morphology which correlated well with their lack of ampicillin resistance upon transfer plating.

Construction and expression of an ethanol production operon in Gram-positive bacteria

Pyruvate decarboxylase (PDC), an enzyme central to homoethanol fermentation, catalyses the non-oxidative decarboxylation of pyruvate to acetaldehyde with release of carbon dioxide. PDC enzymes from diverse organisms have different kinetic properties, thermal stability and codon usage that are likely to offer unique advantages for the development of desirable Gram-positive biocatalysts for use in the ethanol industry. To examine this further, pdc genes from bacteria to yeast were expressed in the Gram-positive host Bacillus megaterium. The PDC activity and protein levels were determined for each strain. In addition, the levels of pdc-specific mRNA transcripts and stability of recombinant proteins were assessed. From this analysis, the pdc gene of Gram-positive Sarcina ventriculi was found to be the most advantageous for engineering high-level synthesis of PDC in a Gram-positive host. This gene was thus selected for transcriptional coupling to the alcohol dehydrogenase gene (adh) of Geobacillus stearothermophilus. The resulting Gram-positive ethanol production operon was expressed at high levels in B. megaterium. Extracts from this recombinant were shown to catalyse the production of ethanol from pyruvate.

Purification and characterization of a furfural reductase (FFR) from Escherichia coli strain LYO1—An enzyme important in the detoxification of furfural during ethanol production

Furfural, an inhibitor of ethanol production from hemicellulose acid hydrolysates, is reductively detoxified to furfuryl alcohol by the ethanologenic bacterium Escherichia coli strain LYO1. Furfural reductase was purified 106-fold from this bacterium to approximately 50% homogeneity. It has a native molecular mass of 135 kDa, determined by gel filtration, and subunit molecular mass of approximately 68 kDa, determined by denaturing gel electrophoresis, indicating the holoenzyme is a dimer of two similar if not identical subunits. The enzyme shows strong activity from pH 4 to 8 (optimum pH 7.0), relatively high temperature tolerance (50-55 degrees C), and an apparent Km and Vmax for furfural of 1.5x10(-4)M and 28.5 micromol/min/mg of protein, respectively. It catalyzes the essentially irreversible reduction of furfural with NADPH, is specific for NADPH as cofactor, and is relatively specific for the reduction of furfural and benzaldehyde; 2-acetylfuran, xylose, and glucose were not reduced, while acetaldehyde was reduced at a rate 25-fold lower than furfural. This is the first description of a furfural reductase which, based upon size and substrate specificity, appears to represent a new type of alcohol-aldehyde oxido-reductase. The conversion of relatively toxic furfural to less toxic furfuryl alcohol suggests a beneficial role for this enzyme in mitigating furfural toxicity encountered during ethanol production from lignocellulosic biomass.

Recombinant production of Zymomonas mobilis pyruvate decarboxylase in the haloarchaeon Haloferax volcanii

The unusual physiological properties of archaea (e.g., growth in extreme salt concentration, temperature and pH) make them ideal platforms for metabolic engineering. Towards the ultimate goal of modifying an archaeon to produce bioethanol or other useful products, the pyruvate decarboxylase gene of Zymomonas mobilis (Zm pdc) was expressed in Haloferax volcanii. This gene has been used successfully to channel pyruvate to ethanol in various Gram-negative bacteria, including Escherichia coli. Although the ionic strength of the H. volcanii cytosol differs over 15-fold from that of E. coli, gel filtration and circular dichroism revealed no difference in secondary structure between the ZmPDC protein isolated from either of these hosts. Like the E. coli purified enzyme, ZmPDC from H. volcanii catalyzed the nonoxidative decarboxylation of pyruvate. A decrease in the amount of soluble ZmPDC protein was detected as H. volcanii transitioned from log phase to late stationary phase that was inversely proportional to the amount of pdc-specific mRNA. Based on these results, proteins from non-halophilic organisms can be actively synthesized in haloarchaea; however, post-transcriptional mechanisms present in stationary phase appear to limit the amount of recombinant protein expressed.

Pyruvate formate lyase and acetate kinase are essential for anaerobic growth of Escherichia coli on xylose

During anaerobic growth of bacteria, organic intermediates of metabolism, such as pyruvate or its derivatives, serve as electron acceptors to maintain the overall redox balance. Under these conditions, the ATP needed for cell growth is derived from substrate-level phosphorylation. In Escherichia coli, conversion of glucose to pyruvate yields 2 net ATPs, while metabolism of a pentose, such as xylose, to pyruvate only yields 0.67 net ATP per xylose due to the need for one (each) ATP for xylose transport and xylulose phosphorylation. During fermentative growth, E. coli produces equimolar amounts of acetate and ethanol from two pyruvates, and these reactions generate one additional ATP from two pyruvates (one hexose equivalent) while still maintaining the overall redox balance. Conversion of xylose to acetate and ethanol increases the net ATP yield from 0.67 to 1.5 per xylose. An E. coli pfl mutant lacking pyruvate formate lyase cannot convert pyruvate to acetyl coenzyme A, the required precursor for acetate and ethanol production, and could not produce this additional ATP. E. coli pfl mutants failed to grow under anaerobic conditions in xylose minimal medium without any negative effect on their survival or aerobic growth. An ackA mutant, lacking the ability to generate ATP from acetyl phosphate, also failed to grow in xylose minimal medium under anaerobic conditions, confirming the need for the ATP produced by acetate kinase for anaerobic growth on xylose. Since arabinose transport by AraE, the low-affinity, high-capacity, arabinose/H+ symport, conserves the ATP expended in pentose transport by the ABC transporter, both pfl and ackA mutants grew anaerobically with arabinose. AraE-based xylose transport, achieved after constitutively expressing araE, also supported the growth of the pfl mutant in xylose minimal medium. These results suggest that a net ATP yield of 0.67 per pentose is only enough to provide for maintenance energy but not enough to support growth of E. coli in minimal medium. Thus, pyruvate formate lyase and acetate kinase are essential for anaerobic growth of E. coli on xylose due to energetic constraints.

Reduction of furfural to furfuryl alcohol by ethanologenic strains of bacteria and its effect on ethanol production from xylose

The ethanologenic bacteria Escherichia coli strains KO11 and LYO1, and Klebsiella oxytoca strain P2, were investigated for their ability to metabolize furfural. Using high performance liquid chromatography and 13C-nuclear magnetic resonance spectroscopy, furfural was found to be completely biotransformed into furfuryl alcohol by each of the three strains with tryptone and yeast extract as sole carbon sources. This reduction appears to be constitutive with NAD(P)H acting as electron donor. Glucose was shown to be an effective source of reducing power. Succinate inhibited furfural reduction, indicating that flavins are unlikely participants in this process. Furfural at concentrations >10 mM decreased the rate of ethanol formation but did not affect the final yield. Insight into the biochemical nature of this furfural reduction process may help efforts to mitigate furfural toxicity during ethanol production by ethanologenic bacteria.

Engineering a homo-ethanol pathway in Escherichia coli: increased glycolytic flux and levels of expression of glycolytic genes during xylose fermentation

Replacement of the native fermentation pathway in Escherichia coli B with a homo-ethanol pathway from Zymomonas mobilis (pdc and adhB genes) resulted in a 30 to 50% increase in growth rate and glycolytic flux during the anaerobic fermentation of xylose. Gene array analysis was used as a tool to investigate differences in expression levels for the 30 genes involved in xylose catabolism in the parent (strain B) and the engineered strain (KO11). Of the 4,290 total open reading frames, only 8% were expressed at a significantly higher level in KO11 (P < 0.05). In contrast, over half of the 30 genes involved in the catabolism of xylose to pyruvate were expressed at 1.5-fold- to 8-fold-higher levels in KO11. For 14 of the 30 genes, higher expression was statistically significant at the 95% confidence level (xylAB, xylE, xylFG, xylR, rpiA, rpiB, pfkA, fbaA, tpiA, gapA, pgk, and pykA) during active fermentation (6, 12, and 24 h). Values at single time points for only four of these genes (eno, fbaA, fbaB, and talA) were higher in strain B than in KO11. The relationship between changes in mRNA (cDNA) levels and changes in specific activities was verified for two genes (xylA and xylB) with good agreement. In KO11, expression levels and activities were threefold higher than in strain B for xylose isomerase (xylA) and twofold higher for xylulokinase (xylB). Increased expression of genes involved in xylose catabolism is proposed as the basis for the increase in growth rate and glycolytic flux in ethanologenic KO11.