Engineering Bacillus subtilis for isobutanol production by heterologous Ehrlich pathway construction and the biosynthetic 2-ketoisovalerate precursor pathway overexpression

Development of a CRISPR-Cas9 Tool Kit for Comprehensive Engineering of Bacillus subtilis

The establishment of a clustered regularly interspaced short palindromic repeat (CRISPR)-Cas9 system for strain construction in Bacillus subtilis is essential for its progression toward industrial utility. Here we outline the development of a CRISPR-Cas9 tool kit for comprehensive genetic engineering in B. subtilis In addition to site-specific mutation and gene insertion, our approach enables continuous genome editing and multiplexing and is extended to CRISPR interference (CRISPRi) for transcriptional modulation. Our tool kit employs chromosomal expression of Cas9 and chromosomal transcription of guide RNAs (gRNAs) using a gRNA transcription cassette and counterselectable gRNA delivery vectors. Our design obviates the need for multicopy plasmids, which can be unstable and impede cell viability. Efficiencies of up to 100% and 85% were obtained for single and double gene mutations, respectively. Also, a 2.9-kb hyaluronic acid (HA) biosynthetic operon was chromosomally inserted with an efficiency of 69%. Furthermore, repression of a heterologous reporter gene was achieved, demonstrating the versatility of the tool kit. The performance of our tool kit is comparable with those of systems developed for Escherichia coli and Saccharomyces cerevisiae, which rely on replicating vectors to implement CRISPR-Cas9 machinery.

IMPORTANCE

In this paper, as the first approach, we report implementation of the CRISPR-Cas9 system in Bacillus subtilis, which is recognized as a valuable host system for biomanufacturing. The study enables comprehensive engineering of B. subtilis strains with virtually any desired genotypes/phenotypes and biochemical properties for extensive industrial application.

Engineering alternative isobutanol production platforms

A synthetic inducible operon (IbPSO) expressing alsS, ilvC, ilvD and kivD genes encoding a pathway capable to transform pyruvate into 2-isobutyraldehyde has been designed and two recombinant plasmids named pIZIbPSO and p424IbPSO were constructed. The IbPSO containing plasmids can generate in a single transformation event new recombinant isobutanol producer strains and are useful for testing as suitable hosts wild type bacteria in different culture media. In this way we found that Shimwellia blattae (p424IbPSO) was able to produce in flasks up to 6 g l(-1) of isobutanol using glucose as carbon source. Moreover, for the first time, we have demonstrated that isobutanol can be produced from sucrose using Escherichia coli W (ATCC9367) transformed with pIZIbPSO. These robust recombinant strains were also able to produce isobutanol from a raw carbon source like hydrolysed lignocellulosic biomass.

Production of C4 and C5 branched-chain alcohols by engineered Escherichia. coli

Higher alcohols, longer chain alcohols, contain more than 3 carbon atoms, showed close energy advantages as gasoline, and were considered as the next generation substitution for chemical fuels. Higher alcohol biosynthesis by native microorganisms mainly needs gene expression of heterologous keto acid decarboxylase and alcohol dehydrogenases. In the present study, branched-chain α-keto acid decarboxylase gene from Lactococcus lactis subsp. lactis CICC 6246 (Kivd) and alcohol dehydrogenases gene from Zymomonas mobilis CICC 41465 (AdhB) were transformed into Escherichia coli for higher alcohol production. SDS-PAGE results showed these two proteins were expressed in the recombinant strains. The resulting strain was incubated in LB medium at 37 °C in Erlenmeyer flasks and much more 3-methyl-1-butanol (104 mg/L) than isobutanol (24 mg/L) was produced. However, in 5 g/L glucose-containing medium, the production of two alcohols was similar, 156 and 161 mg/L for C4 (isobutanol) and C5 (3-methyl-1-butanol) alcohol, respectively. Effects of fermentation factors including temperature, glucose content, and α-keto acid on alcohol production were also investigated. The increase of glucose content and the adding of α-keto acids facilitated the production of C4 and C5 alcohols. The enzyme activities of pure Kivd on α-ketoisovalerate and α-ketoisocaproate were 26.77 and 21.24 μmol min(-1) mg(-1), respectively. Due to its ability on decarboxylation of α-ketoisovalerate and α-ketoisocaproate, the recombinant E. coli strain showed potential application on isoamyl alcohol and isobutanol production.

Eliminating the isoleucine biosynthetic pathway to reduce competitive carbon outflow during isobutanol production by Saccharomyces cerevisiae

Background

Isobutanol is an important biorefinery target alcohol that can be used as a fuel, fuel additive, or commodity chemical. Baker’s yeast, Saccharomyces cerevisiae, is a promising organism for the industrial manufacture of isobutanol because of its tolerance for low pH and resistance to autolysis. It has been reported that gene deletion of the pyruvate dehydrogenase complex, which is directly involved in pyruvate metabolism, improved isobutanol production by S. cerevisiae. However, the engineering strategies available for S. cerevisiae are immature compared to those available for bacterial hosts such as Escherichia coli, and several pathways in addition to pyruvate metabolism compete with isobutanol production.

Results

The isobutyrate, pantothenate or isoleucine biosynthetic pathways were deleted to reduce the outflow of carbon competing with isobutanol biosynthesis in S. cerevisiae. The judicious elimination of these competing pathways increased isobutanol production. ILV1 encodes threonine ammonia-lyase, the enzyme that converts threonine to 2-ketobutanoate, a precursor for isoleucine biosynthesis. S. cerevisiae mutants in which ILV1 had been deleted displayed 3.5-fold increased isobutanol productivity. The ΔILV1 strategy was further combined with two previously established engineering strategies (activation of two steps of the Ehrlich pathway and the transhydrogenase-like shunt), providing 11-fold higher isobutanol productivity as compared to the parent strain. The titer and yield of this engineered strain was 224 ± 5 mg/L and 12.04 ± 0.23 mg/g glucose, respectively.

Conclusions

The deletion of competitive pathways to reduce the outflow of carbon, including ILV1 deletion, is an important strategy for increasing isobutanol production by S. cerevisiae.

Detailed structure-function correlations of Bacillus subtilis acetolactate synthase

Isobutanol is deemed to be a next-generation biofuel and a renewable platform chemical.1 Non-natural biosynthetic pathways for isobutanol production have been implemented in cell-based and in vitro systems with Bacillus subtilis acetolactate synthase (AlsS) as key biocatalyst.2-6 AlsS catalyzes the condensation of two pyruvate molecules to acetolactate with thiamine diphosphate and Mg(2+) as cofactors. AlsS also catalyzes the conversion of 2-ketoisovalerate into isobutyraldehyde, the immediate precursor of isobutanol. Our phylogenetic analysis suggests that the ALS enzyme family forms a distinct subgroup of ThDP-dependent enzymes. To unravel catalytically relevant structure-function relationships, we solved the AlsS crystal structure at 2.3 Å in the presence of ThDP, Mg(2+) and in a transition state with a 2-lactyl moiety bound to ThDP. We supplemented our structural data by point mutations in the active site to identify catalytically important residues.

Metabolic engineering of microorganisms for the production of higher alcohols

Due to the increasing concerns about limited fossil resources and environmental problems, there has been much interest in developing biofuels from renewable biomass. Ethanol is currently used as a major biofuel, as it can be easily produced by existing fermentation technology, but it is not the best biofuel due to its low energy density, high vapor pressure, hygroscopy, and incompatibility with current infrastructure. Higher alcohols, including 1-propanol, 1-butanol, isobutanol, 2-methyl-1-butanol, and 3-methyl-1-butanol, which possess fuel properties more similar to those of petroleum-based fuel, have attracted particular interest as alternatives to ethanol. Since microorganisms isolated from nature do not allow production of these alcohols at high enough efficiencies, metabolic engineering has been employed to enhance their production. Here, we review recent advances in metabolic engineering of microorganisms for the production of higher alcohols.

Model-driven redox pathway manipulation for improved isobutanol production in Bacillus subtilis complemented with experimental validation and metabolic profiling analysis

To rationally guide the improvement of isobutanol production, metabolic network and metabolic profiling analysis were performed to provide global and profound insights into cell metabolism of isobutanol-producing Bacillus subtilis. The metabolic flux distribution of strains with different isobutanol production capacity (BSUL03, BSUL04 and BSUL05) drops a hint of the importance of NADPH on isobutanol biosynthesis. Therefore, the redox pathways were redesigned in this study. To increase NADPH concentration, glucose-6-phosphate isomerase was inactivated (BSUL06) and glucose-6-phosphate dehydrogenase was overexpressed (BSUL07) successively. As expected, NADPH pool size in BSUL07 was 4.4-fold higher than that in parental strain BSUL05. However, cell growth, isobutanol yield and production were decreased by 46%, 22%, and 80%, respectively. Metabolic profiling analysis suggested that the severely imbalanced redox status might be the primary reason. To solve this problem, gene udhA of Escherichia coli encoding transhydrogenase was further overexpressed (BSUL08), which not only well balanced the cellular ratio of NAD(P)H/NAD(P)⁺, but also increased NADH and ATP concentration. In addition, a straightforward engineering approach for improving NADPH concentrations was employed in BSUL05 by overexpressing exogenous gene pntAB and obtained BSUL09. The performance for isobutanol production by BSUL09 was poorer than BSUL08 but better than other engineered strains. Furthermore, in fed-batch fermentation the isobutanol production and yield of BSUL08 increased by 11% and 19%, up to the value of 6.12 g/L and 0.37 C-mol isobutanol/C-mol glucose (63% of the theoretical value), respectively, compared with parental strain BSUL05. These results demonstrated that model-driven complemented with metabolic profiling analysis could serve as a useful approach in the strain improvement for higher bio-productivity in further application.

Increased isobutanol production in Saccharomyces cerevisiae by eliminating competing pathways and resolving cofactor imbalance

Background

Isobutanol is an important target for biorefinery research as a next-generation biofuel and a building block for commodity chemical production. Metabolically engineered microbial strains to produce isobutanol have been successfully developed by introducing the Ehrlich pathway into bacterial hosts. Isobutanol-producing baker’s yeast (Saccharomyces cerevisiae) strains have been developed following the strategy with respect to its advantageous characteristics for cost-effective isobutanol production. However, the isobutanol yields and titers attained by the developed strains need to be further improved through engineering of S. cerevisiae metabolism.

Results

Two strategies including eliminating competing pathways and resolving the cofactor imbalance were applied to improve isobutanol production in S. cerevisiae. Isobutanol production levels were increased in strains lacking genes encoding members of the pyruvate dehydrogenase complex such as LPD1, indicating that the pyruvate supply for isobutanol biosynthesis is competing with acetyl-CoA biosynthesis in mitochondria. Isobutanol production was increased by overexpression of enzymes responsible for transhydrogenase-like shunts such as pyruvate carboxylase, malate dehydrogenase, and malic enzyme. The integration of a single gene deletion lpd1Δ and the activation of the transhydrogenase-like shunt further increased isobutanol levels. In a batch fermentation test at the 50-mL scale from 100 g/L glucose using the two integrated strains, the isobutanol titer reached 1.62 ± 0.11 g/L and 1.61 ± 0.03 g/L at 24 h after the start of fermentation, which corresponds to the yield at 0.016 ± 0.001 g/g glucose consumed and 0.016 ± 0.0003 g/g glucose consumed, respectively.

Conclusions

These results demonstrate that downregulation of competing pathways and metabolic functions for resolving the cofactor imbalance are promising strategies to construct S. cerevisiae strains that effectively produce isobutanol.

Microbial synthesis of n-butanol, isobutanol, and other higher alcohols from diverse resources

Microbial production of fuel and chemical feedstock is a promising approach to solving energy and environmental problems. n-Butanol, isobutanol and other higher alcohols are of particular interest because they can serve as both fuel and chemical feedstock. Alternative resources such as CO2, syngas, waste protein, and lignocellulose are currently being investigated for their potential to produce these compounds. Except for lignocellulose, utilization of such alternative resource has not been examined extensively. This review aims to summarize the development of metabolic pathways for efficient synthesis of these higher alcohols and the current status of microbial strain development for the conversion of diverse resources into higher alcohols.

Next generation biofuel engineering in prokaryotes

Next-generation biofuels must be compatible with current transportation infrastructure and be derived from environmentally sustainable resources that do not compete with food crops. Many bacterial species have unique properties advantageous to the production of such next-generation fuels. However, no single species possesses all characteristics necessary to make high quantities of fuels from plant waste or CO2. Species containing a subset of the desired characteristics are used as starting points for engineering organisms with all desired attributes. Metabolic engineering of model organisms has yielded high titer production of advanced fuels, including alcohols, isoprenoids, and fatty acid derivatives. Technical developments now allow engineering of native fuel producers, as well as lignocellulolytic and autotrophic bacteria, for the production of biofuels. Continued research on multiple fronts is required to engineer organisms for truly sustainable and economical biofuel production.

Compartmentalization of metabolic pathways in yeast mitochondria improves the production of branched-chain alcohols

Efforts to improve the production of a compound of interest in Saccharomyces cerevisiae have mainly involved engineering or overexpression of cytoplasmic enzymes. We show that targeting metabolic pathways to mitochondria can increase production compared with overexpression of the enzymes involved in the same pathways in the cytoplasm. Compartmentalization of the Ehrlich pathway into mitochondria increased isobutanol production by 260%, whereas overexpression of the same pathway in the cytoplasm only improved yields by 10%, compared with a strain overproducing enzymes involved in only the first three steps of the biosynthetic pathway. Subcellular fractionation of engineered strains revealed that targeting the enzymes of the Ehrlich pathway to the mitochondria achieves greater local enzyme concentrations. Other benefits of compartmentalization may include increased availability of intermediates, removing the need to transport intermediates out of the mitochondrion and reducing the loss of intermediates to competing pathways.

Structure-guided engineering of Lactococcus lactis alcohol dehydrogenase LlAdhA for improved conversion of isobutyraldehyde to isobutanol

We have determined the X-ray crystal structures of the NADH-dependent alcohol dehydrogenase LlAdhA from Lactococcus lactis and its laboratory-evolved variant LlAdhA(RE1) at 1.9Å and 2.5Å resolution, respectively. LlAdhA(RE1), which contains three amino acid mutations (Y50F, I212T, and L264V), was engineered to increase the microbial production of isobutanol (2-methylpropan-1-ol) from isobutyraldehyde (2-methylpropanal). Structural comparison of LlAdhA and LlAdhA(RE1) indicates that the enhanced activity on isobutyraldehyde stems from increases in the protein's active site size, hydrophobicity, and substrate access. Further structure-guided mutagenesis generated a quadruple mutant (Y50F/N110S/I212T/L264V), whose KM for isobutyraldehyde is ∼17-fold lower and catalytic efficiency (kcat/KM) is ∼160-fold higher than wild-type LlAdhA. Combining detailed structural information and directed evolution, we have achieved significant improvements in non-native alcohol dehydrogenase activity that will facilitate the production of next-generation fuels such as isobutanol from renewable resources.

Recombinant Bacillus subtilis that grows on untreated plant biomass

Lignocellulosic biomass is a promising feedstock to produce biofuels and other valuable biocommodities. A major obstacle to its commercialization is the high cost of degrading biomass into fermentable sugars, which is typically achieved using cellulolytic enzymes from Trichoderma reesei. Here, we explore the use of microbes to break down biomass. Bacillus subtilis was engineered to display a multicellulase-containing minicellulosome. The complex contains a miniscaffoldin protein that is covalently attached to the cell wall and three noncovalently associated cellulase enzymes derived from Clostridium cellulolyticum (Cel48F, Cel9E, and Cel5A). The minicellulosome spontaneously assembles, thus increasing the practicality of the cells. The recombinant bacteria are highly cellulolytic and grew in minimal medium containing industrially relevant forms of biomass as the primary nutrient source (corn stover, hatched straw, and switch grass). Notably, growth did not require dilute acid pretreatment of the biomass and the cells achieved densities approaching those of cells cultured with glucose. An analysis of the sugars released from acid-pretreated corn stover indicates that the cells have stable cellulolytic activity that enables them to break down 62.3% ± 2.6% of the biomass. When supplemented with beta-glucosidase, the cells liberated 21% and 33% of the total available glucose and xylose in the biomass, respectively. As the cells display only three types of enzymes, increasing the number of displayed enzymes should lead to even more potent cellulolytic microbes. This work has important implications for the efficient conversion of lignocellulose to value-added biocommodities.

Cytosolic re-localization and optimization of valine synthesis and catabolism enables inseased isobutanol production with the yeast Saccharomyces cerevisiae

BACKGROUND

The branched chain alcohol isobutanol exhibits superior physicochemical properties as an alternative biofuel. The yeast Saccharomyces cerevisiae naturally produces low amounts of isobutanol as a by-product during fermentations, resulting from the catabolism of valine. As S. cerevisiae is widely used in industrial applications and can easily be modified by genetic engineering, this microorganism is a promising host for the fermentative production of higher amounts of isobutanol.

RESULTS

Isobutanol production could be improved by re-locating the valine biosynthesis enzymes Ilv2, Ilv5 and Ilv3 from the mitochondrial matrix into the cytosol. To prevent the import of the three enzymes into yeast mitochondria, N-terminally shortened Ilv2, Ilv5 and Ilv3 versions were constructed lacking their mitochondrial targeting sequences. SDS-PAGE and immunofluorescence analyses confirmed expression and re-localization of the truncated enzymes. Growth tests or enzyme assays confirmed enzymatic activities. Isobutanol production was only increased in the absence of valine and the simultaneous blockage of the mitochondrial valine synthesis pathway. Isobutanol production could be even more enhanced after adapting the codon usage of the truncated valine biosynthesis genes to the codon usage of highly expressed glycolytic genes. Finally, a suitable ketoisovalerate decarboxylase, Aro10, and alcohol dehydrogenase, Adh2, were selected and overexpressed. The highest isobutanol titer was 0.63 g/L at a yield of nearly 15 mg per g glucose.

CONCLUSION

A cytosolic isobutanol production pathway was successfully established in yeast by re-localization and optimization of mitochondrial valine synthesis enzymes together with overexpression of Aro10 decarboxylase and Adh2 alcohol dehydrogenase. Driving forces were generated by blocking competition with the mitochondrial valine pathway and by omitting valine from the fermentation medium. Additional deletion of pyruvate decarboxylase genes and engineering of co-factor imbalances should lead to even higher isobutanol production.

Improved 2-methyl-1-propanol production in an engineered Bacillus subtilis by constructing inducible pathways

High-level constitutive gene expression can result in cellular metabolic imbalance and limit production. To circumvent these problems, a P(alsSD)-controlled auto-inducible 2-ketoisovalerate biosynthetic pathway and a P(spac)-controlled IPTG-inducible Ehrlich pathway were constructed in Bacillus subtilis to modulate gene expression. Based on the precise gene expression characteristics of the two inducible pathways, the optimal IPTG induction time point and dose for 2-methyl-1-propanol biosynthesis were determined as 9.5 h and 300 μM, respectively. Under the optimized conditions, strain BSUΔL-03 with inducible pathways produced up to 3.83 ± 0.46 g 2-methyl-1-propanol/l, which was about 60 % higher than BSUL04 with constitutive pathways.

Rational improvement of the engineered isobutanol-producing Bacillus subtilis by elementary mode analysis

Background

Isobutanol is considered as a leading candidate for the replacement of current fossil fuels, and expected to be produced biotechnologically. Owing to the valuable features, Bacillus subtilis has been engineered as an isobutanol producer, whereas it needs to be further optimized for more efficient production. Since elementary mode analysis (EMA) is a powerful tool for systematical analysis of metabolic network structures and cell metabolism, it might be of great importance in the rational strain improvement.

Results

Metabolic network of the isobutanol-producing B. subtilis BSUL03 was first constructed for EMA. Considering the actual cellular physiological state, 239 elementary modes (EMs) were screened from total 11,342 EMs for potential target prediction. On this basis, lactate dehydrogenase (LDH) and pyruvate dehydrogenase complex (PDHC) were predicted as the most promising inactivation candidates according to flux flexibility analysis and intracellular flux distribution simulation. Then, the in silico designed mutants were experimentally constructed. The maximal isobutanol yield of the LDH- and PDHC-deficient strain BSUL05 reached 61% of the theoretical value to 0.36 ± 0.02 C-mol isobutanol/C-mol glucose, which was 2.3-fold of BSUL03. Moreover, this mutant produced approximately 70 % more isobutanol to the maximal titer of 5.5 ± 0.3 g/L in fed-batch fermentations.

Conclusions

EMA was employed as a guiding tool to direct rational improvement of the engineered isobutanol-producing B. subtilis. The consistency between model prediction and experimental results demonstrates the rationality and accuracy of this EMA-based approach for target identification. This network-based rational strain improvement strategy could serve as a promising concept to engineer efficient B. subtilis hosts for isobutanol, as well as other valuable products.

A selection platform for carbon chain elongation using the CoA-dependent pathway to produce linear higher alcohols

Production of green chemicals and fuels using metabolically engineered organisms has been a promising alternative to petroleum-based production. Higher chain alcohols (C4-C8) are of interest because they can be used as chemical feedstock as well as fuels. Recently, the feasibility of n-hexanol synthesis using Escherichia coli has been demonstrated by extending the modified Clostridium CoA-dependent n-butanol synthesis pathway, thereby elongating carbon chain length via reactions in reversed β-oxidation, (or β-reduction). Here, we developed an anaerobic growth selection platform that allows selection or enrichment of enzymes for increased synthesis of C6 and C8 linear alcohols. Using this selection, we were able to improve the carbon flux towards the synthesis of C6 and C8 acyl-CoA intermediates. Replacement of the original enzyme Clostridium acetobutylicum Hbd with Ralstonia eutropha homologue PaaH1 increased production of n-hexanol by 10-fold. Further directed evolution by random mutagenesis of PaaH1 improved n-hexanol and n-octanol production. This anaerobic growth selection platform may be useful for selecting enzymes for production of long-chain alcohols and acids using this CoA-dependent pathway.

Recent progress in synthetic biology for microbial production of C3-C10 alcohols

The growing need to address current energy and environmental problems has sparked an interest in developing improved biological methods to produce liquid fuels from renewable sources. While microbial ethanol production is well established, higher-chain alcohols possess chemical properties that are more similar to gasoline. Unfortunately, these alcohols (except 1-butanol) are not produced efficiently in natural microorganisms, and thus economical production in industrial volumes remains a challenge. Synthetic biology, however, offers additional tools to engineer synthetic pathways in user-friendly hosts to help increase titers and productivity of these advanced biofuels. This review concentrates on recent developments in synthetic biology to produce higher-chain alcohols as viable renewable replacements for traditional fuel.

An in silico platform for the design of heterologous pathways in nonnative metabolite production

Background

Microorganisms are used as cell factories to produce valuable compounds in pharmaceuticals, biofuels, and other industrial processes. Incorporating heterologous metabolic pathways into well-characterized hosts is a major strategy for obtaining these target metabolites and improving productivity. However, selecting appropriate heterologous metabolic pathways for a host microorganism remains difficult owing to the complexity of metabolic networks. Hence, metabolic network design could benefit greatly from the availability of an in silico platform for heterologous pathway searching.

Results

We developed an algorithm for finding feasible heterologous pathways by which nonnative target metabolites are produced by host microorganisms, using Escherichia coli, Corynebacterium glutamicum, and Saccharomyces cerevisiae as templates. Using this algorithm, we screened heterologous pathways for the production of all possible nonnative target metabolites contained within databases. We then assessed the feasibility of the target productions using flux balance analysis, by which we could identify target metabolites associated with maximum cellular growth rate.

Conclusions

This in silico platform, designed for targeted searching of heterologous metabolic reactions, provides essential information for cell factory improvement.

Cell-free biosystems for biomanufacturing

Although cell-free biosystems have been used as a tool for investigating fundamental aspects of biological systems for more than 100 years, they are becoming an emerging biomanufacturing platform in the production of low-value biocommodities (e.g., H(2), ethanol, and isobutanol), fine chemicals, and high-value protein and carbohydrate drugs and their precursors. Here we would like to define the cell-free biosystems containing more than three catalytic components in a single reaction vessel, which although different from one-, two-, or three-enzyme biocatalysis can be regarded as a straightforward extension of multienzymatic biocatalysis. In this chapter, we compare the advantages and disadvantages of cell-free biosystems versus living organisms, briefly review the history of cell-free biosystems, highlight a few examples, analyze any remaining obstacles to the scale-up of cell-free biosystems, and suggest potential solutions. Cell-free biosystems could become a disruptive technology to microbial fermentation, especially in the production of high-impact low-value biocommodities mainly due to the very high product yields and potentially low production costs.

Current knowledge on isobutanol production with Escherichia coli, Bacillus subtilis and Corynebacterium glutamicum

Due to steadily rising crude oil prices great efforts have been made to develop designer bugs for the fermentative production of higher alcohols, such as 2-methyl-1-butanol, 3-methyl-1-butanol and 2-Methyl-1-propanol (isobutanol), which all possess quality characteristics comparable to traditional oil based fuels. The common metabolic engineering approach uses the last two steps of the Ehrlich pathway, catalyzed by 2-ketoacid decarboxylase and an alcohol dehydrogenase converting the branched chain 2-ketoacids of L-isoleucine, L-leucine, and L-valine into the respective alcohols. This strategy was successfully used to engineer well suited and industrially employed bacteria, such as Escherichia coli, Bacillus subtilis and Corynebacterium glutamicum for the production of higher alcohols. Among these alcohols, isobutanol is currently the most promising one regarding final titer and yield. This article summarizes the current knowledge and achievements on isobutanol production with E. coli, B. subtilis and C. glutamicum regarding the metabolic engineering approaches and process conditions.