cAMP receptor protein (CRP)-mediated resistance/tolerance in bacteria: mechanism and utilization in biotechnology

Engineering Pseudomonas putida KT2440 for Dipicolinate Production via the Entner-Doudoroff Pathway

Dipicolinic acid (DPA), a cyclic diacid, has garnered significant interest due to its potential applications in antimicrobial agents, antioxidants, chelating reagents, and polymer precursors. However, its natural bioproduction is limited since DPA is only accumulated in Bacillus and Clostridium species during sporulation. Thus, heterologous production by engineered strains is of paramount importance for developing a sustainable biological route for DPA production. Pseudomonas putida KT2440 has emerged as a promising host for the production of various chemicals thanks to its robustness, metabolic versatility, and genetic tractability. The dominant Entner-Doudoroff (ED) pathway for glucose metabolism in this strain offers an ideal route for DPA production due to the advantage of NADPH generation and the naturally balanced flux between glyceraldehyde-3-phosphate and pyruvate, which are both precursors for DPA synthesis. In this study, DPA production via the ED pathway was in silico designed in P. putida KT2440. The systematically engineered strain produced dipicolinate with a titer of 11.72 g/L from glucose in a 5 L fermentor. This approach not only provides a sustainable green route for DPA production but also expands our understanding of the metabolic potential of the ED pathway in P. putida KT2440.

The cAMP receptor protein gene contributes to growth, stress resistance, and colonization of Actinobacillus pleuropneumoniae

Porcine infectious pleuropneumonia (PCP) is a severe disease of porcine caused by Actinobacillus pleuropneumoniae (APP). The spread of PCP remains a threat to the porcine farms and has been known to cause severe economic losses. The cAMP receptor protein (CRP) serves as a pivotal player in helping bacteria adapt to shifts in their environment, particularly when facing the challenges posed by bacterial infections. In this study, we investigated the role of CRP in APP. Our results revealed that crp mutant (Δcrp) strains were more sensitive to acidic and osmotic stress resistance and had lower biofilm formation ability than wild-type (WT) strains. Furthermore, the Δcrp strains showed deficiencies in anti-phagocytosis, adhesion, and invasion upon interaction with host cells. Mice infected with the Δcrp strains demonstrated reduced bacterial loads in their lungs compared to those infected with the WT strains. This study reveals the pivotal role of crp gene expression in regulating pleuropneumonia growth, stress resistance, iron utilization, biofilm formation, phagocytosis, adhesion, invasion and colonization. Our discoveries offer novel perspectives on understanding the development and progression of APP infections.

Multisite Mutation of the Escherichia coli cAMP Receptor Protein: Enhancing Xylitol Biosynthesis by Activating Xylose Catabolism and Improving Strain Tolerance

The bioproduction of xylitol from hemicellulose hydrolysate has good potential for industrial development. However, xylitol productivity has always been limited due to corncob hydrolysate toxicity and glucose catabolic repression. To address these challenges, this work selected the S83 and S128 amino acid residues of the cyclic AMP receptor protein (CRP) as the modification target. By introducing multisite mutation in CRP, this approach successfully enhanced xylose catabolism and improved the strain's tolerance to corncob hydrolysate. The resulting mutant strain, designated as CPH (CRP S83H-S128P), underwent fermentation in a 20 L bioreactor with semicontinuous feeding of corncob hydrolysate. Remarkably, xylitol yield and xylitol productivity for 41 h fermentation were 175 and 4.32 g/L/h, respectively. Therefore, multisite CRP mutation was demonstrated as an efficient global regulatory strategy to effectively improve xylitol productivity from lime-pretreated corncob hydrolysates.

Designing glucose utilization "highway" for recombinant biosynthesis

cAMP receptor protein (CRP) is known as a global regulatory factor mainly mediating carbon source catabolism. Herein, we successfully engineered CRP to develop microbial chassis cells with improved recombinant biosynthetic capability in minimal medium with glucose as single carbon source. The obtained best-performing cAMP-independent CRPmu9 mutant conferred both faster cell growth and a 133-fold improvement in expression level of lac promoter in presence of 2% glucose, compared with strain under regulation of CRPwild-type. Promoters free from "glucose repression" are advantageous for recombinant expression, as glucose is a frequently used inexpensive carbon source in high-cell-density fermentations. Transcriptome analysis demonstrated that the CRP mutant globally rewired cell metabolism, displaying elevated tricarboxylic acid cycle activity; reduced acetate formation; increased nucleotide biosynthesis; and improved ATP synthesis, tolerance, and stress-resistance activity. Metabolites analysis confirmed the enhancement of glucose utilization with the upregulation of glycolysis and glyoxylate-tricarboxylic acid cycle. As expected, an elevated biosynthetic capability was demonstrated with vanillin, naringenin and caffeic acid biosynthesis in strains regulated by CRPmu9. This study has expanded the significance of CRP optimization into glucose utilization and recombinant biosynthesis, beyond the conventionally designated carbon source utilization other than glucose. The Escherichiacoli cell regulated by CRPmu9 can be potentially used as a beneficial chassis for recombinant biosynthesis.

Structural and functional diversity of bacterial cyclic nucleotide perception by CRP proteins

Cyclic AMP (cAMP) is a ubiquitous second messenger synthesized by most living organisms. In bacteria, it plays highly diverse roles in metabolism, host colonization, motility, and many other processes important for optimal fitness. The main route of cAMP perception is through transcription factors from the diverse and versatile CRP-FNR protein superfamily. Since the discovery of the very first CRP protein CAP in Escherichia coli more than four decades ago, its homologs have been characterized in both closely related and distant bacterial species. The cAMP-mediated gene activation for carbon catabolism by a CRP protein in the absence of glucose seems to be restricted to E. coli and its close relatives. In other phyla, the regulatory targets are more diverse. In addition to cAMP, cGMP has recently been identified as a ligand of certain CRP proteins. In a CRP dimer, each of the two cyclic nucleotide molecules makes contacts with both protein subunits and effectuates a conformational change that favors DNA binding. Here, we summarize the current knowledge on structural and physiological aspects of E. coli CAP compared with other cAMP- and cGMP-activated transcription factors, and point to emerging trends in metabolic regulation related to lysine modification and membrane association of CRP proteins.

Genetically Engineering Escherichia coli to Produce Xylitol from Corncob Hydrolysate without Lime Detoxification

Before fermentation with hemicellulosic hydrolysate as a substrate, it is generally necessary to detoxify the toxic substances that are harmful to microorganism growth. Cyclic AMP receptor protein (CRP) is a global regulator, and mutation of its key sites may have an important impact on E. coli virulence tolerance. Using corncob hydrolysate without ion-exchange or lime detoxification as the substrate, shake flask fermentation experiments showed that CRP mutant IS5-dG (I112L, T127G, A144T) produced 18.4 g/L of xylitol within 34 h, and the OD₆₀₀ was 9.7 at 24 h; these values were 41.5% and 21.3% higher than those of the starting strain, IS5-d, respectively. This mutant produced 82 g/L of xylitol from corncob hydrolysate without ion-exchange or lime detoxification during fed-batch fermentation in a 15-L bioreactor, with a productivity of 1.04 g/L/h; these values were 173% and 174% higher than the starting strain, respectively. To our knowledge, this is the highest xylitol concentration and productivity produced by microbial fermentation using completely non-detoxified hemicellulosic hydrolysate as the substrate to date. This study also showed that alkali neutralization, high temperature sterilization, and fermentation of the hydrolysate had important effects on the xylose loss rate and xylitol production.

Refactoring transcription factors for metabolic engineering

Due to the ability to regulate target metabolic pathways globally and dynamically, metabolic regulation systems composed of transcription factors have been widely used in metabolic engineering and synthetic biology. This review introduced the categories, action principles, prediction strategies, and related databases of transcription factors. Then, the application of global transcription machinery engineering technology and the transcription factor-based biosensors and quorum sensing systems are overviewed. In addition, strategies for optimizing the transcriptional regulatory tools' performance by refactoring transcription factors are summarized. Finally, the current limitations and prospects of constructing various regulatory tools based on transcription factors are discussed. This review will provide theoretical guidance for the rational design and construction of transcription factor-based metabolic regulation systems.

cAMP Receptor Protein Positively Regulates the Expression of Genes Involved in the Biosynthesis of Klebsiella oxytoca Tilivalline Cytotoxin

Klebsiella oxytoca is a resident of the human gut. However, certain K. oxytoca toxigenic strains exist that secrete the nonribosomal peptide tilivalline (TV) cytotoxin. TV is a pyrrolobenzodiazepine that causes antibiotic-associated hemorrhagic colitis (AAHC). The biosynthesis of TV is driven by enzymes encoded by the aroX and NRPS operons. In this study, we determined the effect of environmental signals such as carbon sources, osmolarity, and divalent cations on the transcription of both TV biosynthetic operons. Gene expression was enhanced when bacteria were cultivated in tryptone lactose broth. Glucose, high osmolarity, and depletion of calcium and magnesium diminished gene expression, whereas glycerol increased transcription of both TV biosynthetic operons. The cAMP receptor protein (CRP) is a major transcriptional regulator in bacteria that plays a key role in metabolic regulation. To investigate the role of CRP on the cytotoxicity of K. oxytoca, we compared levels of expression of TV biosynthetic operons and synthesis of TV in wild-type strain MIT 09-7231 and a Δcrp isogenic mutant. In summary, we found that CRP directly activates the transcription of the aroX and NRPS operons and that the absence of CRP reduced cytotoxicity of K. oxytoca on HeLa cells, due to a significant reduction in TV production. This study highlights the importance of the CRP protein in the regulation of virulence genes in enteric bacteria and broadens our knowledge on the regulatory mechanisms of the TV cytotoxin.

Establishment of a Biosensor-based High-Throughput Screening Platform for Tryptophan Overproduction

With the flexibility to fold into complex structures, RNA is well-suited to act as a cellular sensor to recognize environmental fluctuations and respond to changes by regulating the corresponding genes. In this study, we established a high-throughput screening platform to screen tryptophan high-producing strains from a large repertoire of candidate strains. This platform consists of a tryptophan-specific aptamer-based biosensor and fluorescence-activated droplet sorting technology. One mutant strain, with a 165.9% increase in Trp titer compared with the parental strain, was successfully screened from a random mutagenesis library. Sequencing results revealed that a total of 10 single-nucleotide polymorphisms were discovered in the genome of the mutant strain, among which CRP(T29K) was proven to significantly increase Trp production through improving the strain's tolerance of the harsh environment during the stationary phase of the fermentation process. Our results indicate that this strategy has great potential for improving the production of other amino acids in Escherichia coli.

Integrating CRISPR-Enabled Trackable Genome Engineering and Transcriptomic Analysis of Global Regulators for Antibiotic Resistance Selection and Identification in Escherichia coli

The growing threat of antimicrobial resistance poses a serious threat to public health care and motivates efforts to understand the means by which resistance acquisition occurs and how this can be combatted. To address these challenges, we expedited the identification of novel mutations that enable complex phenotypic changes that result in improved tolerance to antibiotics by integrating CREATE and transcriptomic analysis of global regulators. The results give us a better understanding of the mechanisms of resistance to tetracycline antibiotics and aminoglycoside antibiotics and also indicate that the method may be used for quickly identifying resistance-related mutations.

It is important to expedite our understanding of antibiotic resistance to address the increasing numbers of fatalities and environmental pollution due to the emergence of antibiotic resistance and multidrug-resistant strains. Here, we combined the CRISPR-enabled trackable genome engineering (CREATE) technology and transcriptomic analysis to investigate antibiotic tolerance in Escherichia coli. We developed rationally designed site saturation mutagenesis libraries targeting 23 global regulators to identify fitness-conferring mutations in response to diverse antibiotic stresses. We identified seven novel mutations that confer resistance to the ribosome-targeting antibiotics doxycycline, thiamphenicol, and gentamicin in E. coli. To the best of our knowledge, these mutations that we identified have not been reported previously during treatment with the indicated antibiotics. Transcriptome sequencing-based transcriptome analysis was further employed to evaluate the genome-wide changes in gene expression in E. coli for SoxR G121P and cAMP receptor protein (CRP) V140W reconstructions, and improved fitness in response to doxycycline and gentamicin was seen. In the case of doxycycline, we speculated that SoxR G121P significantly increased the expression of genes involved in carbohydrate metabolism and energy metabolism to promote cell growth for improved adaptation. In the CRP V140W mutant with improved gentamicin tolerance, the expression of several amino acid biosynthesis genes and fatty acid degradation genes was significantly changed, and these changes probably altered the cellular energy state to improve adaptation. These findings have important significance for understanding such nonspecific mechanisms of antibiotic resistance and developing new antibacterial drugs.

IMPORTANCE The growing threat of antimicrobial resistance poses a serious threat to public health care and motivates efforts to understand the means by which resistance acquisition occurs and how this can be combatted. To address these challenges, we expedited the identification of novel mutations that enable complex phenotypic changes that result in improved tolerance to antibiotics by integrating CREATE and transcriptomic analysis of global regulators. The results give us a better understanding of the mechanisms of resistance to tetracycline antibiotics and aminoglycoside antibiotics and also indicate that the method may be used for quickly identifying resistance-related mutations.

Effect of cAMP Receptor Protein Gene on Growth Characteristics and Stress Resistance of Haemophilus parasuis Serovar 5

Haemophilus parasuis (HPS), a member of the family Pasteurellaceae, is a common bacteria in the upper respiratory tract of pigs but under certain circumstances can cause serious systemic disease (Glasser's disease) characterized by severe infection of the upper respiratory tract, fibrinous polyserositis, polyarthritis, and meningitis. cAMP receptor protein (CRP) is among the most important global regulators, playing a vital role in adapting to environmental changes during the process of bacterial infection. In order to investigate the function of the crp gene in the growth characteristics of H. parasuis serovar 5 (HPS5) and its ability to overcome adverse environmental stresses, a crp mutant strain (Δcrp) was constructed and verified. In this study, we found that the crp gene was involved in growth rate, biofilm formation, stress tolerance, serum resistance, and iron utilization. Compared with the wild type, both the growth rate of the crp mutant and its resistance to osmotic pressure decreased significantly. Similar phenomena were also found in biofilm formation and iron utilization. However, the resistance to heat shock and serum complement of the crp mutant were enhanced. This study aimed to reveal the function in growth characteristics and stress resistance of the crp gene in HPS5. Whether it relates to virulence requires additional in-depth research.

Combination of the CRP mutation and ptsG deletion in Escherichia coli to efficiently synthesize xylitol from corncob hydrolysates

The biotechnology-based production of xylitol has received widespread attention because it can use cheap and renewable lignocellulose as a raw material, thereby decreasing costs and pollution. The simultaneous use of various sugars in lignocellulose hydrolysates is a primary prerequisite for efficient xylitol production. In this study, a ΔptsG and crp* combinatorial strategy was used to generate Escherichia coli W3110 strain IS5-dI, which completely eliminated glucose repression and simultaneously used glucose and xylose. This strain produced 164 g/L xylitol from detoxified corncob hydrolysates during a fed-batch fermentation in a 15-L bioreactor, which was 14.7% higher than the xylitol produced by the starting strain, IS5-d (143 g/L), and the xylitol productivity was 3.04 g/L/h. These results represent the highest xylitol concentration and productivity reported to date for bacteria and hemicellulosic sugars. Additionally, strain IS5-dG, which differs from IS5-dI at CRP amino acid residue 127 (I127G), was tolerant to the toxins in corncob hydrolysates. In a fed-batch fermentation experiment involving a 15-L bioreactor, IS5-dG produced 137 g/L xylitol from non-detoxified corncob hydrolysates, with a productivity of 1.76 g/L/h. On the basis of these results, we believe that IS5-dI and IS5-dG may be useful host strains for the industrial-scale production of xylitol from detoxified or non-detoxified corncob hydrolysates.

Conservation and diversity of radiation and oxidative stress resistance mechanisms in Deinococcus species

Deinococcus bacteria are famous for their extreme resistance to ionising radiation and other DNA damage- and oxidative stress-generating agents. More than a hundred genes have been reported to contribute to resistance to radiation, desiccation and/or oxidative stress in Deinococcus radiodurans. These encode proteins involved in DNA repair, oxidative stress defence, regulation and proteins of yet unknown function or with an extracytoplasmic location. Here, we analysed the conservation of radiation resistance-associated proteins in other radiation-resistant Deinococcus species. Strikingly, homologues of dozens of these proteins are absent in one or more Deinococcus species. For example, only a few Deinococcus-specific proteins and radiation resistance-associated regulatory proteins are present in each Deinococcus, notably the metallopeptidase/repressor pair IrrE/DdrO that controls the radiation/desiccation response regulon. Inversely, some Deinococcus species possess proteins that D. radiodurans lacks, including DNA repair proteins consisting of novel domain combinations, translesion polymerases, additional metalloregulators, redox-sensitive regulator SoxR and manganese-containing catalase. Moreover, the comparisons improved the characterisation of several proteins regarding important conserved residues, cellular location and possible protein–protein interactions. This comprehensive analysis indicates not only conservation but also large diversity in the molecular mechanisms involved in radiation resistance even within the Deinococcus genus.

Engineered global regulator H-NS improves the acid tolerance of E. coli

Background

Acid stress is often encountered during industrial fermentation as a result of the accumulation of acidic metabolites. Acid stress increases the intracellular acidity and can cause DNA damage and denaturation of essential enzymes, thus leading to a decrease of growth and fermentation yields. Although acid stress can be relieved by addition of a base to the medium, fermentations with acid-tolerant strains are generally considered much more efficient and cost-effective.

Results

In this study, the global regulator H-NS was found to have significant influence on the acid tolerance of E. coli. The final OD₆₀₀ of strains overexpressing H-NS increased by 24% compared to control, when cultured for 24 h at pH 4.5 using HCl as an acid agent. To further improve the acid tolerance, a library of H-NS was constructed by error-prone PCR and subjected to selection. Five mutants that conferred a significant growth advantage compared to the control strain were obtained. The final OD₆₀₀ of strains harboring the five H-NS mutants was enhanced by 26–53%, and their survival rate was increased by 10- to 100-fold at pH 2.5. Further investigation showed that the improved acid tolerance of H-NS mutants coincides with the activation of multiple acid resistance mechanisms, in particular the glutamate- and glutamine-dependent acid resistance system (AR2). The improved acid tolerance of H-NS mutants was also demonstrated in media acidified by acetic acid and succinic acid, which are common acidic fermentation by-products or products.

Conclusions

The results obtained in this work demonstrate that the engineering of H-NS can enhance the acid tolerance of E. coli. More in general, this study shows the potential of the engineering of global regulators acting as repressors, such as H-NS, as a promising method to obtain phenotypes of interest. This approach could expand the spectrum of application of global transcription machinery engineering.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0966-z) contains supplementary material, which is available to authorized users.

Comparative transcriptional profiling of tildipirosin-resistant and sensitive Haemophilus parasuis

Numerous studies have been conducted to examine the molecular mechanism of Haemophilus parasuis resistance to antibiotic, but rarely to tildipirosin. In the current study, transcriptional profiling was applied to analyse the variation in gene expression of JS0135 and tildipirosin-resistant JS32. The growth curves showed that JS32 had a higher growth rate but fewer bacteria than JS0135. The cell membranes of JS32 and a resistant clinical isolate (HB32) were observed to be smoother than those of JS0135. From the comparative gene expression profile 349 up- and 113 downregulated genes were observed, covering 37 GO and 63 KEGG pathways which are involved in biological processes (11), cellular components (17), molecular function (9), cellular processes (1), environmental information processing (4), genetic information processing (9) and metabolism (49) affected in JS32. In addition, the relative overexpression of genes of the metabolism pathway (HAPS_RS09315, HAPS_RS09320), ribosomes (HAPS_RS07815) and ABC transporters (HAPS_RS10945) was detected, particularly the metabolism pathway, and verified with RT-qPCR. Collectively, the gene expression profile in connection with tildipirosin resistance factors revealed unique and highly resistant determinants of H. parasuis to macrolides that warrant further attention due to the significant threat of bacterial resistance.

C-terminal dimerization of apo-cyclic AMP receptor protein validated in solution

Although cyclic AMP receptor protein (CRP) has long served as a typical example of effector-mediated protein allostery, mechanistic details into its regulation have been controversial due to discrepancy between the known crystal structure and NMR structure of apo-CRP. Here, we report that the recombinant protein corresponding to its C-terminal DNA-binding domain (CDD) forms a dimer. This result, together with structural information obtained in the present NMR study, is consistent with the previous crystal structure and validates its relevance also in solution. Therefore, our findings suggest that dissociation of the CDD may be critically involved in cAMP-induced allosteric activation of CRP.

The Resistome: A Comprehensive Database of Escherichia coli Resistance Phenotypes

The microbial ability to resist stressful environmental conditions and chemical inhibitors is of great industrial and medical interest. Much of the data related to mutation-based stress resistance, however, is scattered through the academic literature, making it difficult to apply systematic analyses to this wealth of information. To address this issue, we introduce the Resistome database: a literature-curated collection of Escherichia coli genotypes-phenotypes containing over 5,000 mutants that resist hundreds of compounds and environmental conditions. We use the Resistome to understand our current state of knowledge regarding resistance and to detect potential synergy or antagonism between resistance phenotypes. Our data set represents one of the most comprehensive collections of genomic data related to resistance currently available. Future development will focus on the construction of a combined genomic-transcriptomic-proteomic framework for understanding E. coli's resistance biology. The Resistome can be downloaded at https://bitbucket.org/jdwinkler/resistome_release/overview .

Quantifying complexity in metabolic engineering using the LASER database

We previously introduced the LASER database (Learning Assisted Strain EngineeRing, https://bitbucket.org/jdwinkler/laser_release) (Winkler et al. 2015) to serve as a platform for understanding past and present metabolic engineering practices. Over the past year, LASER has been expanded by 50% to include over 600 engineered strains from 450 papers, including their growth conditions, genetic modifications, and other information in an easily searchable format. Here, we present the results of our efforts to use LASER as a means for defining the complexity of a metabolic engineering "design". We evaluate two complexity metrics based on the concepts of construction difficulty and novelty. No correlation is observed between expected product yield and complexity, allowing minimization of complexity without a performance trade-off. We envision the use of such complexity metrics to filter and prioritize designs prior to implementation of metabolic engineering efforts, thereby potentially reducing the time, labor, and expenses of large-scale projects. Possible future developments based on an expanding LASER database are then discussed.

Molecular Underpinnings of Nitrite Effect on CymA-Dependent Respiration in Shewanella oneidensis

Shewanella exhibit a remarkable versatility of respiration, with a diverse array of electron acceptors (EAs). In environments where these bacteria thrive, multiple EAs are usually present. However, we know little about strategies by which these EAs and their interaction affect ecophysiology of Shewanella. In this study, we demonstrate in the model strain, Shewanella oneidensis MR-1, that nitrite, not through nitric oxide to which it may convert, inhibits respiration of fumarate, and probably many other EAs whose reduction depends on quinol dehydrogenase CymA. This is achieved via the repression of cyclic adenosine monophosphate (cAMP) production, a second messenger required for activation of cAMP-receptor protein (Crp) which plays a primary role in regulation of respiration. If nitrite is not promptly removed, intracellular cAMP levels drop, and this impairs Crp activity. As a result, the production of nitrite reductase NrfA, CymA, and fumarate reductase FccA is substantially reduced. In contrast, nitrite can be simultaneously respired with trimethylamine N-oxide, resulting in enhanced biomass.

Recent progress in biobutanol tolerance in microbial systems with an emphasis on Clostridium

Biobased production of butanol promises a more sustainable route for industrial production. However, butanol toxicity remains a barrier for achieving high product titers. Investigation into butanol stress has shed some light on its modes of toxicity. Unfortunately, there still remain significant shortfalls in our understanding of the complex interactions of butanol with cells. To address this knowledge gap, a diverse range of tools have been employed to gain a better understanding of the adverse effects of butanol on the cell. These findings have lead to the identification of possible molecular mechanisms associated with butanol tolerance, which can be harnessed for future strain development efforts. This review focuses on recent efforts to address the toxicity of butanol in microbial producers and offers some perspectives on the future direction of this research sector.

Biofuels and bio-based chemicals from lignocellulose: metabolic engineering strategies in strain development

Interest in developing a sustainable technology for fuels and chemicals has unleashed tremendous creativity in metabolic engineering for strain development over the last few years. This is driven by the exceptionally recalcitrant substrate, lignocellulose, and the necessity to keep the costs down for commodity products. Traditional methods of gene expression and evolutionary engineering are more effectively used with the help of synthetic biology and -omics techniques. Compared to the last biomass research peak during the 1980s oil crisis, a more diverse range of microorganisms are being engineered for a greater variety of products, reflecting the broad applicability and effectiveness of today's gene technology. We review here several prominent and successful metabolic engineering strategies with emphasis on the following four areas: xylose catabolism, inhibitor tolerance, synthetic microbial consortium, and cellulosic oligomer assimilation.