Thermotolerant genes essential for survival at a critical high temperature in thermotolerant ethanologenic Zymomonas mobilis TISTR 548

Metagenomic analysis of a thermophilic bacterial consortium and its use in the bioremediation of a petroleum-contaminated soil

Biodegradation is difficult at high temperatures due to the limited capacity of microorganisms to survive and function outside their optimum temperature range. Here, a thermophilic petroleum-degrading consortium was enriched from compost at a temperature of 55 °C. 16S rDNA and metagenomic techniques were used to analyze the composition of the consortium and the mechanisms of degradation. The consortium degraded 17000 mg total petroleum hydrocarbons (TPHs) L-1 with a degradation efficiency of 81.5% in 14 days. The consortium utilized a range of substrates such as n-hexadecane, n-docosane, naphthalene and pyrene and grew well over a wide range of pH (4-10) and salinity (0-90 g L-1). The hydrocarbon-degrading extremophilic consortium contained, inter alia, (relative abundance >1%) Caldibacillus, Geobacillus, Mycolicibacterium, Bacillus, Chelatococcus, and Aeribacillus spp. Metagenomic analysis was conducted to discover the degradation and environmental tolerance functional genes of the consortium. Two alkane hydroxylase genes, alkB and ladA, were found. A microcosm study shows that the consortium promoted the bioremediation of soil TPHs. The results indicate that the consortium may be a good candidate for the high-temperature bioremediation of petroleum-contaminated soils.

Molecular basis and functional development of membrane-based microbial metabolism

My research interest has so far been focused on metabolisms related to the "membrane" of microorganisms, such as the respiratory chain, membrane proteins, sugar uptake, membrane stress and cell lysis, and fermentation. These basic metabolisms are important for the growth and survival of cell, and their knowledge can be used for efficient production of useful materials. Notable achievements in research on metabolisms are elucidation of the structure and function of membrane-bound glucose dehydrogenase as a primary enzyme in the respiratory chain, elucidation of ingenious expression regulation of several operons or by divergent promoters, elucidation of stress-induced programed-cell lysis and its requirement for survival during a long-term stationary phase, elucidation of molecular mechanism of survival at a critical high temperature, elucidation of thermal adaptation and its limit, isolation of thermotolerant fermenting yeast strains, and development of high-temperature fermentation and green energy production technologies. These achievements are described together in this review.

Impact of temperature on the virulence of Streptococcus agalactiae in Indonesian aquaculture: A better vaccine design is required

Due to their poikilothermic nature, fish are very sensitive to changes in temperature. Due to climate change, the average global temperature has increased by 1.5°C in the last century, which may have caused an increase in farmed fish mortality recently. Predictions using the model estimate that a 1°C increase in temperature could cause 3%-4% and 4%-6% mortality due to infectious diseases in organisms living in warm and temperate waters, respectively. There is a need to determine whether there is a relationship between increasing environmental temperature and disease virulence. This review examines the influence and impact of increasing temperatures due to climate change on the physiology and pathogenicity of Streptococcus agalactiae, which causes streptococcosis in tilapia and causes significant economic losses. Changes in the pathogenicity of S. agalactiae, especially its virulence properties due to increasing temperature, require changes in the composition design of the fish vaccine formula to provide better protection through the production of protective antibodies.

Implication of amino acid metabolism and cell surface integrity for the thermotolerance mechanism in the thermally adapted acetic acid bacterium Acetobacter pasteurianus TH-3

IMPORTANCE

Acetobacter pasteurianus, an industrial vinegar-producing strain, is suffered by fermentation stress such as fermentation heat and/or high concentrations of acetic acid. By an experimental evolution approach, we have obtained a stress-tolerant strain, exhibiting significantly increased growth and acetic acid fermentation ability at higher temperatures. In this study, we report that only the three gene mutations of ones accumulated during the adaptation process, ansP, dctD, and glnD, were sufficient to reproduce the increased thermotolerance of A. pasteurianus. These mutations resulted in cell envelope modification, including increased phospholipid and lipopolysaccharide synthesis, increased respiratory activity, and cell size reduction. The phenotypic changes may cooperatively work to make the adapted cell thermotolerant by enhancing cell surface integrity, nutrient or oxygen availability, and energy generation.

Molecular mechanism of engineered Zymomonas mobilis to furfural and acetic acid stress

Acetic acid and furfural (AF) are two major inhibitors of microorganisms during lignocellulosic ethanol production. In our previous study, we successfully engineered Zymomonas mobilis 532 (ZM532) strain by genome shuffling, but the molecular mechanisms of tolerance to inhibitors were still unknown. Therefore, this study investigated the responses of ZM532 and its wild-type Z. mobilis (ZM4) to AF using multi-omics approaches (transcriptomics, genomics, and label free quantitative proteomics). Based on RNA-Seq data, two differentially expressed genes, ZMO_RS02740 (up-regulated) and ZMO_RS06525 (down-regulated) were knocked out and over-expressed through CRISPR-Cas technology to investigate their roles in AF tolerance. Overall, we identified 1865 and 14 novel DEGs in ZM532 and wild-type ZM4. In contrast, 1532 proteins were identified in ZM532 and wild-type ZM4. Among these, we found 96 important genes in ZM532 involving acid resistance mechanisms and survival rates against stressors. Furthermore, our knockout results demonstrated that growth activity and glucose consumption of mutant strains ZM532∆ZMO_RS02740 and ZM4∆ZMO_RS02740 decreased with increased fermentation time from 42 to 55 h and ethanol production up to 58% in ZM532 than that in ZM532∆ZMO_RS02740. Hence, these findings suggest ZMO_RS02740 as a protective strategy for ZM ethanol production under stressful conditions.

Mutations in degP and spoT Genes Mediate Response to Fermentation Stress in Thermally Adapted Strains of Acetic Acid Bacterium Komagataeibacter medellinensis NBRC 3288

An acetic acid bacterium, Komagataeibacter medellinensis NBRC 3288, was adapted to higher growth temperatures through an experimental evolution approach in acetic acid fermentation conditions, in which the cells grew under high concentrations of ethanol and acetic acid. The thermally adapted strains were shown to exhibit significantly increased growth and fermentation ability, compared to the wild strain, at higher temperatures. Although the wild cells were largely elongated and exhibited a rough cell surface, the adapted strains repressed the elongation and exhibited a smaller cell size and a smoother cell surface than the wild strain. Among the adapted strains, the ITO-1 strain isolated during the initial rounds of adaptation was shown to have three indel mutations in the genes gyrB, degP, and spoT. Among these, two dispensable genes, degP and spoT, were further examined in this study. Rough cell surface morphology related to degP mutation suggested that membrane vesicle-like structures were increased on the cell surface of the wild-type strain but repressed in the ITO-1 strain under high-temperature acetic acid fermentation conditions. The ΔdegP strain could not grow at higher temperatures and accumulated a large amount of membrane vesicles in the culture supernatant when grown even at 30°C, suggesting that the degP mutation is involved in cell surface stability. As the spoT gene of ITO-1 lost a 3′-end of 424 bp, which includes one (Act-4) of the possible two regulatory domains (TGS and Act-4), two spoT mutant strains were created: one (ΔTGSAct) with a drug cassette in between the 5′-half catalytic domain and 3′-half regulatory domains of the gene, and the other (ΔAct-4) in between TGS and Act-4 domains of the regulatory domain. These spoT mutants exhibited different growth responses; ΔTGSAct grew better in both the fermentation and non-fermentation conditions, whereas ΔAct-4 did only under fermentation conditions, such as ITO-1 at higher temperatures. We suggest that cell elongation and/or cell size are largely related to these spoT mutations, which may be involved in fermentation stress and thermotolerance.

Microbial adaptation to different environmental conditions: molecular perspective of evolved genetic and cellular systems

Microorganisms are ubiquitous on Earth and can inhabit almost every environment. In a complex heterogeneous environment or in face of ecological disturbance, the microbes adjust to fluctuating environmental conditions through a cascade of cellular and molecular systems. Their habitats differ from cold microcosms of Antarctica to the geothermal volcanic areas, terrestrial to marine, highly alkaline zones to the extremely acidic areas and freshwater to brackish water sources. The diverse ecological microbial niches are attributed to the versatile, adaptable nature under fluctuating temperature, nutrient availability and pH of the microorganisms. These organisms have developed a series of mechanisms to face the environmental changes and thereby keep their role in mediate important ecosystem functions. The underlying mechanisms of adaptable microbial nature are thoroughly investigated at the cellular, genetic and molecular levels. The adaptation is mediated by a spectrum of processes like natural selection, genetic recombination, horizontal gene transfer, DNA damage repair and pleiotropy-like events. This review paper provides the fundamentals insight into the microbial adaptability besides highlighting the molecular network of microbial adaptation under different environmental conditions.

Investigation of the impact of a broad range of temperatures on the physiological and transcriptional profiles of Zymomonas mobilis ZM4 for high-temperature-tolerant recombinant strain development

The model ethanologenic bacterium Zymomonas mobilis has many advantages for diverse biochemical production. Although the impact of temperature especially high temperature on the growth and ethanol production of Z. mobilis has been reported, the transcriptional profiles of Z. mobilis grown at different temperatures have not been systematically investigated. In this study, Z. mobilis wild-type strain ZM4 was used to study the effect of a broad range of temperatures of 24, 30, 36, 40, and 45 °C on cell growth and morphology, glucose utilization and ethanol production, as well as the corresponding global gene expression profiles using RNA-Seq-based transcriptomics. In addition, a recombinant Z. mobilis strain expressing reporter gene EGFP (ZM4_EGFP) was constructed to study the effect of temperature on heterologous protein expression at different temperatures. Our result demonstrated that the effect of temperature on the growth and morphology of ZM4 and ZM4_EGFP were similar. The biomass of these two strains decreased along with the temperature increase, and an optimal temperature range is needed for efficient glucose utilization and ethanol production. Temperatures lower or higher than normal temperature investigated in this work was not favorable for the glucose utilization and ethanol production as well as the expression of exogenous protein EGFP based on the results of flow cytometry and Western blot. Temperature also affected the transcriptional profiles of Z. mobilis especially under high temperature. Compared with ZM4 cultured at 30 °C, 478 genes were up-regulated and 481 genes were down-regulated at 45 °C. The number of differentially expressed genes of ZM4 cultured at other temperatures (24, 36 or 40 °C) was relatively small though compared with those at 30 °C. Since temperature usually increases during the fermentation process, and heat tolerance is one of the important robustness traits of industrial strains, candidate genes related to heat resistance based on our RNA-Seq result and literature report were then selected for genetics study using the strategies of plasmid overexpression of candidate gene or replacement of the native promoter of candidate gene by an inducible Ptet promoter. The genetics studies indicated that ZMO0236, ZMO1335, ZMO0994, operon groESL, and cspL, which encodes Mrp family chromosome partitioning ATPase, flavoprotein WrbA, an uncharacterized protein, chaperonin Cpn10 and GroEL, and an exogenous cold shock protein, respectively, were associated with heat tolerance, and recombinant strains over-expressing these genes can improve their heat tolerance. Our work thus not only explored the effects of temperature on the expression of exogenous gene EGFP and endogenous genes, but also selected and confirmed several genes associated with heat tolerance in Z. mobilis, which provided a guidance on identifying candidate genes associated with phenotypic improvement through systems biology strategy and genetics studies for other microorganisms.

Compost Samples from Different Temperature Zones as a Model to Study Co-occurrence of Thermophilic and Psychrophilic Bacterial Population: a Metagenomics Approach

In this study, using a metagenomic approach, we explore the bacterial diversity of compost sites categorized based on their ambient temperatures. The two sites were Reckong Peo in the lower Himalayas and Tambaram in the southern region of the country, namely, CPR and CT. Following assembly of the raw reads from shotgun metagenomics, similarity hits were generated using NCBI BLAST + and SILVA database. A total of 1463 and 1483 species were annotated from CPR and CT. A species-level annotation was performed using a python-based literature search pipeline revealing their growth characteristics. Thermophiles Thermomonospora curvata and Thermus scotoductus were among the prominent species in CT. CPR too was seen abundant with Acidothermus cellulolyticus and Moorella thermoacetica, constituting 10% of the population. Nearly 3% of the identified species in the site CPR were psychrophilic. Although found higher in CPR, psychrophilic species were identified in CT too. Flavobacterium and Psychrobacter spp. were present in both sites without any significant changes in their relative distribution contrary to the thermophilic species abundance (z = - 4.3). Akin to the sequenced samples, database-derived metagenomes also showed similar distribution of thermophiles and psychrophiles. Identifying such peculiar prevalence of extremophiles can be central to understanding extended growth temperatures.

Precise promoter integration improves cellulose bioconversion and thermotolerance in Clostridium cellulolyticum

Lignocellulose has been used for production of sustainable biofuels and value-added chemicals. However, the low-efficiency bioconversion of lignocellulose greatly contributes to a high production cost. Here, we employed CRISPR-Cas9 editing to improve cellulose degradation efficiency by editing a regulatory element of the cip-cel gene cluster in Clostridium cellulolyticum. Insertion of a synthetic promoter (P4) and an endogenous promoter (P2) in the mspI-deficient parental strain (Δ2866) created chromosomal integrants, P4-2866 and P2-2866, respectively. Both engineered strains increased the transcript abundance of downstream polycistronic genes and enhanced in vitro cellulolytic activities of isolated cellulosomes. A high cellulose load of 20 g/L suppressed cellulose degradation in the parental strain in the first 150 h fermentation; whereas P4-2866 and P2-2866 hydrolyzed 29% and 53% of the cellulose, respectively. Both engineered strains also demonstrated a greater growth rate and a higher cell biomass yield. Interestingly, the Δ2866 parental strain demonstrated better thermotolerance than the wildtype strain, and promoter insertion further enhanced thermotolerance. Similar improvements in cell growth and cellulose degradation were reproduced by promoter insertion in the wildtype strain and a lactate production-defective mutant (LM). P2 insertion in LM increased ethanol titer by 65%. Together, the editing of regulatory elements of catabolic gene clusters provides new perspectives on improving cellulose bioconversion in microbes.

Enhancement of Thermal Resistance by Metal Ions in Thermotolerant Zymomonas mobilis TISTR 548

The thermal resistance of fermenting microbes is a key characteristic of stable fermentation at high temperatures. Therefore, the effects of various metal ions on the growth of Zymomonas mobilis TISTR 548, a thermotolerant ethanologenic bacterium, at a critical high temperature (CHT) were examined. Addition of Mg2+ and K+ increased CHT by 1°C, but the effects of the addition of Mn2+, Ni2+, Co2+, Al3+, Fe3+, and Zn2+ on CHT were negligible. To understand the physiological functions associated with the addition of Mg2+ or K+, cell morphology, intracellular reactive oxygen species (ROS) level, and ethanol productivity were investigated at 39°C (i.e., above CHT). Cell elongation was repressed by Mg2+, but not by K+. Addition of both metals reduced intracellular ROS level, with only K+ showing the highest reduction strength, followed by both metals and only Mg2+. Additionally, ethanol productivity was recovered with the addition of both metals. Moreover, the addition of Mg2+ or K+ at a non-permissive temperature in 26 thermosensitive, single gene-disrupted mutants of Z. mobilis TISTR 548 revealed that several mutants showed metal ion-specific growth improvement. Remarkably, K+ repressed growth of two mutants. These results suggest that K+ and Mg2+ enhance cell growth at CHT via different mechanisms, which involve the maintenance of low intracellular ROS levels.

Improvement of Thermotolerance of Zymomonas mobilis by Genes for Reactive Oxygen Species-Scavenging Enzymes and Heat Shock Proteins

Thermotolerant genes, which are essential for survival at a high temperature, have been identified in three mesophilic microbes, including Zymomonas mobilis. Contrary to expectation, they include only a few genes for reactive oxygen species (ROS)-scavenging enzymes and heat shock proteins, which are assumed to play key roles at a critical high temperature (CHT) as an upper limit of survival. We thus examined the effects of increased expression of these genes on the cell growth of Z. mobilis strains at its CHT. When overexpressed, most of the genes increased the CHT by about one degree, and some of them enhanced tolerance against acetic acid. These findings suggest that ROS-damaged molecules or unfolded proteins that prevent cell growth are accumulated in cells at the CHT.

Zymomonas mobilis metabolism: Novel tools and targets for its rational engineering

Zymomonas mobilis is an α-proteobacterium that interests the biofuel industry due to its perfect ethanol fermentation yields. From its first description as a bacterial isolate in fermented alcoholic beverages to date, Z. mobilis has been rigorously studied in directions basic and applied. The Z. mobilis powerful Entner-Doudoroff glycolytic pathway has been the center of rigorous biochemical studies and, aside from ethanol, it has attracted interest in terms of high-added-value chemical manufacturing. Energetic balances and the effects of respiration have been explored in fundamental directions as also in applications pursuing strain enhancement and the utilization of alternative carbon sources. Metabolic modeling has addressed the optimization of the biochemical circuitry at various conditions of growth and/or substrate utilization; it has been also critical in predicting desirable end-product yields via flux redirection. Lastly, stress tolerance has received particular attention, since it directly determines biocatalytical performance at challenging bioreactor conditions. At a genetic level, advances in the genetic engineering of the organism have brought forth beneficial manipulations in the Z. mobilis gene pool, e.g., knock-outs, knock-ins and gene stacking, aiming to broaden the metabolic repertoire and increase robustness. Recent omic and expressional studies shed light on the genomic content of the most applied strains and reveal landscapes of activity manifested at ambient or reactor-based conditions. Studies such as those reviewed in this work, contribute to the understanding of the biology of Z. mobilis, enable insightful strain development, and pave the way for the transformation of Z. mobilis into a consummate organism for biomass conversion.

Metabolic Engineering of Bacterial Respiration: High vs. Low P/O and the Case of Zymomonas mobilis

Respiratory chain plays a pivotal role in the energy and redox balance of aerobic bacteria. By engineering respiration, it is possible to alter the efficiency of energy generation and intracellular redox state, and thus affect the key bioprocess parameters: cell yield, productivity and stress resistance. Here we summarize the current metabolic engineering and synthetic biology approaches to bacterial respiratory metabolism, with a special focus on the respiratory chain of the ethanologenic bacterium Zymomonas mobilis. Electron transport in Z. mobilis can serve as a model system of bacterial respiration with low oxidative phosphorylation efficiency. Its application for redox balancing and relevance for improvement of stress tolerance are analyzed.

Physiological effects of overexpressed sigma factors on fermentative stress response of Zymomonas mobilis

Zymomonas mobilis is a bacterium of industrial interest due to its high ethanol productivity and high tolerance to stresses. Although the physiological parameters of fermentation are well characterized, there are few studies on the molecular mechanisms that regulate the response to fermentative stress. Z. mobilis ZM4 presents five different sigma factors identified in the genome annotation, but the absence of sigma 38 leads to the questioning of which sigma factors are responsible for its mechanism of fermentative stress response. Thus, in this study, factors sigma 32 and sigma 24, traditionally related to heat shock, were tested for their influence on the response to osmotic and ethanol stress. The overexpression of these sigma factors in Z. mobilis ZM4 strain confirmed that both are associated with heat shock response, as described in other bacteria. Moreover, sigma 32 has also a role in the adaptation to osmotic stress, increasing both growth rate and glucose influx rate. The same strain that overexpresses sigma 32 also showed a decrease in ethanol tolerance, suggesting an antagonism between these two mechanisms. It was not possible to conclude if sigma 24 really affects ethanol tolerance in Z. mobilis, but the overexpression of this sigma factor led to a decrease in ethanol productivity.

Capacity for survival in global warming: Adaptation of mesophiles to the temperature upper limit

The Intergovernmental Panel on Climate Change recommends keeping the increase in temperature to less than a two-degree increase by the end of the century, but the direct impact of global warming on ecosystems including microbes has not been investigated. Here we performed thermal adaptation of two species and three strains of mesophilic microbes for improvement of the survival upper limit of temperature, and the improvement was evaluated by a newly developed method. To understand the limitation and variation of thermal adaptation, experiments with mutators and by multiple cultures were performed. The results of experiments including genome sequencing and analysis of the characteristics of mutants suggest that these microbes bear a genomic potential to endure a 2–3°C rise in temperature but possess a limited variation of strategies for thermal adaptation.

New technologies provide more metabolic engineering strategies for bioethanol production in Zymomonas mobilis

Bioethanol has been considered as a potentially renewable energy source, and metabolic engineering plays an important role in the production of biofuels. As an efficient ethanol-producing bacterium, Zymomonas mobilis has garnered special attention due to its high sugar uptake, ethanol yield, and tolerance. Different metabolic engineering strategies have been used to establish new metabolic pathways for Z. mobilis to broaden its substrate range, remove competing pathways, and enhance its tolerance to ethanol and lignocellulosic hydrolysate inhibitors. Recent advances in omics technology, computational modeling and simulation, system biology, and synthetic biology contribute to the efficient re-design and manipulation of microbes via metabolic engineering at the whole-cell level. In this review, we summarize the progress of some new technologies used for metabolic engineering to improve bioethanol production and tolerance in Z. mobilis. Some successful examples of metabolic engineering used to develop strains for ethanol production are described in detail. Lastly, some important strategies for future metabolic engineering efforts are also highlighted.

Prediction and characterization of promoters and ribosomal binding sites of Zymomonas mobilis in system biology era

BACKGROUND

Zymomonas mobilis is a model bacterial ethanologen with many systems biology studies reported. Besides lignocellulosic ethanol production, Z. mobilis has been developed as a platform for biochemical production through metabolic engineering. However, identification and rigorous understanding of the genetic origins of cellular function, especially those based in non-coding region of DNA, such as promoters and ribosomal binding sites (RBSs), are still in its infancy. This knowledge is crucial for the effective application of Z. mobilis to new industrial applications of biotechnology for fuels and chemicals production.

RESULTS

In this study, we explored the possibility to systematically predict the strength of promoters based on systems biology datasets. The promoter strength was clustered based on the expression values of downstream genes (or proteins) from systems biology studies including microarray, RNA-Seq and proteomics. Candidate promoters with different strengths were selected for further characterization, which include 19 strong, nine medium, and ten weak ones. A dual reporter-gene system was developed which included appropriate reporter genes. These are the opmCherry reporter gene driven by the constitutive PlacUV5 promoter for calibration, and EGFP reporter gene driven by candidate promoters for quantification. This dual reporter-gene system was confirmed using the inducible promoter, Ptet, which was used to determine the strength of these predicted promoters with different strengths. In addition, the dual reporter-gene system was applied to determine four synthetic RBSs with different translation initiation rates based on the prediction from bioinformatics server RBS calculator. Our results showed that the correlations between the prediction and experimental results for the promoter and RBS strength are relatively high, with R ² values more than 0.7 and 0.9, respectively.

CONCLUSIONS

This study not only identified and characterized 38 promoters and four RBSs with different strengths for future metabolic engineering in Z. mobilis, but also established a flow cytometry-based dual reporter-gene system to characterize genetic elements including, but not limited to the promoters and RBSs studied in this work. This study also suggested the feasibility of predicting and selecting candidate genetic elements based on omics datasets and bioinformatics tools. Moreover, the dual reporter-gene system developed in this study can be utilized to characterize other genetic elements of Z. mobilis, which can also be applied to other microorganisms.

Aerobic catabolism and respiratory lactate bypass in Ndh-negative Zymomonas mobilis

Ability to ferment in the presence of oxygen increases the robustness of bioprocesses and opens opportunity for novel industrial setups. The ethanologenic bacterium Zymomonas mobilis performs rapid and efficient anaerobic ethanol fermentation, yet its respiratory NADH dehydrogenase (Ndh)-deficient strain (ndh-) is known to produce ethanol with high yield also under oxic conditions. Compared to the wild type, it has a lower rate of oxygen consumption, and an increased expression of the respiratory lactate dehydrogenase (Ldh). Here we present a quantitative study of the product spectrum and carbon balance for aerobically growing ndh-. Ldh-deficient and Ldh-overexpressing ndh- strains were constructed and used to examine the putative role of the respiratory lactate bypass for aerobic growth and production. We show that aerobically growing ndh- strains perform fermentative metabolism with a near-maximum ethanol yield, irrespective of their Ldh expression background. Yet, Ldh activity strongly affects the aerobic product spectrum in glucose-consuming non-growing cells. Also, Ldh-deficiency hampers growth at elevated temperature (42 °C) and delays the restart of growth after 10-15 h of aerobic starvation.

Characterization of a thermo-adapted strain of Zymomonas mobilis for ethanol production at high temperature

A thermo-adapted strain of Zymomonas mobilis designated ZM AD41 that capable of growth and ethanol production at high temperature was obtained using the thermal stress adaptation technique. This thermo-adapted strain exhibited approximately 1.8- and 27-fold higher growth rate than the wild-type at 39 °C and 41 °C, respectively. It was more resistant to stress induced by acetic acid at 200 mM and hydrogen peroxide (H₂O₂) at 0.4 mM and produced approximately 1.8- and 38.6-fold higher ethanol concentrations than the wild-type at 39 °C and 41 °C, respectively. Moreover, it had better sedimentation performance during ethanol fermentation at high temperature than the wild-type. Based on the growth performance, heat, acetic acid and H₂O₂ stress treatments, sedimentation characteristics, and ethanol fermentation capability, Z. mobilis ZM AD41 was a good candidate for ethanol production at high temperature.

Advances and prospects in metabolic engineering of Zymomonas mobilis

Biorefinery of biomass-based biofuels and biochemicals by microorganisms is a competitive alternative of traditional petroleum refineries. Zymomonas mobilis is a natural ethanologen with many desirable characteristics, which makes it an ideal industrial microbial biocatalyst for commercial production of desirable bioproducts through metabolic engineering. In this review, we summarize the metabolic engineering progress achieved in Z. mobilis to expand its substrate and product ranges as well as to enhance its robustness against stressful conditions such as inhibitory compounds within the lignocellulosic hydrolysates and slurries. We also discuss a few metabolic engineering strategies that can be applied in Z. mobilis to further develop it as a robust workhorse for economic lignocellulosic bioproducts. In addition, we briefly review the progress of metabolic engineering in Z. mobilis related to the classical synthetic biology cycle of "Design-Build-Test-Learn", as well as the progress and potential to develop Z. mobilis as a model chassis for biorefinery practices in the synthetic biology era.