The isolation of pentose-assimilating yeasts and their xylose fermentation potential

A catalog of ethanol-producing microbes in humans

Aim: Endogenous ethanol production emerges as a mechanism of nonalcoholic steatohepatitis, obesity, diabetes and auto-brewery syndrome. Methods: To identify ethanol-producing microbes in humans, we used the NCBI taxonomy browser and the PubMed database with an automatic query and manual verification. Results: 85 ethanol-producing microbes in human were identified. Saccharomyces cerevisiae, Candida and Pichia were the most represented fungi. Enterobacteriaceae was the most represented bacterial family with mainly Escherichia coli and Klebsiella pneumoniae. Species of the Lachnospiraceae and Clostridiaceae family, of the Lactobacillales order and of the Bifidobacterium genus were also identified. Conclusion: This catalog will help the study of ethanol-producing microbes in human in the pathophysiology, diagnosis, prevention and management of human diseases associated with endogenous ethanol production.

Current status of metabolic engineering of microorganisms for bioethanol production by effective utilization of pentose sugars of lignocellulosic biomass

Lignocellulosic biomass, consisting of homo- and heteropolymeric sugars, acts as a substrate for the generation of valuable biochemicals and biomaterials. The readily available hexoses are easily utilized by microbes due to the presence of transporters and native metabolic pathways. But, utilization of pentose sugar viz., xylose and arabinose are still challenging due to several reasons including (i) the absence of the particular native pathways and transporters, (ii) the presence of inhibitors, and (iii) lower uptake of pentose sugars. These challenges can be overcome by manipulating metabolic pathways/glycosidic enzymes cascade by using genetic engineering tools involving inverse-metabolic engineering, ex-vivo isomerization, Adaptive Laboratory Evolution, Directed Metabolic Engineering, etc. Metabolic engineering of bacteria and fungi for the utilization of pentose sugars for bioethanol production is the focus area of research in the current decade. This review outlines current approaches to biofuel development and strategies involved in the metabolic engineering of different microbes that can uptake pentose for bioethanol production.

Isolation of xylose-utilizing yeasts from oil palm waste for xylitol and ethanol production

The energy crisis triggers the use of energy sources that are renewable, such as biomass made from lignocellulosic materials, to produce various chemical compounds for food ingredients and biofuel. The efficient conversion of lignocellulosic biomass into products with added value involves the activity of microorganisms, such as yeasts. For the conversion, microorganisms must be able to use various sugars in lignocellulosic biomass, including pentose sugars, especially xylose. This study aims to isolate xylose-utilizing yeasts and analyze their fermentation activity to produce xylitol and ethanol, as well as their ability to grow in liquid hydrolysate produced from pretreated lignocellulosic biomass. Nineteen yeast isolates could grow on solid and liquid media using solely xylose as a carbon source. All isolates can grow in a xylose medium with incubation at 30 °C, 37 °C, 42 °C, and 45 °C. Six isolates, namely SLI (1), SL3, SL6, SL7, R5, and OPT4B, were chosen based on their considerable growth and high xylose consumption rate in a medium with 50 g/L xylose with incubation at 30 °C for 48 h. Four isolates tested, namely SLI (1), SL6, SL7, and R5, can produce xylitol in media containing xylose carbon sources. The concentration of xylitol produced was determined using high-pressure liquid chromatography (HPLC), and the results ranged from 5.0 to 6.0 g/L. Five isolates tested, namely SLI (1), SL6, SL3, R5, and OPT4B, can produce ethanol. The ethanol content produced was determined using gas chromatography (GC), with concentrations ranging from 0.85 to 1.34 g/L. Three isolates, namely SL1(1), R5, and SL6, were able to produce xylitol and ethanol from xylose as carbon sources and were also able to grow on liquid hydrolyzate from pretreated oil palm trunk waste with the subcritical water method. The three isolates were further analyzed using the 18S rDNA sequence to identify the species and confirm their phylogenetic position. Identification based on DNA sequence analysis revealed that isolates SL1(1) and R5 were Pichia kudriavzevii, while isolate SL6 was Candida xylopsoci. The yeast strains isolated from this study could potentially be used for the bioconversion process of lignocellulosic biomass waste to produce value-added derivative products.

Xylose consumption and ethanol production by Pichia guilliermondii and Candida oleophila in the presence of furans, phenolic compounds, and organic acids commonly produced during the pre-treatment of plant biomass

For 2G ethanol production, pentose fermentation and yeast tolerance to lignocellulosic hydrolyzate components are essential to improve biorefinery yields. Generally, physicochemical pre-treatment methodologies are used to facilitate access to cellulose and hemicellulose in plant material, which consequently can generate microbial growth inhibitory compounds, such as furans, weak acids, and phenolic compounds. Because of the unsatisfactory yield of wild-type Saccharomyces cerevisiae during pentose fermentation, the search for xylose-fermenting yeasts tolerant to microbial growth inhibitors has gained attention. In this study, we investigated the ability of the yeasts Pichia guilliermondii G1.2 and Candida oleophila G10.1 to produce ethanol from xylose and tolerate the inhibitors furfural, 5-hydroxymethylfurfural (HMF), acetic acid, formic acid, ferulic acid, and vanillin. We demonstrated that both yeasts were able to grow and consume xylose in the presence of all single inhibitors, with greater growth limitation in media containing furfural, acetic acid, and vanillin. In saline medium containing a mixture of these inhibitors (2.5-3.5 mM furfural and HMF, 1 mM ferulic acid, 1-1.5 mM vanillin, 10-13 mM acetic acid, and 5-7 mM formic acid), both yeasts were able to produce ethanol from xylose, similar to that detected in the control medium (without inhibitors). In future studies, the proteins involved in the transport of pentose and tolerance to these inhibitors need to be investigated.

Non-conventional yeast strains: Unexploited resources for effective commercialization of second generation bioethanol

The conventional yeast (Saccharomyces cerevisiae) is the most studied yeast and has been used in many important industrial productions, especially in bioethanol production from first generation feedstock (sugar and starchy biomass). However, for reduced cost and to avoid competition with food, second generation bioethanol, which is produced from lignocellulosic feedstock, is now being investigated. Production of second generation bioethanol involves pre-treatment and hydrolysis of lignocellulosic biomass to sugar monomers containing, amongst others, d-glucose and D-xylose. Intrinsically, S. cerevisiae strains lack the ability to ferment pentose sugars and genetic engineering of S. cerevisiae to inculcate the ability to ferment pentose sugars is ongoing to develop recombinant strains with the required stability and robustness for commercial second generation bioethanol production. Furthermore, pre-treatment of these lignocellulosic wastes leads to the release of inhibitory compounds which adversely affect the growth and fermentation by S. cerevisae. S. cerevisiae also lacks the ability to grow at high temperatures which favour Simultaneous Saccharification and Fermentation of substrates to bioethanol. There is, therefore, a need for robust yeast species which can co-ferment hexose and pentose sugars and can tolerate high temperatures and the inhibitory substances produced during pre-treatment and hydrolysis of lignocellulosic materials. Non-conventional yeast strains are potential solutions to these problems due to their abilities to ferment both hexose and pentose sugars, and tolerate high temperature and stress conditions encountered during ethanol production from lignocellulosic hydrolysate. This review highlights the limitations of the conventional yeast species and the potentials of non-conventional yeast strains in commercialization of second generation bioethanol.

Enhanced cellulosic ethanol production via fed-batch simultaneous saccharification and fermentation of sequential dilute acid-alkali pretreated sugarcane bagasse

This study reports high gravity fed-batch simultaneous saccharification and fermentation (FB-SSF) of sequentially pretreated sugarcane bagasse (SCB) for enhanced bioethanol by employing multiple inhibitor tolerant Kluyveromyces marxianusJKH5 C60. FB-SSF with intermittent feeding of SCB (total 20 % solid loading) and enzyme (total dose of 20 FPU/g) at 6 and 12 h resulted in superior bioethanol production at42 °C. Under optimizedlab-scaleFB-SSF, the maximum ethanoltiter, efficiency and productivities were73.4 ± 1.2 g/L,78 % and 3.0 g/L/h, respectively, after 72 h in presence of inhibitors (acetic acid, furfural, and vanillin at 3, 1, and 1 g/L, respectively). Furthermore, pentose rich dilute acid hydrolysate of SCB was subjected to fermentation by Pichia stipitis NCIM 3499, resulting in ethanol titer of 6.8 g/L. Overall ethanol yield during the developed process was 260.1 g/kg native SCB, which proves industrial potential of the developed bioethanol conversion process.

Electrospinning Composites as Carriers of Natural Pigment: Screening of Polymeric Blends

Several studies have already demonstrated that electrospinning is an excellent tool for forming nano/microfibers. However, the number of parameters affecting the formation of the structures has become a great challenge, including the polymeric solutions’ rheological properties, directly affecting the morphology of the fibers formed. The present work aimed to produce polymeric composites and determine their rheological properties, comparing them to the morphology of the fibers formed by electrospinning. Also, to evaluate their potential use as the carriers of natural pigments. To this end, a distinct combination of solutions containing Chitosan/Gelatin, Chitosan/poly(ethylene) oxide (PEO) and Zein/PEO was produced and submitted to electrospinning. The sample containing zein manufactured the structures smaller in diameter (201.3 ± 58.6 nm) among those studied. Besides, it was observed that adding PEO to the solutions impacts the increase in viscosity and shear thinning behavior, guaranteeing uniformity in the structures formed. Natural pigments were successfully incorporated into the chosen zein/PEO solution, and it was observed that adding these compounds led to changes in the rheological characteristics, as expected. Nevertheless, it was possible to produce uniform fibers with diameters ranging from 665.68 ± 249.56 to 2874.44 ± 1187.40 nm, opening the possibility of using these natural pigments in biotechnological processes.

Nature's Most Fruitful Threesome: The Relationship between Yeasts, Insects, and Angiosperms

The importance of insects for angiosperm pollination is widely recognized. In fact, approximately 90% of all plant species benefit from animal-mediated pollination. However, only recently, a third part player in this story has been properly acknowledged. Microorganisms inhabiting floral nectar, among which yeasts have a prominent role, can ferment glucose, fructose, sucrose, and/or other carbon sources in this habitat. As a result of their metabolism, nectar yeasts produce diverse volatile organic compounds (VOCs) and other valuable metabolites. Notably, some VOCs of yeast origin can influence insects' foraging behavior, e.g., by attracting them to flowers (although repelling effects have also been reported). Moreover, when insects feed on nectar, they also ingest yeast cells, which provide them with nutrients and protect them from pathogenic microorganisms. In return, insects serve yeasts as transportation and a safer habitat during winter when floral nectar is absent. From the plant's point of view, the result is flowers being pollinated. From humanity's perspective, this ecological relationship may also be highly profitable. Therefore, prospecting nectar-inhabiting yeasts for VOC production is of major biotechnological interest. Substances such as acetaldehyde, ethyl acetate, ethyl butyrate, and isobutanol have been reported in yeast volatomes, and they account for a global market of approximately USD 15 billion. In this scenario, the present review addresses the ecological, environmental, and biotechnological outlooks of this three-party mutualism, aiming to encourage researchers worldwide to dig into this field.

Integrated bioinformatics, modelling, and gene expression analysis of the putative pentose transporter from Candida tropicalis during xylose fermentation with and without glucose addition

The transport of substrates across the cell membrane plays an essential role in nutrient assimilation by yeasts. The establishment of an efficient microbial cell factory, based on the maximum use of available carbon sources, can generate new technologies that allow the full use of lignocellulosic constituents. These technologies are of interest because they could promote the formation of added-value products with economic feasibility. In silico analyses were performed to investigate gene sequences capable of encoding xylose transporter proteins in the Candida tropicalis genome. The current study identified 11 putative transport proteins that have not yet been functionally characterized. A phylogenetic tree highlighted the potential C. tropicalis xylose-transporter proteins CtXUT1, CtXUT4, CtSTL1, CtSTL2, and CtGXT2, which were homologous to previously characterized and reported xylose transporters. Their expression was quantified through real-time qPCR at defined times, determined through a kinetic analysis of the microbial growth curve in the absence/presence of glucose supplemented with xylose as the main carbon source. The results indicated different mRNA expression levels for each gene. CtXUT1 mRNA expression was only found in the absence of glucose in the medium. Maximum CtXUT1 expression was observed in intervals of the highest xylose consumption (21 to 36 h) that corresponded to consumption rates of 1.02 and 0.82 g/L/h in the formulated media, with xylose as the only carbon source and with glucose addition. These observations indicate that CtXUT1 is an important xylose transporter in C. tropicalis. KEY POINTS: • Putative xylose transporter proteins were identified in Candida tropicalis; • The glucose concentration in the cultivation medium plays a key role in xylose transporter regulation; • The transporter gene CtXUT1 has an important role in xylose consumption by Candida tropicalis.

Past, Present, and Future Perspectives on Whey as a Promising Feedstock for Bioethanol Production by Yeast

Concerns about fossil fuel depletion and the environmental effects of greenhouse gas emissions have led to widespread fermentation-based production of bioethanol from corn starch or sugarcane. However, competition for arable land with food production has led to the extensive investigation of lignocellulosic sources and waste products of the food industry as alternative sources of fermentable sugars. In particular, whey, a lactose-rich, inexpensive byproduct of dairy production, is available in stable, high quantities worldwide. This review summarizes strategies and specific factors essential for efficient lactose/whey fermentation to ethanol. In particular, we cover the most commonly used strains and approaches for developing high-performance strains that tolerate fermentation conditions. The relevant genes and regulatory systems controlling lactose utilization and sources of new genes are also discussed in detail. Moreover, this review covers the optimal conditions, various feedstocks that can be coupled with whey substrates, and enzyme supplements for increasing efficiency and yield. In addition to the historical advances in bioethanol production from whey, this review explores the future of yeast-based fermentation of lactose or whey products for beverage or fuel ethanol as a fertile research area for advanced, environmentally friendly uses of industrial waste products.

Engineered Saccharomyces cerevisiae for lignocellulosic valorization: a review and perspectives on bioethanol production

The biorefinery concept, consisting in using renewable biomass with economical and energy goals, appeared in response to the ongoing exhaustion of fossil reserves. Bioethanol is the most prominent biofuel and has been considered one of the top chemicals to be obtained from biomass. Saccharomyces cerevisiae, the preferred microorganism for ethanol production, has been the target of extensive genetic modifications to improve the production of this alcohol from renewable biomasses. Additionally, S. cerevisiae strains from harsh industrial environments have been exploited due to their robust traits and improved fermentative capacity. Nevertheless, there is still not an optimized strain capable of turning second generation bioprocesses economically viable. Considering this, and aiming to facilitate and guide the future development of effective S. cerevisiae strains, this work reviews genetic engineering strategies envisioning improvements in ^2nd generation bioethanol production, with special focus in process-related traits, xylose consumption, and consolidated bioprocessing. Altogether, the genetic toolbox described proves S. cerevisiae to be a key microorganism for the establishment of a bioeconomy, not only for the production of lignocellulosic bioethanol, but also having potential as a cell factory platform for overall valorization of renewable biomasses.

Ethanol Production and Other Bioproducts by Galactomyces geotrichum from Sugarcane Bagasse Hydrolysate

This study investigated the enzymatic saccharification of alkaline-pretreated sugarcane bagasse (PSB) and the bioconversion of simple sugars from hydrolysates to ethanol and other bioproducts by the yeast Galactomyces geotrichum. The effects of percentage of dry substrate (3 and 10% w/v) and time of hydrolysis (24 and 72 h) in the content of released sugars were evaluated. The concentrations of monosaccharides and total reducing sugars (TRS) were calculated by high-performance liquid chromatography (HPLC) and by 3.5-dinitrosalicylic acid (DNS) method, respectively. The highest concentrations of TRS, glucose and xylose (73.96, 31.78 and 10.85 g/L, respectively) were obtained after the saccharification of 10% of PSB with Cellic CTec3 multi-enzyme cocktail (10 FPU/g cellulose) during 72 h (hydrolysate IV). G. geotrichum UFVJM-R150 fermented both glucose and xylose from the hydrolysates. The most efficient ethanol production was obtained after the fermentation of hydrolysate IV (9.99 g/L of ethanol, volumetric productivity-Q_P of 0.42 g/L.h and yield of ethanol as a function of the substrate-Y_P/S of 0.27 g_ethanol/g_sugar). Besides ethanol, G. geotrichum was also able to produce other high-value chemicals such as isoamyl alcohol and galacturonic acid. This is the first report of the potential of the yeast G. geotrichum to fermentate sugarcane bagasse hydrolysates with the production of important bioproducts to further use by biorefineries.

Xylitol Production from Lignocellulosic Pentosans: A Rational Strain Engineering Approach toward a Multiproduct Biorefinery

Kluyveromyces marxianus IIPE453 can utilize biomass-derived fermentable sugars for xylitol and ethanol fermentation. In this study, the xylitol production in the native strain was improved by overexpression of endogenous d-xylose reductase gene. A suitable expression cassette harboring the gene of interest was constructed and incorporated in the native yeast. qPCR analysis demonstrated the 2.1-fold enhancement in d-xylose reductase transcript levels in the modified strain with 1.62-fold enhancement in overall xylitol yield without affecting its ethanol fermenting capacity. Material balance analysis on 2 kg of sugar cane bagasse-derived fermentable sugars illustrated an excess of 58.62 ± 0.15 g of xylitol production by transformed strain in comparison to the wild variety with similar ethanol yield. The modified strain can be suitably used as a single biocatalyst for multiproduct biorefinery application.

Review of Second Generation Bioethanol Production from Residual Biomass

In the context of climate change and the depletion of fossil fuels, there is a great need for alternatives to petroleum in the transport sector. This review provides an overview of the production of second generation bioethanol, which is distinguished from the first generation and subsequent generations of biofuels by its use of lignocellulosic biomass as raw material. The structural components of the lignocellulosic biomass such as cellulose, hemicellulose and lignin, are presented along with technological unit steps including pretreatment, enzymatic hydrolysis, fermentation, distillation and dehydration. The purpose of the pretreatment step is to increase the surface area of carbohydrate available for enzymatic saccharification, while minimizing the content of inhibitors. Performing the enzymatic hydrolysis releases fermentable sugars, which are converted by microbial catalysts into ethanol. The hydrolysates obtained after the pretreatment and enzymatic hydrolysis contain a wide spectrum of sugars, predominantly glucose and xylose. Genetically engineered microorganisms are therefore needed to carry out co-fermentation. The excess of harmful inhibitors in the hydrolysate, such as weak organic acids, furan derivatives and phenol components, can be removed by detoxification before fermentation. Effective saccharification further requires using exogenous hemicellulases and cellulolytic enzymes. Conventional species of distiller's yeast are unable to ferment pentoses into ethanol, and only a very few natural microorganisms, including yeast species like Candida shehatae, Pichia (Scheffersomyces) stipitis, and Pachysolen tannophilus, metabolize xylose to ethanol. Enzymatic hydrolysis and fermentation can be performed in a number of ways: by separate saccharification and fermentation, simultaneous saccharification and fermentation or consolidated bioprocessing. Pentose-fermenting microorganisms can be obtained through genetic engineering, by introducing xylose-encoding genes into metabolism of a selected microorganism to optimize its use of xylose accumulated in the hydrolysate.

Xylose transport in yeast for lignocellulosic ethanol production: Current status

Lignocellulosic ethanol has been considered as an alternative transportation fuel. Utilization of hemicellulosic fraction in lignocelluloses is crucial in economical production of lignocellulosic ethanol. However, this fraction has not efficiently been utilized by traditional yeast Saccharomyces cerevisiae. Genetically modified S. cerevisiae, which can utilize xylose, has several limitations including low ethanol yield, redox imbalance, and undesired metabolite formation similar to native xylose utilizing yeasts. Besides, xylose uptake is a major issue, where sugar transport system plays an important role. These genetically modified and wild-type yeast strains have further been engineered for improved xylose uptake. Various techniques have been employed to facilitate the xylose transportation in these strains. The present review is focused on the sugar transport machineries, mechanisms of xylose transport, limitations and how to deal with xylose transport for xylose assimilation in yeast cells. The recent advances in different techniques to facilitate the xylose transportation have also been discussed.