Ammonium nanochelators in conjunction with arginine-specific enzymes in amperometric biosensors for arginine assay.

Amino acid L-arginine (Arg), usually presented in food products and biological liquids, can serve both as a useful indicator of food quality and an important biomarker in medicine. The biosensors based on Arg-selective enzymes are the most promising devices for Arg assay. In this research, three types of amperometric biosensors have been fabricated. They exploit arginine oxidase (ArgO), recombinant arginase I (ARG)/urease, and arginine deiminase (ADI) coupled with the ammonium-chelating redox-active nanoparticles. Cadmium-copper nanoparticles (nCdCu) as the most effective nanochelators were used for the development of ammonium chemosensors and enzyme-coupled Arg biosensors. The fabricated enzyme/nCdCu-containing bioelectrodes show wide linear ranges (up to 200 µM), satisfactory storage stabilities (14 days), and high sensitivities (A⋅M-1⋅m-2) to Arg: 1650, 1700, and 4500 for ADI-, ArgO- and ARG/urease-based sensors, respectively. All biosensors have been exploited to estimate Arg content in commercial juices. The obtained data correlate well with the values obtained by the reference method. A hypothetic scheme for mechanism of action of ammonium nanochelators in electron transfer reaction on the arginine-sensing electrodes has been proposed.

Efficient production of bacterial antibiotics aminoriboflavin and roseoflavin in eukaryotic microorganisms, yeasts.

BACKGROUND: Actinomycetes Streptomyces davaonensis and Streptomyces cinnabarinus synthesize a promising broad-spectrum antibiotic roseoflavin, with its synthesis starting from flavin mononucleotide and proceeding through an immediate precursor, aminoriboflavin, that also has antibiotic properties. Roseoflavin accumulation by the natural producers is rather low, whereas aminoriboflavin accumulation is negligible. Yeasts have many advantages as biotechnological producers relative to bacteria, however, no recombinant producers of bacterial antibiotics in yeasts are known. RESULTS: Roseoflavin biosynthesis genes have been expressed in riboflavin- or FMN-overproducing yeast strains of Candida famata and Komagataella phaffii. Both these strains accumulated aminoriboflavin, whereas only the latter produced roseoflavin. Aminoriboflavin isolated from the culture liquid of C. famata strain inhibited the growth of Staphylococcus aureus (including MRSA) and Listeria monocytogenes. Maximal accumulation of aminoriboflavin in shake-flasks reached 1.5 mg L- 1 (C. famata), and that of roseoflavin was 5 mg L- 1 (K. phaffii). Accumulation of aminoriboflavin and roseoflavin by K. phaffii recombinant strain in a bioreactor reached 22 and 130 mg L- 1, respectively. For comparison, recombinant strains of the native bacterial producer S. davaonensis accumulated near one-order less of roseoflavin while no recombinant producers of aminoriboflavin was reported at all. CONCLUSIONS: Yeast recombinant producers of bacterial antibiotics aminoriboflavin and roseoflavin were constructed and evaluated.

Metabolic engineering of non-conventional yeasts for construction of the advanced producers of biofuels and high-value chemicals.

Non-conventional yeasts, i.e. yeasts different from Saccharomyces cerevisiae, represent heterogenous group of unicellular fungi consisting of near 1500 species. Some of these species have interesting and sometimes unique properties like ability to grow on methanol, n-alkanes, ferment pentose sugars xylose and l-arabinose, grow at high temperatures (50°Ð¡ and more), overproduce riboflavin (vitamin B2) and others. These unique properties are important for development of basic science; moreover, some of them possess also significant applied interest for elaboration of new biotechnologies. Current paper represents review of the recent own results and of those of other authors in the field of non-conventional yeast study for construction of the advanced producers of biofuels (ethanol, isobutanol) from lignocellulosic sugars glucose and xylose or crude glycerol (Ogataea polymorpha, Magnusiomyces magnusii) and vitamin B2 (riboflavin) from glucose and cheese whey (Candida famata).

Regulatory effect of moderate Jiang-flavour baijiu (Chinese liquor) dosage on organ function and gut microbiota in mice.

Chinese baijiu, an ancient fermented alcoholic beverage, contains ethanol and a variety of compounds. One of the most popular types of Chinese baijiu is Jiang-flavor baijiu. To investigate the effects of Jiang-flavor baijiu on organ function and gut microbiota, we developed a moderate drinking mouse model and studied its effects on the liver, kidney biomarkers, memory function, and gut microbiota. The results showed that ethanol caused more hepatic steatosis, liver and kidney damage, and memory impairment than Jiang-flavour baijiu consumption. Furthermore, Jiang-flavor baijiu altered the gut microbiota by increasing the abundance of beneficial taxa such as Lactobacillus and Akkermansia, whereas ethanol increased the abundance of harmful bacteria such as Prevotella and Mucispirillum. Our findings provide preliminary evidence that moderate dose Jiang-flavor baijiu regulates gut microbiota and organ function and provide a theoretical foundation for future research on the positive health effects of particular varieties of Chinese baijiu.

Construction of the advanced flavin mononucleotide producers in the flavinogenic yeast Candida famata.

Flavin mononucleotide (FMN, riboflavin-5'-phosphate) is flavin coenzyme synthesized in all living organisms from riboflavin (vitamin B2 ) after phosphorylation in the reaction catalyzed by riboflavin kinase. FMN has several applications mostly as yellow colorant in food industry due to 200 times better water solubility as compared to riboflavin. Currently, FMN is produced by chemical phosphorylation of riboflavin, however, final product contains up to 25% of flavin impurities. Microbial overproducers of FMN are known, however, they accumulate this coenzyme in glucose medium. Current work shows that the recombinant strains of the flavinogenic yeast Candida famata with overexpressed FMN1 gene coding for riboflavin kinase in the recently isolated by us advanced riboflavin producers due to overexpression of the structural and regulatory genes of riboflavin synthesis and of the putative exporter of riboflavin from the cell, synthesized elevated amounts of FMN in the media not only with glucose but also in lactose and cheese whey. Activation of FMN accumulation on lactose and cheese whey was especially strong in the strains which expressed the gene of transcription activator SEF1 under control of the lactose-induced LAC4 promoter. The accumulation of this coenzyme by the washed cells of the best recombinant strain achieved 540 mg/L in the cheese whey supplemented only with ammonium sulfate during 48 h in shake flask experiments.

The Komagatella phaffii ACG1 gene, encoding ß-1,6-N-acetylglucosaminyltransferase, is involved in the autophagy of cytosolic and peroxisomal proteins.

The methylotrophic yeast Komagataella phaffii is considered one of the most effective producers of recombinant proteins of industrial importance. Effective producers should be characterized by the maximal reduction of degradation of the cytosolic recombinant proteins. The mechanisms of degradation of cytosolic proteins in K. phaffii have not been elucidated; however, data suggest that they are partially degraded in the autophagic pathway. To identify factors that influence this process, a developed system for the selection of recombinant strains of K. phaffii with impaired autophagic degradation of the heterologous model cytosolic protein (yeast ß-galactosidase) was used for insertional tagging of the genes involved in cytosolic proteins degradation. In one of the obtained strains, the insertion cassette disrupted the open reading frame of the gene encoding ß-1,6-N-acetylglucosaminyltransferase. A recombinant strain with deletion of this gene was also obtained. The rate of degradation of the ß-galactosidase enzyme was two times slower in the insertion mutant and 1.5 times slower in the deletion strain as compared to the parental strain with native ß-1,6-N-acetylglucosaminyltransferase. The rate of degradation of native K. phaffii cytosolic and peroxisomal enzymes, formaldehyde dehydrogenase, formate dehydrogenase, and alcohol oxidase, respectively, showed similar trends to that of ß-galactosidase-slower degradation in the deletion and insertional mutants as compared to the wild-type strain, but faster protein degradation relative to the strain completely defective in autophagy. We conclude that K. phaffii gene designated ACG1, encoding ß-1,6-N-acetylglucosaminyltransferase, is involved in autophagy of the cytosolic and peroxisomal proteins.

Cheese whey supports high riboflavin synthesis by the engineered strains of the flavinogenic yeast Candida famata.

BACKGROUND: Riboflavin is a precursor of FMN and FAD which act as coenzymes of numerous enzymes. Riboflavin is an important biotechnological commodity with annual market sales exceeding nine billion US dollars. It is used primarily as a component of feed premixes, a food colorant, a component of multivitamin mixtures and medicines. Currently, industrial riboflavin production uses the bacterium, Bacillus subtilis, and the filamentous fungus, Ashbya gossypii, and utilizes glucose and/or oils as carbon substrates. RESULTS: We studied riboflavin biosynthesis in the flavinogenic yeast Candida famata that is a genetically stable riboflavin overproducer. Here it was found that the wild type C. famata is characterized by robust growth on lactose and cheese whey and the engineered strains also overproduce riboflavin on whey. The riboflavin synthesis on whey was close to that obtained on glucose. To further enhance riboflavin production on whey, the gene of the transcription activator SEF1 was expressed under control of the lactose-induced promoter of the native ß-galactosidase gene LAC4. These transformants produced elevated amounts of riboflavin on lactose and especially on whey. The strain with additional overexpression of gene RIB6 involved in conversion of ribulose-5-phosphate to riboflavin precursor had the highest titer of accumulated riboflavin in flasks during cultivation on whey. Activation of riboflavin synthesis was also obtained after overexpression of the GND1 gene that is involved in the synthesis of the riboflavin precursor ribulose-5-phosphate. The best engineered strains accumulated 2.5 g of riboflavin/L on whey supplemented only with (NH4)2SO4 during batch cultivation in bioreactor with high yield (more than 300 mg/g dry cell weight). The use of concentrated whey inhibited growth of wild-type and engineered strains of C. famata, so the mutants tolerant to concentrated whey were isolated. CONCLUSIONS: Our data show that the waste of dairy industry is a promising substrate for riboflavin production by C. famata. Possibilities for using the engineered strains of C. famata to produce high-value commodity (riboflavin) from whey are discussed.

The role of hexose transporter-like sensor hxs1 and transcription activator involved in carbohydrate sensing azf1 in xylose and glucose fermentation in the thermotolerant yeast Ogataea polymorpha.

BACKGROUND: Fuel ethanol from lignocellulose could be important source of renewable energy. However, to make the process feasible, more efficient microbial fermentation of pentose sugars, mainly xylose, should be achieved. The native xylose-fermenting thermotolerant yeast Ogataea polymorpha is a promising organism for further development. Efficacy of xylose alcoholic fermentation by O. polymorpha was significantly improved by metabolic engineering. Still, genes involved in regulation of xylose fermentation are insufficiently studied. RESULTS: We isolated an insertional mutant of O. polymorpha with impaired ethanol production from xylose. The insertion occurred in the gene HXS1 that encodes hexose transporter-like sensor, a close homolog of Saccharomyces cerevisiae sensors Snf3 and Rgt2. The role of this gene in xylose utilization and fermentation was not previously elucidated. We additionally analyzed O. polymorpha strains with the deletion and overexpression of the corresponding gene. Strains with deletion of the HXS1 gene had slower rate of glucose and xylose consumption and produced 4 times less ethanol than the wild-type strain, whereas overexpression of HXS1 led to 10% increase of ethanol production from glucose and more than 2 times increase of ethanol production from xylose. We also constructed strains of O. polymorpha with overexpression of the gene AZF1 homologous to S. cerevisiae AZF1 gene which encodes transcription activator involved in carbohydrate sensing. Such transformants produced 10% more ethanol in glucose medium and 2.4 times more ethanol in xylose medium. Besides, we deleted the AZF1 gene in O. polymorpha. Ethanol accumulation in xylose and glucose media in such deletion strains dropped 1.5 and 1.8 times respectively. Overexpression of the HXS1 and AZF1 genes was also obtained in the advanced ethanol producer from xylose. The corresponding strains were characterized by 20-40% elevated ethanol accumulation in xylose medium. To understand underlying mechanisms of the observed phenotypes, specific enzymatic activities were evaluated in the isolated recombinant strains. CONCLUSIONS: This paper shows the important role of hexose sensor Hxs1 and transcription factor Azf1 in xylose and glucose alcoholic fermentation in the native xylose-fermenting yeast O. polymorpha and suggests potential importance of the corresponding genes for construction of the advanced ethanol producers from the major sugars of lignocellulose.

The impact of transcription factors Znf1, Sip4, Adr1, Tup1, and Hap4 on xylose alcoholic fermentation in the engineered yeast Saccharomyces cerevisiae.

Lignocellulosic biomass is an attractive sustainable platform for fuel ethanol production. Xylose is a second after glucose most abounded sugar in lignocellulosic hydrolysates. Effective conversion of xylose to ethanol is one of key prerequisite for the development of an efficient conversion of biomass to ethanol. Engineered Saccharomyces cerevisiae strains are able to xylose fermentation. However, the yield and productivities of xylose fermentation remains lower in comparison with glucose fermentation. In this work, we studied impact of transcription factors Znf1, Sip4, Adr1, Tup1, and Hap4 on xylose catabolism. We have isolated znf1Δ, adr1Δ, tup1Δ and hap4Δ mutants, and strains overexpressing SIP4, ADR1 and HAP4 genes on the background of xylose-fermenting strain of S. cerevisiae aiming to explore involvement of these transcription factors in regulation of xylose growth and fermentation. It was shown that hap4Δ reveal 1.8-fold increase of ethanol production from xylose as compared to that of parental strain. The hap4Δ mutant accumulates 10.38 g l-1 of ethanol with an overall ethanol yield reaching 0.41 g g-1 of consumed xylose. While the other constructed strains revealed a decrease in ethanol production from this pentose.

The role of Mig1, Mig2, Tup1 and Hap4 transcription factors in regulation of xylose and glucose fermentation in the thermotolerant yeast Ogataea polymorpha.

Glucose is a preferred carbon source for most living organisms. The metabolism and regulation of glucose utilization are well studied mostly for Saccharomyces cerevisiae. Xylose is the main pentose sugar released from the lignocellulosic biomass, which has a high potential as a renewable feedstock for bioethanol production. The thermotolerant yeast Ogataea (Hansenula) polymorpha, in contrast to S. cerevisiae, is able to metabolize and ferment not only glucose but also xylose. However, in non-conventional yeasts, the regulation of glucose and xylose metabolism remains poorly understood. In this study, we characterize the role of transcriptional factors Mig1, Mig2, Tup1 and Hap4 in the natural xylose-fermenting yeast O. polymorpha. The deletion of MIG1 had no significant influence on ethanol production either from xylose or glucose, however the deletion of both MIG1 and MIG2 reduced the amount of ethanol produced from these sugars. The deletion of HAP4-A and TUP1 genes resulted in increased ethanol production from xylose. Inversely, the overexpression of HAP4-A and TUP1 genes reduced ethanol production during xylose alcoholic fermentation. Thus, HAP4-A and TUP1 are involved in repression of xylose metabolism and fermentation in yeast O. polymorpha and their deletion could be a viable strategy to improve ethanol production from this pentose.

Recent Advances in Construction of the Efficient Producers of Riboflavin and Flavin Nucleotides (FMN, FAD) in the Yeast Candida famata.

The approaches used by the authors to design the Candida famata strains capable to overproduce riboflavin, flavin mononucleotide (FMN), and flavin adenine dinucleotide (FAD) are described. The metabolic engineering approaches include overexpression of SEF1 gene encoding positive regulator of riboflavin biosynthesis, IMH3 (coding for IMP dehydrogenase) orthologs from another species of flavinogenic yeast Debaryomyces hansenii, and the homologous genes RIB1 and RIB7 encoding GTP cyclohydrolase II and riboflavin synthase, the first and the last enzymes of riboflavin biosynthesis pathway, respectively. Overexpression of the above mentioned genes in the genetically stable riboflavin overproducer AF-4 obtained by classical selection resulted in fourfold increase of riboflavin production in shake flask experiments.Overexpression of engineered enzymes phosphoribosyl pyrophosphate synthetase and phosphoribosyl pyrophosphate amidotransferase catalyzing the initial steps of purine nucleotide biosynthesis enhances riboflavin synthesis in the flavinogenic yeast C. famata even more.Recombinant strains of C. famata containing FMN1 gene from D. hansenii encoding riboflavin kinase under control of the strong constitutive TEF1 promoter were constructed. Overexpression of the FMN1 gene in the riboflavin-producing mutant led to the 30-fold increase of the riboflavin kinase activity and 400-fold increase of FMN production in the resulting recombinant strains which reached maximally 318.2 mg/L.FAD overproducing strains of C. famata were also constructed. This was achieved by overexpression of FAD1 gene from D. hansenii in C. famata FMN overproducing strain. The 7- to 15-fold increase in FAD synthetase activity as compared to the wild-type strain and FAD accumulation into cultural medium were observed. The maximal FAD titer 451.5 mg/L was achieved.

Overexpression of Riboflavin Excretase Enhances Riboflavin Production in the Yeast Candida famata.

Many microorganisms are capable of riboflavin oversynthesis and accumulation in a medium, suggesting that they efficiently excrete riboflavin. The mechanisms of riboflavin efflux in microorganisms remain elusive. Candida famata are representatives of a group of so-called flavinogenic yeast species that overproduce riboflavin (vitamin B2) in response to iron limitation. The riboflavin overproducers of this yeast species have been obtained by classical mutagenesis and metabolic engineering. Overproduced riboflavin accumulates in the cultural medium rather than in the cells suggesting existence of the special mechanisms involved in riboflavin excretion. The appropriate protein and gene have not been identified in yeasts till recently. At the same time, the gene BCRP (breast cancer resistance protein) has been identified in mammal mammary glands. Several homologs of the mammal BCRP gene encoding putative riboflavin efflux protein (excretase) were identified in the flavinogenic yeasts Debaryomyces hansenii and C. famata. Here we evaluate the yeast homologs of BCRP with respect to improvement of a riboflavin production by C. famata. The closest homologs from D. hansenii or C. famata were expressed under the control of TEF1 promoter of these yeasts in the wild-type and riboflavin-overproducing strains of C. famata. Resulted transformants overexpressed the corresponding genes (designated as DhRFE and CfRFE) and produced 1.4- to 6-fold more riboflavin as compared to the corresponding parental strains. They also were characterized by overexpression of RIB1 and RIB6 genes which encode the first and the last structural enzymes of riboflavin synthesis and exhibited elevated specific activity of GTP cyclohydrolase II. Thus, overexpression of yeast homolog of mammal gene BCRP may be useful to increase the riboflavin yield in a riboflavin production process using a recombinant overproducing C. famata strain or other flavinogenic microorganisms.

Flavocytochrome b2 of the Methylotrophic Yeast Ogataea polymorpha: Construction of Overproducers, Purification, and Bioanalytical Application.

Flavocytochrome b2 (EC 1.1.2.3; L-lactate cytochrome: c oxidoreductase, FC b2) from the thermotolerant methylotrophic yeast Ogataea polymorpha is a thermostable enzyme-prospective for a highly selective L-lactate analysis in the medicine, nutrition sector, and quality control of commercial products. Here we describe the construction of FC b2 producers by overexpression of the CYB2 gene O. polymorpha, encoding FC b2, under the control of a strong alcohol oxidase promoter in the frame of plasmid for multicopy integration with the next transformation of recipient strain O. polymorpha C-105 (gcr1 catX) impaired in the glucose repression and devoid of catalase activity. The selected recombinant strain O. polymorpha "tr1" (gcr1 catX CYB2), characterized by eightfold increased FC b2 activity compared to the initial strain, was used as a source of the enzyme. For purification of FC b2 a new method of affinity chromatography was developed and purified preparations of the enzyme were used for the construction of the highly selective enzymatic kits and amperometric biosensor for L-lactate analysis in human liquids and foods.

Insertional tagging of the Scheffersomyces stipitis gene HEM25 involved in regulation of glucose and xylose alcoholic fermentation.

Amid known microbial bioethanol producers, the yeast Scheffersomyces (Pichia) stipitis is particularly promising in terms of alcoholic fermentation of both glucose and xylose, the main constituents of lignocellulosic biomass hydrolysates. However, the ethanol yield and productivity, especially from xylose, are still insufficient to meet the requirements of a feasible industrial technology; therefore, the construction of more efficient S. stipitis ethanol producers is of great significance. The aim of this study was to isolate the insertional mutants of S. stipitis with altered ethanol production from glucose and xylose and to identify the disrupted gene(s). Mutants obtained by random insertional mutagenesis were screened for their growth abilities on solid media with different sugars and for resistance to 3-bromopyruvate. Of more than 1,300 screened mutants, 17 were identified to have significantly changed ethanol yields during the fermentation. In one of the best fermenting strains (strain 4.6), insertion was found to occur within the ORF of a homolog to the Saccharomyces cerevisiae gene HEM25 (YDL119C), encoding a mitochondrial glycine transporter required for heme synthesis. The role of HEM25 in heme accumulation, respiration, and alcoholic fermentation in the yeast S. stipitis was studied using strain 4.6, the complementation strain Comp-a derivative from the 4.6 strain with expression of the WT HEM25 allele and the deletion strain hem25Δ. As hem25Δ produced lower amounts of ethanol than strain 4.6, we assume that the phenotype of strain 4.6 may be caused not only by HEM25 disruption but additionally by some point mutation.

Fructose-1,6-bisphosphatase degradation in the methylotrophic yeast Komagataella phaffii occurs in autophagy pathway.

Many enzymes of methanol metabolism of methylotrophic yeasts are located in peroxisomes whereas some of them have the cytosolic localization. After shift of methanol-grown cells of methylotrophic yeasts to glucose medium, a decrease in the activity of cytosolic (formaldehyde dehydrogenase, formate dehydrogenase, and fructose-1,6-bisphosphatase [FBP]) along with peroxisomal enzymes of methanol metabolism is observed. Mechanisms of inactivation of cytosolic enzymes remain unknown. To study the mechanism of FBP inactivation, the changes in its specific activity of the wild type strain GS200, the strain with the deletion of the GSS1 hexose sensor gene and strain defected in autophagy pathway SMD1163 of Komagataella phaffii with or without the addition of the MG132 (proteasome degradation inhibitor) were investigated after shift of methanol-grown cells in glucose medium. Western blot analysis showed that inactivation of FBP in GS200 occurred due to protein degradation whereas inactivation in the strains SMD1163 and gss1Δ was negligible in such conditions. The effect of the proteasome inhibitor MG132 on FBP inactivation was insignificant. To confirm FBP degradation pathway, the recombinant strains with GFP-labeled Fbp1 of K. phaffii and red fluorescent protein-labeled peroxisomes were constructed on the background of GS200 and SMD1163. The fluorescent microscopy analysis of the constructed strains was performed using the vacuolar membrane dye FM4-64. Microscopic data confirmed that Fbp1 degrades by autophagy pathway in K. phaffii. K. phaffii transformants, which express heterologous ß-galactosidase under FLD promoter, have been constructed.

Pentose metabolism and conversion to biofuels and high-value chemicals in yeasts.

Pentose sugars are widespread in nature and two of them, D-xylose and L-arabinose belong to the most abundant sugars being the second and third by abundance sugars in dry plant biomass (lignocellulose) and in general on planet. Therefore, it is not surprising that metabolism and bioconversion of these pentoses attract much attention. Several different pathways of D-xylose and L-arabinose catabolism in bacteria and yeasts are known. There are even more common and really ubiquitous though not so abundant pentoses, D-ribose and 2-deoxy-D-ribose, the constituents of all living cells. Thus, ribose metabolism is example of endogenous metabolism whereas metabolism of other pentoses, including xylose and L-arabinose, represents examples of the metabolism of foreign exogenous compounds which normally are not constituents of yeast cells. As a rule, pentose degradation by the wild-type strains of microorganisms does not lead to accumulation of high amounts of valuable substances; however, productive strains have been obtained by random selection and metabolic engineering. There are numerous reviews on xylose and (less) L-arabinose metabolism and conversion to high value substances; however, they mostly are devoted to bacteria or the yeast Saccharomyces cerevisiae. This review is devoted to reviewing pentose metabolism and bioconversion mostly in non-conventional yeasts, which naturally metabolize xylose. Pentose metabolism in the recombinant strains of S. cerevisiae is also considered for comparison. The available data on ribose, xylose, L-arabinose transport, metabolism, regulation of these processes, interaction with glucose catabolism and construction of the productive strains of high-value chemicals or pentose (ribose) itself are described. In addition, genome studies of the natural xylose metabolizing yeasts and available tools for their molecular research are reviewed. Metabolism of other pentoses (2-deoxyribose, D-arabinose, lyxose) is briefly reviewed.

100 Years Later, What Is New in Glycerol Bioproduction?

Industrial production of glycerol by yeast, which began during WWI in the so-called Neuberg fermentation, was the first example of metabolic engineering. However, this process, based on bisulfite addition to fermentation liquid, has many drawbacks and was replaced by other methods of glycerol production. Osmotolerant yeasts and other microorganisms that do not require addition of bisulfite to steer cellular metabolism towards glycerol synthesis have been discovered or engineered. Because the glycerol market is expected to reach 5 billion US$ by 2024, microbial fermentation may again become a promising way to produce glycerol. This review summarizes some problems and perspectives on the production of glycerol by natural or engineered eukaryotic and prokaryotic microorganisms.