Growth and autolysis of the kefir yeast Kluyveromyces marxianus in lactate culture.

Kluyveromyces marxianus is a yeast that could be identified from kefir and can use a broad range of substrates, such as glucose and lactate, as carbon sources. The lactate produced in kefir culture can be a substrate for K. marxianus. However, the complexity of the kefir microbiota makes the traits of K. marxianus difficult to study. In this research, we focused on K. marxianus cultured with lactate as the sole carbon source. The optimal growth and released protein in lactate culture were determined under different pH conditions, and the LC-MS/MS-identified proteins were associated with the tricarboxylic acid cycle, glycolysis pathway, and cellular stress responses in cells, indicating that autolysis of K. marxianus had occurred under the culture conditions. The abundant glyceraldehyde-3-phosphate dehydrogenase 1 (GAP1) was cocrystallized with other proteins in the cell-free fraction, and the low transcription level of the GAP1 gene indicated that the protein abundance under autolysis conditions was dependent on protein stability. These results suggest that lactate induces the growth and autolysis of K. marxianus, releasing proteins and peptides. These findings can be fundamental for K. marxianus probiotic and kefir studies in the future.

Synergistic effects of the early administration of Lactobacillus kefiranofaciens DN1 and Kluyveromyces marxianus KU140723-05 on the inhibition of Salmonella Enteritidis colonization in young chickens.

In this study, we aimed to assess the feasibility of the lactic acid bacterium Lactobacillus kefiranofaciens DN1 (LKF_DN1) and the yeast Kluyveromyces marxianus KU140723-05 (KMA5), recently isolated from kefir, as probiotics. Specifically, we evaluated the effect of early administration of these 2 microbes on the inhibition of Salmonella Enteritidis (SE) colonization in neonatal chicks. We also examined the effects of exposure of chicks to probiotics before SE exposure on the reduction in the number of gut SE. A total of 108 1-day-old specific-pathogen-free male layer chicks were used for 3 independent experiments. The experimental chicks were randomly divided into 6 groups (negative control: basal diet [BD] without probiotics and SE; positive control: BD; probiotic group [PG] 1: BD + LKF_DN1; PG2: BD + KMA5; PG3: BD + LKF_DN1 + KMA5; and PG4: BD+ a commercial product IDF-7), all of which, except negative control, were coadministered with SE strain resistant to rifampicin (SERR). We found that the administration of LKF_DN1 and/or KMA5 reduced the number of viable cells of the SERR strain in chicks by up to 1.90 log10, relative to positive control chicks. Compared with late administration (day [D] 10 and D11), early administration (D1 and D2) of the probiotics was more effective in reducing SERR cell numbers in the gut. Furthermore, we detected no significant difference in the reduction of gut SERR cell numbers in chicks from the same groups exposed to the probiotics at D10 and D11 before and after administration with SERR. Collectively, our findings indicate that, as dietary additives, LKF_DN1 and KMA5 showed potential probiotic activity in chicks. Moreover, the combination of the lactic acid bacteria and/or yeast strain was found to rapidly reduce SE numbers in the chick gut and showed a prolonged inhibitory effect against SE colonization. We, thus, propose that the administration of these 2 probiotics, as early as possible after hatching, would be considerably effective in controlling SE colonization in the guts of chicks.

In Vitro Studies on Therapeutic Potential of Probiotic Yeasts Isolated from Various Sources.

The present study investigates the therapeutic properties of probiotic yeasts viz. Yarrowia lipolytica VIT-MN01, Kluyveromyces lactis VIT-MN02, Lipomyces starkeyi VIT-MN03, Saccharomycopsis fibuligera VIT-MN04 and Brettanomyces custersianus VIT-MN05. The antimutagenic activity of probiotic yeasts against the mutagens viz. Benzo[a]pyrene (B[a]P), and Sodium azide (SA) was tested. S. fibuligera VIT-MN04 showed highest antimutagenicity (75%). Binding ability on the mutagen acridine orange (AO) was tested and L. starkeyi VIT-MN03 was able to bind AO effectively (88%). The probiotic yeasts were treated with the genotoxins viz. 4-Nitroquinoline 1-Oxide (NQO) and Methylnitronitrosoguanidine (MNNG). The prominent changes in UV shift confirmed the reduction in genotoxic activity of S. fibuligera VIT-MN04 and L. starkeyi VIT-MN03, respectively. Significant viability of probiotic yeasts was noted after being exposed to mutagens and genotoxins. The adhesion capacity and anticancer activity were also assessed using Caco-2 and IEC-6 cell lines. Adhesion ability was found to be more in IEC-6 cells and remarkable antiproliferative activity was noted in Caco-2 cells compared to normal cells. Further, antagonistic activity of probiotic yeasts was investigated against S. typhimurium which was found to be more in S. fibuligera VIT-MN04 and L. starkeyi VIT-MN03. The inhibition of α-glucosidase and α-amylase activity confirmed the antidiabetic activity of probiotic yeasts. Antioxidant activity was also tested using standard assays. Therefore, based on the results, it can be concluded that probiotic yeasts can serve as potential therapeutic agents for the prevention and treatment of colon cancer, type 2 diabetes and gastrointestinal infections.

Probiotic Potential, Antioxidant Activity, and Phytase Production of Indigenous Yeasts Isolated from Indigenous Fermented Foods.

While many bacteria have been used as probiotics by industries, only two yeasts, Saccharomyces cerevisiae var. boulardii and Kluyveromyces fragilis (B0399), have been used for this purpose. In the present work, a total of 116 yeasts isolated from Brazilian indigenous fermented food, cocoa fermentation, and kefir were in vitro characterized for probiotic attributes. From 116 isolates, 36 were tolerant to gastrointestinal conditions evaluated by tolerance to pH 2.0, bile salts (0.3% w/v), and 37 °C temperature. From those, 15 isolates showed a similar or higher percentage (P < 0.05) of hydrophobicity, autoaggregation, and coaggregation with E. coli than the reference strain S. boulardii. All these strains showed a high percentage of adhesion to Caco-2 cells (> 63%) and antioxidant activity (ranging from 18 to 62%). Phytate hydrolysis was evaluated for these yeasts and 13 strains showed positive results, which is important for nutrient availability in plant-based foods. These results are important insights for characterization of novel probiotic yeast strains as well as to aggregate functional value to these food products.

Dielectric property measurements as a method to determine the physiological state of Kluyveromyces marxianus and Saccharomyces cerevisiae stressed with furan aldehydes.

Cell physiology parameters are essential aspects of biological processes; however, they are difficult to determine on-line. Dielectric spectroscopy allows the on-line estimation of viable cells and can provide important information about cell physiology during culture. In this study, we investigated the dielectric property variations in Kluyveromyces marxianus SLP1 and Saccharomyces cerevisiae ERD yeasts stressed by 5-hydroxymethyl-2-furaldehyde and 2-furaldehyde during aerobic growth. The dielectric properties of cell permittivity, specific membrane capacitance (Cm), and intracellular conductivity (σIn) were considerably affected by furan aldehydes in the same way that the cell population, viability, cell size, substrate consumption, organic acid production, and respiratory parameters were. The yeasts stressed with furan aldehydes exhibited three physiological states (φ): adaptation, replicating, and nonreplicating states. During the adaptation state, there were small and stable signs of permittivity, Cm, and σIn; additionally, no cell growth was observed. During the replicating state, cell growth was restored, and the cell viability increased; in addition, the permittivity and σIn increased rapidly and reached their maximum values, while the Cm decreased. In the nonreplicating state, the permittivity and σIn were stable, and Cm decreased to its minimum value. Our results demonstrated that knowing dielectric properties allowed us to obtain information about the physiological state of the cells under control and stressed conditions. Since the permittivity, Cm, and σIn are directly associated with the physiological state of the yeast, these results should contribute to a better understanding of the stress response of yeasts and open the possibility to on-line monitor and control the physiological state of the cell in the near future.

Survival of Kluyveromyces lactis and Torulaspora delbrueckii to simulated gastrointestinal conditions and their use as single and mixed inoculum for cheese production.

The demand for new probiotic products has shown recent increases alongside a growing interest in studying starter cultures of cheeses. This study thus aims to evaluate the ability to survive under simulated gastrointestinal conditions and impact of Torulaspora delbrueckii B14 and Kluyveromyces lactis B10 as single and mixed inocula for cheese production. These two yeast strains were subjected to simulated gastrointestinal tracts and tested for self-aggregation, hydrophobicity, pathogen inhibition, antibiotic resistance, and ß-galactosidase production. The yeast strains were also assessed for their ability to survive in different NaCl concentrations (2.5%, 5%, and 10% w/v), multiple temperatures (4 °C and 40 °C), and used as single and mixed starter cultures for cheese production. Yeasts population levels were monitored by YPD plating and MALDI-TOF and metabolites were analyzed by HPLC and GC-MS over the course of the 21 days cheese maturation process. T. delbrueckii B14 and K. lactis B10 both showed >80% viability after the passage through the simulated gastrointestinal tract, had self-aggregation rates >90%, and displayed ß-galactosidase activities of 0.35 U/g and 0.53 U/g, respectively. Both yeasts survived at 2.5%, 5%, and 10% NaCl for 21 days and showed growth at 4 °C. In cheese, the single inoculum of K. lactis B10 and mixed inoculum showed the highest levels of lactose consumption. HS-SPME GC-MS analysis of cheese samples allowed the identification of 38 volatile compounds. The highest concentrations of most of these compounds were observed after 21 days of maturation for the cheese produced with mixed inoculum. The most abundant acids detected were hexanoic and decanoic acid; the most abundant alcohols were 2,3-butanediol, 2-phenylethanol and isoamyl alcohol, and the most prevalent ester compounds were isoamyl acetate and phenethyl acetate. Our results therefore show that T. delbrueckii B14 and K. lactis B10 are interesting yeasts for further studies in the context of probiotics and positively impact the composition of desirable volatile compounds in cheeses, particularly when used as mixed inoculum.

Probiotic properties and fatty acid composition of the yeast Kluyveromyces lactis M3. In vivo immunomodulatory activities in gilthead seabream (Sparus aurata).

The aim of this study was to analyze the probiotic potential, fatty acid composition and immunostimulant activities of Kluyveromyces lactis M3 isolated from a hypersaline sediment. For this purpose, K. lactis M3 resistance to different pH, salinities and bile, as well as its antioxidant capability were assayed. Furthermore, total fatty acid composition of the yeast was determined where the dominant fatty acids were palmitic, palmitoleic, oleic and linoleic acids. K. lactis M3 showed no cytotoxic effects on peripheral blood leukocytes. During an in vivo experiment in gilthead seabream (Sparus aurata), dietary K. lactis M3 supplemented at 0.55 or 1.1% of the basal diet enhanced bactericidal activity against Vibrio parahaemolyticus N16, V. harveyi Lg 16/00, and V. anguillarum CECT 43442 compared to fish fed commercial diet (control group). Finally, nitric oxide production, peroxidase activity and skin mucus lectin union levels strongly increased in fish fed K. lactis M3 with respect to the control group. The results suggested that the yeast K. lactis M3 had exhibited high antioxidant capability, and its dietary administration at 0.55 or 1% basal diet had immunostimulant activity for gilthead seabream. For all these reasons, it should be considered an appropriate probiotic candidate for the aquaculture fish industry.

Biocontrol effect of Kluyveromyces lactis on aflatoxin expression and production in Aspergillus parasiticus.

Aspergillus parasiticus is one of the most common fungi able to produce aflatoxins, which are naturally occurring carcinogenic substances. This study evaluated the effects of the safe yeast, Kluyveromyces lactis, on fungal growth, aflatoxin production and expression of aflR gene in A. parasiticus. Antifungal susceptibility was evaluated by exposing A. parasiticus to different amounts of K. lactis, and aflatoxin production was measured using high-performance liquid chromatography. Expression of the aflR gene was determined by measuring the cognate aflR mRNA level by quantitative real-time reverse-transcription polymerase chain reaction assay. The growth of A. parasiticus was inhibited by 7 days of incubation at 30°C with a minimum population of 1.5 × 105 CFU/ml of K. lactis, which also suppressed expression of the A. parasiticus aflR gene, reducing the total production of aflatoxins by 97.9% and aflatoxins B1, B2, G1 and G2 by 97.8, 98.6, 98 and 94%, respectively. Accordingly, K. lactis could be considered as a potential biocontrol agent against toxigenic molds in food and animal feed.

Elevated temperatures do not trigger a conserved metabolic network response among thermotolerant yeasts.

BACKGROUND: Thermotolerance is a highly desirable trait of microbial cell factories and has been the focus of extensive research. Yeast usually tolerate only a narrow temperature range and just two species, Kluyveromyces marxianus and Ogataea polymorpha have been described to grow at reasonable rates above 40 °C. However, the complex mechanisms of thermotolerance in yeast impede its full comprehension and the rare physiological data at elevated temperatures has so far not been matched with corresponding metabolic analyses. RESULTS: To elaborate on the metabolic network response to increased fermentation temperatures of up to 49 °C, comprehensive physiological datasets of several Kluyveromyces and Ogataea strains were generated and used for 13C-metabolic flux analyses. While the maximum growth temperature was very similar in all investigated strains, the metabolic network response to elevated temperatures was not conserved among the different species. In fact, metabolic flux distributions were remarkably irresponsive to increasing temperatures in O. polymorpha, while the K. marxianus strains exhibited extensive flux rerouting at elevated temperatures. CONCLUSIONS: While a clear mechanism of thermotolerance is not deducible from the fluxome level alone, the generated data can be valued as a knowledge repository for using temperature to modulate the metabolic activity towards engineering goals.

Increased flux in acetyl-CoA synthetic pathway and TCA cycle of Kluyveromyces marxianus under respiratory conditions.

Yeasts are extremely useful, not only for fermentation but also for a wide spectrum of fuel and chemical productions. We analyzed the overall metabolic turnover and transcript dynamics in glycolysis and the TCA cycle, revealing the difference in adaptive pyruvate metabolic response between a Crabtree-negative species, Kluyveromyces marxianus, and a Crabtree-positive species, Saccharomyces cerevisiae, during aerobic growth. Pyruvate metabolism was inclined toward ethanol production under aerobic conditions in S. cerevisiae, while increased transcript abundances of the genes involved in ethanol metabolism and those encoding pyruvate dehydrogenase were seen in K. marxianus, indicating the augmentation of acetyl-CoA synthesis. Furthermore, different metabolic turnover in the TCA cycle was observed in the two species: malate and fumarate production in S. cerevisiae was higher than in K. marxianus, irrespective of aeration; however, fluxes of both the reductive and oxidative TCA cycles were enhanced in K. marxianus by aeration, implying both the cycles contribute to efficient electron flux without producing ethanol. Additionally, decreased hexokinase activity under aerobic conditions is expected to be important for maintenance of suitable carbon flux. These findings demonstrate differences in the key metabolic trait of yeasts employing respiration or fermentation, and provide important insight into the metabolic engineering of yeasts.

The hypoxic transcription factor KlMga2 mediates the response to oxidative stress and influences longevity in the yeast Kluyveromyces lactis.

Hypoxia is defined as the decline of oxygen availability, depending on environmental supply and cellular consumption rate. The decrease in O2 results in reduction of available energy in facultative aerobes. The response and/or adaptation to hypoxia and other changing environmental conditions can influence the properties and functions of membranes by modifying lipid composition. In the yeast Kluyveromyces lactis, the KlMga2 gene is a hypoxic regulatory factor for lipid biosynthesis-fatty acids and sterols-and is also involved in glucose signaling, glucose catabolism and is generally important for cellular fitness. In this work we show that, in addition to the above defects, the absence of the KlMGA2 gene caused increased resistance to oxidative stress and extended lifespan of the yeast, associated with increased expression levels of catalase and SOD genes. We propose that KlMga2 might also act as a mediator of the oxidative stress response/adaptation, thus revealing connections among hypoxia, glucose signaling, fatty acid biosynthesis and ROS metabolism in K. lactis.

Survival of Kluyveromyces marxianus with stigmasterol as subjected to freezing stress.

Kluyveromyces marxianus is an aerobic yeast and is interested to be applied in many industries. This research was aimed to study the effect of the sterol alternative to ergosterol on the freezing stress of K. marxianus UBU1-11, a thermotolerant yeast. The 0-9 mgL-1 stigmasterol were added to the YM broth and applied for culturing. The growth of all conditions were not interfered by the addition of stigmasterol. The intra-cellular sterol content was detected in the medium with 5 mgL-1 stigmasterol and higher, where the maximum content was 0.32 mg g-1 cell dry weight. After frozen and thawed, the cultures contained stigmasterol had significantly higher viability than those without. It was found that the amount of stigmasterol contained in cells did not affect the number of survival. The stigmasterol provided a significant protection to the yeast cell when subjected to slow freezing. It also increased the survival rate of the culture subjected to subzero temperature storage.

Functional analysis of Mig1 and Rag5 as expressional regulators in thermotolerant yeast Kluyveromyces marxianus.

To analyze the glucose repression mechanism in the thermotolerant yeast Kluyveromyces marxianus, disrupted mutants of genes for Mig1 and Rag5 as orthologs of Mig1 and Hxk2, respectively, in Saccharomyces cerevisiae were constructed, and their characteristics were compared with those of the corresponding mutants of S. cerevisiae. MIG1 mutants of both yeasts exhibited more resistance than the corresponding parental strains to 2-deoxyglucose (2-DOG). Histidine was found to be essential for the growth of Kmmig1, but not that of Kmrag5, suggesting that MIG1 is required for histidine biosynthesis in K. marxianus. Moreover, Kmrag5 and Schxk2 were more resistant than the corresponding MIG1 mutant to 2-DOG, and only the latter increased the utilization speed of sucrose in the presence of glucose. Kmrag5 exhibited very low activities for gluco-hexokinase and hexokinase and, unlike Schxk2, showed very slow growth and a low level of ethanol production in a glucose medium. Furthermore, Kmrag5, but not Kmmig1, exhibited high inulinase activity in a glucose medium and exhibited greatly delayed utilization of accumulated fructose in the medium containing both glucose and sucrose. Transcription analysis revealed that the expression levels of INU1 for inulinase and GLK1 for glucokinase in Kmrag5 were higher than those in the parental strain; the expression level of INU1 in Kmmig1 was higher, but the expression levels of RAG1 for a low-affinity glucose transporter in Kmmig1 and Kmrag5 were lower. These findings suggest that except for regulation of histidine biosynthesis, Mig1 and Rag5 of K. marxianus play similar roles in the regulation of gene expression and share some functions with Mig1 and Hxk2, respectively, in S. cerevisiae.

Application of the Severity Factor and HMF Removal of Red Macroalgae Gracilaria verrucosa to Production of Bioethanol by Pichia stipitis and Kluyveromyces marxianus with Adaptive Evolution.

Gracilaria verrucosa, red seaweed, is a promising biomass for bioethanol production due to its high carbohydrate content. The optimal hyper thermal (HT) acid hydrolysis conditions are 12% (w/v) G. verrucosa with 0.2 M H2SO4 at 130 °C for 15 min, with a severity factor of 1.66. This HT acid hydrolysis produces 50.7 g/L monosaccharides. The maximum monosaccharide concentration of 58.0 g/L was achieved with 96.6% of the theoretical monosaccharide production from 120 g dry weight/L G. verrucosa slurry after HT acid hydrolysis and enzymatic saccharification. Fermentation was carried out by removing an inhibitory compound and via yeast adaptation to galactose. Both Pichia stipitis and Kluyveromyces marxianus adapted to galactose were excellent producers, with the ethanol yield (YEtOH) of 0.50 and 29.0 g/L ethanol production. However, the bioethanol productivity with Pichia stipitis adapted to galactose is higher than that with Kluyveromyces marxianus adapted to galactose, being 0.81 and 0.35 g/L/h, respectively. The results from this study can be applied to industrial scale bioethanol production from seaweed.

Erg6 gene is essential for stress adaptation in Kluyveromyces lactis.

We investigated the effect of Kluyveromyces lactis ERG6 gene deletion on plasma membrane function and showed increased susceptibility of mutant cells to salt stress, cationic drugs and weak organic acids. Contrary to Saccharomyces cerevisiae, Klerg6 mutant cells exhibited increased tolerance to tunicamycin. The content of cell wall polysacharides did not significantly vary between wild-type and mutant cells. Although the expression of the NAD+-dependent glycerol 3-phosphate dehydrogenase (KlGPD1) in the Klerg6 mutant cells was only half of that in the parental strain, it was induced in the presence of calcofluor white. Also, cells exposed to this drug accumulated glycerol. The absence of KlErg6p led to plasma membrane hyperpolarization but had no statistically significant influence on the plasma membrane fluidity. We propose that the phenotype of Klerg6 mutant cells to a large extent was a result of the reduced activity of specific plasma membrane proteins that require proper lipid composition for full activity.

Thermotolerant Yeast Kluyveromyces marxianus Reveals More Tolerance to Heat Shock than the Brewery Yeast Saccharomyces cerevisiae.

The thermotolerant yeast Kluyveromyces marxianus, growing at high temperature (45℃) , showed stronger survival under heat shock at 50℃ than the brewing yeast Saccharomyces cerevisiae, which was unable to grow at 45℃. The survival rate of K. marxianus decreased to 10% during heat shock at 50℃ for 20 min, and to less than 0.01% at 60℃ for 20 min. Cells with damaged cellular membranes were infrequently observed at 50℃ and had decreased significantly from heat shock at 60℃. The metabolic activity of K. marxianus was retained at 50℃, whereas that of S. cerevisiae was not. The trehalose content of K. marxianus was approximately two times that of S. cerevisiae. These results suggest that K. marxianus protects itself from heat shock-induced damage through the use of trehalose (a protective molecule in S. cerevisiae) as well as other different factors.

Dietary supplemental Kluyveromyces marxianus alters the serum metabolite profile in broiler chickens.

Metabolomics is used to evaluate the bioavailability of food components, as well as to validate the metabolic changes associated with food consumption. This study was conducted to investigate the effects of the dietary supplement Kluyveromyces marxianus on the serum metabolite profile in broiler chickens. A total of 240 1-d-old broilers were divided into 2 groups with 8 replicates. Birds were fed basal diets without or with K. marxianus supplementation (5 × 1010 CFU kg-1 of diet). Serum samples were collected on d 21 and were analyzed by high-performance liquid chromatography with quadrupole time-of flight/mass spectrometry. The results showed that supplemental K. marxianus altered the concentrations of a variety of metabolites in the serum. Thereinto, a total of 39 metabolites were identified at higher (P < 0.05) concentrations while 21 metabolites were identified at lower (P < 0.05) concentrations in the treatment group as compared with the control. These metabolites were primarily involved with the regulation of amino acids and carbohydrate metabolism. Further metabolic pathway analysis revealed that glutamine and glutamate metabolism was the most relevant and critical pathway identified from these two groups. The activated pathway may partially interpret the beneficial effects of K. marxianus. Overall, the present research could promote our understanding of the probiotic action of K. marxianus and provide new insight into the design and application of K. marxianus-containing functional foods.

Genome-wide prediction of CRISPR/Cas9 targets in Kluyveromyces marxianus and its application to obtain a stable haploid strain.

Kluyveromyces marxianus, a probiotic yeast, is important in industrial applications because it has a broad substrate spectrum, a rapid growth rate and high thermotolerance. To date, however, there has been little effort in its genetic engineering by the CRISPR/Cas9 system. Therefore, we aimed at establishing the CRISPR/Cas9 system in K. marxianus and creating stable haploid strains, which will make genome engineering simpler. First, we predicted the genome-wide target sites of CRISPR/Cas9 that have been conserved among the eight sequenced genomes of K. marxianus strains. Second, we established the CRISPR/Cas9 system in the K. marxianus 4G5 strain, which was selected for its high thermotolerance, rapid growth, a pH range of pH3-9, utilization of xylose, cellobiose and glycerol, and toxin tolerance, and we knocked out its MATα3 to prevent mating-type switching. Finally, we used K. marxianus MATα3 knockout diploid strains to obtain stable haploid strains with a growth rate comparable to that of the diploid 4G5 strain. In summary, we present the workflow from identifying conserved CRISPR/Cas9 targets in the genome to knock out the MATα3 genes in K. marxianus to obtain a stable haploid strain, which can facilitate genome engineering applications.

Cell regeneration and cyclic catalysis of engineered Kluyveromyces marxianus of a D-psicose-3-epimerase gene from Agrobacterium tumefaciens for D-allulose production.

D-Allulose as a low-energy and special bioactive monosaccharide sugar is essential for human health. In this study, the D-psicose-3-epimerase gene (DPEase) of Agrobacterium tumefaciens was transferred into thermotolerant Kluyveromyces marxianus to decrease the production cost of D-allulose and reduce the number of manufacturing procedures. The cell regeneration of K. marxianus and cyclic catalysis via whole-cell reaction were investigated to achieve the sustainable application of K. marxianus and the consumption of residual D-fructose. Results showed that DPEase, encoding a 33 kDa protein, could be effectively expressed in thermotolerant K. marxianus. The engineered K. marxianus produced 190 g L-1 D-allulose with 750 g L-1 D-fructose as a substrate at 55 °C within 12 h. Approximately 100 g of residual D-fructose was converted into 34 g of ethanol, and 15 g of the engineered K. marxianus cells was regenerated after fermentation at 37 °C for 21 h. The purity of D-allulose of more than 90% could be obtained without isolating it from D-allulose and D-fructose mixture through residual D-fructose consumption. This study provided a valuable pathway to regenerate engineered K. marxianus cells and achieve cyclic catalysis for D-allulose production.

Fatty acid addition and thermotolerance of Kluyveromyces marxianus.

Membrane fatty acid composition has an important role in yeast stress resistance, particularly in temperature tolerance. Most studies investigating temperature and membrane fatty acids use the yeast Saccharomyces cerevisiae without considering other yeasts, such as Kluyveromyces marxianus, which has physiological differences and industrial advantages with respect to S. cerevisiae. One of the primary traits of K. marxianus is its thermotolerance. The effect of fatty acid addition (oleic acid, linoleic acid, linolenic acid and araquidic acid) on the thermotolerance of the K. marxianus strain SLP1 was evaluated. SLP1 yeast exhibited temperature tolerance of up to 50°C; at 55°C, viability was reduced significantly, probably due to an increase in the generation of reactive oxygen chemical species. Externally added fatty acids were incorporated in the yeast membrane, increasing their proportion to approximately 70%, thereby changing membrane fluidity. SLP1 cells supplemented with polyunsaturated fatty acids decreased cell thermotolerance and increased the degree of lipoperoxidation, while arachidic acid addition exhibited a tendency to increase yeast thermotolerance.