Effect of inorganic salt stress on the thermotolerance and ethanol production at high temperature of Pichia kudriavzevii

In Vitro Evaluation of the Technological and Probiotic Potential of Pichia kudriavzevii Strains Isolated from Traditional Fermented Foods

The isolation of endogenous yeast strains from traditionally fermented food products to use as functional starter cultures has become more popular for improved food safety, quality, and beneficial health effects. In this study, 107 Pichia kudriavzevii strains were isolated from sourdough, shalgam, tarhana, artisanal Tulum cheese, and yogurt. The strains were identified by DNA fingerprinting using iPBS-PCR method before technological and probiotic characterization. The multivariate statistical approach revealed that five strains were most promising in terms of technological characterization, including different harsh growth conditions. These strains were also examined in terms of probiotic properties with a commercial S. cerevisiae var boulardii MYA-796 strain. The multivariate statistical analyses indicated that P. kudriavzevii 5S5 were most promising in in vitro probiotic properties such as surviving in human GI conditions, adhering to intestinal cell lines, and exhibiting high hydrophobicity. Therefore, it seems to be a great starter candidate for the production of functional fermented food products.

Fungal Diversity Profiles in Pit Mud Samples from Chinese Strong-Flavour Liquor Pit

Pit mud, a specific fermented soil, is an essential material for the fermentation of Chinese strong-flavour liquor. However, few studies to date have sought to characterize the spatial profiles of pit mud fungal communities in fermentation cellars from Chinese strong-flavour liquor distilleries. In this analysis, differences in fungal community structures and physicochemical properties in pit mud samples from different spatial positions within fermentation cellars were analyzed, revealing unique characteristic multidimensional pit mud fungal community profiles. Penicillium roqueforti, Pichia kudriavzevii, Aotearoamyces nothofagi, Penicillium robsamsonii, Alternaria arborescens, Trichosporon insectorum, Seltsamia ulmi, Trichosporon coremiiforme, Malassezia restricta were dominant in the pit mud samples form the upper cellar wall, whereas Metarhizium frigidum, Calonectria pseudoreteaudii, Penicillium clavigerum, Fusarium equiseti, Simplicillium chinense, Aspergillus intermedius, Trichosporon coremiiforme, Fusarium circinatum, Alternaria radicina, Aspergillus heterocaryoticus were predominant in the middle cellar wall. Alternaria radicina, Cladosporium chasmanthicola, Alternaria helianthiinficiens, Penicillium argentinense, Antarctomyces psychrotrophicus, and Trichosporon inkin are majorly present in the down cellar wall layer. Bipolaris axonopicola, Ramgea ozimecii, Penicillium argentinense, Calonectria queenslandica, Metarhizium robertsii, and Penicillium roqueforti were identified as the dominant fungi in pit mud samples from the cellar bottom. Additionally, Alternaria destruens and Alternaria doliconidium are present at notably high levels in all layers of pit mud samples. Moisture, pH, PO₄^3-, acetic acid, humus, K⁺, Mg²⁺, Ca²⁺, butyric acid, and caproic acid levels in these different pit mud positions exhibited a rising incremental pattern from the upper wall layer to the bottom layer, whereas lactic acid levels were significantly lower in the bottom pit mud layer relative to these other layers. Moisture, pH, and NH₄⁺-N were identified as the three most significant factors associated with fungal community composition through a redundancy analysis. Overall, these findings may offer a theoretical foundation for future efforts to improve or standardize artificial pit mud.

Investigation of stress tolerance of Pichia kudriavzevii for high gravity bioethanol production from steam-exploded wheat straw hydrolysate

This study investigated a newly isolated thermotolerant strain of Pichia kudriavzevii with respect to its stress tolerance and fermentation performance. Response surface methodology was applied to evaluate the combined effects of furfural, osmotic and thermal stress on ethanol yield. The proposed model shows that P. kudriavzevii has a natural resistance against multiple stress factors. Further evolutionary adaptation of the isolated strain in lignocellulosic hydrolysates improved the ethanol yield by ≥ 24 %. The adapted strain HYPK213_ELA was able to produce ethanol from wheat straw hydrolysates at a high solid loading of 37 %ww-1 at 40 °C and anaerobic conditions. The highest ethanol concentration of 56.8 ± 1.0 gL-1 was reached at 40°C with an inoculum size of 2.5 × 106cellsmL-1. The results show that Pichia kudriavzevii has the potential to enable high gravity bioethanol production under conditions where most yeast strains are unable to grow.

Salt stress improves thermotolerance and high-temperature bioethanol production of multi-stress-tolerant Pichia kudriavzevii by stimulating intracellular metabolism and inhibiting oxidative damage

BACKGROUND

High-temperature bioethanol production benefits from yeast thermotolerance. Salt stress could induce obvious cross-protection against heat stress of Pichia kudriavzevii, contributing to the improvement of its thermotolerance and bioethanol fermentation. However, the underlying mechanisms of the cross-protection remain poorly understood.

RESULTS

Salt stress showed obvious cross-protection for thermotolerance and high-temperature ethanol production of P. kudriavzevii observed by biomass, cell morphology and bioethanol production capacity. The biomass and ethanol production of P. kudriavzevii at 45 °C were, respectively, improved by 2.6 and 3.9 times by 300 mmol/L NaCl. Metabolic network map showed that salt stress obviously improved the key enzymes and intermediates in carbohydrate metabolism, contributing to the synthesis of bioethanol, ATP, amino acids, nucleotides, and unsaturated fatty acids, as well as subsequent intracellular metabolisms. The increasing trehalose, glycerol, HSPs, and ergosterol helped maintain the normal function of cell components. Heat stress induced serious oxidative stress that the ROS-positive cell rate and dead cell rate, respectively, rose from 0.5% and 2.4% to 28.2% and 69.2%, with the incubation temperature increasing from 30 to 45 °C. The heat-induced ROS outburst, oxidative damage, and cell death were obviously inhibited by salt stress, especially the dead cell rate which fell to only 20.3% at 300 mmol/L NaCl. The inhibiting oxidative damage mainly resulted from the abundant synthesis of GSH and GST, which, respectively, increased by 4.8 and 76.1 times after addition of 300 mmol/L NaCl. The improved bioethanol production was not only due to the improved thermotolerance, but resulted from the up-regulated alcohol dehydrogenases and down-regulated aldehyde dehydrogenases by salt stress.

CONCLUSION

The results provide a first insight into the mechanisms of the improved thermotolerance and high-temperature bioethanol production of P. kudriavzevii by salt stress, and provide important information to construct genetic engineering yeasts for high-temperature bioethanol production.

Ethanol Production from Wheat Straw Hydrolysate by Issatchenkia Orientalis Isolated from Waste Cooking Oil

The interest in using non-conventional yeasts to produce value-added compounds from low cost substrates, such as lignocellulosic materials, has increased in recent years. Setting out to discover novel microbial strains that can be used in biorefineries, an Issatchenkia orientalis strain was isolated from waste cooking oil (WCO) and its capability to produce ethanol from wheat straw hydrolysate (WSHL) was analyzed. As with previously isolated I. orientalis strains, WCO-isolated I. orientalis KJ27-7 is thermotolerant. It grows well at elevated temperatures up to 42 °C. Furthermore, spot drop tests showed that it is tolerant to various chemical fermentation inhibitors that are derived from the pre-treatment of lignocellulosic materials. I. orientalis KJ27-7 is particularly tolerant to acetic acid (up to 75 mM) and tolerates 10 mM formic acid, 5 mM furfural and 10 mM hydroxymethylfurfural. Important for biotechnological cellulosic ethanol production, I. orientalis KJ27-7 grows well on plates containing up to 10% ethanol and media containing up to 90% WSHL. As observed in shake flask fermentations, the specific ethanol productivity correlates with WSHL concentrations. In 90% WSHL media, I. orientalis KJ27-7 produced 10.3 g L⁻¹ ethanol within 24 h. This corresponds to a product yield of 0.50 g g⁻¹ glucose (97% of the theoretical maximum) and a volumetric productivity of 0.43 g L⁻¹ h⁻¹. Therefore, I. orientalis KJ27-7 is an efficient producer of lignocellulosic ethanol from WSHL.

Osmotolerance as a determinant of microbial ecology: A study of phylogenetically diverse fungi

Osmotic stress induced by high solute concentration can prevent fungal metabolism and growth due to alterations in properties of the cytosol, changes in turgor, and the energy required to synthesize and retain compatible solutes. We used germination to quantify tolerance/sensitivity to the osmolyte KCl (0.1-4.5 M, in 0.1 M increments) for 71 strains (40 species) of ecologically diverse fungi. These include 11 saprotrophic species (17 strains, including two xerophilic species), five mycoparasitic species (five strains), six plant-pathogenic species (13 strains), and 19 entomopathogenic species (36 strains). A dendrogram obtained from cluster analyses, based on KCl inhibitory concentrations 50 % and 90 % calculated by Probit Analysis, revealed three groups of fungal isolates accordingly to their osmotolerance. The most-osmotolerant group (Group 3) contained the majority of saprotrophic fungi, and Aspergillus niger (F19) was the most tolerant. The highly xerophilic Aspergillus montevidense and Aspergillus pseudoglaucus were the second- and third-most tolerant species, respectively. All Aspergillus and Cladosporium species belonged to Group 3, followed by the entomopathogens Colletotrichum fioriniae, Simplicillium lanosoniveum, and Trichothecium roseum. Group 2 exhibited a moderate osmotolerance, and included plant-pathogens such as Colletotrichum and Fusarium, mycoparasites such as Clonostachys spp, some saprotrophs such as Mucor and Penicillium spp., and some entomopathogens such as Isaria, Lecanicillium, Mariannaea, Simplicillium, and Torrubiella. Group 1 contained the osmo-sensitive strains: the rest of the entomopathogens and the mycoparasitic Gliocladium and Trichoderma. Although stress tolerance did not correlate with their primary ecological niche, classification of these 71 fungal strains was more closely aligned with their ecology than with their phylogenetic relatedness. We discuss the implications for both microbial ecology and fungal taxonomy.

Acid stress induces cross-protection for cadmium tolerance of multi-stress-tolerant Pichia kudriavzevii by regulating cadmium transport and antioxidant defense system

The toxicity of cadmium (Cd) is the major limitation to its removal using microorganisms. The Cd tolerance of Pichia kudriavzevii was obviously enhanced by acid stress based on multi-stress cross-protection. RNA-Seq showed that most differentially expressed genes (DEGs) in the Pentose phosphate pathway, Citrate cycle (TCA cycle), Glycolysis/Gluconeogenesis, Peroxisome and Glutathione metabolism were up-regulated by acid stress. The up-regulated expression of genes related to ATP synthesis (GOR1, ALD5, ADH4, ADH6, MDH2, IDH1, IDH2, and ATP19) and Cd transport (GSTY2, GTO2, GLO2, and YOR1), and the improvement of intracellular GSH level and GST activity, reduced the Cd toxicity towards P. kudriavzevii. Cd efflux by YOR1 played a key role in the decline of intracellular Cd level. Acid stress obviously improved the gene expression levels and activities of antioxidant enzymes (SOD, POD, and CAT), which inhibited the Cd-induced ROS outburst and oxidative damage of proteins and membrane lipids. In addition, the enhanced expression of HSP12 protected P. kudriavzevii from the damage of Cd stress. These results provide some important clues to reconstruct robust strains using for Cd removal in aquatic environments.

Cadmium detoxification induced by salt stress improves cadmium tolerance of multi-stress-tolerant Pichia kudriavzevii

Heavy metal tolerance of microorganisms is the basis of heavy metal removal by growing cells. In this study, a cross-protection effect generated by salt stress significantly enhanced the cadmium tolerance of multi-stress-tolerant Pichia kudriavzevii. Comparative transcriptome analysis using RNA-Seq linked with physiological and biochemical observation was used to elucidate the underlying mechanisms of the improved cadmium tolerance. The expression of cadmium transport related genes (GSTY2, GLR1, GLO2, YCF1 and YOR1), GSH content and GST activity were elevated by salt stress, suggesting enhanced cadmium conjugation and detoxification in yeast cells. The inhibited cadmium uptake by ZRT1 and enhanced cadmium efflux by YOR1 contributed to the decrease in the intracellular cadmium concentration. The improved expression of antioxidant enzyme genes (SOD1, SOD2, SOD6, CAT1 and PRXIID), along with the enhanced activities of antioxidant enzymes (SOD, CAT and POD) resulted in a decrease in cadmium-induced ROS production, protein carbonylation, lipid peroxidation and cell death. The abundant expression of heat shock protein genes (HSP12, HSP10 and SSC1) and genes related to trehalose synthesis (TPS1 and TSL1) induced by salt stress protected yeast cells against complex stress conditions, contributing to the improved cadmium tolerance. These findings will be useful to develop cadmium-tolerant yeasts for cadmium removal by growing cells.