Exponential growth phase cells of the osmotolerant yeast Debaryomyces hansenii are extremely resistant to dehydration stress

Physiological characterization of polyextremotolerant yeasts from cold environments of Patagonia

Yeasts from cold environments have a wide range of strategies to prevent the negative effects of extreme conditions, including the production of metabolites of biotechnological interest. We investigated the growth profile and production of metabolites in yeast species isolated from cold environments. Thirty-eight strains were tested for their ability to grow at different temperatures (5-30 °C) and solute concentrations (3-12.5% NaCl and 50% glucose). All strains tested were able to grow at 5 °C, and 77% were able to grow with 5% NaCl at 18 °C. We were able to group strains based on different physicochemical/lifestyle profiles such as polyextremotolerant, osmotolerant, psychrotolerant, or psychrophilic. Five strains were selected to study biomass and metabolite production (glycerol, trehalose, ergosterol, and mycosporines). These analyses revealed that the accumulation pattern of trehalose and ergosterol was related to each lifestyle profile. Also, our findings would suggest that mycosporines does not have a role as an osmolyte. Non-conventional fermentative yeasts such as Phaffia tasmanica and Saccharomyces eubayanus may be of interest for trehalose production. This work contributes to the knowledge of non-conventional yeasts with biotechnological application from cold environments, including their growth profile, metabolites, and biomass production under different conditions.

Physiological and genetic regulation of anhydrobiosis in yeast cells

Anhydrobiosis is a state of living organisms during which their metabolism is reversibly delayed or suspended due to a high degree of dehydration. Yeast cells, which are widely used in the food industry, may be induced into this state. The degree of viability of yeast cells undergoing the drying process also depends on rehydration. In an attempt to explain the essence of the state of anhydrobiosis and clarify the mechanisms responsible for its course, scientists have described various cellular compounds and structures that are responsible for it. The structures discussed in this work include the cell wall and plasma membrane, vacuoles, mitochondria, and lysosomes, among others, while the most important compounds include trehalose, glycogen, glutathione, and lipid droplets. Various proteins (Stf2p; Sip18p; Hsp12p and Hsp70p) and genes (STF2; Nsip18; TRX2; TPS1 and TPS2) are also responsible for the process of anhydrobiosis. Each factor has a specific function and is irreplaceable, detailed information is presented in this overview.

Machine learning aided experimental approach for evaluating the growth kinetics of Candida antarctica for lipase production

A hybrid machine learning (ML) aided experimental approach was proposed in this study to evaluate the growth kinetics of Candida antarctica for lipase production. Different ML models were trained and optimized to predict the growth curves at various substrate concentrations. Results on comparison demonstrate the superior performance of the Gradient boosting regression (GBR) model in growth curves prediction. GBR-based growth kinetics was found to be matching well with the results of the conventional experimental approach while significantly reducing the experimental effort, time, and resources by ∼ 50%. Further, the activity and enzyme kinetics of lipase produced in this study was investigated on hydrolysis of p-nitrophenyl butyrate resulting in a maximum lipase activity of 24.07 U at 44 h. The robustness and significance of developed kinetic models were ensured through detailed statistical analysis. The application of the proposed hybrid approach can be extended to any other microbial process.

Preservation of non-Saccharomyces yeasts: Current technologies and challenges

There is a recent and growing interest in the study and application of non-Saccharomyces yeasts, mainly in fermented foods. Numerous publications and patents show the importance of these yeasts. However, a fundamental issue in studying and applying them is to ensure an appropriate preservation scheme that allows to the non-Saccharomyces yeasts conserve their characteristics and fermentative capabilities by long periods of time. The main objective of this review is to present and analyze the techniques available to preserve these yeasts (by conventional and non-conventional methods), in small or large quantities for laboratory or industrial applications, respectively. Wine fermentation is one of the few industrial applications of non-Saccharomyces yeasts, but the preservation stage has been a major obstacle to achieve a wider application of these yeasts. This review considers the preservation techniques, and clearly defines parameters such as culturability, viability, vitality and robustness. Several conservation strategies published in research articles as well as patents are analyzed, and the advantages and disadvantages of each technique used are discussed. Another important issue during conservation processes is the stress to which yeasts are subjected at the time of preservation (mainly oxidative stress). There is little published information on the subject for non-Saccharomyces yeast, but it is a fundamental point to consider when designing a preservation strategy.

Anhydrobiosis in Yeasts: Changes in Mitochondrial Membranes Improve the Resistance of Saccharomyces cerevisiae Cells to Dehydration–Rehydration

Anhydrobiosis is a unique state of live organisms in which their metabolism is temporary reversibly suspended as the result of strong dehydration of their cells. This state is widely used currently during large-capacity production of active dry baker’s yeast. Other strains of the yeast Saccharomyces cerevisiae, as well as other yeast species that could potentially find use in modern biotechnology, are not resistant to dehydration–rehydration treatments. To improve their resistance, the main factors that influence cell survival during such treatment need to be revealed. This study showed the importance of mitochondria for yeast cell survival during transfer into anhydrobiosis, a factor that was strongly underestimated until this study. It was revealed that the external introduction inside yeast cells of 50 μM of lithocholic acid (LCA), an agent that induces changes in glycerophospholipids in mitochondrial membranes, in combination with 1% DMSO, may improve the survival rate of dehydrated cells. The influence of LCA upon yeast cell resistance to dehydration–rehydration was not linked with changes in the state of the cells’ plasma membrane.

Comparative study of Saccharomyces cerevisiae wine strains to identify potential marker genes correlated to desiccation stress tolerance

The most diffused formulation of starter for winemaking is active dry yeast (ADY). ADYs production process is essentially characterized by air-drying stress, a combination of several stresses, including thermal, hyperosmotic and oxidative and cell capacity to counteract such multiple stresses will determine its survival. The molecular mechanisms underlying cell stress response to desiccation have been mostly studied in laboratory and commercial yeast strains, but a growing interest is currently developing for indigenous yeast strains which represent a valuable and alternative source of genetic and molecular biodiversity to be exploited. In this work, a comparative study of different Saccharomyces cerevisiae indigenous wine strains, previously selected for their technological traits, has been carried out to identify potentially relevant genes involved in desiccation stress tolerance. Cell viability was evaluated along desiccation treatment and gene expression was analyzed by real-time PCR before and during the stress. Our data show that the observed differences in individual strain sensitivity to desiccation stress could be associated to specific gene expression over time. In particular, either the basal or the stress-induced mRNA levels of certain genes, such as HSP12, SSA3, TPS1, TPS2, CTT1 and SOD1, result tightly correlated to the strain survival advantage. This study provides a reliable and sensitive method to predict desiccation stress tolerance of indigenous wine yeast strains which could be preliminary to biotechnological applications.

Anhydrobiosis in yeast: FT-IR spectroscopic studies of yeast grown under conditions of severe oxygen limitation

Anhydrobiosis is a unique state of living organisms when metabolism is temporarily and reversibly delayed in response to the extreme desiccation of cells. The production of dry active preparations of yeast grown under anaerobic conditions is not currently possible because preparations are extremely sensitive to the dehydration procedure, though they could be very helpful in different biotechnological processes, including bioethanol production. To characterize mechanisms responsible for such sensitivity to the dehydration procedure, Fourier transform infrared spectroscopy was used to study the composition of aerobically grown yeast Saccharomyces cerevisiae resistant to dehydration and grown under conditions of severe oxygen limitation and sensitive to dehydration. Results indicated that significantly lower amounts of lipids in cells, grown under conditions of severe oxygen limitation, may be related to the mechanisms of sensitivity. Dehydration of both resistant and sensitive S. cerevisiae cells was accompanied by similar changes in main cellular compounds. Amounts of nucleic acids and proteins decreased slightly, whereas that of lipids and carbohydrates increased. Artificially reduced sensitivity to dehydration in S. cerevisiae cells, grown under conditions of severe oxygen limitation, led to the increase in the lipid concentration. The chemical composition of S. cerevisiae membranes is proposed to dictate the resistance to dehydration in resistant and sensitive cells.

Bioremediation of petroleum-contaminated soil by a combined system of biostimulation-bioaugmentation with yeast

This paper presents a study of the effect of a combined biostimulation-bioaugmentation treatment applied to a clay-loam soil contaminated with 16,300 mg/kg of total petroleum hydrocarbons (TPH), which comprised 51% saturated hydrocarbons and 31% aromatic hydrocarbons. The bioaugmentation was performed with yeast Candida tropicalis SK21 isolated from petroleum-contaminated soil. The strain was able to grow in a pH range of 3-9 in liquid culture, and the optimum pH was found to be 6 for both growth and biosurfactant production. At pH 6, 96% and 42% of TPH were degraded by the strain at the initial diesel oil concentrations of 0.5% and 5% (v/v), respectively. The remediation via inoculating the yeast removed 83% of TPH in 180 days while the experiment with the indigenous microorganisms alone removed 61%. Microbial enumeration showed that the yeast SK21 could grow good in the soil. It was also found that dehydrogenase and polyphenoloxidase activities in soil were remarkably enhanced by the inoculation of the yeast.

Bioethanol production from brown seaweed, Undaria pinnatifida, using NaCl acclimated yeast

Strain improvement of Pichia angophorae KCTC 17574 was successfully carried out for bioethanol fermentation of seaweed slurry with high salt concentration. P. angophorae KCTC 17574 was cultured under increasing salinity from five practical salinity unit (psu, ‰) to as high as 100 psu for 723 h. The seaweed, Undaria pinnatifida (sea mustard, Miyuk), was fermented to produce bioethanol using high-salt acclimated yeast. The pretreatment of U. pinnatifida was optimized using thermal acid hydrolysis to obtain a high monosaccharide yield. Optimal pretreatment conditions of 75 mM H(2)SO(4) and 13 % (w/v) slurry at 121 °C for 60 min were determined using response surface methodology. A maximum monosaccharide content of 28.65 g/L and the viscosity of 33.19 cP were obtained. The yeasts cultured under various salinity concentrations were collected and inoculated to the pretreated seaweed slurry after the neutralization using 5 N NaOH. The pretreated slurry was fermented with the inoculation of 0.1 g dcw/L of P. angophorae KCTC 17574 strain obtained at 90 psu. The maximum ethanol concentration of 9.42 g/L with 27 % yield of theoretical case of ethanol production from total carbohydrate of U. pinnatifida was obtained.

Freeze-Drying Preservation of Yeast Adjunct Cultures for Cheese Production

Four yeast strains: Yarrowia lipolytica PII6a, Candida famata MI1a, C. kefyr PII1b, C. sphaerica FII7a, adjunct cultures for cheese production were preserved by freeze-drying. The effect of process parameters and cryoprotective agents on cell survival and stability of growth characteristics was evaluated. Among three lyophilisation protocols, the process with three-step drying at temperatures of -38°C, -20°C and -10°C assured the highest cell viability. The survival of yeast strains in the presence of multicomponent cryoprotective agents containing skimmed milk, trehalose and sodium glutamate in three combinations (MT, MG, MTG) was significantly higher than in the presence of those agents used alone. The best agent for Y. lipolytica, C. kefyr and C. sphaerica appeared to be MT, while for C. famata - MG. Cell viability of yeast strains directly after freeze-drying was in the range of 74-80% and was relatively stable during one-year storage except C. famata. Initial yeast growth patterns were very well preserved in most of the preparations during 6 months of storage.

[Studies of viability and vitality after freezing of the probiotic yeast Saccharomyces boulardii: physiological preconditioning effect]

The aim of this study was to evaluate the vitality and viability of the probiotic yeast Saccharomyces boulardii after freezing/thawing and the physiological preconditioning effect on these properties. The results indicate that the specific growth rate (0.3/h(-1)) and biomass (2-3 x10(8)cells/ml) of S. boulardii obtained in flasks shaken at 28 degrees C and at 37 degrees C were similar. Batch cultures of the yeast in bioreactors using glucose or sugar-cane molasses as carbon sources, reached yields of 0.28 g biomass/g sugar consumed, after 10h incubation at 28 degrees C; the same results were obtained in fed batch fermentations. On the other hand, in batch cultures, the vitality of cells recovered during the exponential growth phase was greater than the vitality of cells from the stationary phase of growth. Vitality of cells from fed-batch fermentations was similar to that of stationary growing cells from batch fermentations. Survival to freezing at -20 degrees C and subsequent thawing of cells from batch cultures was 0.31% for cells in exponential phase of growth and 11.5% for cells in stationary phase. Pre-treatment of this yeast in media with water activity (a(w)) 0.98 increased the survival to freezing of S. boulardii cells stored at -20 degrees C for 2 months by 10 fold. Exposure of the yeast to media of reduced a(w) and/or freezing/thawing process negatively affected cell vitality. It was concluded that stress conditions studied herein decrease vitality of S. boulardii. Besides, the yeast strain studied presented good tolerance to bile salts even at low pH values.

Comparative analysis of trehalose production by Debaryomyces hansenii and Saccharomyces cerevisiae under saline stress

The comparative analysis of growth, intracellular content of Na+ and K+, and the production of trehalose in the halophilic Debaryomyces hansenii and Saccharomyces cerevisiae were determined under saline stress. The yeast species were studied based on their ability to grow in the absence or presence of 0.6 or 1.0 M NaCl and KCl. D. hansenii strains grew better and accumulated more Na+ than S. cerevisiae under saline stress (0.6 and 1.0 M of NaCl), compared to S. cerevisiae strains under similar conditions. By two methods, we found that D. hansenii showed a higher production of trehalose, compared to S. cerevisiae; S. cerevisiae active dry yeast contained more trehalose than a regular commercial strain (S. cerevisiae La Azteca) under all conditions, except when the cells were grown in the presence of 1.0 M NaCl. In our experiments, it was found that D. hansenii accumulates more glycerol than trehalose under saline stress (2.0 and 3.0 M salts). However, under moderate NaCl stress, the cells accumulated more trehalose than glycerol. We suggest that the elevated production of trehalose in D. hansenii plays a role as reserve carbohydrate, as reported for other microorganisms.