Screening of Saccharomyces cerevisiae and Torulaspora delbrueckii strains in relation to their effect on malolactic fermentation

The use of Torulaspora delbrueckii in the alcoholic fermentation (AF) of grape must is increasingly studied and used in the wine industry. In addition to the organoleptic improvement of wines, the synergy of this yeast species with the lactic acid bacterium Oenococcus oeni is an interesting field of study. In this work, 60 strain combinations were compared: 3 strains of Saccharomyces cerevisiae (Sc) and 4 strains of Torulaspora delbrueckii (Td) in sequential AF, and four strains of O. oeni (Oo) in malolactic fermentation (MLF). The objective was to describe the positive or negative relationships of these strains with the aim of finding the combination that ensures better MLF performance. In addition, a new synthetic grape must has been developed that allows the success of AF and subsequent MLF. Under these conditions, the Sc-K1 strain would be unsuitable for carrying out MLF unless there is prior inoculation with Td-Prelude, Td-Viniferm or Td-Zymaflore always with the Oo-VP41 combination. However, from all the trials performed, it appears that the combinations of sequential AF with Td-Prelude and Sc-QA23 or Sc-CLOS, followed by MLF with Oo-VP41, reflected a positive effect of T. delbrueckii compared to inoculation of Sc alone, such as a reduction in L-malic consumption time. In conclusion, the obtained results highlight the relevance of strain selection and yeast-LAB strain compatibility in wine fermentations. The study also reveals the positive effect on MLF of some T. delbrueckii strains.

Metabolomic charactetization of yeast cells after dehydration stress

In this study, we analyzed the metabolite features of the yeasts Saccharomyces cerevisiae, Naumovia castellii, and Saccharomyces mikatae. The three species are closely related genetically but differ in their tolerance of desiccation stress. Specifically, we determined whether certain metabolites correlated with cell viability after stress imposition. The metabolomics profiles of these strains were compared before cell desiccation and after cell rehydration. In S. mikatae, the presence of lysine or glutamine during rehydration led to a 20% increase in survival whereas during dehydration the levels of both amino acids in this yeast were drastically reduced.

Transcriptomic and proteomic analysis of Oenococcus oeni PSU-1 response to ethanol shock

The correct development of malolactic fermentation depends on the capacity of Oenococcus oeni to survive under harsh wine conditions. The presence of ethanol is one of the most stressful factors affecting O. oeni performance. In this study, the effect of ethanol addition (12% vol/vol) on O. oeni PSU-1 has been evaluated using a transcriptomic and proteomic approach. Transcriptomic analysis revealed that the main functional categories of the genes affected by ethanol were metabolite transport and cell wall and membrane biogenesis. It was also observed that some genes were over-expressed in response to ethanol stress (for example, the heat shock protein Hsp20 and a dipeptidase). Proteomic analysis showed that several proteins are affected by the presence of ethanol. Functions related to protein synthesis and stability are the main target of ethanol damage. In some cases the decrease in protein concentration could be due to the relocation of cytosolic proteins in the membrane, as a protective mechanism. The omic approach used to study the response of O. oeni to ethanol highlights the importance of the cell membrane in the global stress response and opens the door to future studies on this issue.

ATG18 and FAB1 are involved in dehydration stress tolerance in Saccharomyces cerevisiae

Recently, different dehydration-based technologies have been evaluated for the purpose of cell and tissue preservation. Although some early results have been promising, they have not satisfied the requirements for large-scale applications. The long experience of using quantitative trait loci (QTLs) with the yeast Saccharomyces cerevisiae has proven to be a good model organism for studying the link between complex phenotypes and DNA variations. Here, we use QTL analysis as a tool for identifying the specific yeast traits involved in dehydration stress tolerance. Three hybrids obtained from stable haploids and sequenced in the Saccharomyces Genome Resequencing Project showed intermediate dehydration tolerance in most cases. The dehydration resistance trait of 96 segregants from each hybrid was quantified. A smooth, continuous distribution of the anhydrobiosis tolerance trait was found, suggesting that this trait is determined by multiple QTLs. Therefore, we carried out a QTL analysis to identify the determinants of this dehydration tolerance trait at the genomic level. Among the genes identified after reciprocal hemizygosity assays, RSM22, ATG18 and DBR1 had not been referenced in previous studies. We report new phenotypes for these genes using a previously validated test. Finally, our data illustrates the power of this approach in the investigation of the complex cell dehydration phenotype.

Membrane lipid variability in Saccharomyces cerevisiae wine strains rehydrated in the presence of metabolic activators

Slight variations in lipid composition of wine yeast membranes can alter some essential functions including selective nutrient transport and ion permeability. The absence of oxygen during alcoholic fermentation inhibits fatty acid desaturation and sterol biosynthesis, thereby reducing the stress resistance of yeast cells. In this work, membrane lipids in two commercial active dry yeast strains rehydrated in the presence of three activators (ergosterol, tetrahydrofolic acid, and manganese) were studied. Each was assayed at three different concentrations. The effect of these activators on the phospholipid, neutral lipid, and fatty acid contents in cell membranes was assessed. Also, cell viability and fermentation kinetics were determined. Ergosterol was found to shorten the lag phase and improve cell viability and membrane lipid composition; tetrahydrofolic acid raised neutral lipid levels; and manganese(II) increased cell viability and modified phospholipid composition and linoleic acid concentration. All activators interacted with yeasts in a strain-dependent way.

Functional analysis to identify genes in wine yeast adaptation to low-temperature fermentation

AIMS

  To identify genes and proteins involved in adaptation to low-temperature fermentations in a commercial wine yeast.

METHODS AND RESULTS

  Nine proteins were identified as representing the most significant changes in proteomic maps during the first 24 h of fermentation at low (13°C) and standard temperature (25°C). These proteins were mainly involved in stress response and in glucose and nitrogen metabolism. Transcription analysis of the genes encoding most of these proteins within the same time frame of wine fermentation presented a good correlation with proteomic data. Knockout and overexpressing strains of some of these genes were constructed and tested to evaluate their ability to start the fermentation process. The strain overexpressing ILV5 improved its fermentation activity in the first hours of fermentation. This strain showed a quicker process of mitochondrial degeneration, an altered intracellular amino acid profile and laxer nitrogen catabolite repression regulation.

CONCLUSIONS

  The proteomic and transcriptomic analysis is useful to detect key molecular adaptation mechanisms of biotechnological interest for industrial processes. ILV5 gene seems to be important in wine yeast adaptation to low-temperature fermentation.

SIGNIFICANCE AND IMPACT OF THE STUDY

  This study provides information that might help improve the future performance of wine yeast, either by genetic modification or by adaptation during industrial production.

The STF2p hydrophilin from Saccharomyces cerevisiae is required for dehydration stress tolerance

The yeast Saccharomyces cerevisiae is able to overcome cell dehydration; cell metabolic activity is arrested during this period but restarts after rehydration. The yeast genes encoding hydrophilin proteins were characterised to determine their roles in the dehydration-resistant phenotype, and STF2p was found to be a hydrophilin that is essential for survival after the desiccation-rehydration process. Deletion of STF2 promotes the production of reactive oxygen species and apoptotic cell death during stress conditions, whereas the overexpression of STF2, whose gene product localises to the cytoplasm, results in a reduction in ROS production upon oxidative stress as the result of the antioxidant capacity of the STF2p protein.

Xylose utilization and short-chain fatty acid production by selected components of the intestinal microflora of a rodent pollinator (Aethomys namaquensis)

Namaqua rock mice (Aethomys namaquensis) consume nectar xylose when visiting Protea flowers. Whole-animal metabolism studies suggest that the gastrointestinal microflora plays an important role in xylose metabolism in A. namaquensis. We collected caecal contents under anaerobic conditions, cultured caecal microflora both aerobically and anaerobically, and assessed caecal microbial xylose utilization using a (14)C-xylose incubation assay. All four mice sampled hosted culturable caecal micro-organisms that tested positive for xylose utilization. These were classified by 16S rRNA based taxonomy as: Bacillus subtilis, Bacillus pumilus, Bacillus licheniformis, Shigella boydii, Arthrobacter sp. and members of the fungal genera Aspergillus and Penicillium. Cultures of these isolates were then analyzed by gas chromatography to determine the types and quantities of short-chain fatty acids produced by xylose fermentation. These results are discussed in the context of other studies of gut microflora in vertebrates.