Changes of trehalose content and expression of relative genes during the bioethanol fermentation by Saccharomyces cerevisiae

Food yeasts: occurrence, functions, and stress tolerance in the brewing of fermented foods

With the rapid development of systems biology technology, there is a deeper understanding of the molecular biological mechanisms and physiological characteristics of microorganisms. Yeasts are widely used in the food industry with their excellent fermentation performances. While due to the complex environments of food production, yeasts have to suffer from various stress factors. Thus, elucidating the stress mechanisms of food yeasts and proposing potential strategies to improve tolerance have been widely concerned. This review summarized the recent signs of progress in the variety, functions, and stress tolerance of food yeasts. Firstly, the main food yeasts occurred in fermented foods, and the taxonomy levels are demonstrated. Then, the main functions of yeasts including aroma enhancer, safety performance enhancer, and fermentation period reducer are discussed. Finally, the stress response mechanisms of yeasts and the strategies to improve the stress tolerance of cells are reviewed. Based on sorting out these related recent researches systematically, we hope that this review can provide help and approaches to further exert the functions of food yeasts and improve food production efficiency.

Characterizing phenotypic diversity of trehalose biosynthesis mutants in multiple wild strains of Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, trehalose-6-phospahte synthase (Tps1) and trehalose-6-phosphate phosphatase (Tps2) are the main proteins catalyzing intracellular trehalose production. In addition to Tps1 and Tps2, 2 putative regulatory proteins with less clearly defined roles also appear to be involved with trehalose production, Tps3 and Tsl1. While this pathway has been extensively studied in laboratory strains of S. cerevisiae, we sought to examine the phenotypic consequences of disrupting these genes in wild strains. Here we deleted the TPS1, TPS2, TPS3, and TSL1 genes in 4 wild strains and 1 laboratory strain for comparison. Although some tested phenotypes were not shared between all strains, deletion of TPS1 abolished intracellular trehalose, caused inability to grow on fermentable carbon sources and resulted in severe sporulation deficiency for all 5 strains. After examining tps1 mutant strains expressing catalytically inactive variants of Tps1, our results indicate that Tps1, independent of trehalose production, is a key component for yeast survival in response to heat stress, for regulating sporulation, and growth on fermentable sugars. All tps2Δ mutants exhibited growth impairment on nonfermentable carbon sources, whereas variations were observed in trehalose synthesis, thermosensitivity and sporulation efficiency. tps3Δ and tsl1Δ mutants exhibited mild or no phenotypic disparity from their isogenic wild type although double mutants tps3Δ tsl1Δ decreased the amount of intracellular trehalose production in all 5 strains by 17-45%. Altogether, we evaluated, confirmed, and expanded the phenotypic characteristics associated trehalose biosynthesis mutants. We also identified natural phenotypic variants in multiple strains that could be used to genetically dissect the basis of these traits and then develop mechanistic models connecting trehalose metabolism to diverse cellular processes.

NTH2 1271_1272delTA Gene Disruption Results in Salt Tolerance in Saccharomyces cerevisiae

Trehalose is a common energy reservoir, and its accumulation results in osmotic protection. This sugar can accumulate through its synthesis or slow degradation of the reservoir by trehalase enzymes. Saccharomyces cerevisiae contains two neutral trehalases, NTH1 and NTH2, responsible for 75% and 25% of the enzymatic metabolism. We were interested in the loss-of-function of both enzymes with CRISPR/Cas9. The later NTH2 was of great importance since it is responsible for minor metabolic degradation of this sugar. It was believed that losing its functionality results in limited osmotic protection. We constructed an osmotolerant superior yeast capable of growing in 0.85 M NaCl after independent nth2 1271_1272delTA mutation by CRISPR/Cas9 technology, compared with nth1 893_894insT and wild type. We suggest that this yeast model could give clues to breeding commercial yeast resulting in non-GMO salinity-tolerant strains.

Effects of Lactobacillus plantarum on the ethanol tolerance of Saccharomyces cerevisiae

The bioethanol fermentation by Saccharomyces cerevisiae is often challenged by bacterial contamination, especially lactic acid bacteria (LAB). LAB can inhibit the growth S. cerevisiae by secreting organic acids and competing for nutrients and physical space. However, the range of favorable effects attributed to LAB during bioethanol fermentation, and their associated mechanisms of regulation, are not fully understood. This study was performed to clarify the effects of Lactobacillus plantarum, an important contaminative LAB in bioethanol fermentation, on the mechanism of ethanol tolerance in S. cerevisiae. The results showed that the presence of L. plantarum increased the ethanol tolerance of S. cerevisiae by promoting or inhibiting various metabolic processes in the yeast cells: The metabolism of trehalose, ergosterol, certain amino acids, proton pumps, stress response transcriptional activators, and heat shock proteins were all promoted; amounts of intracellular monounsaturated fatty acids and the accumulation of reactive oxygen species were inhibited. Furthermore, the maintenance of the acquired higher ethanol tolerance of S. cerevisiae was dependent on the coexistence of L. plantarum. These results suggested a complex relationship existed between S. cerevisiae and the contaminating LAB that might also play a beneficial role during fermentation by promoting the ethanol tolerance of yeast. The results from this study suggested that the extent of controlling bacterial contamination on bioethanol fermentation efficiency should be given careful consideration. KEY POINTS: • L. plantarum improved the ethanol tolerance of S. cerevisiae. • L. plantarum regulated the ethanol tolerance-related metabolism of yeast cells. • L. plantarum coexistence facilitated maintenance of ethanol tolerance in yeast cells.

Transcriptome profiling of Issatchenkia orientalis under ethanol stress

Issatchenkia orientalis, a non-Saccharomyces yeast that can resist a wide variety of environmental stresses, has potential use in winemaking and bioethanol production. Little is known about gene expression or the physiology of I. orientalis under ethanol stress. In this study, high-throughput RNA sequencing was used to investigate the transcriptome profile of I. orientalis in response to ethanol. 502 gene transcripts were differentially expressed, of which 451 were more abundant, and 51 less abundant, in cells subjected to 4 h of ethanol stress (10% v/v). Annotation and statistical analyses suggest that multiple genes involved in ergosterol biosynthesis, trehalose metabolism, and stress response are differentially expressed under these conditions. The up-regulation of molecular chaperones HSP90 and HSP70, and also genes associated with the ubiquitin–proteasome proteolytic pathway suggests that ethanol stress may cause aggregation of misfolded proteins. Finally, ethanol stress in I. orientalis appears to have a nitrogen starvation effect, and many genes involved in nutrient uptake were up-regulated.

Intracellular metabolic changes in Saccharomyces cerevisiae and promotion of ethanol tolerance during the bioethanol fermentation process

During the batch bioethanol fermentation process, although Saccharomyces cerevisiae cells are challenged by accumulated ethanol, our previous work showed that the ethanol tolerance of S. cerevisiae increased as fermentation time increased.