1. Introduction
Yeasts are microorganisms known for their role in fermentation processes. In other words, they transform complex substances from the environment into totally new ones that are needed in various fields. However, along the way, the microorganisms face several types of stresses, starting with osmotic stress, and as the food in the environment is consumed, stress occurs due to the lack of nutrients and the increasing amount of resulted ethanol [
1]. The most important yeasts used in industry are the
Saccharomyces cerevisiae species. But using them so intensively exposes them to a series of factors which can affect their activity during the fermentation process. Extreme values of pH or temperature, lack of nutrients in the environment, and metabolic reaction products are just a few examples. All the elements presented lead to different types of stress, such as oxidative, ethanol, saline or osmotic [
2].
All the stress formed in the culture medium produces harmful effects on the yeast cells. As a result, their growth and metabolism are slowed down, and the lag phase is often extended. To prevent these events, at the cellular level, the processes of transcription regulation or stimulation of certain genes take place. Such genes encode both proteins and enzymes, which act in such a way as to improve the microorganism’s tolerance to stress [
1,
3]. Yeast cells can respond in a non-specific way to environmental conditions, so that their development is not disturbed during the time required for a specific response, through an evolutionary adaptation, namely the general stress response [
4].
This paper emphasizes some types of cellular stress acting on Saccharomyces cerevisiae species. It also evaluates how S. cerevisiae cultures use their defense mechanisms to increase their tolerance to stressors. Understanding the aspects related to the action of cellular stress on yeasts and their response is important from a biotechnological point of view, being helpful for the optimization of industrial fermentation processes. This can reduce the amount of yeast cells used, and increase the quality for the final products.
2. Saccharomyces cerevisiae Species
Saccharomyces cerevisiae is a unicellular eukaryotic microorganism with a wide distribution in trees, soil, and fruits. Research has revealed that it can also be located in the urinary, respiratory or gastrointestinal tracts of healthy individuals [
5,
6,
7,
8,
9,
10,
11,
12,
13]. As a facultative anaerobic microorganism, it is able to develop both in the presence and absence of oxygen. Depending on the oxygenation conditions, the yeast cell uses different biochemical mechanisms. Thus, in the absence of oxygen, but with a carbohydrate substrate, the energy required for cell development is obtained through glycolysis. In the presence of molecular oxygen and in the absence of sugars, respiration takes place at the mitochondrial level, with energy being obtained through oxidative phosphorylation [
14].
In biotechnology,
S. cerevisiae has been used for several decades to obtain alcohol by fermentation in the beer and wine industries, helping the dough rise during the baking process, and also plays a notable role in obtaining biofuels [
15]. In terms of its widespread use for ethanol production,
S. cerevisiae is very advantageous, as it has a non-pathogenic nature, has the ability to grow on inexpensive substrates, and to withstand high concentrations of alcohol (up to 10%
v/
v) [
16].
S. cerevisiae could be a suitable option for obtaining other compounds, such as butanol (up to 2.5 mg/L with a 2% galactose carbon source, using ESY7 strain which overexpresses the native thiolase and HbCoA dehydrogenase), but there are still obstacles to overcome, in terms of expressing heterologous biosynthetic pathways [
16,
17].
Over the years, researchers were able to sequence its entire genome, discovering about 6000 genes. The data acquisition process was carried out through “omic” technologies, molecular biology techniques and traditional biochemistry. All information is currently included in the “
Saccharomyces Genome Database” [
18,
19,
20,
21,
22,
23,
24]. Yeast cell morphology has a relatively simple ellipsoidal shape and the sequences of genome leads to the easy production of mutants. A database for the morphological characterization of
S.cerevisiae (SCMD) associates individual mutants of the
S. cerevisiae genome with secondary gene annotation and with protein sequences [
25]. Research on intracellular organelles of
S. cerevisiae, specifically mitochondria, has shown that the most frequently occurring mutant is the respiratory deficiency (‘petite’) mutation, in which the mitochondria are incapable of synthesizing certain proteins and become partially unable to function aerobically. Consequently, the yeast cells can no longer metabolize non-fermentable carbon sources, like lactate, ethanol or glycerol. Furthermore, the cell becomes less tolerant to stress factors, such as ethanol and osmotic pressure [
26].
S. cerevisiae grows in colonies when in adequate conditions, but under different stressors, yeast cells can alter their growth pattern to produce complex structures leading to a change in colony morphology and also at a genetic level. Therefore, different genes were identified to be involved in complex colony morphology, components of MAP kinase cascade and the Ras-cMAP-PKA pathway, which are the most researched pathways in eukaryotic organisms. Under different nutrient conditions, some strains develop complex colony morphology morphotypes, included in the following categories: spokes (OS17 strain), concentric rings (YJM224 strain), lacy (YJM311 strain), coralline (NKY292 strain), mountainous (PMY348 strain), and irregular (BY4743 strain) [
27].
The importance of
Saccharomyces cerevisiae yeast cells is given by certain proteins that are considered to be similar to human ones. Thus, yeast cells are used in various studies to analyze biochemical processes that have similar effects in human cells, like aging, cellular stress, and drug screening [
13,
28,
29,
30,
31]. Another role for this microorganism is its involvement in the molecular mechanisms of response to oxidative stress and in the correlation between reactive oxygen species and aging processes at the cellular level.
S. cerevisiae is both genetically and reproducibly safe to grow due to short-lived generations of between 1.5 and 3 h. From this point of view, the microorganism is intensively used as an experimental model for the study of certain genetic and cellular aspects [
32].
4. The Behavior of Saccharomyces cerevisiae in Stressors Action
Once environmental stress acts on microorganisms, it produces changes at a genetic and molecular level. For example, yeast cells produce much higher amounts of trehalose in response to heat stress and the disaccharide acts to stabilize plasma membranes. Also, some genes that have a role in the synthesis of ergosterols are able to exert thermotolerance properties in yeasts [
2].
An overview of the yeast adaptive response in osmotic, ethanol, and oxidative stress is presented in
Figure 3 [
62,
70].
Yeast cells adapt to stress by activating or inhibiting the genes responsible for actions against it. An alternative is epigenetic mechanisms by which yeasts quickly adapt to cellular stress. These processes involve an organization of chromatin into histones, DNA modifications or changes in transcription patterns [
13,
71,
72,
73]. Certain responses of yeast cells to environmental stress are shown in
Table 2.
Due to problems with nutrient-poor environments or poor parameters control during fermentation, the conversion of sugar to alcohol is often slowed down. Apart from these causes, there are also certain factors that cause damage during the process of obtaining alcohol. Osmotic stress, ethanol toxicity or certain pH and temperature values can affect the growth of yeast cells, but also their metabolism. For example, an ethanol concentration greater than 10% (
v/
v) and a temperature above 35 °C greatly reduce the viability of microorganisms [
2].
Zinc is an important micronutrient in the growth process of
S. cerevisiae cells. About 3% of their proteome requires zinc to function properly. In other words, 105 proteins use it as a cofactor, and another 360 need it to maintain structural stability through binding domains where it is a key factor. The trace element also enters into the structure of some proteins called zinc fingers, these being the largest family of proteins that bind nucleic acids and that have an important function in regulating transcription [
74].
Table 3 lists some examples and how they act against cellular stress in the yeast
S. cerevisiae.
Different yeasts from the class
Saccharomycetes have been analyzed and it has been observed that each exhibits distinct defense techniques against osmotic and salt stress. For example,
Zygosaccharomyces rouxii and
S. cerevisiae can export Na
+ cations out of the cell or into vacuoles. In contrast,
Debaryomyces hausenii accumulates Na
+ ions in the cell without suffering intoxication [
75].
Table 4 highlights the cation transport systems in several yeast species. For yeast species where a gene is present, a “+” was marked in the corresponding box. In the opposite sense, a “−” was entered.
Thanks to the success of sequencing the S288c strain from
S. cerevisiae, a coordination system between the extracellular environment and the changes occurring in the cell could be created. The mechanism is based on receiving information from the environment, transmitting it internally, and adapting it with the genetic information of the cell to create an appropriate response. The system was later used in the yeast fermentation process to optimize it at high salt concentrations [
54,
76].
In one study,
S. cerevisiae cells were subjected to a pulsed electric field to observe its effect on them. In addition, they were also exposed to heat and the first strand of the complementary DNA obtained from the RNA of the cells was analyzed. The genes of interest were those responding to oxidative and thermal stress. Heat stress led to the activation of the
MSP104 gene, which encodes the heat shock protein. In contrast, the expression of the oxidative stress response gene
GLR1 was inhibited. On the other hand, the pulsed electric field exhibited the opposite, stimulating the
GLR1 gene and repressing
MSP104 [
77].
During an industrial process, it is important that the chosen microorganism has increased tolerance to the stress that may occur after a certain period of time. Thus, specialists resort to physiological and genetic methods to improve the resistance of the chains of
Saccharomyces used. One of them refers to adaptive evolution, which involves a gradual and prolonged exposure to certain values of environmental factors. In this way, the microorganism develops a higher capacity to face the difficult conditions that may occur until the end of fermentation [
2].
Table 5 mentions some actions that can be carried out to reduce the stress that occurs during fermentation.
The genetic modifications made to improve the chains of
S. cerevisiae also play an important role. Mutagenesis, hybridization, and protoplast fusion are some of the classic techniques used to increase stress resistance. As for newer methods, gene editing technologies show promising results in this area. With the help of one of these, the desired genes can be deleted and inserted as precisely as possible by clustered and regularly interspersed palindromic repeats (CRISPR) and by CRISPR nuclease associated with protein 9 (Cas9). Following application, tolerance to ethanol, acetic acid, and temperature were increased considerably. In the culture medium, stress can occur by itself under the action of certain factors, but it can also be induced to manipulate the cells to produce as much of the final product as possible.
Table 6 shows some examples of how types of stress are used to influence yeasts [
2].
Some of the common genes responsible for yeast cells’ adaptation to different stresses are presented in
Table 7 [
78,
79].
In order to adapt to the conditions to which they are exposed, especially to stress, yeast cells go through certain changes at the proteome level [
35,
80]. A set of proteins that produce positive effects on
S. cerevisiae is represented by Msn2 and Msn4 (Msn 2/4). They are able to stop the mutation of the
SNF1 gene and regulate some stress responses. Through cycles of phosphorylation and dephosphorylation, proteins are activated to produce certain effects. For example, ethanol tolerance and fermentation improvement in the yeast
S. cerevisiae can be influenced after the phosphorylation of specific serine residues of Msn 2/4. Of course, proteins can also act in a less desirable way on
S. cerevisiae. They can decrease their growth rate by overwriting the binding domain of their DNA [
81]. The phosphorylation processes controlling Msn2/4 are represented by low protein kinase A (PKA) activity and low nitrogen and glucose concentrations. Instead, dephosphorylation relies on intense PKA activity and high glucose concentration. In addition to these two, the functions of Msn2/4 are also influenced by the nuclear and cytoplasmic localization of the reactions taking place under the catalysis of the protein kinase Tpk1-3, specific for the cAMP-PKA signaling pathway. In other words, it can be said that this pathway is responsible for the thermotolerance of
S. cerevisiae cells, the influence of some of the cellular processes and the accumulation of stress-resistant substances (glycerol, trehalose, and glycogen). Also, both under normal conditions and under cellular stress, the cAMP-PKA pathway can identify and respond to high or low amounts of nitrogen and glucose [
64,
82]. In the case of the yeast
S. cerevisiae, the cAMP-PKA signaling pathways contain elements such as Msn2/4 (transcription activating proteins), Bcy1-2 (regulatory subunit of PKA), Mep2 (ammonium permease/nitrogen sensor) or Grl1 (sensing glucose, activator of adenylate cyclase). In cAMP-PKA signaling, the DNA binding domain of Msn2/4 has the ability to recognize promoter sequences (AG4 and C4T). At high glucose concentrations, Bcy1 is removed from the catalytic subunits Tpk1-3, and thus, an increase in Tpk1-3 activity results. At the same time, Msn2/4 phosphorylation occurs, followed by expulsion from the cell nucleus [
82].
Another signaling pathway present in
S. cerevisiae cells is that of nitrogen or TORC1 (target of rapamycin 1). It includes Sch9 (protein kinase), TORC1 (is the key regulator, stimulated by nitrogen, activates Sch9 by phosphorylation), Deh1, and Dal80 (represses transcription) and Gat1, Glu3, and Msn 2/4 (activates transcription). The role of the TORC1 signaling pathway is to phosphorylate Glu3, Gat1, and Msn 2/4 and to inhibit the phosphorylation of Deh1 and Dal80. These actions only occur in the presence of high concentrations of nitrogen and amino acids. As long as the nitrogen signaling pathway is active, the expression of the genes responsible for inducing cellular stress is stopped [
83,
84].
5. Conclusions
In the fermentation process, yeast cells are continuously and simultaneously subjected to different types of cellular stress, which determines a constant cellular adaptation to environmental conditions. This paper focuses on the specific response to each type of stress that occurs in the cell—osmotic, oxidative or due to the presence of ethanol—by means of the arduous biochemical and transcription regulation or stimulation of certain genes processes. Under the action of various stressors, yeast cells can modify their colony growth morphological architecture and changes occur at a genetic level. The response to cellular stress involves the action of multiple genes, such as SOD1, SOD2, TSA1, GSH1, GSH2, GLR1, and CTT1 (oxidative stress), GPD1, GPD2, and HSP12 (osmotic stress), and HSP104, GUP1, GPP1, GPP2, GPD1, GAT1, and OLE1 (ethanol stress). Further molecular biology and genetics studies are necessary for the complete understanding of the cellular regulation mechanisms under stress conditions. This information could lead to the optimal use of Saccharomyces cerevisiae species in industrial fermentation processes and to an increase in the bioproducts’ quality. In addition, deciphering the physiological and genetic response of stress mitigation in yeast cells can also lead to countering the aging effects on human cells.