## 3 Types of Cellular Stress
### Osmotic Stress--Stress Due to Environmental Salts  
_S. cerevisiae_ is a microorganism with a high sensitivity to environmental salts. High concentrations can limit water activity, thus leading to cell growth problems [53; 54]. Over time, cells have developed a tolerance to high salt concentrations, thus being able to adapt to water fluctuations in the environment. At the same time, this tolerance has an important role in the processes of maintaining or restructuring biological, physiological or morphological functions [53; 55; 56]. Several strains of _S. cerevisiae_ were analyzed to study salt resistance. Following exposure to different molar concentrations of K\({}^{+}\) (from 0.025 M to 2 M), cells were found to lose viability at 1.5 M K\({}^{+}\). It was also observed that _S. cerevisiae_ strains exhibited a decrease in cell density ranging from 37 to 65% compared to the reference cultures, where they all had approximately the same density. Also, although the concentration of K\({}^{+}\) increased, some strains of _S. cerevisiae_ were able to consume all the glucose in the medium and convert ethanol to acetic acid. In addition to K\({}^{+}\), the microorganisms were also exposed to different concentrations of Na\({}^{+}\). Some _S. cerevisiae_ strains showed a 79% decrease in growth rate at only 0.5 M Na\({}^{+}\), while others had a 59% decrease at 2 M Na\({}^{+}\). In the case of sodium use, the cell density of _S. cerevisiae_ chains was between 25% and 68% compared to the reference values [53]. Some studies have shown, following the analysis of the microorganism _Deinococcus radiodurans_, that it has a gene which increases its tolerance to environmental stress. This was isolated and then combined with the genetic material of a strand of _S. cerevisiae_. After exposure to different salt concentrations (2%, 3%, 4%, 5%, and 7%), it was observed that normal strands of _S. cerevisiae_ can withstand a maximum salt content in the medium of 5%. However, the strand of _S. cerevisiae_ that contained the _pprl_ gene, originating from the microorganism _Deinococcus radiodurans_, tolerated the salt concentration of 7% very well [16]. Another recent study investigated how the adhesion and cytotoxicity of positively charged polystyrene nanoparticles from two yeast cultures are affected in the presence of the NaCl salt. The nanocarriers were exposed for 24 h to NaCl concentrations ranging from 5 to 600 mM and temperatures from 4 to 25 \({}^{\circ}\)C. The yeast strains used were _Saccharomyces cerevisiae_ and _Schizosaccharomyces pombe_. At the end of the research, it was observed that, for the survival of the two cultures, at a temperature of 4 \({}^{\circ}\)C, NaCl concentrations of 100 mM for _S. pombe_ and 150 mM for _S. cerevisiae_ were necessary. In the case of _S. cerevisiae_, the degree of adhesion decreased with increasing NaCl concentration at a temperature of 25 \({}^{\circ}\)C and an exposure time of less than 4 h. Thus, at NaCl values higher than 150 mM, the adhesion became almost insignificant. In terms of cytotoxicity, the higher the degree of adhesion at the beginning, the more pronounced the cell death. Regarding _S. pombe_, the degree of adhesion increased at the same time as the NaCl concentration. Thus, it was observed that the cells coated with nanoparticles had a mortality reduced by 50% at an exposure time of less than 4 h [57]. Water activity describes the chemical yield of free water in a solution. One of its uses is to help classify microorganisms into: halophile, halotolerant, and halosensitive. Halophile microorganisms have the ability to grow in environments with high salinity, while halo-sensitive ones are drastically affected by such amounts. At their core, halotolerant microorganisms are those that do not require certain amounts of salt to grow and can be viable over a wide range of salinity values in the environment. In order to be classified into one of the categories above, yeasts must have a certain value for water activity.