The effect of hyperosmotic shock on cell volume, vacuole volume, and intracellular pH (pH(i)) of individual cells of Saccharomyces cerevisiae and Zygosaccharomyces mellis was investigated. After transfer from a high water activity (a(w)) medium to low a(w) media, the growth latency periods of Z. mellis were shorter than those of S. cerevisiae. These results demonstrate that Z. mellis manages hyperosmotic shock better than S. cerevisiae. As a response to acute hyperosmotic shock, i.e. the first minute of perfusion with hypertonic buffers, the vacuoles shrank and the pH(i) decreased in both yeasts. Furthermore, in the presence of glucose, vacuole shrinkage and intracellular acidification were more pronounced in S. cerevisiae than in Z. mellis. These results may be explained by the fact that the S. cerevisiae cells shrank more than the Z. mellis cells as a response to acute hyperosmotic shock. In the presence of glucose, the vacuoles and the cells of both S. cerevisiae and Z. mellis shrank simultaneously and in proportion to a minimum level during acute hyperosmotic shock, and remained constant at this level throughout the experiment (11 min). These results indicate that vacuoles do not act as water reserves in yeasts after acute hyperosmotic shock. Finally, Z. mellis was able to maintain its pH(i) near normal physiological levels after acute hyperosmotic shock, whereas S. cerevisiae was not. These results suggest that pH(i) regulation may be important for the ability of yeasts to manage hyperosmotic shock.
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