Effect of Osmotic Stress on the Ultrastructure and Viability of the Yeast Saccharomyces cerevisiae

Abstract

Exposure of the yeast Saccharomyces cerevisiae to hypertonic solutions of non-permeating compounds resulted in cell shrinkage, without plasmolysis. The relationship between cell volume and osmolality was non-linear; between 1 and 4 osM there was a plateau in cell volume, with apparently a resistance to further shrinkage; beyond 4 osM cell volume was reduced further. The loss of viability of S. cerevisiae after hypertonic stress was directly related to the reduction in cell volume in the shrunken state. The plasma membrane is often considered to be the primary site of osmotic injury, but on resuspension from a hypertonic stress, which would have resulted in a major loss of viability, all cells were osmotically responsive. The effects of osmotic stress on mitochondrial activity and structure were investigated using the fluorescent probe rhodamine 123. The patterns of rhodamine staining were altered only after extreme stress and are assumed to be a pathological feature rather than a primary cause of injury. Changes in the ultrastructure of the cell envelope were examined by freeze-fracture and scanning electron microscopy. In shrunken cells the wall increased in thickness, the outer surface remained unaltered, whilst the cytoplasmic side buckled with irregular projections into the cytoplasm. On return to isotonic solutions these structural alterations were reversible, suggesting a considerable degree of plasticity of the wall. However, the rate of enzyme digestion of the wall may have been modified, indicating that changes in wall structure persist.

The Effects of Osmotic Pressure and Ethanol on Yeast Viability and Morphology

“…These data demonstrated that effects of osmotic stress on mean cell volume of yeast cells of lager and ale strains were inter-strain dependent. The effect of osmotic stress on mean cell volume was in agreement with the findings of Morris et al 28 with the exception of strain LCC 70. They provided evidence that increases in wort osmotic pressure produced shrinkage in the yeast cells.…”

“…Data originating from the ESEM in situ studies were consistent with those of Morris et al 28 ( Figure 1, micrographs A through C). Although, their studies differed with respect to the S. cerevisiae strain employed, the concentrations and types of hypertonic solutions used and the duration of the experiments.…”

“…It should be noted that during exposure to hypertonic solutions of NaCl and glycerol, shrinkage of yeast cells occurred. However, in contrast to the results being reported here, were the findings of Morris et al 28 comparing the surface of shrunken cells fixed in 3 osM-NaCl to those of cells fixed in isotonic medium. There were no marked differences in morphology, such as crenation of the outside envelope on cells exposed to hypertonic solutions compared to those fixed in isotonic solutions.…”

“…The results were expressed using the Levene's Test, which determines equal variances of any continuous distribution. 28 . These researchers also reported shrinkage of yeast cells and differences in the organization of the cell envelope upon examining the freeze fracture images of shrunken cells.…”

“…Yeast responds to the effects of solute concentration in the growth medium in many ways. Yeast cells alter their cell volume in response to osmotic challenges, decreasing volume in response to hypertonic stress and increasing volume in the presence of hypotonic stresses 28 . Rose 34 proposed that this decrease in cell volume was caused by the equilibration of the intracellular solute concentration with that of the environment by osmotic water loss.…”