The osmotic stress response of ale and lager yeast

Brewing ale (Saccharomyces cerevisiae) and lager (Saccharomyces cerevisiae (syn S. pastorianus) yeast populations are subject to biological, chemical and physical stress during the brewing process. Osmotic stress is evident at discreet points throughout this industrial process, yet the impact of this stress on yeast tolerance, defence and repair has not been investigated. Haploid strains of Saccharomyces cerevisiae exhibit different tolerances to hyper-osmotic stress in which the solute concentration of the surrounding medium exceeds the cell's cytoplasm. The osmotolerance of ale and lager yeast to sorbitol and NaCI-induced osmotic stress has not been previously investigated but was observed to be strain, growth phase and solute dependent, reflecting inherent genetic differences in the management of this stress in all cases. It has been postulated that the yeast vacuole plays a central role in the maintenance of physiological activity during osmotic stress. The gross morphological changes occurring in this organelle during osmotic stress were examined. The vacuole of brewing yeast strains was demonstrated to exhibit a highly fragmented morphology, independent of strain, growth phase and osmotic stress. The fragmentation of the yeast vacuole could not be correlated to one external parameter, and was therefore not observed to be a good biomarker of osmotic stress. Saccharomyces cerevisiae exhibits a series of responses to osmotic stress. Cells may demonstrate an innate ability to withstand osmotic stress. In most instances however, osmotolerance is achieved when cells accumulate compatible solutes in order to increase intracellular osmolarity thus promoting retention of cellular water and as a result maintenance of turgor and viability. Yeast cells preferentially accumulate the polyhydric alcohol glycerol, although a detailed study on glycerol accumulation in brewing yeast strains during osmotic stress has not been previously reported. Intracellular compatible solute accumulation was observed to be complex in brewing yeast strains. Analysis of the cytosolic pool of amino acids demonstrated that there were discreet compositional changes during osmotic stress, however no single amino acid was hyper-accumulated. Glycerol accumulation could not be correlated to the intensity of osmotic stress, however the response observed was strain, growth-phase and solute type dependent. PCR detection revealed that the elucidated genes involved in glycerol biosynthesis were present in all production brewing strains examined. Sequence analysis of the GPDJ gene (encoding a cytoplasmic glycerol-3-phosphate dehydrogenase) revealed point mutations in the sequence of the SCB2 (lager) strain sufficient to affect the primary structure of Gpdlp but not its' functionality. In the ale strain SCB8 no such differences occurred. The unusual glycerol profile observed with both strains appeared to be a function of GPDJ and Gpd 1p production. However it was noted that glycerol export may have also contributed to this phenomenon