[Effect of flocculence of a self-flocculating yeast on its tolerance to ethanol and the mechanism].

Investigation was undertaken for the purpose of examining any possible correlation between flocculence of a self-flocculating fusant of Schizosaccharomyces pombe mutant and Saccharomyces cerevisiae mutant (called fusant SPSC for short) and the tolerance of this strain to ethanol. When exposed to 18% (V/V) ethanol for 7 h at 30 degrees C, 52%, 37% and 9% of viability levels remained for the cells of fusant SPSC and its two parental strains, Sch. pombe mutant and S. cerevisiae mutant respectively. Analysis of phospholipid fatty acid composition of plasma membrane showed that the content of palmitic acid of each flocculating yeast (fusant SPSC or Sch. pombe mutant) was around 2-fold higher than that of free S. cerevisiae mutant, with remarkably lower contents of palmitoleic and oleic acids than the latter. When 0.1 mol/L sodium citrate was initially included in the medium in which cells of each flocculating yeast were grown, free cells rather than aggregates were finally obtained. Furthermore, the content of palmitic acid in the phospholipid fatty acid composition of the plasma membranes of the free cells of each flocculating yeast was found to decrease significantly, with a marked increase in the contents of palmitoleic and oleic acids. As a result, the characteristics of the phospholipid fatty acid composition of the plasma membranes of the free cells of each flocculating yeast were similar to those of S. cerevisiae mutant. Meanwhile, the disappearance of flocculence of each flocculating yeast caused by the action of sodium citrate brought about a steeply decreased tolerance of the free cells to ethanol, thus being equivalent to that of S. cerevisiae mutant. These data suggest that the stronger ethanol tolerance of each flocculating yeast is related to the higher content of palmitic acid in the phospholipid fatty acid composition of the plasma membranes. Thus, the enhancement by flocculence on the tolerance of yeast cells to ethanol as well as its mechanism are first reported in this work.