Microbial water relations: features of the intracellular composition of sugar-tolerant yeasts

Several factors contributed to differences in intracellular composition between sugar-tolerant (osmophilic) and nontolerant species of yeast. One such factor was the difference in accumulation of those nonelectrolytes whose uptake was not dominated by vigorous metabolism. In such cases (lactose and glycerol), the sugar-tolerant species had a much lower capacity for the solute than did the nontolerant species. Sucrose uptake was consistently different between all sugar-tolerant strains on the one hand and all nontolerant strains on the other. The difference was attributable in part to metabolism of sucrose by the nontolerant yeasts. The major difference between the two types of yeast, however, was the presence of one or more polyhydric alcohols at high concentrations within each of the sugar-tolerant strains but none of the nontolerant strains. In most cases the major polyol was arabitol. The solute concentration (and, hence, water availability) of the growth medium affected both the amount of arabitol produced by Saccharomyces rouxii and the proportion retained by the yeast after brief washing with water at 0 C. When the yeast was suspended in a buffer at 30 C, the polyol leaked out at a slow, constant, reproducible rate. The polyene antibiotic amphotericin B caused rapid release of polyol by the yeast, the rate being proportional to amphotericin concentration. Contact of the yeast with glucose (1 mM) caused an extremely rapid ejection of polyol which lasted less than 40 s. Some implications of these results are discussed, as is the role of the polyol as a compatible solute in determining the water relations of the yeast.