Improving water stress tolerance of the biocontrol yeast Candida sake grown in molasses-based media by physiological manipulation

The biocontrol agent Candida sake was cultured on either an unmodified molasses-based medium (water activity, a(w) 0.996) or on water stressed media produced by the addition of glycerol, glucose, NaCl, sorbitol, or proline to 0.98, and 0.96 a(w) for 24, 48, and 72 h, to study their impact on subsequent cell viability, and on concentrations of endogenous sugars (trehalose and glucose) and polyols (glycerol, erythritol, arabitol, and mannitol). The viability of cells of different ages cultured on these media was evaluated on NYDA medium with freely available water (a(w) 0.995), and on medium modified with polyethylene glycol to a(w) 0.95. Regardless of solute used, viable counts of cells grown on molasses-based medium (a(w) 0.98) were equal to or higher than those obtained from the medium with water freely available. The amino acid proline stimulated growth at 10% concentration. In contrast, water stress induced by addition of NaCl, glucose, or sorbitol at a(w) 0.96 caused a significant reduction in viable counts. Older cultures were more resistant to water stress. Glycerol and arabitol were the main solutes accumulated by C. sake cells in response to lowered a(w). Intracellular concentration of these polyols depended more on the solute used to adjust the a(w) than on the a(w) itself. Candida sake was more resistant to water stress with higher intracellular concentration of glycerol and erythritol.