IMPROVING ECOLOGICAL FITNESS AND ENVIRONMENTAL-STRESS TOLERANCE OF THE BIOCONTROL YEAST CANDIDA SAKE BY MANIPULATION OF INTRACELLULAR SUGARALCOHOL AND SUGAR CONTENT

Candida sake was cultured on nutrient yeast dextrose broth, which was diluted and/or modified by the addition of either glycerol, glucose to 0.96 or trehalose to 0.97 water activity (a(w)) to modify endogenous sugar alcohol and sugar content. Sugar alcohols (glycerol, erythritol, arabitol and mannitol) and sugars (trehalose and glucose) were extrac ted from the yeast cells and quantified using HPLC. Total polyol and s ugar content varied significantly between treatments. The total intrac ellular concentrations in NYDB medium were significantly increased in NYDB + glucose media. The major intracellular polyols/sugars in cells grown on unmodified NYDB were arabitol, trehalose and glucose with sma ll amounts of glycerol and erythritol. This was changed by reducing a( w) of the growth medium, particularly with glucose or glycerol. The ma jor polyols in C. sake cells grown on glucose-modified media were arab itol and the low mol wt polyol glycerol, with smaller amounts of gluco se. In glycerol-amended full strength normal and diluted media, glycer ol was the major intracellular polyol with lower amounts of the other polyols and sugars. The viability of the yeast cells with modified pol yols/sugars was significantly improved at lowered a(w) level (0.935 an d 0.95) when compared to unmodified yeast cells. Improvements in water stress tolerance was better in yeast cells grown for 48 than 24 h, es pecially in those from NYDB modified with either glucose, glycerol or trehalose. Such modifications of endogenous reserves were also shown t o preserve or improve the biocontrol potential of the yeast against Pe nicillium expansum rot of apples. Thus physiological manipulation of i ntracellular polyol and sugar content may provide a means for developi ng biocontrol agents with improved ecological fitness in field environ ments.