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 (aw)
to modify endogenous sugar alcohol and sugar content. Sugar alcohols
(glycerol, erythritol, arabitol and mannitol) and sugars (trehalose and
glucose) were extracted from the yeast cells and quantified
using HPLC. Total polyol and sugar content varied significantly between
treatments. The total intracellular 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 small amounts of glycerol
and erythritol. This was changed by reducing aw of the
growth medium, particularly with glucose or glycerol. The major polyols
in C. sake cells grown on glucose-modified media were
arabitol and the low mol wt polyol glycerol, with smaller amounts of glucose.
In glycerol-amended full strength normal and diluted
media, glycerol was the major intracellular polyol with lower amounts of
the other polyols and sugars. The viability of the yeast
cells with modified polyols/sugars was significantly improved at lowered
aw level (0·935 and 0·95) when compared
to unmodified
yeast cells. Improvements in water stress tolerance was better in yeast
cells grown for 48 than 24 h, especially in those from NYDB
modified with either glucose, glycerol or trehalose. Such modifications
of endogenous reserves were also shown to preserve or
improve the biocontrol potential of the yeast against Penicillium expansum
rot of apples. Thus physiological manipulation of
intracellular polyol and sugar content may provide a means for developing
biocontrol agents with improved ecological fitness in
field environments.