Year

1988

Degree Name

Doctor of Philosophy

Department

Department of Biology

Abstract

When growing exponentially in batch culture Saccharomyces cerevisiae passed through a phase in which about one cell in 1 s u r v i v e d a water stress imposed by plating onto low water activity (a^) agar. This phenomenon was called water stress plating hypersensitivity. Stationary phase cultures were resistant to a water stress of the same magnitude. Most of the other yeast species tested did not display plating hypersensitivity, with the exception of Candida krusei and a Kloeckera isolate. The difference between viable count on high a^ agar and low a^ agar (the difference between the log values being called the plating discrepancy) was diminished in exponentially growing S. cerevisiae if the yeast was incubated in a glucose solution of intermediate a^ before being plated onto the stressing agar. Addition of glycine betaine and glycerol to the stressing agar, two substances known to improve the survival of some microorganisms at low a^, did not improve the viability of S. cerevisiae. The proportion of the population able to grow on the stressing agar was dependent on the agar a^. If the agar contained 5% NaCI (mass/vol.) or less, the counts on salt and high a^ agars were the same. The duration of the plating hypersensitivity period during the growth cycle of S. cerevisiae could be prolonged by incubating the culture at 15°C rather than 30°C and by increasing the glucose content of the culture medium. Growth under anaerobic conditions also prolonged the duration of the hypersensitivity period. The nature of the carbon source in liquid culture did not influence greatly the response of S. cerevisiae to a water stress. When ethanol or maltose instead of glucose was the sole carbon and energy source, S. cerevisiae still displayed water stress plating hypersensitivity.

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