Printed in Great Britain Growth Rate Patterns of the So-called Osmophilic and Non-osmophilic Yeasts

The growth rates of nine yeasts, conventionally classified as ' osmophilic' and six yeasts, conventionally classified as ' non-osmophilic ' were measured at 30 " in media adjusted to known water activities with polyethylene glycol. The organisms were less tolerant of low water activities (a,) in the presence of polyethylene glycol than in the presence of sugars. There was no significant difference between average minimal water activities supporting growth of the two groups of yeasts in polyethylene glycol, although differences in sugar tolerance were conspicuous. There was good correlation, however, between quantitative water relations of the organisms in polyethylene glycol and qualitative observations of ability to grow at specified sugar concentrations. The osmophilic organisms grew about half as fast as the non-osmophilic organisms at their respective optimal water activities. The osmophils had relatively broad water activity optima; the non-osmophils had sharp optima. Neither group of yeasts had a general requirement for a water activity lower than that of the basal medium (o.997aW) in order to grow, although there were two osmophilic strains which would not grow in the basal medium at 30". The adjective ' osmophilic ' is considered to be inaccurate; until the physiology of these yeasts is better understood we propose that they be designated simply as ' sugar tolerant '. The quantitative results have provided a basis for selecting sugar tolerant organisms for biochemical and physiological investigations