Adaptation in a Yeast Unable to Ferment Glucose

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Abstract
Ascospores of Saccharomyces cerevisiae were grown on a complete synthetic agar medium at pH 6 and with 1% Na-lactate added instead of glucose as a carbon source. Subcultures were made in duplicate from > 400 individual colonies to liquid media of the same composition, one containing Na-lactate and the other containing 2% glucose as a C source. 12 subcultures failed to grow on glucose. However, in subsequent transfers from lactate to glucose media 11 of these reverted relatively quickly to glucose utilization. The 12th subculture (derived from the hybrid S. microellipsoideus N.R.R.L. No. Y-1350 X S. cerevisiae (Mrak 93)) also adapted to glucose utilization under the following conditions: (1) frequently after prolonged incubation (1 to 4 weeks) on the synthetic medium with glucose or mixtures of glucose and lactate added (2) occasionally in very old lactate cultures without added glucose and (3) invariably within 1 to 2 days in a natural medium with yeast extract. The adaptation to growth on glucose occurred at widely different times and to different quantitative extents in replicate cultures. The culture was maintained unadapted to glucose over a 4 months period by frequent transfer on the lactate medium. The growth of the unadapted strain was inhibited by the presence of glucose and to less extent the growth of the adapted strain was inhibited by lactate.