^g_2 THE YEAST CELL 



The non-glucose culture was isolated by suspending spores of 

 the hybrid in distilled water, heated at 58 degrees C. for four min- 

 utes to kill vegetative cells, and then plating on agar media con- 

 taining organic acids in concentrations of 1 per cent substituted 

 for glucose in Burkholder's synthetic medium including vitamins 

 and with alkali added to make the initial pH of the media approxi- 

 mately 6. After four days' incubation at 30 degrees C. individual 

 colonies were transferred to tubes containing liquid media of the 

 same composition. After a few days sufficient growth occurred in 

 these tubes to make possible loop transfers to tubes containing the 

 standard synthetic medium with 2 per cent glucose. These tubes were 

 incubated under CO2 at 30 degrees C. From over four hundred such 

 transfers from individual colonies, twelve were found which grew 

 on the organic acid substrates, i.e., on lactate, succinate, or malate, 

 or in combinations of salts of these acids, but which failed to grow 

 when subsequently transferred to glucose. Subcultures were main- 

 tained on lactate medium and were tested repeatedly for their abili- 

 ty to grow on glucose either in the presence or absence of atmos- 

 pheric oxygen. It was found that all but two of the twelve organisms 

 quickly reverted to glucose utilization, and these were not further 

 investigated. Nearly all the subcultures of one of the two remaining 

 isolates reverted to sugar utilization within a week and generally 

 within a few days after exposure to glucose in liquid medium. With 

 rare exceptions, reisolations from the reverted cultures yielded 

 only glucose utilizing organisms. 



The remaining culture, isolate No. 97, was carried for three 

 months on a lactate medium and was kept stable with respect to non- 

 utilization of glucose by frequent transfers to new medium. It was 

 unable under the conditions of these experiments either to ferment 

 or respire glucose (at least at a significant rate). The same holds 

 for fructose and sucrose. As will be seen below, however, this sta- 

 bility would not be maintained in standard culture practice. 



Adaptation to Glucose Utilization 



In tests of the stability of non -glucose utilization, loop trans- 

 fers were made from the lactate medium to synthetic medium with 

 glucose. Many such transfers of strain No. 97 after standing for 

 some weeks would show a sudden rapid growth and fermentation; 

 i.e., a reversion to glucose utilization occurred in individual tubes 

 after different long periods of incubation in contact with the sub- 

 strate. 



On the other hand, it was observed that the addition of glucose 

 to the organic acid substrates tended to repress the growth of the 

 organism. Thus glucose not only is not assimilated by the unadapted 

 organism but it actually inhibits the utilization of the organic acids. 



