16-8 THE YEAST CELL 



The final total yield of a culture is primarily determined by the 

 glucose content of the medium. Thus the largest yields on the av- 

 erage were obtained on 4 per cent glucose, and approximately the 

 same yields were eventually obtained on mixtures with lactate. In 

 the case of 1 per cent glucose the addition of lactate may give sig- 

 nificant increases. Mean values for the total yields of four sets of 

 strain No. 97 measured after between 600 and 800 hours and the 

 results obtained with a set of strain No. 322 after 142 hours are 

 shown in Table 16-1. Many factors affect the final yields. Most 

 important of these is perhaps the different extents of fermentation 

 versus assimilation in growth on the available substrate occurring 

 in the individual cultures. Furthermore, the long growth periods 

 and different growth rates introduce variable changes in the medium 

 leading to modifications in yields, partial autolysis, presence of 

 ghost cells and perhaps even reutilization of substrate by new types. 

 These difficulties do not arise with the glucose adapted strain, where 

 the relation of final yield to glucose content is very definite except 

 in so far as the presence of 2 per cent lactate inhibits the growth 

 on glucose. The cultures were not kept long enough to determine 

 whether this lag in yields on 2 per cent lactate was ultimately re- 

 covered. 



To summarize: Inoculation in glucose or glucose -lactate mix- 

 tures does not result in immediate growth as inoculation from lac- 

 tate to lactate does. The delay is the incubation period required to 

 adapt to the utilization of glucose. The incubation period may be 

 shortened by additions of lactate, but when lactate is present the 

 total growth and the growth rate is generally less and soon suffers 

 a temporary stoppage. After this stoppage a secondary increase 

 in growth occurs with a correspondingly slower rate of utilization 

 of the remaining glucose and lactate. 



Adaptation to glucose on lactate medium. The above re- 

 sults show that adaptation to glucose utilization occurs on synthetic 

 medium with added glucose. It may also occur after very long peri- 

 ods on lactate media without added glucose. Thus, of some 50 cul- 

 tures which have been tested for glucose adaptation after 600 or 

 more hours of incubation on 1 and | per cent lactate media, 3 and 

 possibly several others were adapted at least to slow growth on glu- 

 cose. It is not certain, however, that this adaptation has occurred 

 in the absence of the substrate. More likely, some glucose has been 

 produced in the cells as a result of hydrolysis of trehalose and per- 

 haps of other polysaccharides so that a condition comparable with 

 that of added glucose obtains in old cultures of the unadapted strain. 



Adaptat ion on standard peptone glucose broth. It was 

 found that transfers of culture No. 97 from lactate to standard glu- 

 cose broth containing yeast extract and peptone (in contrast to the 

 behavior on synthetic medium) grew rapidly and as a rule exhibited 



