22-33 



THE YEAST CELL 



concentrations of pantothenate while in very low concentrations the 

 adapted cells make up the bulk of the population. It may be noted 

 from table 22-7 that the total number of adapted cells in the mixed 

 cultures at completion of growth increases as the concentration of 

 pantothenate decreases. The ratio of the two types of cells in the 

 final growth obtained was determined by plating samples on plus and 

 minus pantothenate plates and the total number of cells estimated 

 from baemocytometer counts. 



Table 22-7 

 Analysis of Mixed Cultures 



If we refer to the growth rates of these two cultures grown sep- 

 arately, it is apparent from comparison of figure 22-4 showing growth 

 rates for pantothenate -dependent strain 2154 alone and figure 22-5 

 for adapted 2154 alone, that the rate of growth of the dependent strain 

 varies extremely with concentration of pantothenate while the rate of 

 the independent strain varies only slightly. The unadapted dependent 

 strain begins to grow sooner and grows more rapidly than the adapted 

 strain at high concentrations. (See also figure 22-10). In low concen- 

 trations of pantothenate the adapted strain grows more rapidly. In 

 the intermediate concentrations the growth rates of the two strains 

 are approximately similar. Hence, in mixed cultures, while there is 

 a possibility of competition between the cells of the two strains in all 

 concentrations, it is effective only at intermediate concentrations, 

 because at high concentrations the unadapted cells almost completely 

 overgrow the adapted, while at low concentrations the adapted pre- 

 dominate. 



The fact that mixtures of strains at intermediate concentrations 

 do not reach maximum growth may be accounted for in two ways. 

 (1) On limiting concentrations of pantothenate the dependent cells 

 may not grow as efficiently as they do with sufficient pantothenate 

 so that they exhaust one or more of the essential nutrients more 

 rapidly than at optimal pantothenate concentrations and thus are un- 

 able to attain as high total growth as in the presence of sufficient 

 pantothenate. When dependent cells are grown on limiting concen- 



