■U200.UI00. U50 



TIME 



Fig. 22-10 Idealized Growth Curves Produces by Adapted an^ 

 Unadapted Ce ] 1-s Inoculated into Media Containing Various Concen- 

 trations of Pantothenate. The numbers indicate the concentrations 

 of pantothenate in micrograms per liter; U and A, respectively, 

 indicate unadapted and adapted inocula. At concentrations of 200, 

 100, and 50 micrograms, the unadapted inocula give practically 

 identical growth curves. When unadapted inocula are introduced in- 

 to tubes containing 20, 10, and 5 micrograms per liter, the total 

 final growth is roughly proportional to the amount of pantothenate 

 present and no further growth occurs in the tubes. One can assume 

 that adapted cells are almost certainly present in the U 20, U 10, 

 and U 5 tubesv, but are unable to grow either because the inefficient 

 growth of the unadapted cells has exhausted the nutrients or pro- 

 duced some inhibitory substance. 



When adapted inocula are introduced into duplicate tubes con- 

 taining from 200 to 2 micrograms of pantothenate per liter, the 

 growth curves virtually superimpose, indicating that the growth 

 rate of adapted cells is limited by factors other than pantothenate 

 above 2 micrograms. The adapted organism exhibits a slightly slow- 

 er growth rate in the absence of added pantothenate or in one microgram 

 of pantothenate per liter, so that addition of pantothenate does 

 have a slight effect in increasing growth rate. The growth curve 

 of the adapted cells has both a longer lag and a slower rate than 

 that of the unadapted cells given adequate pantothenate. Presum- 

 ably, the long lag and the slow rate both depend on the synthe- 

 sizing load carried by the adapted organism. 



Curves U 2, U 1, and UO show the growth achieved by the un- 

 adapted cultures inoculated into media containing 2, 1, and no 

 added micrograms of pantothenate, respectively. These curves are 

 characterized at first by a long period of slow growth of unadapted 

 cells and later by a sharp upward slope consisting of growth by the 

 "adapted", mutant. The mutation to pantothenate synthesis occurs 

 some time before the beginning of the upward swing. The cells dur- 

 ing the primary adaptation, although growing inefficiently and cap- 

 able of producing inhibitory substances, or of exhausting the medium, 



22-35 



