HOROWITZ AND LEUPOLD 



Table 1 

 Statistics of E. colt Study 



No. of irradiated cells 

 No. of surviving cells 

 Secondary colonies 

 isolated 

 No. of temperature 

 mutants 

 Type D40 

 Type 140 



1.7 X 109 (approx.) 

 2.4 X 10« (approx.) 



2157 



161 



124 



37 



The remaining 124 mutants, those 

 which grow on complete medium at 

 40°, were tested by the auxanographic 

 method to determine their growth re- 

 quirements. Seventv-nine per cent of 

 these mutants were classifiable in this 

 way. A variety of requirements was 

 found (Table 2), indicating again that 



Table 2 



Syntheses Known To Be Affected 

 in E. colt Temperature Mutants 



temperature mutation is random with 

 respect to the classes of syntheses 

 which can be affected. A number of 

 substances are conspicuous by their ab- 

 sence from this list, notably trypto- 

 phane and p-aminobenzoic acid. It has 

 not been excluded, however, that re- 

 quirements for these substances are 

 present among the mutants which 

 were not classifiable in the auxano- 

 graphic test. 



215 



Several other points of interest in 

 connection with the E. coli study 

 should be mentioned. These concern 

 the selective forces operating in 

 densely populated versus lightly popu- 

 lated Petri plates. It is obvious that in 

 a method like this, in which there may 

 be many hundred of colonies per (15 

 cm.) plate, we are not actually isolat- 

 ing the mutants in a minimal medium, 

 but in a minimal medium plus or minus 

 whatever the hundreds of wild type 

 colonies add to or subtract from it. 

 This is quite clearly shown in the 

 relative yield of secondary colonies 

 and temperature mutants per number 

 of survivors. In our experiments the 

 total number of colonies (i.e., sur- 

 vivors) per plate varied from 100 to 

 1,700. In sparsely populated plates the 

 relative yield of both secondary col- 

 onies and temperature mutants was 

 much higher than in densely popu- 

 lated plates. In Figure 1 the numbers 

 of secondary colonies and tempera- 

 ture mutants per million survivors are 

 plotted against the total number of 

 colonies per plate. At the lowest den- 

 sities, with populations of the order of 

 100 colonies per plate, yields of 3,400 

 and 6,500 secondary colonies per mil- 

 lion survivors have been recorded. At 

 higher densities, the yield decreases 

 systematically and rapidly, reaching 

 values between 200 and 800 secondary 

 colonies per million survivors at pop- 

 ulation densities of 1,200 to 1,700 col- 

 onies per plate. The yield of tempera- 

 ture mutants per million survivors is 

 roughly one-tenth that of all second- 

 ary colonies, and it exhibits the same 

 systematic trend. The difference be- 

 tween these two curves expresses the 

 fact that approximately 90 per cent 

 of the secondary colonies are wild 

 types which, for one reason or an- 

 other, started to grow late; partially 

 blocked, slowly growingr biochemical 

 mutants; and completely blocked bio- 



