70 



W. Howard Hughes 



selection of populations starting with a single cell. The 

 differences between the two lower populations is even greater 

 than appears since many of the "colonies" given an arbitrary 

 diameter of 1 actually failed to divide at all. 



This method can be applied to the wide spectrum anti- 

 biotics. Streptomycin has been used, since Demerec had 



Rl 



Rll 



ON 8 UNITS 



ON lO UNITS 



ON 12 UNITS 



DIAMETER OF COLONIES 

 Fig. 10. Stages in the selection of streptomycin 

 resistance. 



chosen this to contrast with penicillin in his original paper 

 (Demerec, 1948). Klebsiella pneumoniae was used in the 

 present series of experiments, as it forms a neat colony which 

 is easy to measure. The steps in re-selection for resistance are 

 given in Fig. 10. At each level of antibiotic, up to 12 sub- 

 cultures of the largest colonies were necessary before the 

 diameter of the strain on normal medium was reached. At 

 this point the new strain was able to survive and divide, at 



