28 



Discussion 



Eagle: Although I agree with Dr. Pontecorvo in general, I think the 

 question that Prof. Hinshelwood and Dr. Dean have raised is specifically 

 whether the development of resistance to antibiotics is adaptational or 

 mutational. We have some data, very similar to those presented by 

 Dr. Pollock, in relation to this point. We made a serious effort to show 

 that the development of resistance was indeed the result of adaptation, 

 but were reluctantly forced to the conclusion that all our data were 



10^ 



10= 



10" - 



O I03 



I02 





NUMBER RESISTANT 

 TO 0.036 fig./cc. 



6 8 10 



TIME IN HOURS 

 Fig. 1 (Eagle). Development of resistance to penicillin in broth 

 cultures of Micrococcus pyogenes (concentration of penicillin in 

 medium, 0-036 (jig./ml.). 



consistent with a rare mutational event, followed by selection. There 

 was no conclusive evidence for adaptation. 



With staphylococci and penicillin we get precisely the curves which 

 Dr. Pollock showed. As indicated in Fig. 1, at an appropriate concentra- 

 tion of penicillin in a fluid medium, the total number of viable organisms 

 at first falls steeply, after which the survivors apparently begin to 

 remultiply. Actually, however, throughout the entire period, resistant 

 organisms present ab initio have been multiplying, and eventually take 

 over the population. 



