Discussion 61 



resistance, or any other inheritable change in bacteria, can be brought 

 about through a directive environmental influence. 



Pontecorvo: Those changes, of any type, that we are actually able to 

 demonstrate as genetic are of course a very selective sample. Any 

 change which is either not big enough or not sharp enough or perhaps 

 too much subject to environmental variation cannot be tested; at least 

 it is very difficult to subject such changes to the test of segregation, 

 which is the only one that we can use for an unquestionable distinction. 



Hinshelwood: Dr. Demerec, what is your opinion about the fact that 

 the sensitive cells had grown before the ones that you termed resistant ? 



Demerec: The reason is that we used a very low concentration of 

 streptomycin, a concentration that did not kill any of the bacteria 

 plated on it, as I have shown by comparing the numbers of colonies 

 obtained on plates containing streptomycin and on control plates without 

 it. Therefore, the sensitive bacteria were able to divide, but they divided 

 slowly and formed a very thin layer of gro^vlh. We assume that during 

 those divisions mutants appeared which were better adapted to gro^vth 

 on a medium containing this low concentration of streptomycin, and that 

 thus we had the secondary colonies originating. 



Hinshelwood: We have done experiments on the crossing of yeasts and 

 examined the galactose fermentation, and we got clearcut Mendelian 

 phenomena, superimposed on which there were adaptive developments 

 of the segregated characters themselves, so that both adaptive and clear- 

 cut Mendelian phenomena were present in the same system. We have 

 also done experiments on the crossing of bacterial strains of certain 

 drug-resistant cells and got an almost continuous spectrum of resistance 

 between the levels of the parents. This may, of course, be ascribed to 

 the fact that the mutation steps are very small, but even so it is hard to 

 regard the result as evidence of a sharp segregation. However, in refer- 

 ence to Dr. Demerec's opening remarks, who has ever said that in sexual 

 recombination or in the segregation phenomena of Mendel there was 

 other than transmission of basic material and information? And who 

 would ever have thought of attributing sexual recombination and segre- 

 gation to adaptation ? I have never heard of anybody who would do it. 



Demerec: I agree with you. Fortunately, there is no serious disagree- 

 ment at present about the mechanism of Mendelian inheritance. Un- 

 fortunately, however, until very recently we did not know about sexual 

 recombination in bacteria. And when our first reports on genetic 

 research with bacteria were published, the question was raised by 

 bacteriologists and geneticists: Have bacteria a genetic mechanism 

 similar to that found in higher organisms? Proof of the existence of 

 such a mechanism came only recently. 



Lederberg: In any experiment where bacteria grow in the presence of 

 a drug it is possible to attribute special significance to this and to say 

 that perhaps the drug played some role in inducing and fixing mutation. 

 That type of discussion was quite profitable ten years ago. At the present 

 time we have a number of sharply delineated cases where, on the one 

 hand, we can demonstrate the occurrence of structural changes leading 

 to resistance in the absence of the drug, and these are mutations; on the 



