Discussion 73 



Yudkin: Have these strains which gradually progress toward resist- 

 ance always been derived in the presence of the drug ? 



Hughes: In the particular case of streptomycin they were so derived. 

 They were then grown out away from the drug, and tested again. On 

 the other hand, I believe it would be possible to pick from the sensitive 

 side, for instance in chloramphenicol ; but in streptomycin one can only 

 pick the large colonies, not the small, because in streptomycin a micro- 

 colony of about 16 organisms will be dead within 4 hours. 



Yudkin : That would still be in the presence of the drug ; but I wonder 

 if it would be possible to select toward sensitivity in the absence of drug. 



Lederberg: What were the population sizes? 



Hughes: You have an overnight subculture; 10 ml. building up to the 

 full population which you would expect of an overnight culture. Then 

 you start again from that. 



Eagle: How do you interpret these results? In the specific case of 

 antibiotic resistance, are you suggesting that in these cultures there is a 

 tremendously broad spectrum of mutated organisms, differing in degree 

 of resistance, or are you suggesting that these are indeed adaptive 

 changes ? 



Hughes: I believe that every time a bacterial cell divides, just as every 

 time you sow a seed of any other plant, you get individuals. I believe 

 that all bacteria in a culture differ from all the other bacteria in a culture ; 

 and that is the only explanation that I can see for the very wide differ- 

 ences ; and that the so-called mutant is the product of killing off 99 • 999 

 per cent of them and taking the one that survives. Then, it is quite 

 impossible to tell whether you have selected from a very broad spectrum 

 or whether you have got a genuine mutant. I don't deny the existence 

 of mutants, I am sure they exist. But I do say that there is a great deal 

 of variation which, if you are going to use the term "mutant" for it, 

 means that every time a bacterial cell divides, the two daughter cells 

 are mutants. I don't think we should use the term in that way. I think 

 that something very much more clearcut should happen — an entirely 

 new characteristic, not a gradation — to merit the term "mutant". I 

 think they are all individuals. 



Lederberg: We are all prepared to accept the conclusions which have 

 just been stated. But I wonder if the situation might not be slightly more 

 complex. Firstly, that there might be individual responses on the part 

 of different cells to a drug like penicillin is almost to be expected and you 

 certainly have given us a very beautiful demonstration of it. As your 

 work shows, there is reason to believe that the action of penicillin is 

 concerned with the manufacture of cell walls, and it would be no surprise 

 if the rate at which new walls form were related to the amount already 

 there. One would not be surprised to see that there are individual 

 responses on the part of individual cells. I just raise the question, not 

 because the conclusion you give is in any sense unlikely, but this does 

 mean that every cell division gives rise to a transmissible difference 

 between the two cells. It seems quite likely that when you do finally 

 make measures of the norm of reaction of two clones, that may reflect 

 to some extent mutations which are transmissible. But is it certain 



