162 Discussion 



yeast cell is the same as the one used to induce mutation. To do this one 

 extracts with HCl-ethanol, then one gets rid of the extracting medium. 

 There is no change in the spectrum (measured at different pH's) and 

 there is no appreciable change in the biological activity. One must, 

 however, have taken special precautions to ensure preservation of eufla- 

 vine. The work must be done in the dark, at constant pH, and in a well 

 defined chemical medium. 



Walker: Is the same chemical species present over the pH range in 

 which you carried out your first experiment, Prof. Yudkin ? 



Yudkin: No, probably not. We wanted to know whether the change 

 which allows bacteria to grow in high concentrations of proflavine is a 

 change in the cells or a change in the medium. We think we have estab- 

 lished that it is a change in the medium, and that it is simply a matter 

 ofpH. 



Slonimski: Albert and his collaborators have shown that 12 years ago; 

 but this is quite an interesting phenomenon in itself because firstly, you 

 have a competitive ratio of 1 hydrogen ion per 500 to 1000 ions of 

 acridine, which is quite surprising; and secondly, this is not always 

 a question of dissociation of acridine. Marcovich (1953, Ann. Inst. 

 Pasteur, 85, 199) has shown with euflavine (which has a pKa of more 

 than 12, while proflavine has 9) that the antimutagenic effect of H+ ions 

 had a pKa around 5 • 5, i.e. it cannot be explained by the dissociation of 

 the drug. 



Davis: If one is concerned with determining the mechanism by which 

 a resistant strain becomes sensitive on growth in the absence of the 

 drug, I am not sure I understand the rationale underlying the extensive 

 experiments of Yudkin, and of Dean and Hinshelwood, designed to 

 reveal how many culture passages are necessary to bring about this 

 return of sensitivity. It is known that in mixtures of resistant mutants 

 and the sensitive parental strain one can find differences in relative rate 

 of growth that lead to selection — in favour respectively of the resistant 

 strain in the presence of the drug and the sensitive strain in the absence 

 of the drug. Hence, whether a resistant strain becomes sensitive in the 

 absence of the drug after 5 passages — or only after 500 — this observa- 

 tion per se does not help us to decide whether the change was based on 

 mutation and selection or on a physiological adaptation. 



Yudkin: Speaking solely from our own point of view, what we were 

 trying to do was to see whether we got the same phenomenon as Hin- 

 shelwood. We wanted to know what would happen in the Hinshelwood 

 conditions, but measuring resistance in the more orthodox way of sur- 

 vival curves. We concluded that you don't seem to get anywhere with 

 that sort of experimentation. 



Dean: In their experiments, Davies, Hinshelwood and Pryce (1945, 

 Trans. Faraday Soc, 41, 163) were investigating the adaptation of 

 Bact. laciis aerogenes to proflavine. They trained organisms at certain 

 concentrations, and then did lag-concentration curves with these strains, 

 and found that the resistance was continuously graded to conform to the 

 concentration at which training has been carried out. Then the interest- 

 ing question arose : Is this resistance stable ? The conclusion is that the 



