Discussion 161 



DISCUSSION 



Dean: We have frequently said that permanence of training is never 

 absolute. Our general thesis is that the longer the strain is trained the 

 more stable does the adaptation become ; you can eventually reverse it if 

 you subculture it for a sulFiciently long time in a drug-free medium, but 

 it may take a very long time (Hinshelwood, C. (1953), J. chem. Soc, 

 p. 1947; Dean, A. C. R., and Hinshelwood, C. (1954), Proc. roy. Soc.,.B, 

 142, 45). 



With regard to the accelerated adaptation experiments, I have quoted 

 cases where I got the concentration up to 43 mg./l. without any change 

 in the pH of the medium ; the pH was 7 at the beginning and 7 at the end. 

 I have also done a set where the concentration was gradually increased 

 up to 63 mg./l., and there pH began at 7 and ended at 6-8. But if a 

 control was put on at pH 6-8 it did not grow. Furthermore, I think 

 nobody w^ould question that one would get variations in adaptability 

 during the gro%\i:h cycle, at least with proflavine. In most of the training 

 experiments of Davies, Hinshelwood and Pryce, training was done by 

 subculture in the logarithmic phase. 



Yudkin : The point I was making about reversion was that there seemed 

 to us, at any rate, to be no relationship between the number of times a 

 culture has been trained and the degree of reversion ; and that very large 

 numbers of subcultures in the absence of the drug may produce degrees 

 of reversion no different from those produced by a few subcultures. It 

 would not be useful to discuss the pH experiment; our findings are just 

 different. I don't see how the pH stayed constant because in fact there 

 is a continuous fall in pH, as I have shown. You have a glucose medium 

 and the culture is growing in logarithmic phase, and I find it difficult 

 to imagine that the pH stayed constant while the experiment went on. 

 We used exactly the same medium as you. I should emphasize that 

 what we are discussing has two aspects : one is the cultural phase, and 

 the other is the cell-division phase. 



Walker: If proflavine is added to a culture at the beginning, is it still 

 proflavine after two days ? 



Dean: It is oxidized in the presence of light and air. One has to 

 protect against that. 



Rose: With these antibacterial agents that have amino groups present, 

 one imagines that the amino groups could easily be replaced by hydroxy! 

 through the action of deaminases which would give substances that are 

 only very feebly antibacterial. 



Yudkin: In the induction experiments where they were left in con- 

 tact with proflavine for several days, after taking them out of proflavine 

 or its degradation product, as the case may be, there was then a continu- 

 ing large increase in the number of resistant cells. 



Rose: They may be resistant now because they have the ability to 

 remove one or both amino groups. 



Yudkin: I think that may be the basis of resistance. 



Slonimski: It has been possible to show with euflavine (2:8 diamino- 

 iV-methylacridine) that the compound which can be extracted from the 



DRUG RES, 6 



