248 Discussion 



Knox: We plan to do that. 



Dean: Can j^ou get some indication of the rate of decay of the drug 

 and maintain the drug concentration at a constant level ? 



Knox: This is also something that we plan to do. 



Westergaard: This may not be relevant to Prof. Knox's problem, but 

 when you block catalase, you do increase the concentration of peroxide 

 and thereby the concentration of a mutagen, so you certainly introduce 

 a new variable which you have not taken into account. 



Knox: One of the possibilities is, that if isoniazid does interfere with 

 peroxide destruction, and peroxide is accumulating, then the peroxide 

 itself acts as a mutagenic agent; but I don't think that will explain the 

 deficiency of the resistant mutants which have never been in contact 

 with isoniazid. 



Hayes: In view of the fact that isoniazid is a killing drug, is it not very 

 likely that the pseudomutants are in fact persistorsand that the difference 

 between the 3-day culture and the other is simply that virtually all the 

 cells of the 3-day culture are actively growing when the drug is added 

 and, therefore, susceptible to it? Perhaps if you added some non-specific, 

 purely bacteriostatic substance, which does not kill but which simply 

 stops growth, you could clear this up. 



Knox: Schaefer has shown that if you add glucose to a medium of 

 tubercle bacilli, isoniazid immediately becomes much more bactericidal. 

 Oxygenation probably has the same effect; it is fairly bactericidal in 

 some media, at any rate. In the Fisher type of medium in which this 

 pseudomutant phenomenon does not occur, it seems to be fairly bacter- 

 icidal. Streptomycin, on the other hand, does not show this phenomenon. 

 There seems to be a great controversy as to whether streptomycin is or is 

 not bactericidal for tubercle bacilli. 



Lederberg: Is there any explicit evidence that the rate of bactericide 

 by isoniazid depends on the rate at which the cells are grown ? 



Knox: I think Schaefer's work proves that. 



Lederberg: Then there may be a superficial resemblance to some other 

 things that Dr. Eagle described, in that if there were both a bacterio- 

 static and a bactericidal effect, on the part of isoniazid, those cells which 

 happen to be first inhibited in their growth would be temporarily pro- 

 tected from the bactericidal effect. This might account for the zone in 

 which you can get an increase of either in the presence of isoniazid. 

 This would then be very much subject to external conditions. This 

 bears on the second point, that some more attention should be paid to 

 these cases where there are diffusible effects from sensitive cells to 

 resistant ones in the same population. This is a hopeful point that may 

 have bearing on practical problems of chemotherapy. If we understood 

 those better we would have at least one approach to coping with drug- 

 resistant organisms. It seems not impossible that, in Dr. Eagle's case, 

 these resistant mutants are resistant because of some block or some 

 deficiency in a metabolic process, and that this has been repaired by the 

 products from sensitive cells. One would like to know how specific that is. 



Pollock: We have done a few experiments along these lines with peni- 

 cillin resistance and penicillin dependence of Staphylococcus. We made 



