Discussion 41 



cannot get in one step a high increase in resistance to chloramphenicol, 

 and you must therefore work on what occurs in nature. In the case of 

 chloramphenicol, in fact, some trouble was encountered and indirect 

 selection took a little longer. The level of resistance of the particular 

 mutant selected indirectly was such that it did not give 100 per cent 

 survival with the concentration of the drug that would kill all of the 

 sensitives, it gave only 12-20 per cent survival. 



Hinshelwood: As regards chloramphenicol resistance, by means of 

 growing mass cultures in different biochemical media (in different sugars, 

 broth and synthetic medium and so on) we can change what may be 

 called the natural level of resistance in the range from 10 to 20 or 30 parts 

 per million; and in the same way resistance to proflavine and certain 

 other drugs changes. On the other hand, the resistance can be raised to 

 800 or 1000 by the direct action of the drug, so it is a very low-grade 

 resistance indeed which is selected in your experiment. There is a great 

 contrast, in any case, between chloramphenicol and streptomycin where 

 the degree of resistance rises abruptly after adaptation, by whatever 

 means, to quite a low concentration of streptomycin. 



We think that there is a distinction between types of resistance. We 

 think, moreover, that streptomycin resistance may sometimes be due to 

 the lack of the power of cells to take up the drug. 



Cavalli-Sforza : I think that with chloramphenicol you have to work in 

 those conditions, because you cannot get higher resistance in just one 

 step. It is another genetic system. You have not got any single gene 

 capable of giving high resistance at once. You could get higher resis- 

 tance only by repeating the entire process of selection on first-step 

 mutants, and so on. 



Hinshelwood: Still, when mutants are selected up to a certain point in 

 your environment, the further steps should be coming in quite freely. 



Cavalli-Sforza: The later cycles of an indirect selection experiment 

 may be easier than the first ones ; but if not, you can change the method 

 if you want to, for instance you could go over to replica plating. 



Hinshelwood: You would not be inclined to entertain the idea that 

 there are really two types of adaptation ? 



Cavalli-Sforza: What was done here was to test one hypothesis. The 

 present method can only disprove the genetical hypothesis; if it does 

 disprove it, it leaves the field open for the alternative one, but it has not 

 done that. 



Hinshelwood: Have these resistant cells obtained by the indirect 

 method been tested for streptomycin adsorption ? That would be a very 

 interesting datum to have. 



Cavalli-Sforza: I don't think they have been tested. 



Dean: Would you consider, if you got no selection, that the genetical 

 hypothesis is disproved for a particular drug and a particular organism ? 

 For instance, if one did an experiment with a certain drug and a certain 

 organism and found no selection at all, would you consider that as 

 disproving the genetical hypothesis ? 



Cavalli-Sforza : Of course ; but you have to be very careful, in using 

 indirect selection, about those shortcomings that I mentioned, for 



