192 ROLLIN D. HOTCHKISS AND AuDREY H. EVANS 



General Remarks 



By way of more general observation, it should be pointed 

 out that there may be many other drug systems for which 

 storage of end-product metabolites, or inhibitors, or their 

 presence in the medium can give temporary independence 

 from drug-inhibition. In such systems, many cell divisions 

 could occur in the presence of the drug. If, during these ter- 

 minal divisions any adaptive process reduces the requirement 

 for end-metabolite, then of course an adaptive resistance will 

 have been developed. If these terminal divisions have 

 allowed the development of a genetic mutation towards 

 resistance, then that too may have seemed to be favoured by 

 the presence of drug. In general, it should however have 

 appeared in the control populations of equivalent size, and the 

 actual observation would only be an increased proportion of 

 resistant cells among the survivors when drug is present. 

 The familiar variance test should be capable of showing that 

 the presence or absence of the mutant is determined by chance, 

 and that only its accumulation during the period of terminal 

 divisions of the non-resistants is influenced by drug. 



The danger exists that in such cases not all cells rated as 

 "resistant" will be really so, since merely replacing with 

 fresh medium or shifting the medium or mode of test between 

 the "adaptation" stage and the challenge for resistance can 

 lead to the result that any cells which may have stored much 

 metabolite during "adaptation" will grow for a long time 

 relatively independently of drug when given a new oppor- 

 tunity in the challenge situation. Clearly in such a case it is 

 important to use subtransfers and lineage tests to inquire 

 to what extent a more or less persistent drug resistance has 

 been achieved. 



REFERENCES 



Avery, O. T., MacLeod, C. M., and McCarty, M. (1944). J. exp. Med., 



79, 137. 

 Benzer, S. (1955). Proc. nat. Acad. Sci. Wash., 41, 344. 

 Demerec, M. (1945). Proc. nat. Acad. Sci. Wash., 31, 16. 



