II 



CLONAL PHENOMENA IN BACTERIAL 

 AND VIRAL POPULATIONS 



In the classic days of bacteriology before 1940 changes were 

 always Kable to be found in bacterial cultures. For the most 

 part these changes obtruded only when the bacteriologist 

 found, on returning to a topic that had been dropped for 

 some months, that his standard culture was not behaving in 

 the fashion he had expected. With the development almost 

 simultaneously of potent antibiotics and an interest in bac- 

 terial genetics, these changes have themselves become impor- 

 tant objects of study. The consensus of modern opinion is 

 that most changes are a result of the selective overgrowth of 

 mutants arising independently of the action of the selective 

 agent, and this is the point of view which, with some qualifi- 

 cations, will be adopted. 



It is highly desirable, however, to begin with a brief 

 mention of those conditions in which there is adequate evi- 

 dence that change is not mutational in origin, and of those 

 where a positive effect of the selective agent has been claimed 

 with some cogency. This is particularly important when our 

 main objective is to provide a background for the discussion 

 of clonal phenomena in cell populations within the body. 



I. Adaptive enzyme formation and other adaptive changes 



Workers on adaptive enzyme formation have been able to 

 show directly that the effect of a potent inducer in changing, 

 for example, an Escherichia coli culture not producing /?-galac- 

 tosidase into one which does, is a modification, dependent on 

 pre-existing genetic qualities, but involving a physiological 

 rather than a genetic change. This holds too for the more 

 permanent change induced by penicillin on a suitable strain 



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