CLONAL PHENOMENA 



media supporting growth of the parent. By a rather para- 

 doxical manipulation this quaUty can be used to provide 

 a favourable environment for the mutant. PeniciUin is 

 known to destroy sensitive bacteria only when they are growing, 

 apparently because its harmful effect is to dissociate con- 

 struction of the bacterial wall from enlargement of the con- 

 tents of the cell. Lederberg and Davis in 1948 independently 

 showed that by treating a bacterial culture with penicilHn in 

 an environment in which the parent strain could grow but 

 the type of mutant sought could not, conditions were 

 provided which would allow the survival and subsequent 

 selective isolation of a nutritionally deficient mutant. The 

 usual procedure in seeking a mutant requiring addition of 

 substance X for its growth is to expose a culture to whatever 

 mutagenic agent is being used and then allow the culture to 

 multiply for a few generations in optimal medium in which 

 the deficient mutant can grow. The culture is now washed 

 with minimal medium on which only prototrophic parent 

 can grow and time allowed for the diffusion of any residual X 

 in the mutant cells. The culture is now treated with an 

 appropriate concentration of penicilUn in minimal medium 

 until the parent type is rendered non-viable, and then 

 washed to remove the antibiotic. Colonies developing after 

 plating on optimal medium will be auxotrophs, that is, 

 nutritionally deficient types of one sort or another, and their 

 specific requirements can be determined by standard tests. 



An auxiUary technique of great usefulness is Lederberg's 

 replica method (Lederberg and Lederberg, 1952) by which 

 a very large number of colonies can be tested simultaneously 

 on plates of any desired combination of nutrients. Essentially 

 it is a disc covered with velvet which is first pressed on to the 

 primary plate to charge the filaments of the pile and then 

 transferred, with proper care as to orientation, to as many 

 test plates as desired. The method makes it possible to prove 

 that some mutants at least are pre-existent and are not 

 * produced' by the phage or antibiotic used to test for their 



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