GENETIC IMPLICATIONS OF MUTATIONS IN S. TYPHIMURIUM 



273 



as used in E. coli, stated "... the method as developed so far does not appear 

 to yield quantitative survival of mutants." Such a statement assumes that 

 the penicillin screening may be expected to be complete, which in fact is not 

 true. Rather penicillin acts, as do all antibiotics, in a progressive fashion ac- 

 cording to a typical logarithmic killing curve. If two or more mutant cells 



PERCENT 

 MUTATIONS 

 100 



80 



60 



40 



20 



DATA OF TABLE 1 

 A = I - VllI 

 O - IX- X 



o 



4 6 8 10 12 14 16 

 MINUTES OF RADIATION 



20 



Fig. 17.3 — Graph showing the relation between percentage of mutations isolated and 

 X-ray dosage in minutes (2850 R per minute). 



appear in a growing wild type population, they will increase logarithmically 

 and form smaller less numerous clones. As the penicillin acts, the far more 

 numerous parent clones will be logarithmically reduced in numbers, while 

 the mutant clones exposed will have reached a level which may be main- 

 tained during the 24 hour period of penicillin action. It is clear that if a 

 sample is taken, and plated at any point short of the complete killing oflf of 

 the wild type, we may expect frequencies showing the same order as in the 

 original population, although the mutant percentages are greatly magnified. 



