Indirect Selection and Drug Resistance 37 



be expected to contain a mutant at the beginning of growth. 

 As the sample of 0-125 ml. was made up to 20 ml. with fresh 

 broth, every cell inoculated was allowed to grow into a clone 

 of 160 cells on average. There should then be about 160 

 resistant cells in 5 out of 10 tubes of the 0-125-ml. series. 

 This was found to be true of 4 tubes instead of 5. In other 

 samples, of 0-5 and 0-25 ml., there is evidence that, as would 



Table II 



A Protocol of Quantitative Indirect Selection 



From a culture which has been assayed for streptomycin resistants and 

 found to contain 39-5 resistants per ml., further samples, after addition 

 of fresh broth to a total of 20 ml. and incubation, have given: 



be expected, more than one mutant fell into some tubes (e.g. 

 the first and ninth tube of the 0-25-ml. sample, etc.). 



One can in this way count the number of spontaneous 

 mutants in a culture and compare it with the expected one, 

 if the hypothesis is made that all resistants observed on drug 

 plates are the consequence of spontaneous mutation. These 

 two values are given for each sample in the second-last and 

 third-last lines of Table II. 



Corrections have to be made, however, to account for 

 the statistical distribution of mutants and for the possible 



