MUTATIONS OF BACTERIA 503 



EXPERIMENTAL 



A Test of the Reliability of the Plating Method 



In our experiments we wanted to study the fluctuations of the numbers of 

 resistant bacteria found in cultures of sensitive bacteria. It was therefore 

 necessary to show first that the method of testing did not involve any unrecog- 

 nized variables, which caused the number of resistant colonies to vary from 

 plate to plate or from sample to sample. 



Therefore, parallel platings were made using a series of samples from the 

 same bacterial culture. If our plating method is reliable, fluctuations should 

 in this arrangement be due to random sampling only, and the variance from a 

 series of such samples should be equal to the mean. 



Table 1 gives the results of three such experiments. It .will be seen that in 



Table i 



The number of resistant bacteria in different samples from the same culture. 



all three cases variance and mean agree as well as may be expected. There is 

 therefore no reason to assume that the method of sampling or plating intro- 

 duces any fluctuations into our results besides the sampling error. 



Fluctuations of the Number of Resistant Bacteria in Samples from a 

 Series of Similar Cultures 



As pointed out in the introduction and in the theoretical part, the hy- 

 pothesis of acquired immunity and the hypothesis of mutation lead to radi- 

 cally different predictions regarding the distribution of the number of resistant 

 bacteria in a series of similar cultures. The hypothesis of acquired immunity 

 predicts a variance equal to the average, as in sampling, while the mutation 

 hypothesis predicts a much greater variance. 



Series of five to 100 cultures were set up in parallel with small equal inocula, 

 and were grown until maximum titer was reached. Three kinds of cultures 



