MUTATIONS OF BACTERIAL VIRUSES 



Table 4 



Distribution of the numbers of plaques produced on B7 by a 

 series of similar cultures of virus y. 



93 



EXPERIMENT NO. 



27 



28a 



28b 



28C 



2 8d 



29 



Number of cultures tested 

 Volume of each culture, cc 

 Virus 7 per culture 



20 9 9 9 9 40 



.2 .2 .2 .2 .2 .2 



20X10 8 16X10 8 15X10 8 22X10 7 6X10 7 15X10 7 



this is a Poisson one. The experimental distribution will be derived from the 

 theoretical one by multiplying the mean by the efficiency of plating and will 

 still be a Poisson distribution. 



It is immediately seen in tables 3 and 4 that the fluctuations of the numbers 

 of plaques produced on the resistant bacteria are much higher than could be 

 accounted for by the sampling errors. The variance is generally much higher 

 than the mean, in accord with the expectation from the hypothesis that the 

 new virus arises by mutation from the normal virus. 



Another result evident in tables 3 and 4 is the presence of a large proportion 

 of low values, well below the value of the burst size. This suggests that a bac- 

 terium may liberate a mixture of normal and mutant viruses and seems to ex- 

 clude the possibility that the new virus be produced in full bursts by some ab- 

 normal bacteria, according to hypothesis 3. The new virus particles arise by 

 mutation in the course of the multiplication of normal virus inside the cell. 



It would now be desirable to compare the experimental distribution of the 

 number of mutant virus particles with the distributions to be expected accord- 



