MUTATIONS OF BACTERIAL VIRUSES 9 1 



ror. These numbers should therefore show a Poisson distribution (variance 

 = mean). 



According to hypothesis 2, on the other hand, the new type of virus par- 

 ticles stem from mutations occurring during the growth of normal virus on sen- 

 sitive bacteria, prior to the test. If a mutation occurs before the growth of the 

 virus is completed, the mutant particle will multiply on the normal bacteria 

 and giverise to a clone of mutant particles. The earlier a mutation occurs, the 

 larger the clone will be. If we test a large number of samples, each containing 

 the same amount of normal virus, the result will be different, depending on 

 whether the samples come from the same virus culture or from different virus 

 cultures. If we test different samples of the same culture, we shall again find a 

 Poisson distribution of the number of plaques produced on resistant bacteria. 

 If, however, we test a series of similar virus cultures, all started with a few 

 sensitive bacteria and a few normal virus particles and all containing the same 

 final amount of virus, the numbers of plaques produced will show a distribution 

 with a variance much higher than the mean, because of the presence of clones 

 of mutant particles. 



The situation in the case of bacterial viruses is more complicated than in the 

 case of bacteria. The virus particle multiplies by infecting a sensitive bacter- 

 ium, which, after a latent period, liberates a hundred or more new particles. 

 The mechanism of multiplication of the virus inside the bacterial cell is not 

 known. If the new type of virus arises by mutation during the growth of the 

 normal virus, the distribution of the number of mutant particles will depend 

 on the modalities of the growth of the virus inside the cell. 



As far as hypothesis 3 is concerned, if the new type of virus is produced by 

 some abnormal cells, these may be expected to liberate a full burst of particles 

 of the new type. Therefore, we should find that in each virus culture the par- 

 ticles of the new type occur in clones averaging the burst size. 



The three hypotheses thus lead to different predictions regarding the distri- 

 bution of the numbers of plaques produced on resistant bacteria by a series of 

 similar cultures of virus. Accordingly, experiments were undertaken to deter- 

 mine this distribution. In these experiments, a small number of bacteria of 

 strain B (about io 3 /cc) were added to a broth suspension of either virus a or 

 virus 7 containing io 3 -io 4 particles/cc. The mixture was immediately divided 

 into portions of 0.2 or 0.5 cc and these were incubated at 37°C. Upon incuba- 

 tion, the cultures of virus 7 always remained clear (complete lysis, due to the 

 rarity of the mutation B— iBy). Cultures of virus a generally also gave com- 

 plete lysis; only occasionally a few cultures prepared under such conditions 

 showed secondary growth of resistant bacteria. This is explained by the fact 

 that generally in such cultures complete lysis takes place before the bacteria 

 reach a titer high enough to render the occurrence of mutations B— >Ba likely. 

 If exceptionally a mutation B— »Bai occurs, the bacteria B«i, resistant to both 

 viruses a and a', grow to saturation. If a mutation B— >Ba 2 occurs, the mutant 

 cells grow to saturation if no a'-particle is present. If a'-particles are present, 

 they are adsorbed by the cells Ba 2 when the concentration of the latter is high 

 enough, and the result is a culture containing a large amount of virus a', com- 



229 



