MUTATIONS OF BACTERIAL VIRUSES 87 



homogeneity of the particles of virus y' . It could be imagined that the y '-par- 

 ticles in reproducing gave origin to a certain proportion of particles of type 7, 

 and therefore active on B only. This was excluded by experiments of the fol- 

 lowing type: Virus 7' was plated with cells B in amounts that gave a few hun- 

 dred plaques. After incubation, the contents of each of 20 plaques were picked 

 up with a needle, inoculated into separate samples of a suspension of cells B7, 

 and immediately plated. All the plates showed numerous plaques, proving that 

 all the plaques produced by the suspension of virus y' on B actually contained 

 virus y' . 



Another conceivable possibility was that a certain proportion of the particles 

 in a suspension of virus y' produced virus y' when growing on B, but were not 

 themselves capable of attacking cells of type B7. This possibility was excluded 

 by experiments of the following kind, directed to prove that all particles in a 

 suspension of virus y' can attack B7 in liquid media: A definite amount of virus 

 y' ', after titration with both bacteria B and B7, was added to an excess of grow- 

 ing cells B7. After allowing a few minutes for adsorption (10 minutes, that is, 

 much less than the constant period; see below), the mixture was divided into 

 two portions. One portion was centrifuged for titration of the free virus in the 

 supernatant, the other portion was tested for the number of infective centers, 

 by plating separately with B and with B7. Since no liberation of new virus had 

 yet taken place, each infective center represented either a free virus particle or 

 an infected bacterium. If those particles of virus y' that give plaques on B and 

 not on B7 are actually unable to attack B7, they will remain free and not ap- 

 pear as infective centers on B7. If, on the contrary, they can be adsorbed by 

 B7, they will account for a part of the infected bacteria. As such, they are likely 

 to appear as infective centers on B7, because each infected bacterium later 

 liberates a large number of virus particles, thereby increasing the chances of 

 plaque formation. This will result in an increase of the number of infective cen- 

 ters relative to the original input of virus as measured by plating with B7. 

 Table 1 shows the results of three such experiments. 



It is seen that the number of infective centers on B7 after adsorption by B7 



Table i 

 The number of infective centers of virus y' after adsorption by cells B7. 



EXPERIMENT EXPERIMENT EXPERIMENT 



NO. 13 NO. 14 NO. 16 



PLATING PLATING PLATING PLATING PLATING PLATING 

 WITHB WITH B7 WITHB WITHB7 WITH B WITH By 



Virus input per cc 27X10 6 7X10 6 35X10 6 7X10 6 n X 10 s 3X10* 



Free virus per cc 8X10 6 2.7X10 6 8X10 6 2.5X10 6 2.7X10 6 i.iXio* 



Infective centers after adsorption 



percc 28X10 9 27X10 6 35X10 6 28X10* 11.5X10 8 10.5X10* 



225 



