VIRULENCE OF THE BACTERIOPHAGE 175 



other organisms becomes lost. In this connection we will see later 

 many examples showing that the characters pertaining to each race of 

 the bacteriophage are very stable. 



When, after a number of passages at the expense of a given bacterium, 

 the bacteriophage corpuscle is placed in contact with a bacterium of 

 another species for which some of the corpuscles possess a latent viru- 

 lence, a selection occurs. Only the corpuscles having this virulence 

 multiply; the virulence for the new bacterium is thus increased so that 

 after a few passages the virulence reaches such a degree that complete 

 bacteriophagy is effected. It is necessary, however, to state that this 

 tendency toward an increase in virulence differs very markedly for 

 different races of the bacteriophage. Given two races of the bacterio- 

 phage, both presenting a latent virulence for a single bacterial strain, 

 one may obtain, after a few passages, a high virulence with one of the 

 races while it may require a considerable number of passages before the 

 second race attains a comparable degree of virulence. Indeed, with 

 certain races this may never be accomplished. 



We have seen in the experiments recorded above that even in the case 

 of a bacteriophage having a maximum activity for a given bacterium 

 each corpuscle possesses its own individual virulence. Certain of them 

 are high in virulence, others are less active. We will see that this is 

 also true for the latent virulences which these corpuscles may manifest. 

 This behavior of bacteriophage corpuscles is identical to that of patho- 

 genic bacteria. Since the time of Pasteur we have known that in a pure 

 bacterial culture each individual cell, although possessing the general 

 characteristics of the species, presents "variations" which are peculiar 

 to it. A bacterium may be considered, according to the expression of 

 Maurice Nicolle, as a "mosaic" of properties, each of these properties — 

 vitality, resistance to such and such an agent, virulence for such and 

 such an animal — -being subject to continual variation. And the varia- 

 tions in each of these factors follow different lines with each cell division. 

 Everything would indicate that the situation is precisely the same for 

 bacteriophage corpuscles. Each race of the bacteriophage possesses 

 characters which distinguish it from other races, and each of the cor- 

 puscles of a given race possesses characters which are subject to con- 

 tinual variation. 



11. THE ACQUISITION OF VIRULENCE 



From the very first of my communications upon the subject I have 

 insisted upon the fact that a bacteriophage can acquire, experimentally, 



