334 THE BACTERIOPHAGE AND ITS BEHAVIOR 



the same strain of B. dysenteriae Shiga-Kruse in effecting passages for 

 the purpose of increasing the virulence of these races of the bacterio- 

 phage. It is evident that if the bacteriophage were a bacterial product 

 all of these bacteriophage races ought to have been the same, pre- 

 senting exactly the same properties, since in this case all were derived 

 from the same bacterial substance. Inasmuch as this is not the case, 

 since each bacteriophage possesses its own characters, it follows that 

 each race is definitely individuahstic, and, from this point of view, is 

 quite independent of the bacterium at the expense of which it has mul- 

 tiphed. The bacteriophage is, therefore, an autonomous being. 



II. Everyone admits that the bacterium can acquire a resistance to 

 the bacteriophage. This is an actual immunity transmissible to the 

 descendents throughout a certain number of generations. But in the 

 absence of the bacteriophage this acquired immunity gradually dimin- 

 ishes and is finally lost. And when this has occurred the bacterium is 

 again as susceptible as it was prior to the acquisition of the immunity. 

 This fact can be understood only if the "substance" of the corpuscle 

 differs from the "substance" of the bacterium, that is, it can not be 

 interpreted unless the bacteriophage is an autonomous being, independ- 

 ent of the bacterium. 



III. Under natural conditions "lysogenic" strains of bacteria, from 

 which it is possible to isolate a bacteriophage principle, may occasionally 

 be encountered. Such strains can also be produced experimentally. 

 We have seen above that it is possible to "purify" a bacterial strain by 

 the ordinary bacteriological method of colony isolation. This suggests 

 strongly that the bacteriophage corpuscle behaves like an "impurity" 

 contaminating the bacterial culture. How would one expect by such a 

 simple procedure as isolating individual colonies to purify a bacterial 

 culture of some product which it itself produced? The bacteriophage 

 corpuscle is therefore foreign to the bacterium, and being foreign, is 

 autonomous. 



IV. We have seen that B. typhosus is a heterologous species as regards 

 the bacteriophage, that is, different strains of this bacillus are not all 

 attacked by the same race of the bacteriophage. Each race of Typho- 

 bacteriophage has an affinity for some strains of B. tijphosus and remains 

 entirely inactive toward others. Here, we are deahng, not with an 

 acquired resistance, but with a natural resistance, for this resistance is 

 not lost by a series of successive cultivations in the absence of the bac- 

 teriophage. The majority of strains which are not attacked by one 

 race of the bacteriophage are susceptible to another. 



