178 THE BACTERIOPHAGE AND ITS BEHAVIOR 



for strains 24 and 29, while bacteriophage Wi, through passages with this 

 same strain Sm, has acquired a virulence for strain 1. 



Da Costa Cruz^^^ has applied the same procedure to develop a race 

 which would give bacteriophagy of a strain which was originally not 

 attacked. When isolated this bacteriophage had but a slight virulence 

 for strain 1 of B. typhosus, but it was very active for strain 18. With 

 these two strains as extremes, the virulence for a number of other strains 

 was more or less marked. The first passages were made with strain 18, 

 then with strains which originally were less susceptible. Contacts were 

 thus made involving successive passages with strains which were less 

 and less susceptible. When the series of contacts was completed the 

 race showed a very marked virulence for strain 1. 



It would appear, therefore, that contacts with species more and more 

 remote from the normal host may, in reality, be the device whereby a 

 given race of the bacteriophage acquires a virulence for a bacterium 

 previously insensitive to its action. 



Eliava and myself^^^ have adapted a Staphylo-bacteriophage to 

 bacteriophagy of B. dysenteriae Shiga, but thus far it has been impossible 

 to accomplish the reverse change. 



McKinley,*^^ working with a race of Shiga-bacteriophage* quickly 

 obtained a high virulence for the meningococcus by means of four pas- 

 sages in a bouillon containing glucose and blood. This bacteriophage, 

 after its adaptation to the meningococcus, was found to be virulent for 

 Staphylococcus albus, aureus, and citreus, for M. tetragenus, for Strep- 

 tococcus hemolyticus, and for pneumococci of Groups I, II and III. 



This same Shiga-bacteriophage, without the passages with the menin- 

 gococcus was avinilent for all of these different organisms. 



The virulence of a race of bacteriophage is not fixed. No race is 

 immutable; all vary continually. Some of them lose virulence, al- 

 though very slowly, and others acquire virulences by adaptation. We 

 will see that this behavior of the bacteriophage, here demonstrated 

 experimentally, is but a reproduction of processes occurring in the same 

 way in nature. 



12. BACTERIOPHAGY IN MIXED CULTURES 



If one introduces a bacteriophage possessing multiple virulences into 

 a bacterial suspension prepared by mixing cultures of several different 



* A race which I sent him; indeed, one of the races which is mentioned here in 

 a number of the experiments. It had undergone, at the time when it left my 

 laboratory, about 1200 passages with B. dysenteriae. 



