140 THE BACTERIOPHAGE 



The inhibitive effect is exercised only against the strain of 

 bacteriophage which has been used to inject the animal in the 

 preparation of the antiserum. 5 On the other hand we know that 

 strains of the bacteriophage differ one from the other only in 

 the virulence which they have acquired, by adaptation, for this 

 or that bacterium. It follows that the lysin secreted by the bac- 

 teriophage is different for the different bacteria attacked, since 

 the antilysin neutralized only the lysin of the strain which has 

 served in the treatment of the animal furnishing the serum. 

 Each bacterial species requires for its lysis, then, the production 

 of a specific lysin. 



Virulence for a given bacterium is, therefore, in the last analy- 

 sis, the power possessed by the bacteriophage to secrete a lysin 

 specific for this bacterium. 



The bacterial species are divided into groups. In each group 

 the species which compose it present certain common character- 

 istics. For example, we have the colon group, the typhoid group 

 (B. typhosus and the paratyphoid bacilli), the dysentery group 

 (the Shiga, Flexner, and Hiss types, etc.). These three groups 

 are indeed closely related one to another. On the other hand, 

 we see the Pasteurella group (bacteria of the diverse hemorrhagic 

 septicemias, chicken cholera, barbone, etc.), the staphylococcus 

 group, and so on. Each bacterial species requires that it be 

 attacked through the secretion of a specific lysin and the differ- 

 ence between these specific lysins will be slight in passing from 

 one bacterial species to another in the same group or in a neigh- 

 boring group. The bacteriophage adapts itself rapidly. A 

 single strain is in fact generally active for all the bacteria of the 

 group, or for the organisms of the most closely related ones. 

 On the contrary, adaptation is difficult of acquisition in passing 

 from a bacterium of one group to an organism of a remotely 

 related group. 



Furthermore, the bacteriophage normally parasitizes the 

 intestinal bacteria which constitute its habitual culture medium. 



6 It is evident that if the antibacteriophage-Shiga serum is tested against 

 closely related bacterial types, forms for which the bacteriophage would 

 have a certain activity, an inhibitive effect more or less pronounced will 

 be noted. 



