VIRULENCE OF THE BACTERIOPHAGE 169 



The strains which are not attacked possess a true natural immunity, 

 but this immunity is Hmited, not absolute. As a matter of fact, Janzen 

 and Wolff,"^ working with B. typhosus, have shown that those strains 

 of this bacterium not attacked by a certain bacteriophage are attacked 

 by others. 



It is, however, quite essential that we do not confuse this natural 

 immunity, possessed by certain bacterial strains toward some races of 

 the bacteriophage, with the immunity acquired by a susceptible bac- 

 terium in reacting to the action of a bacteriophage. 



Each race of the bacteriophage which has a virulence for a single bac- 

 terial strain belonging to a heterogeneous species possesses an individual 

 range of virulences. This range varies from one race to another. Cer- 

 tain bacteriophage races will attack only a few strains of the heterogene- 

 ous bacterium; other races will attack a large number, and there are 

 some which will attack all. With respect to the last, the bacterial 

 species will be homogeneous. 



Furthermore, each susceptible strain will be attacked with greater 

 or less intensity, that is to say, the violence of the reaction will vary. 

 Nevertheless, in these cases it is always possible to increase the virulence 

 by means of passages with the strain that was at first but slightly 

 attacked. 



Since all combinations of these variables are encountered and since 

 there are all degrees of virulence for diverse strains, and since also there 

 are differences in the intensity with which each strain is attacked it is 

 obviously practically impossible to isolate two races of the bacteriophage 

 possessing identical characters (d'Herelle^^^). 



A very typical case is that of the Staphylo-bacteriophage, where 

 races may be isolated which are virulent for but a single strain of the 

 staphylococcus. It is of some significance that the races of Staphylo- 

 bacteriophage isolated from vaccinal lymph* are of this type. On the 

 other hand I have isolated races of Staphylo-bacteriophage from the 

 pus of abscesses, undergoing resolution, which were virulent for a number 

 of staphylococcus strains. Gratia-" has isolated a race (race H) which 

 is virulent for all strains of the staphylococcus, including strains of the 



* As we will see, in some specimens of vaccinal lymph a bacteriophage-bac- 

 terium symbiosis between a staphylococcus and a bacteriophage may be found. 

 The symbiotic organisms are continuously reinoculated at the same time as the 

 ultravirus of vaccinia from calf to calf. This represents a true final adaptation 

 of the bacteriophage to parasitism of the staphylococcus with which it has been 

 associated in a parasitic relationship for a great many generations. 



