192 THE BACTERIOPHAGE AND ITS BEHAVIOR 



bacteriophage and remain susceptible to another and this quite without 

 regard to the virulence of the bacteriophage against which the resistance 

 is acquired. 



Unquestionably this observation is correct, but it is not legitimate to 

 base too broad generahzations upon it. Everything depends upon the 

 respective characters of the race of bacteriophage and the strain of 

 bacteria. In my experience, it has seemed that the matter of virulence 

 usually exerts the greater influence, but to this there are many excep- 

 tions, particularly when the bacterium involved belongs to a hetero- 

 geneous species. A bacterium which has acquired a resistance to a 

 bacteriophage of low virulence may remain susceptible to another bac- 

 teriophage of high virulence. But a bacterium which has acquired a 

 resistance to a bacteriophage of very high virulence is usually resistant 

 to bacteriophagy by other races which are of lower virulence. 



In cases where two races of the bacteriophage have approximately 

 the same degree of virulence a bacterium may become resistant to one 

 of them and remain susceptible to the attack of the other. But even 

 here, the matter of virulence often plays a role as is proved by the fact 

 that if a bacterium acquires a refractory state toward a bacteriophage of 

 maxunum virulence this bacterium usually resists the action of all other 

 races of the bacteriophage even if they are likewise endowed with a 

 maximum virulence. 



An experiment illustrative of these facts may be given. The 

 Staphylo-bacteriophage v has a maximum virulence for but a single 

 strain of the staphylococcus, Staphylococcus albus V. Staphylo- 

 bacteriophage h also possesses a maximum virulence, but its action is 

 exercised indiscriminately upon all strains of the staphylococcus. By 

 carrying out the process of bacteriophagy in a bouillon having a pH of 

 6.8, leading thus to secondary cultures, I have obtained a strain of 

 staphylococcus resistant to bacteriophage h. With this bacterium a 

 series of cultures were made in the presence of increasing amounts of 

 bacteriophage h, first incubating them at 37°C., and then at 30°C. 

 Finally a strain of Staphylococcus V was derived which was refractory 

 to bacteriophage h, that is to say, at this time the staphylococcus 

 developed in a normal manner, macroscopically, in a pure suspension of 

 bacteriophage h corpuscles in a medium with a pH of 7.8 and at a tem- 

 perature of 30°C. 



At this time this staphylococcus was found to be refractory to the 

 action of race v also, developing normally insofar as macroscopic obser- 

 vation could reveal in a pure suspension of corpuscles of bacteriophage 

 V with the mediiun of a pH of 6.8 and the temperature at 30°C. 



