342 THE BACTERIOPHAGE AND ITS BEHAVIOR 



rather a variation in the bacterium, which "adapts itself" to produce 

 variable amounts of the "lytic principle?" The obvious reply to this is, 

 that proof being adduced that the bacteriophage corpuscle is a being 

 independent of the bacterium, such an objection falls ipso facto. But 

 there is no necessity to dispose of the question in this summary 

 manner. 



If a bacteriophage of low virulence is inoculated into a bacterial 

 suspension, and if a series of cultures is made, transferring each time a 

 trace of the preceding culture into some sterile bouillon, without either 

 filtration or heating between each passage, will we obtain an increase in 

 the virulence of the bacteriophage? Not at all. Everyone is quite 

 agreed on this point, I believe. And yet, if the conception of Seiffert is 

 correct, under these conditions the bacterium should accustom itself to 

 produce a more and more potent "lytic" principle. 



What is the procedure for obtaining an increase in the virulence of the 

 bacteriophage corpuscle? Simply effecting serial passages, taking care 

 to introduce in each succeeding suspension no bacteria from the preced- 

 ing suspension which have been in contact with the bacteriophage. This 

 we accomphsh either by filtering the suspension where bacteriophagy has 

 occurred, or by heating it at a temperature sufficiently high to destroy 

 the bacteria. Under these conditions we find that with each new pas- 

 sage the virulence of the bacteriophage increases. 



Wherein is the logic in speaking of an adaptation of the bacteria, 

 when, with each passage, the bacteria of the preceding suspension which 

 have been in contact with the bacteriophage are systematically dis- 

 carded? Only the corpuscles suspended in the filtrate are carried along 

 in the passages, only they may possibly become accustomed, — adapt 

 themselves. 



But why, indeed, are we obliged to filter (or heat, although this is far 

 less satisfactory) between each passage? Because the bacterium also, 

 like all hving beings, possesses the power of adaptation. But far from 

 adapting itself to elaborate a ferment directed toward its own destruction 

 its adaptation consists in the acquisition of a resistance to the action of 

 the bacteriophage. 



An abundance of experimental data demonstrates this process of 

 bacterial adaptation, and it shows with equal clearness that the result 

 of the adaptation is exactly the opposite of the production of an "auto- 

 lytic" principle. It is the development of a resistance to bacteriophagy. 

 And meanwhile, the bacteriophage is adapting itself to an attack on the 

 bacteria. 



