188 THE BACTERIOPHAGE AND ITS BEHAVIOR 



attacks the bacterium. Under these circumstances the latter does not 

 acquire a resistance since it is not attacked. Thus Scheidegger^^'^ work- 

 ing with B. coli, has shown that in a bouillon having a pH of 4.5, in 

 which this organism is still able to develop, the bacteriophage remains 

 inert, and the bacterium acquires no resistance. But if such a medium 

 is neutralized bacteriophagy takes place and the colon bacillus 

 develops a resistance. The optimum conditions for development are 

 not the same for bacteriophage corpuscles and for bacteria. Conse- 

 quently the conditions favoring the one or the other of the two antago- 

 nists determines whether the first or the second will finally win out. 



2. THE ORIGIN OF SECONDARY CULTURES 



What is the intimate mechanism of the process that results in the 

 formation of secondary cultures? A 'priori two hypotheses can be for- 

 mulated. Two factors are present, a bacteriophage whose virulence 

 may be attenuated, and a bacterium whose resistance may be aug- 

 mented. Thus, are secondary cultures due to a weakening of the activ- 

 ity of the bacteriophage, or, do there exist in the bacterial suspension 

 certain individual cells which acquire an immunity to the bacteriophage, 

 thus leading to the development of a resistant race? The following 

 experiments clearly settle the question in favor of the last hypothesis. 



In the section treating of the isolation of the bacteriophage we have 

 seen that in the large majority of cases the races which are freshly iso- 

 lated are of too low activity to effect a complete dissolution of a bacterial 

 suspension; cases where the presence of the corpuscles could only be 

 detected by the presence of plaques upon the agar slants. These same 

 races were able to acquire, by successive passages, a very high activity, 

 a potency which enabled them to bring about dissolution of very heavy 

 suspensions. This method of serial passages of the bacteriophage, in 

 which it is forced to develop in vitro at the expense of a given bacterium, 

 corresponds exactly with the method of Pasteur for effecting an enhance- 

 ment in virulence of a bacterial strain by repeated passage through a 

 given animal species. 



This single experiment, repeated a considerable number of times, — ■ 

 in fact, each time that a bacteriophage of low virulence is isolated from 

 the body^ — shows that secondary cultures are not produced by a simple 

 diminution in the virulence of the bacteriophage. Indeed, there is, on 

 the contrary, an enhancement with each passage, even if macroscopic 

 dissolution is not to be seen. For this the following experiment offers 

 direct proof: 



