74 



THE BACTERIOPHAGE 



phage. Indeed, there is, on the contrary, an enhancement with 

 each passage, even if macroscopic lysis is not to be seen. For this 

 the following experiment offers direct proof: 



Experiment XIX. The contents of a tube that gave a secondary culture 

 (as described on page 72) is filtered through infusorial earth and a bougie. 

 Twelve tubes of a Shiga suspension are inoculated, each receiving 0.001 

 cc. of the filtrate. Perfect lysis is seen in all tubes, and in all but one the 

 lysis is permanent. This single tube again becomes turbid after 4 days. 



From this it is clear that the bacteriophage has not lost in 

 virulence, and that secondary cultures can not be ascribed to a 

 change in that direction. The bacteriophage remains virulent, 

 coexisting with bacteria which have become resistant. The 

 secondary cultures, then, are the result of an adaptation undergone 

 by the bacterium which acquires an immunity to its parasite. 



It has already been shown that the number of ultramicrobes 

 inoculated is without influence on the appearance of secondary 

 cultures. The conflict is not one of numbers; it is rather a strug- 

 gle in which the significant factors are virulence on one side and 

 ability to resist on the other. 



Experiment XX. A suspension of B. dysenteriae, 250,000,000 per cubic 

 centimeter, is distributed into 6 tubes and these are inoculated with vari- 

 able quantities of the same bacteriophage culture. The following results 

 are obtained: 



The tubes yielding secondary cultures are distributed at ran- 

 dom throughout the series, showing no fixed relationship to those 

 tubes in which lysis was permanent. 



