CHAPTER V 



Resistance of the Bacteria 

 1. secondary cultures 



Throughout the first three chapters we have studied the typical 

 phenomenon of bacteriophagy as it is brought about through the action 

 of powerful races of the bacteriophage. Here the bacteriophage cor- 

 puscles develop in proportion as the bacteria become dissolved and when 

 the process is once completed that which was a few hours previously a 

 suspension of bacteria has become a suspension of bacteriophage cor- 

 puscles, free of all bacteria. We have also seen that the power of pro- 

 voking bacteriophagy varies among races of the bacteriophage. For 

 some of them the power to reproduce at the expense of the bacteria is 

 considerable, for others it is less, and this reproductive capacity cor- 

 responds exactly to a virulence more or less great. As a rule each race 

 of the bacteriophage will attack bacteria belonging to different species 

 and will have for each of them a different degree of virulence, capable of 

 being increased by adaptation. 



But there is another aspect of the phenomenon. In biology there is 

 a general law to the effect that all living beings react against an agent 

 which attacks them. If we consider the bacteria, endowed with 

 virulence for some animal species, we do not know of any whose viru- 

 lence can be such that an opposing immunity may not possibly be 

 acquired.* The natural law of resistance comes into play in the same 

 way when a bacterium is attacked by a bacteriophage. Bacteria have 

 the capacity to resist, and may thus acquire an immunity to the bacter- 

 iophage corpuscles which attack them (d'Herelle^^^). 



There are races of the bacteriophage of such virulence that under cer- 

 tain determined conditions of temperature and of medium the suscep- 

 tible bacterium is always overcome. Resistance is impossible. Bacteri- 

 ophagy is then complete and the medium becoming clear remains so 

 indefinitely. 



I have obtained, by repeated selection of the most apt corpuscles, 



* This has occurred and will occur in the future. The inescapable result of 

 the possession by a bacterium of an "absolute virulence" for an animal species 

 means the disappearance of this animal species within a short period of time. 



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