CHAPTER II 



The Bacteriophage and the Bacterium 



Virulence of the Bacteriophage. Measure of Virulence. Resistance of 

 the Bacterium. Secondary Cultures. Instability of Mixed Cultures. 

 Characteristics of Mixed Cultures. Resistant Bacteria. Acquisition 

 of Resistance. Production of Anti-lysins by the Bacteria. Multiple 

 Cultures. 



VIRULENCE OF THE BACTERIOPHAGE 



In the preceding chapter we have taken note of the complexity 

 of the phenomenon under study; a complexity resulting from the 

 fact that there are simultaneously three elements which react, 

 the one upon the others, — the medium, the bacteriophage, and 

 the bacterium. Moreover, up to the present we have considered 

 only the simplest case that may be presented, a bacteriophage of 

 maximum virulence before which the bacteria always succumb. 

 Often, the issue is very different, and for two reasons. The 

 activity of the bacteriophage is not fixed, it varies along a scale 

 extending from an action barely capable of detection to a lytic 

 power most intense. The bacterium on its part is not passive; 

 it defends itself. 



We have already stated that the activity of the bacteriophage is 

 a true virulence, in the exact meaning of the word, "the ability 

 which a micro-organism possesses to develop within the body of 

 a host and there to secrete toxic substances. " Just as for 

 each pathogenic bacterial type there is a scale of virulence, so 

 also for each strain of bacteriophage there is a certain virulence. 

 It is possible to exalt or to attenuate the virulence of a given 

 bacterium and the same can be accomplished with the bacterio- 

 phage. Finally, just as higher forms when parasitized by a 

 bacterium defend themselves and are capable of acquiring an 

 immunity to this bacterium, so in like manner the bacterium 

 attacked by a bacteriophage does not remain passive, but strug- 

 gles, and may either be destroyed or acquire an immunity. All 

 the vicissitudes of a conflict between an animal and an attacking 



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