CHAPTER IV 



The Bacteriophage and Immunity 



summary and conclusions 



The bacteriophage, Bacteriophagum intestinale d'Herelle, 1918, 

 an ultramicrobial parasite of bacteria, normally exists in the 

 intestinfal tracts of animals, both vertebrates and invertebrates. 

 The possibility of counting the ultramicrobes is a most important 

 point in the study of the bacteriophage, for it makes it possible 

 to follow its development and to recognize its mode of action 

 in vitro and in vivo. An obligatory parasite, the bacteriophage 

 lives only at the expense of living, normal bacteria, which con- 

 stitute its sole culture medium. Experiments and ultramicro- 

 scopic examination agree in showing that the ultramicrobial 

 bacteriophage penetrates into the interior of the bacterium, there 

 forms a colony of fifteen to twenty-five elements within one to 

 one and one-half hours; whereupon the bacterium bursts and 

 liberates the young ultramicrobes. For its development the 

 bacteriophage utilizes the substance of the bacteria which it 

 dissolves by the aid of the lytic diastases which it secretes. The 

 property possessed by the bacteriophage of secreting a lysin, 

 active for a given bacterium, — that which permits it to penetrate 

 this bacterium and to reproduce there — represents, in the strict 

 sense of the word, its virulence for this bacterium. 



There is but a single species of bacteriophage, common to all 

 animals, capable of acquiring virulence for different bacterial 

 species, probably for all species. 



Just as for each pathogenic bacterium there is a scale of viru- 

 lence for a given animal, so also for each bacteriophage there is 

 an individual virulence. We are able to increase or attenuate 

 the virulence of the pathogenic bacterium, and the same phenom- 

 enon can be obtained with the bacteriophage. The parasitized 

 superior organism defends itself against the bacterial secretions 

 and is able to acquire an antitoxic immunit} r ; the bacterium at- 



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