VIRULENCE OF THE BACTERIOPHAGE 181 



(d'Herelle,^'"). It is also possible to produce, experimentally, an atten- 

 uation in the virulence of a bacteriophage. Attenuation occurs when it 

 is subjected to an appropriate degree of heat (d'Herelle and Pozerski^''"). 

 It may occur spontaneously with the passage of time (Asheshov^^). 



The action of certain races of the bacteriophage is strictly specific; 

 they act upon but a single strain of a given bacterium. Examples of 

 this are afforded by certain races of the Typhoid-bacteriophage (d'Her- 

 elle^^^). Certain bacterial species are homogeneous with regard to the 

 bacteriophage, that is to say, when a race of the bacteriophage is virulent 

 for one strain it is also virulent for all other strains. B. dysenteriae 

 Shiga and B. pestis are examples of homogeneous species. Other bac- 

 terial species are heterogeneous, that is to say, certain strains may be 

 attacked while others are insusceptible (d'Herelle^-'). This natural 

 resistance of certain strains is not absolute inasmuch as a strain may be 

 unattacked by one race of the bacteriophage although it is attacked by 

 others (Janzen and Wolff^^"*). 



Ordinarily a bacteriophage is virulent, not only for bacteria of a given 

 species, but also for bacteria belonging to different species, sometimes 

 to organisms very distantly related (d'Herelle''^^). The range of viru- 

 lence and the intensity of virulence may differ for each race of the 

 bacteriophage and as the result of this it is impossible to conceive of two 

 races as having absolutely identical characteristics (d'Herelle^^^). 



The manner in which virulence for different bacterial species persists 

 in a bacteriophage indicates that virulence is a fairly stable property. 

 Virulence will persist throughout a very great number of passages, all 

 carried out at the expense of a single strain (d'Herelle'^^^). Multiple 

 virulences persist in spite of the fact that the race is subjected to purifi- 

 cation procedures, indicating that they are, therefore, an inherent 

 property of a race derived from a single corpuscle (d'Herelle). 



The persistence of virulence for a bacterium other than that with 

 which the passages are made is not an attribute of all corpuscles but 

 of certain ones only, and the number of these varies but slightly during 

 a series of passages (d'Herelle^-'-). 



It is possible, by experimental adaptation procedures, to cause a race 

 of the bacteriophage to acquire a virulence for an organism against which 

 it was originally completely inactive (d'Herelle^^^'^-^. 



