THE BACTERIOPHAGE AND THE BACTERIUM 69 



EVALUATION OF THE DEGREE OF VIRULENCE 



As will be shown in Part II of this monograph, it is necessary 

 to the study of the processes of immunity associated with the 

 presence of the bacteriophage, to be able to measure as exactly 

 as possible the degree of virulence possessed by each strain of the 

 ultramicrobe. The intensity of the action upon a bacterial sus- 

 pension, or on a culture, in a liquid medium gives an indication 

 of the virulence. A strain of maximum activity causes complete 

 lysis in a relatively short period of time, varying between three and 

 thirty hours. A less active strain causes only a partial lysis. 

 This method of evaluation is, however, very crude and subcultures 

 upon agar provide more precise determinations, particularly 

 when dealing with strains which are but slightly active. 



If we introduce into a bacterial suspension a drop of a filtrate 

 containing a bacteriophage active for the bacterium in the sus- 

 pension, and if we plate upon agar a drop of this material after 

 variable periods of incubation, it is possible to follow the multipli- 

 cation of the ultramicrobes. It is only necessary to count the 

 isolated colonies, which assume, as we have seen, the form of 

 circular plaques. If working with two or more strains of the 

 bacteriophage, it is thus easy to follow the relative rapidity of 

 their multiplication, and by the same fact, to measure their 

 respective powers of growth at the expense of the bacteria para- 

 sitized, that is to say, their virulence. 



The extension of the plaques furnishes a second measure of the 

 rapidity of the multiplication of the ultramicrobes. Each plaque, 

 representing a colony, results from the extension into the culture 

 of the descendants of a single ultramicrobial element, deposited 

 during the plating, at the expense of the bacteria in its environ- 

 ment. The more rapid the multiplication of the bacteriophage 

 the more rapid the extension of the plaque. Thus, the diameter 

 of the plaque permits a valuation of the degree of virulence of the 

 bacteriophage which produces it. 



Experiment XVI. The relative virulence of four strains of an anti- 

 typhoid bacteriophage taken from a single patient convalescent from 

 typhoid fever (Jeanne Del ) at different periods during this convales- 

 cence is determined. 



