THE BACTERIOPHAGE IN DISEASE 175 



= no virulence toward a given bacterium. Normal 

 cultures of the bacterium develop in bouillon 

 or on agar, whatever the quantity of the fil- 

 trate from the feces which had been added. 

 + = weak virulence. The growth in bouillon of 

 the bacterium to which the filtrate has been 

 added is apparently normal. Transfer of this 

 culture to agar gives, after incubation, a culture 

 layer showing a few minute plaques. Some 

 of the bacteriophagous germs have therefore 

 attacked the bacteria and have formed colonies. 

 + + = medium virulence. The culture of the bacterium 

 to which the filtrate has been added is almost 

 normal in bouillon. Transfers of this cul- 

 ture to agar give, after incubation, either a 

 culture layer of the bacterium studded by very 

 numerous colonies of the bacteriophage, pre- 

 senting an appreciable surface area, or of 

 fragments of bacterial culture because of the 

 very great number of bacteriophage colonies. 

 + + + = high virulence. Lysis of a bacterial suspension 

 is obtained but secondary cultures constantly 

 develop. The reinoculations on to agar remain 

 sterile or give only rare colonies of the bacterium. 

 + + + + = extreme virulence. The bouillon suspension 

 shows complete, and, in general, permanent 

 lysis. Inoculations on to agar always remain 

 sterile. 

 Obviously, it would be possible to establish a more detailed 

 scale of virulence. (Moreover, this has been done in the curves 

 which will be given, where the interval between no virulence and 

 extreme virulence has been subdivided into ten steps, in accord- 

 ance with the aspect of the cultures, the number of colonies of 

 the bacteriophage, and the size of the plaques, which bear a rela- 

 tion to its virulence.) Practically, the appreciation is adequate 

 with four steps, particularly in view of the fact of the extreme 

 variability of virulence in the bacteriophage in the body of a 

 single individual from one time to another. 



