142 THE BACTERIOPHAGE AND ITS BEHAVIOR 



whatever the quantity of the filtrate from the feces 

 which had been added. 

 + = weak virulence. The growth in bouillon of the bac- 

 terium to which the filtrate has been added is appar- 

 ently normal. Transfer of this culture to agar gives, 

 after incubation, a culture layer showing a few minute 

 plaques. Some of the bacteriophagous corpuscles 

 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 culture to agar give, after 

 incubation^ either a culture layer of the bacterium 

 studded with very numerous colonies of the bacterio- 

 phage, presenting an appreciable surface area, or of 

 fragments of bacterial culture because of the very 

 great number of bacteriophage colonies. 

 + + + = high virulence. Dissolution 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 dissolution. 

 Inoculations on to agar always remain sterile. 

 Obviously, it would be possible to establish a more detailed scale of 

 virulence. In fact, this has been done in the curves which will be given 

 in Part III of this text, where the interval between no virulence and 

 extreme virulence has been subdivided into ten steps, in accordance 

 with the aspect of the cultures, the number of colonies of the bacterio- 

 phage, and the size of the plaques, which bear a relation 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 bacterio- 

 phage in the body of a single individual from one time to another. 



Perhaps the first method for determining virulence which we should 

 consider is that of Appelmans.^^ Objecting to the method of plaque 

 counting on the grounds that colonies of the bacteriophage upon agar — ■ 

 the plaques — are sometimes difficult to see (an ill-founded objection) 

 and that plaques may be confused with bare spots on the agar due to the 

 method of distribution of the material during the spreading (a difficulty 

 encountered only when the technic is poor) he has proposed a method 



