96 THE BACTERIOPHAGE AND ITS BEHAVIOR 



of substratum capable of diluting this antiseptic substance, and the 

 larger will be the colony. The situation is precisely the same with the 

 bacteriophage ; the more scattered the colonies and the deeper the sub- 

 stratum, the greater the diameter. 



This is also the reason why the plaques are larger, other conditions 

 being equal, when the medium contains less agar. We know that dif- 

 fusibility in an agar substratum is diminished as the percentage of agar 

 in the medium is increased. 



6. ENUMERATION OF BACTERIOPHAGE CORPUSCLES* 



Since we have now presented the evidence proving the corpuscular 

 nature of the bacteriophage we will no longer make use of such vague 

 expressions as bacteriophage "liquid," ''fluid," or "filtrate," but will 

 employ the more precise term "suspension of bacteriophage corpuscles," 

 or even more simply, "bacteriophage suspension." A bacterial sus- 

 pension which has become limpid because the bacteria have disappeared 

 through bacteriophagy, and in which, on the other hand, the bacterio- 

 phage corpuscles have multiplied from the beginning, has, then, become 

 a "bacteriophage corpuscle suspension." 



The evidence adduced above also shows that each plaque represents 

 a colony of bacteriophage corpusclesf and experiment shows that this 

 colony originates in a single corpuscle deposited on the agar in the midst 

 of the bacteria. 



Obviously, this finding provides a method for the enumeration of the 

 bacteriophage corpuscles to be found in a suspension (d'Herelle^^'^'^^^'^-^). 



The experiments described in a preceding section suggest, moreover, 

 a second method. Since the hmiting dilution of a suspension of bac- 

 teriophage distributed in equal fractions in bacterial suspensions, causes 

 bacteriophagy in a certain number of them, the others remaining unat- 

 tacked, it follows that each bacteriophaged suspension must have been 



* We are considering here only the question of the enumeration of the corpus- 

 cles, not that which might be called the "titration" of the bacteriophage; a 

 question much more complex, the study of which is reserved for a later chapter. 



t No concept of the nature of the corpuscle is here involved. This is a question 

 which we will approach when, having accomplished the study of bacteriophagy, 

 we will consider the characters of this corpuscle. The word "colony" is em- 

 ployed because it signifies a "collection of individuals of the same type." As 

 there is on agar a collection of corpuscles of the same type, having for their origin 

 a corpuscle which has multiplied, it is evidently a colony, without predicating 

 whether the corpuscles are living or not. 



