160 THE BACTERIOPHAGE AND ITS BEHAVIOR 



corpuscles were not present in the 1 cc. quantities which each received. 

 As for the 2 cloudy suspensions, they contain a bacteriophage whose 

 virulence is much less than that of the suspensions which had undergone 

 complete bacteriophagy.* 



It is unnecessary to state that the race of Staphylo-bacteriophage 

 utihzed was derived from a single corpuscle ; as a matter of fact it had 

 passed through multiple isolations carried out at different times either 

 by the dilution method or by the plaque method. 



This experiment shows that, considering the end result as well as the 

 rate of the reaction, among 7 corpuscles each possessed a different degree 

 of virulence. We must, therefore, conclude that in a suspension of a 

 race of the bacteriophage every corpuscle possesses a special degree of 

 virulence. The same experiment carried out with Coli-, Typho-, and 

 Shiga-bacteriophages has given in every case comparable results. This 

 finding indicates the reason for the differences in virulence of different 

 races of the bacteriophage as they are found under natural conditions. 



5. INCREASES IN VIRULENCE 



We have seen that a bacteriophage suspension, originating from a 

 single very virulent corpuscle, contains corpuscles possessing a virulence 

 equal to that of the original corpuscle which served as the source of the 

 race, together with others whose virulence is much lower. This simply 

 means that virulence may be attenuated, as is the case with bacteria. 

 But Pasteur showed that the virulence of a bacterium is capable of being 

 increased by successive passages within the body of a susceptible animal. 

 Is this likewise true for the bacteriophage? 



Again the analogy is complete. The virulence of a bacteriophage 

 may be exalted by successive passages in suspensions of a susceptible 

 bacterium (d'Herelle^^"' ^^^). There are several methods by means of 

 which a bacteriophage but slightly active at the time of its isolation 

 may be increased in virulence.^^^ For example, the following procedure 

 will result in such a change in virulence. 



When an agar inoculation has shown that a bouillon suspension con- 

 tains an active bacteriophage this suspension is filtered through infuso- 

 rial earth and then through a bougie. A slightly turbid suspension is 

 prepared, using the bacterial strain against which the bacteriophage has 

 shown some activity, and into this suspension are introduced some four 

 or five drops of the filtrate. After incubation, if dissolution has not 

 been produced, this second bacterial suspension is filtered as before and 



* They contain secondary cultures also. 



