VIRULENCE OF THE BACTERIOPHAGE 173 



species. This affords, then, mathematical proof that multiple viru- 

 lences are really attributes of bacteriophage corpuscles.* 



10. THE MECHANISM OF THE PERSISTENCE OF VIRULENCE 



A bacteriophage which has received more than a thousand passages 

 with B. dysenteriae has a relatively weak action upon the typhoid bacil- 

 lus. This can be demonstrated by spreadings made on agar, with sub- 

 sequent observation of the formation of characteristic plaques. If we 

 introduce into a tube of bouillon about 10 drops of an anti-dysentery 

 bacteriophage and then a small amount of typhoid culture we secure, 

 after incubation for 18 to 24 hours, an apparently normal culture of B. 

 typhosus, but if it is spread upon agar a few plaques are obtained. 



This finding provides us with additional information as to the 

 mechanism of the virulence of the bacteriophage. For here we have 

 introduced into the suspension of B. typhosus several thousand million 

 bacteriophage corpuscles, all virulent for B. dysenteriae. Each plaque 

 to develop on the agar, upon which the suspension of B. typhosus in- 

 oculated with the Shiga-bacteriophage is spread, represents a colony 

 derived from a corpuscle virulent for B. typhosus. And this shows that 

 among several billions of corpuscles virulent for B. dysenteriae there are 

 only a few which are also virulent for B. typhosus. 



Furthermore, by such a procedure it can be shown that the number of 

 corpuscles having a virulence for B. typhosus hardly varies throughout 

 the series of passages with B. dysenteriae, for the number of plaques ob- 

 tained by spreading a suspension of typhoid bacilli inoculated with the 

 suspension of bacteriophaged dysentery bacilli is approximately the 

 same, whether the bacteriophage has undergone 50, 100, 500, or 1000 

 passages, always with B. dysenteriae. Nevertheless, the number of 

 corpuscles active for B. typhosus diminishes, although the aptitude 

 for the bacteriophagy of B. typhosus is lost but very slowly. After 

 1500 passages the virulence was so weak that spreadings upon agar no 

 longer showed plaques, yet plaques appeared after only two passages. 

 After 17 passages with B. typhosus the dissolution of a normal suspension 

 of typhoid bacilli was complete. 



* At least, if it is understood that the bacteriophage is not derived from the 

 bacterium itself. This is a point which we will examine in another chapter, but 

 I believe it wise to state here that there is very conclusive evidence which has 

 not been discussed or contradicted by anyone, which demonstrates that the 

 bacteriophage corpuscle is autonomous, independent of the bacterium which is 

 subject to its action. The evidence supporting this will be presented when we 

 discuss the question of the nature of the bacteriophage. 



