VIRULENCE OF THE BACTERIOPHAGE 155 



form in suspensions where bacteriophagy results from the dissolution of 

 a normal suspension by a large number of corpuscles (a million or more) . 

 Here the virulence corresponds to a multiplication such that for each 

 corpuscle inoculated into the 10 cc, of normal suspension, there is an 

 end result between 5000 and 10,000 million per cubic centimeter. 



High virulence. Here again the phenomenon is the same, with the 

 exception that with races of this type secondary cultures are the rule, 

 quite without regard to the number of corpuscles inoculated. The viru- 

 lence corresponds to an increase whereby a single corpuscle in the course 

 of the action becomes between 1000 and 5000 milHon per cubic 

 centimeter. 



Moderate virulence. Bacteriophagy is never complete when a single 

 corpuscle is inoculated. When the inoculation involves a million or 

 more corpuscles complete bacteriophagy of a normal suspension results. 

 The development of secondary cultures is constant. The virulence 

 here represents an increase in the number of corpuscles to such an extent 

 that for each one originally present there are at the end of the process 

 between 100 and 1000 million per cubic centimeter. 



Weak virulence. Bacteriophagy is never complete whatever may be 

 the number of corpuscles inoculated, even though the suspension con- 

 tains very few bacteria. The opacity of the suspension may be reduced 

 to a greater or less degree. If complete bacteriophagy by any chance 

 takes place the number of corpuscles is always below 100 million per 

 cubic centimeter. 



Very weak virulence. With such strains no clearing of the medium can 

 be observed. The presence of the bacteriophage can be disclosed only 

 by the development of plaques on an agar medium. The rate of in- 

 crease of corpuscles is very low ; after 24 hours only a few hundred per 

 cubic centimeter are to be found. 



3. PURE RACES OF THE BACTERIOPHAGE 



Different races of the bacteriophage may be isolated which possess 

 virulences for bacteria belonging to different species. On the other hand, 

 the degree of virulence for each of the bacteriophages possessing an 

 activity for a given bacterium is in its very nature variable. As a mat- 

 ter of fact, among the hundreds of bacteriophage races which I have 

 isolated I have yet to find two which are exactly alike. ^^^ It is, there- 

 fore, evident that there are under natural conditions an infinite number 

 of different races of bacteriophage corpuscles.* 



* This, of course, has no direct bearing upon the question of the unicity of the 

 bacteriophage; a question which will be discussed in a later chapter. 



