VIRULENCE OF THE BACTERIOPHAGE 



149 



This method is certainly a good one, but it would be still more ac- 

 curate and much more simple if the number of plaques, that is to say 

 the number of bacteriophage corpuscles, was plainly stated. 



Such are the different methods proposed. Can it be assumed that 

 any of them permit a precise evaluation of the virulence of a bacterio- 

 phage? A logical method of titration must take the facts into con- 

 sideration; it must recognize the physical state of the bacteriophage as 

 well as its behavior. We know several facts which should afford a 

 basis for a proper method of titration. Thus, we know (1) that the 

 bacteriophage exists in the form of corpuscles, and (2) that it is possible 

 to enumerate these corpuscles. We also know (3) that the corpuscle 

 possesses a "virulence" in the true sense of the word, that is, it possesses 



SPECIMEN REMOVED 



Immediately .... 

 After 10 minutes 

 After 1 hour. . . 

 After 2 hours.., 

 After 3f hours. . , 

 After 5^ hours. . 

 After 8| hours. . 

 After llf hours. . 

 After 25| hours. . 

 After 7 days . . . 



DILUTIONS PLATED 



the faculty of multiplying at the expense of bacteria; and (4) that the 

 virulence of bacteriophage corpuscles is variable, the degree of virulence 

 of different corpuscles being strictly proportional to the rapidity with 

 which each race multiphes. 



From these facts, based upon the experiments detailed in the preced- 

 ing chapters, it follows that if we inoculate a series of tubes containing 

 identical suspensions of a susceptible bacterium with equal quantities 

 of bacteriophage suspensions derived from different sources, we will 

 find, after a given time, the conditions throughout being the same, that 

 the resulting bacteriophage suspensions will each contain a different 

 number of corpuscles. These numbers will be strictly proportional to 

 the virulence of the different bacteriophages.* 



* It may be noted that if it were possible to effect the same operation with a 

 pathogenic bacterium we could likewise obtain a numerical expression of viru- 



