154 THE BACTERIOPHAGE AND ITS BEHAVIOR 



corpuscles of each of the three races. And since virulence, by common 

 consent, is considered as an expression of the power of multiphcation 

 in a foreign host, the respective virulences of the three bacteriophage 

 races above could be represented by the ratios : 



26 540 7600 . ^ , , „« .„^ 



26 * "26" ■ "26~ " approximately 1 : 20 : 300 



If the increase in the number of bacteriophage corpuscles taking place 

 during bacteriophagy is determined for several bacteriophages of dif- 

 ferent virulences it becomes obvious that the numbers derived will be 

 proportionate to the rate of bacteriophagy with each. Such a deduction 

 is, of course, obligatory. The contrary could not be comprehended, 

 since insofar as the study of bacteriophagy is concerned the single meth- 

 od permitting the recognition of the behavior of the bacteriophage con- 

 sists in determining the virulence by means of the increase in the number 

 of corpuscles. From the practical point of view it is convenient to 

 designate the intensity of the virulence of a bacteriophage by the rate 

 of the phenomenon which it causes. Such a procedure is, indeed, quite 

 logical since there is always a parallelism between the rate at which 

 the corpuscles increase and the rate of bacteriophagy. 



Different degrees of virulence may be designated by the following 

 terms: the determinations being made always in bouillon having a pH 

 of 7.6 to 8.0, and the temperature being between 30 and 32°C. 



Maximum virulence. Single corpuscles inoculated into 10 cc. of a 

 normal suspension (each cubic centimeter containing 250 million bac- 

 teria derived from an 18 to 24 hour culture on agar) cause complete 

 bacteriophagy. The suspension of the bacteriophage resulting remains 

 clear indefinitely provided the suspension is held at a temperature lower 

 than 32°C. This virulence corresponds to a multiplication of such 

 an order that for each corpuscle inoculated the final number is greater 

 than 10,000 million per cubic centimeter. It may be mentioned that 

 this number is apparently fLxed, that is to say, it does not apply to 

 bacteriophagy of any particular bacterial species. Whatever the race 

 of bacteriophage this titre of multiplication is that of a race of maximum 

 virulence. 



Very high virulence. Here the phenomenon is the same as in the above 

 case, except for the fact that the suspension of corpuscles resulting from 

 bacteriophagy when a normal suspension is inoculated with a single 

 corpuscle becomes clouded again through the development of a sec- 

 ondary culture. On the other hand, secondary cultures do not usually 



