180 THE BACTERIOPHAGE AND ITS BEHAVIOR 



''force" — of a bacteriophage is always strictly proportional to the in- 

 tensity of its power of multiplication. The activity of a bacteriophage 

 corresponds, then, to a 'Virulence" in the strict sense of the word 

 (d'Herelle^'i^). 



A method of "titrating" the activity of a race must take the attributes 

 of the bacteriophage into consideration, in particular, the fact that it is 

 corpuscular in nature and that the corpuscle possesses a virulence. 

 Inasmuch as the activity of a bacteriophage for a particular bacterium 

 is causally related to the intensity of its power of multiplication at the 

 expense of this bacterium the only correct method of numerically 

 expressing virulence consists in a determination of the number of 

 corpuscles per cubic centimeter to be found in a suspension after bacte- 

 riophagy is complete. Results are comparable only if the figures repre- 

 senting the intensities of multiplication are derived through experi- 

 ments carried out under comparable conditions. The larger the number 

 of corpuscles found the more intense has been the development, and the 

 higher is the virulence of the race (d'Herelle). Particular emphasis is 

 placed upon this point because of its very great importance; because of 

 its lack of precision the so-called "dilution method" of counting bacterio- 

 phage corpuscles can not be used in the study of bacteriophagy. It 

 represents the greatest single cause of error appearing in the results of 

 those who have, unfortunately, employed it. 



Inasmuch as different races of the bacteriophage show varying de- 

 grees of virulence it is necessary, in experimental procedures, to work 

 with pure races, that is to say, with a race derived entirely from a single 

 corpuscle (BaiP^). There are two methods of purification which permit 

 one to obtain a pure race. The first consists in the removal of corpuscles 

 from a colony or plaque arising from the multiplication of a single cor- 

 puscle (d'Herelle^^"-^^^); the second is the method where dilution is car- 

 ried to the point where but a single corpuscle per unit volume is present 

 (d'Herelle^is-^^'*^). 



In a given suspension of the bacteriophage not all of the corpuscles 

 are endowed with exactly the same properties. Virulence, in particu- 

 lar, is variable for each one of them (d'Herelle^-^- The method of isola- 

 tion by the limiting dilution permits the selection of the most virulent 

 corpuscles as the origin of pure races. This procedure is at one and the 

 same time a method of isolation and of selection (d'Herelle). 



The virulence of a bacteriophage is variable. It can be increased ex- 

 perimentally. Increase in virulence may be obtained by serial passages 

 with a bacterium toward which one desires the virulence to be increased 



