Aug. 19, 1922] THE BACTERIOPHAGE. [ M Jlcl^ Jo-kL. 



antibacteriophagic, and not antibacterial. Therefore the bacteriophage is an 

 autonomous antigen, and consequently must be considered, in the present 

 state of knowledge, as a definite ultramicroscopic virus constituted by cor- 

 puscles, such a virus being necessarily an ultramicrobe. 



Only the hypothesis of a liltrable micro-organism which is an obligatory 

 parasite of bacteria is compatible with the foregoing facts. It now remains to 

 show that this only admissible hypothesis is not contradicted by any experi- 

 mental fact. 



Fourth Fact: The ultramicroscopic corpuscles possess a variable virulence. 



We have seen that the above ultramicroscopic organism, which we have 

 designated by the name of "bacteriophage," is constituted by living corpuscles 

 which multiply and which it is possible to count. Now, experience shows that 

 the lytic activity of these corpuscles varies from one strain to another. For 

 instance, certain strains recently isolated from the organism are unable to 

 provoke the dissolution of a bacterial emulsion, and no matter what number 

 of corpuscles are inoculated into this bacterial emulsion, one can only perceive 

 the presence of these bacteriophagic corpuscles by the production of pin-point 

 bare spaces on agar. On the contrary, with very active strains giving on agar 

 large bare spaces (4 to 5 mm. in diameter), the inoculation of one corpuscle 

 is sufficient to provoke a total dissolution of all the bacteria contained in the 

 emulsion. It is easy to prove that this difference in action is due to a difference 

 in the multiplication of the corpuscles inoculated. In the case of feeble strains 

 reproduction is slow; in the case of strong strains it is rapid. The active 

 particle, therefore, is endowed with a variable virulence, the term "virulence" 

 being taken in its real bacteriological sense of "vegetative power in vivo." To 

 explain this fact one must consider the active corpuscle as a microbe, because 

 only a microbe possesses a variable virulence from strain to strain. Moreover, 

 an enzyme only acts by its quantity; a microbe by its virulence and its toxicity 

 combined. 



Fifth Fact: By successive passages, it is possible to increase the virulence 

 of a feeble strain of bacteriophage. 



For instance, inoculate a feeble strain of bacteriophage into a culture of 

 B. typhosus; filter the mixture after incubation; introduce a drop of this filtrate 

 into a fresh culture of B. typhosus, which is filtered in turn, and so on for a few 

 passages. The virulence is found to increase little by little, and after a certain 

 number of passages the virulence becomes such that a trace of the filtrate is 

 sufficient to provoke the total dissolution of a fresh emulsion of this bacillus. 

 The technique of the exaltation of virulence is really the same as for any 

 microbe: we exalt the virulence of a bacterium for a given animal by serial 

 passages in this animal species; we exalt the virulence of the bacteriophage for 

 a given bacterial species by serial passages in this bacterium. This fact is only 

 compatible with the hypothesis that bacteriophage is a microbial parasite, 

 since adaptation is the prerogative of living beings. 



