174 THE BACTERIOPHAGE 



considerable work, but it must be applied for it is the only proce- 

 dure which will allow of a conclusion. 



We have seen in the course of the preceding chapters that the 

 virulence of a strain of the bacteriophage is rarely limited to any 

 one particular bacterial species, but exercises in general, with 

 variable intensity, its action on several species pertaining to the 

 same group or to closely related groups. 



It would be practically impossible, in view of the length of the 

 operations, to investigate all of the bacteria which may be attacked 

 by a bacteriophage isolated from the stools of a patient at any 

 given time. The procedure must, therefore, be reduced to a 

 systematic examination in each case of the virulence of the 

 bacteriophage toward the particular bacterium involved in a 

 causal relationship. The virulence should be determined for a 

 type strain of this bacterium which has been cultivated for a 

 long time in the laboratory, for the strain derived from the pa- 

 tient himself, and for B. coli. Eventually, the investigation 

 may be extended to bacteria belonging to the same group or to 

 related groups. 



We know that the virulence of different strains of bacteriophage 

 for a given bacterium is far from constant. It varies throughout 

 a scale which goes from zero to an activity such that it is suffi- 

 cient to add only a few germs to a heavy suspension of this 

 bacterium in order to obtain within three or four hours a complete 

 and permanent lysis, all the bacteria being then destroyed. Be- 

 tween these two limits, — no virulence and extreme virulence — 

 all intermediate degrees are possible. A weak virulence we have 

 seen, may be enhanced in vitro, but in so far as the study of im- 

 munity is concerned, the point in which we are interested is the 

 virulence presented by the bacteriophage in the organism at the 

 moment of observation; or the actual virulence of the bacterio- 

 phage contained in the filtrate prepared directly from the feces 

 at any given time during the disease. 



As we have also seen, the appearance of cultures of the bacterio- 

 phage in bouillon or on agar enables us to evaluate its virulence 

 for the bacterium in question. In order to facilitate explanation 

 in the further discussion of the subject we will adopt a scale of 

 virulence fixed as follows: 



