104 THE BACTERIOPHAGE 



encountered in dealing with the bacterium in question will be 

 mentioned. As far as general characteristics are concerned, all 

 are similar, that is, what has been recorded in preceding chapters 

 regarding the method of isolation, the mode of action, the variable 

 virulence, the enhancement of virulence by passage, the resistance 

 of the bacteria, and secondary cultures, applies to all strains of 

 bacteriophage and to all species of bacteria attacked. In all 

 of the experiments mentioned up to the present time the dysen- 

 tery bacillus has been taken as an example, but it has been shown 

 that all such experiments may be repeated with identical results 

 with any strain of bacteriophage active for a definite organism. 

 B. dysenteriae presents practical advantages in experimentation. 

 It is easy to isolate a very active bacteriophage for this organism 

 and bacteriologists can readily procure strains and repeat these 

 experiments without conducting a long series of preliminary 

 investigations. 



B. dysenteriae Shiga 



For this organism it is particularly easy to isolate a very active 

 strain of the bacteriophage. The bacteriophage opposed to this 

 bacillus exists, it may be said to be normally present, in the in- 

 testinal tract of numerous animals, the horse and domestic fowls 

 m particular. It is likewise frequent in man and may acquire 

 a high virulence, not only in convalescence from an attack of 

 dysentery, but in recovery from a variety of pathologic conditions. 

 Up to the present time about 200 strains have been isolated, 

 without finding any two of exactly comparable virulence and of 

 equal extent in their action on the related bacteria of the colon- 

 typhoid-dysentery group. Among these 200 strains, one only, 

 and that of a moderate activity when isolated, has failed to act 

 upon any other bacteria of the group. 



A strain of bacteriophage active for B. dysenteriae Shiga is 

 usually active for B. coli and for B. dysenteriae Flexner and Hiss. 

 From the point of view of the bacteriophage the Shiga type of 

 dysentery bacilli represents a homogeneous species, a bacterio- 

 phage active for one strain being equally active for all others. 

 A bacteriophage very active for one strain of bacilli, at the ex- 

 pense of which it has passed through a number of passages, may 



