VIKULENCE OF THE BACTERIOPHAGE 107 



Certain strains of bacteriophage active for B. dysenteriae "X" 

 were likewise active for other species of dysentery bacilli, others 

 were virulent for only one or two among them. When maintained 

 for several generations at the expense of B. dysenteriae "X" 

 they almost completely lose their activity for other dysentery 

 organisms. 



B. coli 



An anti-coli bacteriophage is extremely frequent in the feces 

 of normal vertebrates and invertebrates, but only exceptionally 

 is it found possessed of any considerable virulence. On the other 

 hand, in recovery from the most varied pathologic conditions 

 very active strains can be isolated. B. coli, particularly when 

 recently isolated, constitutes a heterogeneous species as regards 

 the bacteriophage. In the presence of a bacteriophage possess- 

 ing a very high virulence for certain coli strains, other races are 

 hardly touched, some are even absolutely refractory. When 

 taken from the body, B. coli always shows a degree of resistance. 

 In the intestine it forms with the bacteriophage a mixed culture. 

 On artificial culture media the resistance decreases very slowly 

 with successive transfers. 



With a colon organism of maximum resistance, that is, one 

 which is completely refractory, the colonies on agar are large, 

 white, fluent, exactly like those of the bacillus of Friedlander. 



B. typhosus 



Quite frequently a strain of bacteriophage showing a slight 

 activity for B. typhosus can be isolated from the normal intestine. 

 The isolation of a very active strain is exceptional since such are 

 found only in convalescents. A single strain of bacteriophage 

 may show a very great variation in virulence for different races 

 of B. typhosus, certain races being entirely resistant to a given 

 bacteriophage although they may be very susceptible to other 

 strains of the bacteriophage. In such a case there is a natural 

 resistance, a true natural immunity, a condition which can be 

 demonstrated not only for the typhoid bacillus but also for B. 

 coli, the paratyphoid bacilli, B. proteus, etc. It is because of such 

 reactions that these organisms are spoken of as belonging to 



