VIRULENCE OF THE BACTERIOPHAGE 97 



It is manifestly impossible to make a complete analysis of a 

 strain of bacteriophage, for to do so would necessitate a determina- 

 tion of its activity against all known, and even against unknown, 

 bacterial species. With a given filtrate prepared from the in- 

 testinal contents, we can affirm that a bacteriophage is found 

 there at the moment of testing because of its activity manifested 

 toward a given bacterium. On the contrary, it is not possible 

 to conclude that none is present simply because the tests were 

 negative. Investigating the activity of the intestinal bacterio- 

 phage in a nitrate from the feces of a healthy person a negative 

 result has been obtained in testing against the intestinal bacteria 

 toward which it was expected an activity would be evidenced. 

 The investigation was extended to the most varied bacterial 

 types, and finally a strain of bacteriophage was isolated active 

 against an organism of the Salmonella group (bacillus of hog 

 cholera). This strain of bacteriophage was cultivated in series 

 and an active bacteriophage was thus secured. 



A given strain of bacteriophage will vary from time to time, 

 either in the body, as can be demonstrated by isolating the bac- 

 teriophage each day from the feces of a patient during the course 

 of the disease and during convalescence, or it may vary in vitro, 

 as has been shown in the preceding chapter. 



All combinations of virulence are possible, both as to quantity 

 and to quality; that is to say, in the extent of the action against 

 varied bacterial species, and in the intensity of the action for 

 each of the bacteria attacked. It can be readily seen, in view of 

 the infinite number of combinations possible, that two strains 

 of bacteriophage identical in all respects can not exist. 



PERSISTENCE OF VIRULENCE 



The faculty which a strain of bacteriophage possesses to return 

 to parasitism with a bacterium persists throughout a very great 

 number of passages in vitro along with a bacterium of another 

 species. For example, in 1916 a bacteriophage was isolated which 

 was extremely active for B. dysenteriae Shiga, of but average viru- 

 lence for B. coli, and but slightly active for B. typhosus and the 

 paratyphoid organisms. This strain, which has been used in 

 many experiments, was subjected during the years 1916, 1917, 



