174 THE BACTERIOPHAGE AND ITS BEHAVIOR 



It is probable that by continuing the passages the aptitude for reacting 

 with B. typhosus would have gradually diminished and finally would 

 have been completely lost. This would have represented, then, a strict 

 adaptation to the bacteriophagy of B. dysenteriae. 



As I have said above, I have repeated at different times, with diverse 

 races of the bacteriophage, experiments which show that after a series 

 of passages followed by three successive isolations the virulence of a 

 bacteriophage for several bacterial species persists. Here is such an 

 experiment. 



A race of Typhoid-bacteriophage, isolated in 1918 from the stools of a 

 convalescent from typhoid fever, manifested at its origin a strong viru- 

 lence (complete bacteriophagy of a normal suspension but with almost 

 always the development of secondary cultures) for many strains of B. 

 typhosus and B. coli, for the paratyphoids A and B, and for all strains of 

 B. dysenteriae, Shiga, Flexner, and Hiss. In 1924 I made two or three 

 hundred passages with B. coli, B. typhosus, and B. dysenteriae, alternat- 

 ing the bacterium with which the bacteriophage was placed in contact. 

 A series of a dozen passages were then made with B. dysenteriae to elimi- 

 nate avirulent corpuscles if any were present (in each passage 0.001 

 cc. of the suspension previously dissolved was introduced into 10 cc. 

 of a fresh suspension) . And finally, the race was carried through three 

 successive isolations by selection. This procedure assured two things; 

 first, that all of the corpuscles avirulent for B. dysenteriae must have 

 been ehminated by dilution, and second, that the derived suspension 

 represented the progeny of a single corpuscle. With the final suspension 

 the virulences were the same as those shown immediately after the iso- 

 lation of the race in 1918. This can only mean, therefore, that a single 

 bacteriophage possesses multiple virulences. 



In all of the experiments performed, where consideration was given 

 to this point, it has been possible to show that among the corpuscles 

 comprising a race of the bacteriophage, there are some — many or few, 

 as the case may be — which show a virulence for a bacterium of a species 

 other than that at the expense of which the passages have been made. 

 The persistence of a latent virulence is a function of certain particularly 

 apt corpuscles. In view of the very great number of corpuscles present 

 after bacteriophagy, with the law of large numbers applying to each 

 passage, it follows that the number of corpuscles presenting a latent 

 virulence for a bacterium of another species continues practically the 

 same. It is only after a very great many passages, always at the ex- 

 pense of a single bacterium, that the aptitude for bacteriophagy with 



