NATURE OF BACTERIOPHAGE 



353 



We have seen in an earlier section that when a number of races of the 

 bacteriophage are subjected to passages with a single strain of bacteria 

 each race retains its own characters. Again this same fact appears, since 

 each of the races, No, 1 and No. 3, retains its individuahty following 

 passages with the same strains of B. coli, B. dysenteriae, and B. typhosus. 

 We now find that the passage of a single bacteriophage with bacteria of 

 different strains does modify the characters. In this it behaves just as 

 does a bacterium, as we must recognize if we but recall that Pasteur 



Before the passages: 



Inhibition* 



Dissolution 



Plaque formation . 



After passages with B. typhosus and 

 subsequent plaque isolation: 



Inhibition 



Dissolution 



Plaque formation 



After passages with B. coli and sub- 

 sequent plaque isolation: 



Inhibition 



Dissolution 



Plaque formation 





 



+ + + + 



+ + 



4- 



++++ 



+ + 



+ + 



+ +++ 



+ + + 



+ + 



++++ 





 



++ 



+++ 



++ 



++++ 



+++ 



++ 



++++ 





 



++ 





 



++++ 



+++ 



+ 



* See the section "Appreciation of Virulence' 

 titration adopted by Wolff and Janzen. 



for the details of the method of 



showed that by passage through pigeons the bacillus of swine fever 

 became increasingly virulent, whUe it was attenuated by passages 

 through rabbits. Today we know many instances of exactly compar- 

 able facts. 



The above experiments show further that the capacity for variation 

 involves not only the actually existing virulences of different races of 

 the bacteriophage, but that it also extends to their tendency to preserve 

 their initial virulences throughout a period of development at the 

 expense of other differing bacterial species. 



Even with the bacteriophage, variability is a characteristic phenome- 



