248 THE BACTERIOPHAGE AND ITS BEHAVIOR 



and in man in the pus of buboes at the period of suppuration (d'Her- 

 eUe«2i. 322)^ 



2. HETEROGENEOUS SPECIES 



1. B. typhosus (Eherthella typhi) 



Among the heterogeneous species B. typhosus is the one which is 

 really the best known insofar as its behavior toward the bacteriophage 

 is concerned, thanks to the work of Janzen and Wolff. 



It may be stated, however, that insofar as the phenomenon of bac- 

 teriophagy itself is concerned, including the nature of the resistance of 

 the bacterium, there is nothing to be added nor need we change anything 

 that has been said in the preceding chapters. In these respects B. typho- 

 sus behaves in exactly the same manner as B. dysenteriae. 



We have seen that for homogeneous species any bacteriophage race 

 virulent for one strain is likewise active upon all others provided the 

 strains are normal, that is, provided they present all of the characters 

 of the species. With regard to heterogeneous species it not the same for 

 with the latter a given race of the bacteriophage may prove to be 

 virulent for a number of strains while other strains are absolutely refrac- 

 tory to it. Nevertheless these insusceptible strains may be subject 

 to the action of other races. ^^i 



Some races of the bacteriophage exercise an action limited to a small 

 number of strains, sometimes to but a single one, while other races 

 of the bacteriophage bring about bacteriophagy with a very large num- 

 ber of strains. 



Table 21, taken from the work of Janzen and Wolff,"^'^^^ shows better 

 than any explanation the behavior of the heterogeneous species B. 

 typhosus toward the bacteriophage. 



I would state again (see "Evaluation of Virulence") that Janzen 

 and Wolff determined the virulence of a bacteriophage in the following 

 ways: I. By the dissolution of a suspension of young bacilli in bouillon. 

 II. By inhibition or the interference with growth when sterile bouillon, 

 inoculated with the suspension of the bacteriophage, is planted with 

 the bacteria. III. By plaque formation on agar, obtained^by spreading 

 a drop of suspension I (above) . Four degrees are employed^in express- 

 ing the results of each of these tests: Weak (+), moderate (++), 

 strong (+ + +), and very strong (+ + + +). The results are read after 

 incubation at 37°C. for 24 hours. ■ ' 



Although maintaining its characters through successive passages 

 (we will later discuss this question at length) a race of the bacteriophage 



