230 THE BACTERIOPHAGE 



where indicated, the tests were extended to other bacterial 

 species of particular interest. 



With the first examinations a weak activity (+), especially 

 for B. coli, was not detected or remained doubtful, but when the 

 tests were repeated after several months, using a more satisfac- 

 tory technic, the bacteriophage was clearly demonstrated. As 

 will be seen upon examining the table where the results are re- 

 corded (table 1), some of the examinations remained negative; 

 the bacteriophage appeared to be absent. Would it have been 

 the same if it had been possible to test the filtrate against all 

 of the bacteria which may be found in the intestine? An answer 

 to this question was sought. A specimen taken on July 1st was 

 inactive toward the eight species of bacteria routinely employed, 

 and it was tested against a different series of bacteria, selected 

 at random. The filtrate showed a high activity for an organism 

 of the Salmonella (hog cholera) group. When the same experi- 

 mental tests were repeated on December 1st this filtrate was 

 active for B. enteritidis. 



These two examples are sufficient to show that the absence of 

 the bacteriophage is only apparent. It must be remembered 

 that the ultramicrobe is recognized only by its activity, and con- 

 sequently its presence in a filtrate can be detected only by test- 

 ing it against a bacterium for which it has a definite activity. 

 On the other hand, it is not possible to carry out the examina- 

 tion on strains of all bacteria which may be found normally or 

 occasionally in the intestine. For this there are several reasons, 

 the chief being the difficulty of numbers, for there is no species 

 of bacteria which may not be found in the intestinal tract at one 

 time or another. We have seen further that certain bacterial 

 species are not " homogeneous" as regards the bacteriophage. 

 B. coli is of this group. Certain strains are attacked while 

 others remain unharmed. It would then be necessary to make 

 tests with all varieties of a single species; a new impossibility. 

 Finally, it is known that there exist in the intestine certain bac- 

 teria, revealed by the microscope, which it is impossible to iso- 

 late and cultivate. May the bacteriophage not live in commensal- 

 ism with these bacteria, with which they may form in the in- 

 testine ''mixed cultures?" It may even be that the impossibility 



