418 THE BACTERIOPHAGE AND ITS BEHAVIOR 



One of the most important points to be considered in the discussion 

 of the material which is to make up the remainder of this text, is that 

 we must differentiate between the degree which the diverse virulences 

 may attain by adaptation in vitro, and the degree of virulence possessed 

 by a bacteriophage in the intestine at the time of observation. For our 

 present purposes the latter is the more significant and in what follows, 

 when we refer to a bacteriophage as having a given degree of virulence 

 this will refer to the degree of virulence which the bacteriophage has, 

 or had, at the time of isolation from the stool, or other product. 



The notation adopted for recording the degree of virulence is the 

 simphfied scheme which has been described in the section deaUng with 

 "The Appreciation of Virulence." It will be remembered that, ac- 

 cording to this scheme, 



= no virulence 

 + = weak virulence 

 + + = moderate virulence 

 + + + = strong virulence 

 + + + + = maximum virulence 



When the virulence is spoken of as "weak," "moderate," etc., the 

 meaning is that which has been given to these terms in the section 

 referred to. 



2. THE BACTERIOPHAGE IN HEALTHY MAN 



In order to demonstrate that the intestinal tract of the normal healthy 

 man contains a bacteriophage I have made systematic examinations 

 on a person in a perfect state of health.^'" Throughout a year I collected 

 a specimen of stool every two weeks, and the virulence of the bacterio- 

 phage found upon each occasion was tested against strains of the follow- 

 ing bacterial species: B. coll, B. dysenteriae Shiga, B. dysenteriae 

 Flexner, B. dysenteriae Hiss, B. typhosus, B. paratyphosus A, and B. 

 paratyphosus B. Later, 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, with the same filtrates which had been held during 

 the interval in sealed ampoules, using a more satisfactory technic, the 

 bacteriophage was clearly demonstrated. As will be seen upon examin- 

 ing the table where the results are recorded, 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 



