BEHAVIOR OF BACTERIOPHAGE IN DISEASE 449 



terium is pathogenic for a given organism if it secretes substances toxic 

 for the cells of this organism. It is the more virulent the more capable 

 it is of development at the expense of this organism. The dysentery 

 bacilli are pathogens because of this secretion of toxic substances, for 

 they do not invade the organism, as a rule, but remain localized in 

 the intestine and in the intestinal mucosa. Nevertheless, in the case 



of the woman Desp the Hiss strain was accidentally endowed 



with an extreme virulence, and this solely because the bacteriophage 

 had been overcome. This suggests an idea which we will have occasion 

 to confirm in the following chapters, — that the virulence of a bacterium 

 at any given moment is the greater if its resistance to the bacterio- 

 phage is at this time high. 



The case Desp is exceptional. As a general rule death occurs 



in dysentery, not because of the acquisition by the bacterium of a 

 refractory condition, but by a failure of the bacteriophage to adapt 

 itself to bacteriophagy of the pathogenic organism. In the four cases 

 mentioned above which were fatal, a bacteriophage active for the Shiga 

 bacillus could not be isolated at any period of the disease. 



We may summarize the situation, insofar as bacillary dysentery is 

 concerned, in the following way. The pathogenesis and the pathology 

 of bacillary dysentery are dominated by two factors, operating in 

 different directions; the dysentery bacillus as the pathogenic agent and 

 the bacteriophage as the agent of immunity. The history of a case of 

 dysentery is only the story of the struggle, occurring within the body, 

 between these two factors, and the condition of the patient faithfully 

 reflects the vicissitudes of the struggle. 



In case of a rapid enhancement in the virulence of the intestinal 

 bacteriophage toward a pathogenic bacillus, the latter is unable to 

 develop a resistance and is destroyed in the struggle, so that the disease 

 aborts before the appearance of any symptoms or manifests itself by 

 only a transitory disturbance. 



The increase in the virulence of the bacteriophage for the invading 

 bacterium may be retarded for one of two reasons: — First, as a result of 

 unfavorable intestinal conditions. (We have seen the considerable 

 importance, in vitro, of very slight variations in the reaction of the 

 medium on the development of the bacteriophage.) In accordance 

 with the chemical and physical state of the intestinal contents, one 

 bacterium is favored at the expense of another; the intestinal fermenta- 

 tions, and as a result, the reaction of the medium will vary according 

 to the predominating flora. The development of the bacteriophage is 



