THE BACTERIOPHAGE IN DISEASE 187 



The increase in the virulence of the bacteriophage for the in- 

 vading 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 ultra- 

 microbial bacteriophage.) In accordance with the chemical and 

 physical state of the intestinal contents, one bacterium is favored 

 at the expense of another; the intestinal fermentations, and as a 

 result, the reaction of the medium will vary according to the 

 predominating flora. The development of the bacteriophage is 

 then doubly influenced, first, by a change in the state of the medium 

 itself, and second, by changes in the flora which increase or de- 

 crease, according to circumstances, the bacterial species at the 

 expense of which it normally develops. This of course, necessi- 

 tates variations in virulence in response to the variation in the 

 bacterial species. Moreover, it has been known for a long time 

 that catarrhal diarrhea affects (provoked by the ingestion of un- 

 digestible foodstuffs, of green fruits in particular, or by the "froid 

 au ventre' ' so common in tropical countries) the incidence of 

 certain intestinal diseases — dysentery and cholera among others. 



Second, as a result of a more or less marked degree of resistance 

 to the bacteriophage of the invading bacillus. We have seen 

 that in the course of the disease the pathogenic agent defends 

 itself. Such a bacillus in a state of resistance, ingested by a 

 healthy person will develop in spite of the presence of a bacterio- 

 phage, particularly if the latter is but slightly active, whereas a 

 non-resistant bacillus is destroyed without a struggle. 



In cases of bacillary dysentery, even very severe, but in which 

 the patient improves rapidly, the bacteriophage manifests its 

 presence in a very active manner at the outset, not only for labora- 

 tory strains of the bacillus, but for the strain secured from the 

 patient himself, and this takes place at the moment when the 

 symptoms begin to improve. There may be a rapid increase in 

 the virulence of the bacteriophage without a corresponding resist- 

 ance in the bacterium. 



In cases where the disease is prolonged, two cases may be 

 considered: 



