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 : 



