450 THE BACTERIOPHAGE AND ITS BEHAVIOR 



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

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

 according to circumstances, the bacterial species at the expense of which 

 it normally develops. This of course necessitates 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 undigestible foodstuffs, of green fruits in particular, 

 or by the "froid au ventre" so common in tropical countries) the in- 

 cidence of certain intestinal diseases — dysentery and cholera among 

 others. 



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

 the invading bacillus to the bacteriophage. We have seen that in the 

 course of the disease the pathogenic agent defends itself. Such a bacil- 

 lus in a state of resistance, ingested by a healthy person will develop 

 in spite of the presence of a bacteriophage, particularly if the latter is 

 but slightly active, whereas a non-resistant bacillus is destroyed with- 

 out any very great difficulty. 



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

 patient improves rapidly, the bacteriophage shows its virulence in a 

 very active manner at the outset, not only for laboratory 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 resistance in the bacterium. 



In cases where the disease is prolonged, one of two situations may 

 develop. 



1. The bacteriophage shows no, or but shght, activity as long as the 

 condition of the patient remains stationary. The improvement occurs 

 when the activity of the bacteriophage manifests itself in an energetic 

 manner, not only for the stock cultures of the bacillus but also for the 

 strain derived from the patient. There has been a delay in the adapta- 

 tion, then the sudden acquisition of a high virulence. Recovery takes 

 place promptly, for the pathogenic bacterium is not able to develop a 

 resistance. 



2. At a given moment of the disease the virulence of the bacteriophage 

 manifests a more or less energetic action on the stock bacilli, but on the 

 contrary, the virulence is inappreciable or but very weak for the strain 

 taken from the patient. Here there has been a delay in the adaptation, 

 since the bacteriophage has gradually acquired virulence for the path- 

 ogenic bacillus, but this delay has allowed sufficient time for the creation 



