214 THE BACTERIOPHAGE 



blood of chickens dead of spontaneous natural infection. This 

 is essential because of the loss in virulence of this organism which 

 takes place under artificial cultivation. 



With chickens nos. 5 and 6 the ingestion of the pathogenic 

 bacillus caused a fatal attack of typhosis. The intestinal bac- 

 teriophage at no time manifested an activity for the causative 

 organism. In chickens nos. 1, 2, 3, and 4, on the contrary, the 

 ingestion of the same culture caused no disturbance and their 

 intestinal bacteriophage which for about a month had showed no 

 activity for the bacillus, rapidly recuperated its first activity. 

 It had, therefore, not disappeared from the intestine, although 

 its activity was no longer evident, but when it found itself again 

 in contact in the intestine with the pathogenic organism it rapidly 

 regained its potency. 



This "latent virulence'' may be maintained for a very long 

 time. In this connection I may recall the fact cited of a strain of 

 bacteriophage still possessing after three years and more than 

 1000 passages in vitro, always with the Shiga bacillus, the power 

 to attack B. coli and B. typhosus. It showed a weak power, but 

 was capable of rapid augmentation by transfers at the expense 

 of these organisms. This is exactly what this experiment shows 

 us to take place in vivo in the chicken. 



Can a chicken contract typhosis in spite of the presence of an 

 active bacteriophage in the intestine? It certainly can. As 

 we have seen in many experiments the bacterium may develop 

 a resistance to the action of the bacteriophage and this resistance 

 is one of the factors comprising the virulence of the bacterium. 

 We have then, on the one hand, the bacterium, which when in- 

 troduced into the organism may acquire a resistance to the action 

 of the bacteriophage ranging from zero to absolute resistance, 

 and on the other hand, the bacteriophage, which at the same time 

 may possess a virulence running from zero to extreme activity. 

 Infection occurs, or does not occur, according to whether the 

 algebraic sum of virulence + resistance is in favor of the one or 

 the other of the two organisms present. Once the disease has 

 manifested itself, the virulence of the one and the resistance of 

 the other become increased or attenuated according to the con- 

 ditions of the moment and the aptitudes previously acquired 



