BEHAVIOR OF BACTERIOPHAGE IN EPIDEMICS 495 



In all of these experiments the infections have been made with bouil- 

 lon cultures of B.gallinarum prepared directly from the 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 bacteriophage 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 recu- 

 perated its first activity. It has, 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. typho- 

 sus. 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 introduced into the organism may acquire a 

 resistance to the action of the bacteriophage ranging from zero to abso- 

 lute resistance, and on the other hand, the bacteriophage, which at the 

 same time may possess a virulence running from zero to extreme activ- 

 ity. 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 conditions of the moment and the apti- 

 tudes previously acquired which favor the one or the other of the two 

 agents. The sequence in which the events of this struggle occur deter- 

 mine the issue. 



The observations made in natural disease and the experiments which 



