492 THE BACTERIOPHAGE AND ITS BEHAVIOR 



yard where for about a month there had been an infection presenting 

 the characters of typhosis. From specimens of the feces taken from 

 two healthy animals living in the same yard two races of bacteriophage 

 were isolated, one showing a low virulence (+) for B. pfaffi, the other 

 showing no activity for this bacillus. Towards the end of the month 

 three chickens became sick, recovered after an interval of two or three 

 days, and then the epizootic ceased. Six specimens of feces examined 

 at this time all showed a bacteriophage of high virulence (+ + +) for i?. 

 pfaffi. Against B. gallinanim four were inactive and two showed a 

 weak virulence (+)• 



B. pfaffi was therefore the cause, for when the epizootic broke out 

 three months later the eighty chickens which had survived received a 

 subcutaneous injection of 0.5 cc. of a suspension of the Pfafh-bacterio- 

 phage and the epidemic stopped abruptly and permanently from the 

 time of the injection. We will see later that this abrupt cessation is the 

 rule following immunization by means of a suspension of the 

 bacteriophage. 



These facts can be explained in only one way. A weak or moderate 

 activity of the intestinal bacteriophage for the pathogenic bacterium is 

 sufficient to render the animal resistant to infection. The pathogenic 

 bacteria which are able to penetrate into the intestine are destroyed 

 before they can multiply. But it is not the same once the disease has 

 appeared and the organism is invaded. The animal recovers — and 

 recovery is very rare in typhosis — ^ouly because of a rapid adaptation 

 of the bacteriophage and the acquisition of a high virulence which leads 

 to an intensive destruction. This bacteriophage with exalted virulence 

 is distributed broadcast with the excreta of the recovered or convalescent 

 animals, and persists, indeed, during several months after recovery. 

 This bacteriophage is necessarily ingested by the other animals of the 

 barn-yard which become, in fact, "infected" by an extremely active 

 bacteriophage and by this means acquire a complete protection against 

 the disease, in spite of the presence of the pathogenic organism in the 

 environment, and in spite of its frequent ingestion, an ingestion which 

 serves to maintain the virulence of the bacteriophage. 



These hypotheses are not simply idle speculation, for the interpreta- 

 tion given to these observed facts is confirmed by experiments which 

 provide, in a controlled manner the natural conditions of the epizootic. 

 Furthermore, it will be seen that the role of defense assigned to the 

 bacteriophage is confirmed by the immunization of several thousand 

 animals by the administration of suspensions of an active bacteriophage. 



