496 THE BACTERIOPHAGE AND ITS BEHAVIOR 



confirm the deductions which these observations suggest, show that the 

 bacteriophage protobe is always present in the intestine of the chicken, 

 whether it is healthy or sick, whether it lives in a locality free of infection 

 or in an epizootic zone. 



Against a definite bacterium, B. gallinarum in so far as avian typhosis 

 is concerned, the intestinal bacteriophage may be virulent or avirulent, 

 and in the first case its virulence may be exercised according to a scale 

 which passes from the smallest degree capable of detection to one of 

 extreme activity. 



Virulence of the bacteriophage protobe for B. gallinarum is only 

 observed in an infected locality. The absence of such a virulence is 

 equally the rule with animals which are about to die and with those 

 which have died. 



In a contaminated area animals which harbor in their intestine a 

 bacteriophage endowed with sufficient virulence for the pathogenic 

 bacterium are by this very fact protected against the disease, and they 

 remain so, provided the actual or latent virulence of the bacteriophage 

 is maintained at a level sufficiently high to effect a rapid destruction of 

 the pathogenic bacilli ingested. 



The ingestion of pathogenic bacilH at sufficiently frequent intervals 

 constitutes the principal factor in maintaining the virulence for the 

 given bacterium. Among the factors which contribute to diminishing 

 the virulence or causing the virulence of the bacteriophage for a patho- 

 genic bacterium to disappear, I would place as most significant the intro- 

 duction into the organism of bacteria endowed with a resistance to the 

 action of the bacteriophage. We have clearly seen this fact in the course 

 of the experimental study of the phenomenon of the resistance of bac- 

 teria. Another possible factor, influencing the activity of the bacterio- 

 phage is the reaction of the medium in the intestine, which may vary 

 according to the accidental conditions of the moment, the type of food, 

 etc. The importance of the reaction of the medium for the dissolution 

 of bacteria m vitro has already been demonstrated. 



A bacteriophage which has lost its virulence for the pathogenic bac- 

 terium lacks the power to exercise it because of the absence of this 

 bacterium, but it possesses, nevertheless, a latent virulence. When 

 placed again after a greater or less length of time in the presence of this 

 bacterium it regains its original virulence. 



The fact of the habitual virulence of the intestinal bacteriophage for 

 B. gallinarum in the infected regions indicates the frequency of the 

 ingestion of these bacilli, and consequently the excessive contamination 

 of the environment by the pathogenic organism. 



