THE BACTERIOPHAGE IN DISEASE 215 



favoring the one or the other of the two germs. The sequence in 

 which the events of the struggle unfold determine the issue. 



Conclusions 



The observations made in natural disease and the experiments 

 which confirm the deductions which these observations suggest, 

 show that the bacteriophagous ultramicrobe 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 viru- 

 lent or avirulent, and in the first case its virulence may be ex- 

 ercised according to a scale which passes from the smallest degree 

 capable of detection to one of extreme activity. 



Virulence of the bacteriophagous ultramicrobe for B. galli- 

 narum 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 patho- 

 genic bacterium are by this very fact protected against the dis- 

 ease, and they remain so, provided the actual or latent virulence 

 is maintained at a level sufficiently high to effect a rapid destruc- 

 tion of the pathogenic bacilli ingested. 



The ingestion of pathogenic bacilli 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 pathogenic bacterium to disappear, 

 I would place as most significant the introduction into the or- 

 ganism of bacteria endowed with 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 bacteria. Another possible factor, influencing the activity 

 of the bacteriophage 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 has already been shown for lysis in vitro. 



