274 THE BACTEBIOPHAGE 



They become inagglutinable. They resist phagocytosis. They 

 are endowed with a very great vitality and a very high virulence. 

 Loss in resistance is accompanied by a return to normal form 

 and properties. 



Although the bacteriophage is capable of acquiring a virulence 

 for the bacterium, the bacterium on its side is capable of acquir- 

 ing a resistance to the bacteriophage. The virulence of the one 

 and the resistance of the other are not fixed, but are essentially 

 variables, being enhanced or attenuated according to the in- 

 herited properties of each of the two germs, and according to 

 the circumstances of the moment which favor the one or the other 

 of the two antagonists. These two phenomena dominate the 

 pathogenesis and the pathology of infectious diseases. 



The bacteriophagous ultramicrobe is a normal inhabitant of 

 the intestine, an obligatory parasite, and there maintains itself 

 at the expense of saprophytic bacteria hereditarily endowed with 

 a certain resistance, with which it lives in commensalism. For 

 any bacterium whatever, pathogenic or not when introduced 

 into the intestine, the bacteriophage exalts its virulence toward 

 the invader, and this so much the more rapidly when this bac- 

 terium is lacking in resistance and when the bacteriophage is 

 hereditarily the more adapted to the struggle. The more fre- 

 quent the reinfections by a given bacterium, the more likely is 

 the bacteriophage to quickly acquire a degree of virulence suffi- 

 cient to inhibit all growth. If the bacterium which invades 

 an individual is the agent of an intestinal infection or if the 

 avenue of infection is intestinal, the infectious process is then 

 prevented at its very inception and the disease aborts before 

 morbid symptoms appear. 



The rapid adaptation of the bacteriophage may be delayed or 

 even prevented by unfavorable circumstances. More sensitive 

 than the bacteria to the action of acids and alkalies, the reaction 

 of the medium is a principal factor which influences the develop- 

 ment of the bacteriophage and its power of attack. On the other 

 hand, its activity may be annihilated if the invading germ is de- 

 rived from an organism in which it has been in conflict with the 

 bacteriophage, a conflict which has allowed it to acquire some 

 degree of resistance. In either the one or the other of these cases 



