212 THE BACTERIOPHAGE AND ITS BEHAVIOR 



One finds by experiment that the stability of mixed cultures is the 

 greater as the S5Tiibiosis is of longer duration. In the course of the 

 first passages in symbiosis it is frequently found that the one or the 

 other of the antagonists is overcome, either because the resistance and 

 the virulence were not balanced or because changes in the experimental 

 conditions favored one or the other. Without a modification in the 

 growth conditions in the case of a mixed culture formed by a slightly 

 virulent bacteriophage and a normal bacterium it is usually the bac- 

 terium which survives. In the course of the subcultures the resistance 

 of the more vigorous bacteria increases gradually, they survive and 

 multiply. Sooner or later the corpuscles no longer find bacteria sus- 

 ceptible to attack, or they cease to multiply because of the lack of sus- 

 ceptible organisms, or they are destroyed. In any case after a certain 

 number of subcultures they become eliminated. 



Sometimes a perfect symbiosis develops and it is then permanent. 

 When one finds that after a dozen subcultures (effected each time by 

 introducing 0.001 cc. of the preceding mixed culture into 10 cc. of 

 sterile bouillon) the sjanbiosis continues, it is rare that it can not be 

 maintained indefinitely. Where the symbiosis is stable there is a 

 perfect equilibrium between the virulence and the resistance. 

 \y- As we have seen such symbiotic cultures may occur whatever the 

 absolute degrees of resistance and of virulence, provided the two are 

 balanced. We will see the considerable importance of this fact, siuce it 

 is not solely a test tube experimental phenomena, but one which 

 occurs also in nature.* 



* Arkwright^'' has claimed that from all cultures of intestinal bacilli it is 

 possible to isolate upon agar colonies of different appearance, some susceptible 

 to the bacteriophage, others resistant. The many experiments which I have 

 carried out upon this subject warrant me in contradicting definitely such a state- 

 ment. Indeed, the contribution of Arkwright is absolutely incomprehensible 

 and it would seem that he has no idea of what is in reality the phenomenon of 

 bacteriophagy. 



He says that among the strains of B. dysenteriae Shiga only those which de- 

 velop in the sediment and agglutinate spontaneously in physiological saline are 

 susceptible to attack by the bacteriophage. I am perfectly ready to conduct a 

 demonstration showing that all normal strains of B. dysenteriae Shiga are sus- 

 ceptible to attack by any race of Shiga bacteriophage whatever having a high 

 virulence; races which it is easy to isolate. By normal strains I mean those 

 which present the characters assigned to Bacillus dysenteriae Shiga by the Society 

 of American Bacteriologists. 



The rare strains naturally contaminated by a bacteriophage develop exactly 

 in the form of sedimented growth. They are composed of resistant bacilli and 



