78 THE BACTERIOPHAGE 



With each passage, therefore, we force a bacteriophage of increas- 

 ing virulence to act upon an organism of normal resistance, that 

 is, upon a bacterial suspension lacking resistance at the moment 

 when it is placed in contact with the bacteriophage. Briefly 

 stated, this technic is simply a method of opposing the develop- 

 ment of resistant bacteria by natural selection. 



THE CHARACTERS OF MIXED CULTURES 



Bacteriophage of low virulence 



The means whereby the equilibrium obtaining in mixed cultures 

 can be disturbed have been mentioned. It is of interest to allow 

 the struggle to proceed naturally and to note its issue in a medium 

 where each organism is dependent upon its own resources, that is, 

 to permit the natural selection of such bacteria as are most apt 

 in the struggle. To witness this, it is only necessary to make re- 

 peated transfers in broth without intermediary filtration or heating. 

 In cases where the bacteriophage is of low virulence in vitro the 

 bacterium usually triumphs; its resistance increases little by little, 

 the more vigorous bacilli survive and multiply, and the point is 

 reached where the ultramicrobes no longer find bacteria suscep- 

 tible to invasion. When this occurs the bacteriophage ceases to 

 multiply and gradually becomes eliminated from the culture, 

 until only a normal culture of bacteria remains. 



In some cases the state of equilibrium is more stable and the 

 mixed cultures are able to continue throughout a large number of 

 passages. 



Often these mixed cultures show cultural abnormalities and a 

 partial lysis. The medium may become turbid only to become 

 somewhat cleared later and finally to revert to a turbid condition. 



Experiment XXIII. Bouillon is inoculated with a mixed culture (B. 

 dysenteriae bacteriophage) taken from an agar slant planted thirteen 

 months previously with a secondary culture. The macroscopic appear- 

 ance passes through the following stages: after forty-eight hours, uniform 

 turbidity; after five days, almost completely cleared; after thirteen days, 

 uniformly turbid; after nineteen days, slightly cloudy with some sedimen- 

 tation; after one month, uniformly turbid with sedimentation. This is 

 the final appearance and at this time the bacterium and the bacteriophage 

 coexist in the medium. 



