CHAPTER IV 



The Virulence of the Bacteriophage 



1. variation in the activity of bacteriophage corpuscles 



Among the very first of the facts revealed by my early studies^^" '^^^ 

 was the observation that bacteriophage principles, as isolated from 

 natural sources, presented very considerable differences. Subsequent 

 study has afforded abundant confirmation of this. With regard to their 

 action upon a single bacterial strain different races of the bacteriophage 

 possess differing degrees of activity. Just as there are races which 

 provoke within a few hours a total dissolution of all of the bacteria 

 to be found in a turbid suspension, so also there are other races of so low 

 an activity that their presence can be detected only by the demonstra- 

 tion of the rare and minute plaques which they form upon agar. 



Early in the first chapter the technic for disclosing the presence of 

 the bacteriophage in different types of material was described. If, 

 following this technic, a series of studies are undertaken for the purpose 

 of isolating races of the Shiga-bacteriophage, for example, it will quickly 

 become apparent that when bacterial suspensions, identical except for 

 bacteriophage material, are inoculated with equal quantities of different 

 filtrates a complete dissolution of the bacteria is not always obtained. 

 The following experiment is ample to demonstrate this. 



Inasmuch as the intestinal contents of animals, — horses and fowl, in 

 particular — almost always contain a bacteriophage active against B. 

 dysenteriae Shiga,^^"' procure a dozen specimens of feces from animals of 

 these species. Prepare filtrates according to the method described 

 (Chapter I) for worldng with such materials. At the same time prepare 

 12 tubes from a young agar culture of B. dysenteriae Shiga, each tube 

 containing 10 cc. of a broth suspension having 75 million bacteria per 

 cubic centimeter.* The turbidity of such a suspension is slight, yet the 

 broth is definitely clouded. Add to each of the tubes 5 drops of one of 



* I have shown that for such a study it is preferable to use suspensions con- 

 taining but 75 to 100 million bacteria per cubic centimeter instead of bouillon 

 cultures. 



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