20 THE BACTERIOPHAGE AND ITS BEHAVIOR 



By the words "normal suspension" as applied to bacteria should 

 be understood ''a suspension containing 250,000,000 bacteria per 

 cubic centimeter" prepared from a young agar culture of the bacterium. 



In the course of this text I will often have occasion to speak of "a 

 strain" of such and such a bacterium and likewise of "strains" of the 

 bacteriophage. In order to avoid repetition and possible confusion 

 I will make use of the word "strain" in its usual sense as appHed to 

 bacteria, and in treating of the bacteriophage the word "race" will 

 be used. 



In order to avoid needless circumlocution, in designating the origin 

 of a bacteriophage I shall precede the word bacteriophage by the name 

 of the bacterium which this bacteriophage parasitizes within the 

 body or from which it has been isolated, or against which it manifests 

 its virulence. For example, a "Shiga-bacteriophage" is one which 

 was originally isolated from a case of dysentery due to B. dysenteriae 

 Shiga or it is a bacteriophage virulent for this organism. A "Staphylo- 

 bacteriophage" was isolated from a lesion caused by the staphylococcus. 

 A "Cholera-bacteriophage" was originally derived from a case of 

 cholera. A "Plague-bacteriophage" was derived from a convalescent 

 from plague or from an animal which had resisted this disease. Or, 

 failing this immediate du-ect connection through origin, they may be 

 races of the bacteriophage virulent for the staphylococcus, for Vibrio 

 cliolerae, or for B. pestis. This scheme will be followed in speaking of 

 aU races of bacteriophage. 



I devote these few paragraphs to the matter of terminology in order 

 to facihtate the discussion of the facts to be presented, and, I hope, to 

 aid in their comprehension. It is impossible to be too careful in 

 treating a subject of such extreme complexity, where we have to con- 

 sider always the simultaneous actions and reactions of two Hving beings, 

 and often of three, when we include the evolution of the bacteriophage 

 in nature. And the subject is still further comphcated in that it deals 

 with rudimentary beings; with those whose adaptations to the conditions 

 of the moment are rapidly effected. This fact already dominates the 

 study of bacteria, since from our point of view as human beings, that 

 which interests us most in these other beings is their "virulence" and 

 this is inherently variable. But this faculty of adaptation, one of the 

 principal characteristics of life, is even yet more marked in the bac- 

 teriophage, since this is an elementary living being. 



