88 THE BACTERIOPHAGE AND ITS BEHAVIOR 



phage, acting upon different bacterial species. It is needless to detail 

 them here, for they are all comparable to the one described above. 



The experiments all agree in showing that each plaque originates in 

 a bacteriophage corpuscle which dissolves the bacteria in its environ- 

 ment. But this plaque occupies a certain area. With certain races 

 particularly active for the dysentery bacillus each plaque may attain 

 a diameter of 8 mm., all of the bacteria found within a radius of 4 mm. 

 of the corpuscle being destroyed. Is this dissolution the result of a 

 distant action of the corpuscle deposited on the agar at the time of 

 spreading, or, does the original corpuscle multiply at the expense of the 

 bacterial bodies as it proceeds with their dissolution? It is easy to sub- 

 ject this question to experimental proof and determine which of the two 

 possibilities accords with the facts. 



Touch the margin of a plaque with a platinum wire and then wash it 

 off in a tube of sterile bouillon. Prepare also 10 tubes of a bacterial 

 suspension susceptible to the action of the bacteriophage. Inoculate 

 each of these tubes with a drop of the bouillon in which the needle has 

 been washed off. After incubation, we will find that the 10 suspensions 

 have undergone bacteriophagy. There must have been, then, at least 

 one bacteriophage corpuscle in each drop of the bouillon with which these 

 suspensions were inoculated. This proves, therefore, that the surface 

 of the plaque is covered by bacteriophage corpuscles, and this, in turn, 

 means that the initial corpuscle must have multiplied. 



The plaque represents, then, a colony of bacteriophage corpuscles, 

 the issue of the original corpuscle which was the origin of the plaque. 

 Upon a solid medium, as in a liquid medium, the dissolution of the bac- 

 teria is accompanied by a multipHcation of the bacteriophage corpuscles. 



4. CONDITIONS ESSENTIAL FOR PLAQUE FORMATION 



Before considering the characteristics of agar cultures of the bacterio- 

 phage corpuscles, let us note the conditions of the medium most favor- 

 able for their development. 



First, let us state, but it is not necessary to emphasize this, that all of 

 the conditions, as discussed in the preceding Chapter which bear upon 

 the nutritive quahties of liquid media for the bacteria subjected to the 

 action of the bacteriophage, such as the reaction (pH), are entirely 

 applicable to agar media. On solid media, as in liquid media, the state 

 of the bacteria is important. Young bacteria are most readily attacked, 

 and the critical period is the moment of division. 



With regard to the consistency of the medium, I have shown^^i ^^^t 



