92 THE BACTERIOPHAGE AND ITS BEHAVIOR 



shows that the bacteriophage corpuscle is able, on a solid medium, to 

 dissolve bacteria only if they are found in an extremely thin layer on 

 the surface of the substratum. This is precisely the reason that the 

 plaque, already formed when the bacterial growth has hardly become 

 perceptible, no longer increases in size; the surrounding bacterial 

 layer has become too thick. 



To summarize all of this, the formation of the plaque, and its extent 

 if its formation is possible, depends upon a whole series of factors, of 

 which two appear to be ; first, the greater or less facility with which the 

 products resulting from the action diffuse into the substratum, and 

 second, the respective powers of multiplication of the bacteriophage 

 corpuscle and of the bacterium at the temperature at which the phe- 

 nomenon takes place. 



5. THE CHARACTERS OF PLAQUES 



It is of interest to note further some of the characters of the colonies of 

 the bacteriophage corpuscle on agar. Let us take, as an example, a 

 very active race of Shiga-bacteriophage, although any other race could 

 be taken, acting upon any bacterial species whatever, for in all cases, the 

 formation and the behavior of the plaques are identical. The only 

 variant is the extent; the diameter of the plaque. 



Observation shows that in general, the size of the plaques formed with 

 different bacterial species is the smaller the more rapidly the bacterium 

 grows upon agar and the thicker the layer of growth becomes. Even 

 with extremely active races of the Staphylo-bacteriophage the plaques 

 are always small ; their diameter (for the races studied) does not exceed 

 1.5 mm. The plaque of the Shiga-bacteriophage, on the contrary, may 

 reach a diameter of 8 mm. But even here uniformity does not obtain, 

 for against a single bacterial species the size of the plaque varies. With 

 all other conditions the same, the size of the plaque is related to the 

 race of the bacteriophage which is acting. We will see the cause for 

 this in the next chapter. 



When the surface of the agar remains bare because of the large num- 

 ber of bacteriophagous corpuscles and maintains this appearance 

 indefinitely it has become unsuited for the cultivation of the Shiga 

 bacillus. When inoculated at such a time with a culture of this bacillus, 

 even in a very abundant sowing, not the slightest development can be 

 detected. The medium is, however, normal for another bacterium. 

 If inoculated with the cholera vibrio, for example, the growth will be as 

 luxuriant as if planted upon fresh medium. Hence, if B. dysenteriae 



