200 THE BACTERIOPHAGE AND ITS BEHAVIOR 



bacteriophage corpuscles are removed from the center of a plaque on 

 agar, the corpuscles are virulent while in the periphery of the plaque, at 

 the margin of the bacterial growth an attenuated bacteriophage is 

 found. 



The reason for this attenuation is still the same. Let us recall the 

 manner in which the plaque is formed. A corpuscle is deposited upon 

 the surface of the agar in the midst of many bacteria. This corpuscle 

 parasitizes the bacterium in its immediate vicinity and multiples, and 

 young corpuscles are liberated by the destruction of the parasitized 

 organism. These freed corpuscles in their turn parasitize the bacterial 

 cells with which they come in contact and the process thus continues in 

 this manner. But during this time those bacteria which are found 

 beyond the reach of a corpuscle multiply. The bacterial layer becomes 

 thicker and thicker and consequently more and more difficult to attack. 

 If the corpuscles are very virulent, that is to say, if they are reproducing 

 actively the plaque has reached a diameter of several millimeters by the 

 time the critical period is reached, when the bacterial layer becomes 

 sufficiently dense to "suffocate" the bacteriophage corpuscles.* If the 

 bacteriophage is of low virulence this period is reached when the plaque 

 is small, simply because of the slowness with which the corpuscles have 

 multiphed. When the layer of growth has reached a certain thickness 

 the products resulting from the activity of the bacteriophage corpuscles 

 can rio longer diffuse into the agar.f We have seen already that it is 

 precisely because of the non-diffusion of the products resulting from 

 bacteriophagy that the process on agar is limited. The activity of 

 the corpuscles becomes paralyzed and the bacteria become resistant and 

 acquire an immunity. This is precisely what takes place at the periph- 

 ery of the plaque. The same conditions are to be found there as when 

 corpuscles are inoculated into extremely dense bacterial suspensions, 



* We have seen that a number of factors limit the multiplication of bacterio- 

 phage corpuscles on agar. The first of these is the thickness of the layer of me- 

 dium which to some extent regulates the rapidity with which the products which 

 result from the activities of the bacteriophage and which impede its action, diffuse. 

 Furthermore, the critical moment for the bacterium is that time when it is di- 

 viding. Division is intense when the layer on the agar is very thin, but it is much 

 less active when the layer becomes somewhat thicker for it then contains a num- 

 ber of old bacteria but slightly susceptible to attack. 



t The fact that bacteriophagy takes place perfectly well on gelatin provided 

 that the layer is very thin on a substratum of agar shows beyond possible con- 

 tradiction the effect of diffusible products upon the activity of the bacteriophage 

 corpuscles. 



