CHAPTER II 



The Bacteriophage Corpuscle 



1. bacteriophagy upon solid media 



The phenomenon of bacteriophagy can be demonstrated, not only 

 in a hquid medium, but upon a sohd medium as well, the presence of 

 the active principle being readily revealed by the following simple 

 procedure. 



Take a young culture or a rather cloudy suspension of dysentery 

 bacilli in bouillon. Also, dilute a drop of bacteriophage fluid, that is, a 

 suspension which has been cleared through bacteriophagy, active for 

 the dysentery organism, in a liter of sterile physiological sahne. Inocu- 

 late the bacterial suspension with a drop* of this dilution of bacterio- 

 phage filtrate. The final mixture will then contain a great many bacilli 

 and very little bacteriophage. Finally, remove a drop of this inoculated 

 suspension and spread it over the surface of an agar slant or upon an 

 agar plate. After incubation the agar will be covered by a layer of 

 bacilli, but this layer presents a curious aspect, for throughout its 

 extent there will be spots, perfectly circular in form, where the agar is 

 bare, devoid of all apparent growth (d'Herelle,^'^'^). 



What do these bare areas, these ''plaques" as I have termed them, 

 mean? In the first place it is certain that they bear some relation to 

 the bacteriophage, for they never appear upon plates made with a nor- 

 mal culture of any species of bacteria. But they are always apparent, 

 and always have an identical appearance, when the agar is planted with 

 a culture or a suspension of any bacterial species previously inoculated 

 with a very minute quantity of a bacteriophagic filtrate active for the 

 bacterial species involved (d'Herelle^^^) . The phenomenon is, then, 

 not restricted to certain bacterial species. It is to be observed with 

 any species provided the homologous dissolving principle is present. 



This simple prehminary experiment shows clearly that there is a 

 causal relationship between the presence of the bacteriophage in a cul- 

 ture or suspension and the appearance of the plaques in the layer of 

 growth obtained by seeding such a suspension upon an agar medium. 



* When speaking of a drop, it is to be understood that a normal drop, 0.05 cc. 

 is meant. 



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