THE BACTEEIOPHAGE CORPUSCLE 99 



In general, these two methods for the enumeration of bacteriophage 

 corpuscles do not differ materially from those employed for counting the 

 living bacteria present in a culture. The single difference resides in the 

 fact that, for the bacteria the dilutions are planted in sterile bouillon 

 or upon sterile agar, while for the bacteriophage corpuscles the counts 

 can only be made in the presence of living bacterial cells. It could not 

 be otherwise, for although the bacterium utihzes for its development the 

 nutritive substances present in the medium itself, the bacteriophage 

 corpuscle multipUes only at the expense of the living bacterium, which 

 constitutes the medium within which it multipKes. 



RESUME 



When a suspension of susceptible bacteria, inoculated with a rela- 

 tively large quantity of bacteriophage, is spread upon an agar medium 

 no further growth results: the medium remains bare indefinitely. 

 Furthermore, the surface of this agar remains permanently unsuited to 

 the development of these susceptible organisms, but if it be seeded with a 

 bacterial species insusceptible to the bacteriophage involved a normal 

 culture is obtained, just as though it had been spread upon sterile agar 

 (d'Herelle,3io. ^'i^. 321), 



A suspension, or a culture, of a susceptible bacterium, inoculated with 

 a minute quantity of the bacteriophage principle, spread upon agar 

 gives a layer of bacterial growth studded with bare spots, circular in 

 form, where the agar is free of all traces of growth. These bare spots 

 or "plaques" once formed are unchanging; they do not increase in size 

 nor are they ever covered by the surrounding bacterial growth. The 

 area occupied by the plaque has become unsuited to the growth of 

 susceptible organisms (d'Herelle^^"- ^^-' ^^^). 



The number of plaques is in direct proportion to the quantity of bac- 

 teriophage filtrate inoculated into the suspension spread upon the agar 

 (d'Herelle^i"' ^^i). 



These facts suggest the hypothesis that the bacteriophage exists in 

 the physical state of corpuscles (d'Herelle^^°). 



The corpuscular state is demonstrated by the fact that dilutions at 

 the limit of activity distributed in equal amounts among different sus- 

 pensions of the susceptible bacterium induce bacteriophagy in certain 

 of these suspensions while the phenomenon fails to take place in others. 

 Either bacteriophagy occurs, or it does not occur; there is no intermedi- 

 ary stage. This proves, not only from the biological point of view, 

 but from the point of view of physics as well, that the bacteriophage is 



