THE BACTERIOPHAGE CORPUSCLE 89 



Martin's alkaline bouillon (pH 7.6 to 7.8) containing 2 per cent of agar 

 serves perfectly well for the formation of plaques. More recently 

 Nakamura^'^ has studied comparatively the formation of plaques upon 

 media containing different concentrations of agar. He found that the 

 plaques were the larger as the concentration of agar was reduced. The 

 most favorable medium was made up of bouillon containing 0.4 per cent 

 of agar. Practically, in view of the semi-fluid state of such a medium 

 which makes it rather difficult to work with, a medium composed of 

 bouillon with 0.8 to 1 per cent of agar is the most convenient. 



What is the cause of this reduction in the size of the plaque propor- 

 tionate to the increase in the concentration of agar? Certainly it can 

 not be, as Nakamura has suggested, a result of the inhibitory action of 

 the agar colloid, for if this were the case it would hardly be clear why a 

 medium containing 1 per cent would be very favorable, since the quan- 

 tity of agar colloid here is sufficient to give the medium the character of 

 a solid. Furthermore the fact that bacteriophagy takes place in a 

 liquid medium in the presence of negative colloids, such as colloidal 

 silver, and even in the presence of gelatin up to a certain concentration, 

 shows indeed that this is not the real reason. 



The stronger and stronger inhibition of bacteriophagy in media con- 

 taining increasing quantities of agar is certainly related to the con- 

 sistency of the substratum. 



Products resulting from the dissolution of the bacteria exercise an 

 inhibiting action upon the phenomenon, as is shown by the arrest of 

 bacteriophagy in a liquid medium when a certain number of bacterial 

 cells have been dissolved. Furthermore, it seems to be a general bio- 

 logical law that the accumulation of the products formed during a 

 reaction inhibits a biological process. Such products not only limit the 

 growth of organisms, but also impair the action of ferments. What- 

 ever may be the nature of the bacteriophage corpuscle, an inhibition of 

 the action which it causes, brought about through the accumulation in 

 the medium of the products resulting from this action, is, then, in con- 

 formity with all that is known. It might even be said that if such an 

 inhibition did not occur it would be unique, a new fact. 



From this it is clear that the higher the amount of agar in the sub- 

 stratum the greater is its consistency, and consequently, the slower will 

 be the diffusion into the substratum of the products formed during the 

 dissolution of the bacterial cells. If the consistency is such that these 

 products accumulate in the surface layer of the medium, there is, from 

 the beginning, an arrest of the phenomenon, and, as a result, a reduction 

 in the area of the plaque. Even its formation may be prevented. 



