VIRULENCE OF THE BACTERIOPHAGE 139 



planted upon agar yields many plaques. This can only mean a qualita- 

 tive difference. The many corpuscles present in the second case are 

 not as powerful as the single very active corpuscle. 



The second observation bearing upon the idea of a quahtative dif- 

 ference deals with plaque formation. Each plaque has its origin in a 

 single corpuscle. The plaques vary in diameter with different races of 

 the bacteriophage, and we know definitely that the formation of large 

 plaques corresponds to races of the bacteriophage which cause a com- 

 plete dissolution of the suspension, while those which do not dissolve the 

 bacterial suspension completely yield little plaques. The more potent 

 the corpuscle, the greater the area of the plaque (d'Herelle^'^). 



The idea of quahtative variation among races of the bacteriophage 

 receives additional support from the fact that everyone who has studied 

 the phenomenon is in agreement upon this point. 



But what is the real reason for this variation in activity among diff- 

 erent races of the corpuscular bacteriophage? The following experi- 

 ment contributes the answer.'^^ 



A. Ten cubic centimeters of a suspension of Shiga bacilh are inocu- 

 lated with 1 cc. of a filtrate made directly from the feces of a patient with 

 dysentery. The suspension is held at 37°C. Counts of the corpuscles, 

 made at different times during the incubation, give the following results 

 when 0.01 cc. is plated on agar. 



When plated immediately, IG plaques develop, representing 1600 

 corpuscles per cubic centimeter. The filtrate from the feces therefore 

 contained 16,000 per cubic centimeter. 



After one and one-quarter hours, the count is 40 plaques, or 4000 

 per cubic centimeter. 



After two and one-half hours, a 1:10 dilution gives 42 plaques, or 

 42,000 per cubic centimeter. 



After three and three-quarter hours, a 1:100 dilution gives 18, or 

 180,000 per cubic centimeter. 



After five hours, a 1:1000 dilution gives 4, or 400,000 per cubic 

 centimeter. 



After fourteen hours, the dissolution is not complete, the medium is 

 cloudy and becomes more and more turbid, so that after forty-eight 

 hours it is very turbid. Here there is an abundant culture, but the 

 solution is never complete. The bacteria have, then, acquired a certain 

 resistance which has allowed them to reproduce in spite of the presence 

 of the bacteriophage. 



A result of this kind is usual when the filtrate is prepared from a stool 

 taken shortly before the manifestations of convalescence appear. 



