THE BACTERIOPHAGE CORPUSCLE 97 



inoculated with a single corpuscle (d'Herelle^^°'^^^'^2i)_ j^ simple calcu- 

 lation gives then the number of corpuscles to each cubic centimeter of 

 the suspension. And these two methods of counting should agree with 

 each other, yielding comparable results. 



Applied to a specific case these two methods give the following values. 

 A suspension of the Shiga-bacteriophage is titrated by successive dilu- 

 tions, as in the experiments described previously. The last active 

 dilution is found to be 10"^". Of this 10~^° dilution 10 cc. are distrib- 

 uted, in amounts of 1 cc. each, into 10 suspensions of Shiga bacilli. 

 After incubation, we find that 5 of these suspensions have undergone 

 bacteriophagy, the other 5 have not. 



There were then, 5 bacteriophage corpuscles in the 10 cc. of the 10"^*' 

 dilution. These 5 corpuscles must have been introduced into this 10~^° 

 dilution with the cubic centimeter of the 10~^ dilution which was com- 

 bined with 9 cc. of bouillon to yield the 10"^° dilution. Manifestly, the 

 10 cc. of the 10~^ dilution contained 50 corpuscles. Continuing the 

 same reasoning for the successive decreasing dilutions to the point of 

 the undiluted suspension, we find that the latter contained 5,000 mil- 

 lion bacteriophage corpuscles per cubic centimeter.* 



Let us now take the 10"" dilution of the series of dilutions above. 

 Inoculate a normal suspension of dysentery bacilH with 1 cc. of this 

 dilution. The suspension will then contain the same quantitiy of 

 bacteriophage as the 10~^ dilution of the original bacteriophage. Spread 

 immediately, on 20 agar tubes, one drop (0.05 cc.) to each tube, 1 cc. of 

 the inoculated suspension. After incubation we find that each agar 



* More simply, the 10~' dilution contained 5 corpuscles per cubic centimeter. 

 The initial suspension undiluted contained then 5 X 10', or 5000 million per 

 cubic centimeter. 



The method for counting is the same, indeed, as that which was devised by 

 Miquel for counting bacteriji and this in turn was patterned after the procedure 

 followed by Pasteur for the purification of bacterial cultures (the dilution 

 method). Insofar as the bacteria are concerned, every tube of the medium which 

 receives at least one bacterium gives a culture and every tube which does not 

 receive one remains sterile. With the bacteriopha§,e every bacterial suspension 

 which receives at least one corpuscle undergoes bacteriophagy while the sus- 

 pensions which receive none fail to reveal the phenomenon. 



Obviously it is possible that the tubes which show a growth in the case of the 

 bacterial counts or those which show bacteriophagy in the case of bacteriophage 

 corpuscle counts may not have received a single element but, indeed, some of them 

 may have received two or even more. The result obtained is, therefore, a mini- 

 mum, a minimum which approaches more closely to the true number as the count 

 is based upon a large number of tubes. 



