98 THE BACTERIOPHAGE AND ITS BEHAVIOR 



tube is covered by a growth of B. dysenteriae spotted with plaques. 

 The total number of plaques is 478, distributed among the tubes in the 



following way : 



1 slant shows 19 plaques = 19 



3 slants show 20 plaques = 60 



3 slants show 21 plaques = 63 



4 slants show 23 plaques = 92 

 1 slant shows 24 plaques = 24 

 1 slant shows 25 plaques = 25 

 1 slant shows 26 plaques = 26 

 4 slants show 27 plaques = 108 

 1 slant shows 29 plaques = 29 

 1 slant shows 32 plaques = 32 



Total = 478 



The 478 ''formers of plaques" were found in 1 cc. of the lO"^ dilution. 

 Calculation indicates that the initial suspension of the bacteriophage 

 contained 478 X 10^ or 4.78 X 10^ 



The second procedure, therefore, shows that each cubic centimeter of 

 the undiluted suspension contained 4780 milHons of "plaque formers," 

 whereas the dilution method indicated that this suspension contained 

 5000 milUon corpuscles per cubic centimeter. To all intents and purposes 

 these two figures agree, hence we can conclude that each plaque had its 

 origin in a single bacteriophage corpuscle and that the method of count- 

 ing the plaques forms a means of enumeration of the bacteriophage cor- 

 puscles present in a suspension. 



I have repeated this experiment with two different races of Staphylo- 

 bacteriophage. It is unnecessary to give the protocols of these experi- 

 ments, since it would prolong this section needlessly, but both of them 

 were comparable throughout with that which has just been described. 

 I will only say that the undiluted suspension (that is, a suspension of 

 staphylococci, 250 milHon per cubic centimeter, bacteriophaged and 

 thus transformed into a suspension of bacteriophage corpuscles) of the 

 more powerful race contained 200,000 million (2 X 10^^) corpuscles per 

 cubic centimeter according to the dilution method and 121,000 million 

 (1.21 X 10^0 according to the plaque method. For the second race the 

 number of corpuscles was 6 X 10» by the first method and 8.1 X 10» by 

 the second.* 



* It is quite incorrect to assume that all suspensions of the same race of bac- 

 teriophage always contain the same number of corpuscles, indeed, the case is quite 

 the contrary. The number of corpuscles present after bacteriophagy depends, 

 as we will see in the following chapter, upon the number of bacteria bacterio- 

 phaged. The above experiments are given simply to show the agreement between 

 the two methods of enumerating the corpuscles. 



