THE BACTERIOPHAGE CORPUSCLE 87 



3. THE plaque: a colony of bacteriophage corpuscles 



Each plaque, scattered throughout the bacterial layer upon agar, 

 represents, then, the point where, during the spreading of the suspension, 

 a bacteriophage corpuscle was deposited. This being the case, the 

 number of plaques must be strictly proportional to the number of cor- 

 puscles inoculated into the bacterial suspension. This is what the 

 experiments presented in the first section of this chapter have already 

 shown. The number of plaques, on the other hand, should be entirely 

 independent of the number of bacteria in the suspension. This is 

 shown by the following experiment (d'Herelle^^^). 



Ten tubes, each containing 10 cc. of a suspension of Shiga dysentery 

 bacilH in different concentrations— 100, 200, 300, 400, 500, 600, 700, 800, 

 900, and 1000 million bacilli per cc, — are inoculated with a constant 

 quantity of a bacteriophage filtrate, 5 X 10"". After shaking, 0.02 

 cc. is removed from each of the tubes and is carefully spread upon a 

 corresponding agar slant. After incubation, each of the 10 slants shows 

 a culture layer of bacteria, studded with plaques, and the number of 

 plaques is practically the same in all of the tubes (the actual figures 

 being— 19, 25, 20, 21, 19, 19, 22, 20, 16, and 18). 



Let us repeat this experiment, reversing the order of the factors. Let 

 us use 10 suspensions of Shiga bacilli, all of the same concentration, 200 

 milKon bacilli per cubic centimeter, and let us inoculate these suspen- 

 sions with an increasing quantity of a dilution of bacteriophage filtrate, 

 in such a way that the first tube will receive a millionth of a cc, the 

 second a 900 thousandth, the third an 800 thousandth, the fourth a 

 700 thousandth, and so on up to the tenth tube, which will get a 100 

 thousandth. Shake the tubes vigorously, and spread 0.02 cc from 

 each upon an agar slant. After incubation each of the agar slants has 

 a layer of bacterial growth studded with plaques, but the number of the 

 plaques varies with the quantity of bacteriophage filtrate inoculated into 

 the tubes. Practically the proportions are: 1:10, 1:9, 1:8, 1:7, etc., 

 up to 0,5:1; the actual numbers observed being, 4, 4, 5, 6, 8, 10, 10, 

 14, 19, 42.* 



I have repeated this experiment with different races of the bacterio- 



* The differences between the numbers observed and the numbers theoretically 

 calculated are of the same magnitude as those which we find in counting bacterial 

 colonies plated on agar from serial dilutions of a suspension. The numbers ob- 

 served approach more and more closely to the calculated numbers as the number 

 of tubes counted increases and average figures are obtained. 



