146 THE BACTERIOPHAGE AND ITS BEHAVIOR 



After incubation at 37°C. they note the presence or absence of plaques 

 on the plates, and designate the result by Pi, P2, P3, etc., according 

 to the number of plaques. If plaques are abundant they use the terms 

 many, or very numerous. As for the results in the fluid media; Lo 

 indicates no dissolution, Li means a slow and partial dissolution, L2 

 is a partial dissolution, and L3 impHes a complete solution. 



Here is an example cited by them, showing that a weak bacterio- 

 phage can not be measured by the dilution method. Indeed, it may 

 not even be detected. Expressed according to this scheme, using a bac- 

 teriophage very weakly active for B. paratyphosus B the results were : 



Dilution, 1:3; Pmany, Lo 

 Dilution, l:30;Prare, Lo 

 Dilution, 1:700; Po, Lo 



This method of Beckerich and Hauduroy, based upon the principle 

 which I have advocated ^^^ determines simultaneously both the dissolv- 

 ing power in a fluid medium and the formation of plaques upon a sohd 

 medium, and it permits an approximately accurate titration, disclosing 

 bacteriophage races which would remain undetected by the dilution 

 method alone. Nevertheless, there is one criticism which may be made 

 to the work of Beckerich and Hauduroy. In certain of their titrations 

 with potent Shiga-bacteriophages it appears that the formation of 

 plaques does not always follow when a quantity of suspension, most 

 certainly containing corpuscles, is spread over the agar. I have carried 

 out some thousands of titrations, and I have never observed this sort of a 

 result. It must be due to some error in their experiments which remains 

 undetected. 



Janzen and Wolff^'^^ effect a titration employing the following technic. 



1. One drop of the suspension of bacteriophage to be titrated is in- 

 troduced into a culture of the susceptible bacterium. 



2. A tube of bouillon hghtly seeded with culture is also inoculated 

 with 1 drop of bacteriophage suspension. 



3. Immediately after the mixture is made 1 di'op of culture no. 1 is 

 spread over an agar plate. 



After incubation the results with culture no. 1 are recorded as 

 + + + + (complete dissolution), + ++, ++, or + (partial dissolu- 

 tion), or finally as ± if the dissolution is hardly perceptible. With 

 culture no. 2, (which is a seeded bouillon, not a turbid suspension) no 

 growth is expressed by + + + + , very weak, and weak growth by 

 + + + and ++, and growth but slightly less than the control by -{-. 

 Culture no. 3, designed to show plaque formation on agar, gives results 



