326 



THE BACTERIOPHAGE 



The General Characters of Bacteriophage Lysis. 



The observations of d'Herelle, of Twort, and of other early workers established 

 the following facts in regard to the behaviour of the lytic agent. 



(1) It will pass through filters that hold back all bacteria. 



(2) It acts upon susceptible bacteria in such a way as to bring about 

 their lysis during the phase of active bacterial growth. This lytic action 



may be demonstrated in several 

 different ways. 



(a) A phage-containing filtrate 

 may be added to a broth, or 

 peptone-water, culture of a sus- 

 ceptible bacterium, the addition 

 being made either at the time the 

 broth is inoculated, or during the 

 early stages of bacterial growth. 

 The addition of an active filtrate 

 will result, after a variable period, 

 in a relatively sudden clearing of 

 the turbid, growing culture. This 

 clearing may be partial, or appar- 

 ently complete ; but, even in the 

 latter case, prolonged incubation 

 usually results in renewed bac- 

 terial growth. 



(&) The surface of an agar plate 

 may be thickly inoculated with a 

 susceptible bacterium, so as to 

 give a confluent growth, and a 



few drops of a phage-containing filtrate may be spread over this inoculated 



surface before the plate is incubated. If the filtrate is very active, and has 



not been diluted, no growth may develop over 



the area on which it has been spread, or thera 



may be a few " resistant " colonies (Fig. 48a). 



With a moderate dilution of the filtrate there 



will result irregular, confluent areas of clearing 



(Fig. 48b). With a still greater dilution there 



will be a number of well-separated clear areas, 



each of them circular, or roughly circular, in 



outline (Fig. 48c) . These are the tdclies vierges, 



plaques or plages, of the French authors, the 



Loc/ier of the German. As the phage filtrate is 



stiU further diluted, these plaques become fewer 



and fewer (Fig. 48d), a linear relation existing 



between the degree of dilution and the number 



of plaques, at least over a considerable range. 

 Phage action may also be demonstrated on 



a solid medium by the appearance of " bitten," or " nibbled " colonies, of 



the type shown in Fig. 49, or, in certain rather exceptional instances, by 



Fig. 48. 

 Effect of increasing dilutions of a phage on bacterial 

 growth on an agar plate. 



.*«^ 



Fig. 



Colony of Bad 

 showing 



49. 



coli on agar, 

 ' bitten " seg- 



ment at the periphery, due 

 to the action of a bacterio- 

 phage ( X 8). 



