THE INFECTIVE PROCESS 25 



plaques will be turbid and may be completely overgrown and in- 

 visible on longer incubation. This may also occur if the phage 

 particles are able to convert a considerable proportion of the 

 sensitive bacteria into the lysogenic condition. 



In certain cases plaque development is accompanied by the 

 release of a soluble lytic enzyme of much smaller particle size 

 than the phage and which produces a spreading halo of lysis 

 around the plaque. This lytic substance does not reproduce 

 itself and in certain cases is a capsule hydrolyzing enzyme 

 (Humphries, 1948; Adams and Park, 1956). It will diffuse 

 across bacteria-free areas on the agar to alter the appearance of 

 bacteria that are beyond reach of the more slowly diffusing phage 

 particles (Sertic, 1929a). 



An additional morphological peculiarity of certain plaques 

 should be noted. One sometimes observes on large plaques, 

 which are turbid because of the growth of resistant bacteria, 

 small subsidiary plaques developing on the secondary turbidity. 

 These small plaques are due to host range mutants which are 

 produced during phage growth in the parent plaque. These 

 secondary plaques are quite analogous to papillae on bacterial 

 colonies, and are illustrated by Wahl and Blum-Emerique 

 (1952a). 



The importance of plaques in phage research is two-fold. 

 They furnish a highly accurate and reproducible assay method 

 for counting the number of viable phage particles present in a 

 given sample of phage preparation, and they furnish most of the 

 characters by which hereditary variation in phages is studied. 



