24 BACTERIOPHAGES 



bacterium to initiate the first infectious cycle until late in the de- 

 velopment of the bacterial lawn the result will obviously be a 

 small plaque or perhaps no visible plaque. It is typical of 

 slowly adsorbing phage-bacterium systems that a single plate 

 will show great diversity of plaque sizes, those particles ad- 

 sorbing early producing much larger plaques than those ad- 

 sorbing late (Wahl and Blum-Emerique, 1952a; Sagik, 1954). 

 That this is the correct explanation can be demonstrated by 

 adsorbing the phage particles to bacteria in fluid medium, in- 

 activating or removing the unadsorbed phage, and then plating 

 the infected bacteria. This results in a much greater uniformity 

 of plaque size than when free phage is plated because all plaques 

 start to develop at about the same time. 



Since the diflfusion rate is often a limiting factor, anything 

 that interferes with diffusion will reduce the plaque size. The 

 concentration of agar in the plating medium is irriportant for this 

 reason, a more dilute agar permitting development of larger 

 plaques. The agar layer method of Gratia (1936c) using a 

 diluted agar in the upper layer is a useful means for increasing 

 plaque size over that obtained by the classical spreading tech- 

 nique. 



Another factor which may affect plaque size and morphology 

 is the genetic constitution of the phage particle. Phage T2r"'' 

 when plated on strain B of £". coli produces a small, fuzzy plaque. 

 Phage T2r, derived from T2r''" by mutation, gives a larger 

 plaque with a sharp outline. The difference is due to the 

 phenomenon of "lysis inhibition" (Doermann, 1948a) caused by 

 superinfection with phage during the latent period. These 

 plaque type mutants have been of great value in the study of 

 bacteriophage genetics (Chapter XVIII). 



The clearness or opacity of the plaque is a characteristic 

 feature of the phage-host pair. If the bacterial culture is com- 

 pletely susceptible, containing no resistant bacteria, the resultant 

 plaques will be clear. If a small proportion of resistant bacteria 

 is present the plaques will be clouded by scattered minute 

 colonies. If large numbers of resistant bacteria are present the 



