THE INFECTIVE PROCESS 23 



spots), or "plages" (beaches). In German they are called 

 "Locher" (holes), and in English "plaques" or "clearings." 



The plaque size and morphology are characteristic features for 

 a particular phage-bacterium combination, and may vary for 

 different phages grown on the same bacterial host or for the same 

 phage grown on different bacterial hosts. Also the plaque size 

 and morphology may change as a result of a single step mutation 

 on the part of the phage. 



The plaque size increases during incubation of the inoculated 

 plate, usually reaching a maximum after 8 to 12 hours, although 

 there is sometimes a further slow increase in size on longer in- 

 cubation. Plaque development does not continue indefinitely 

 because it is dependent on active growth of the host bacteria. 

 When bacterial growth slows because of exhaustion of nutrients, 

 the lysis of infected bacteria is interfered with and the burst size 

 is markedly reduced. 



Another factor affecting size of plaques is the size of the in- 

 dividual virus particle. Elford and Andrewes (1932), who 

 measured the particle size of a number of strains of bacteriophage 

 by filtration through collodion membranes, noted that there was 

 a rough inverse relationship between particle size and plaque 

 size. They suggested that a small particle diffuses more rapidly 

 than a large particle and consequently that, during the limited 

 time available for plaque development, a population of small 

 particles will radiate farther from a common center than will a 

 population of large particles. In spite of the fact that this is a 

 valid generalization and that its physical basis is readily under- 

 stood it has been repeatedly criticized and does not invariably 

 hold (Felix, 1953). In fact, plaque size may be limited by 

 many factors other than diffusion. Such factors are the burst 

 size, latent period, and adsorption rate. The meaning of the 

 general correlation between plaque size and phage particle 

 size may be questioned because there is also a tendency for small 

 particle phages to have short latent periods. 



The role of adsorption in affecting size of plaques has been 

 n6ted several times. If a phage particle does not adsorb to a 



