252 G. S. STENT 



3. MorpJiology 



It is also possible to search for incomplete phages by means of the electron 

 optical observation of structures whose morphology bears some resemblance 

 to the characteristic shape of mature bacterial virus particles. Wyckoff (1948) 

 and Hercik (1950a,b,c) obtained electron micrographs of crude lysates of 

 phage-infected bacteria in which, besides the typical tadpole-shaped mature 

 bacteriophages, a variety of objects can be discerned whose form suggests 

 they represent incomplete forms of the virus. Prominent among these are 

 certain circular objects of about the same diameter as the electron-dense head 

 of the mature phage particle, but differing from the latter by having no tail 

 and by being either ring-shaped or "empty." That these rings, or "doughnuts," 

 are mdeed related to mtracellular phage growth could be demonstrated by 

 Levinthal and Fisher (1952), who broke open phage-infected bacteria by 

 explosive decompression at various times after infection and counted the 

 number of various objects visible on their electron micrographs. Levinthal 

 and Fisher found that at early stages of the latent period no structures are 

 apparent inside the infected host cell which cannot also be seen in uninfected 

 control bacteria similarly disrupted by decompression. About 3 minutes 

 before the emergence of morphologically intact phages and the termination 

 of the eclipse, however, the doughnuts make their appearance and then 

 increase in number at about the same rate as the complete phage particles 

 which follow them, until a maximum of about 35 doughnuts per infected 

 bacterium has been attamed. The number of doughnuts then remains at this 

 level, while the number of intact phage particles per cell rises to the neighbor- 

 hood of 100. Anderson et al. (1953) subsequently found that the douglmuts 

 are, in fact, the empty heads of phage particles, which look like flat discs 

 rather than rings when the specimens for electron microscopy are dried by 

 methods which avoid surface tension distortions. The doughnuts are not 

 adsorbed to sensitive bacteria, in agreement with the fact that the empty 

 heads have no tail, the normal attachment organ of the phage. They also do 

 not possess any neutralizing antigens and are, therefore, not identical with the 

 incomplete serum-blocking antigens described in the previous section. They do 

 fix complement in the presence of antiphage serum, however, and contain, 

 therefore, part of the antigenic structures of the phage. The mass of the 

 doughnuts is of the order of 25 % of that of the whole virus and comprises 

 most of the protein of the virus (i.e., about 75 % of the total sulfur) and very 

 little of the nucleic acid (i.e., less than 15 % of the total phosphorus) (DeMars 

 et al., 1953). Further experiments by KeUenberger and Sechaud (1957) 

 confirmed Levinthal and Fisher's kinetic studies on the appearance of empty 

 phage heads and also demonstrated the intracellular presence of certain 

 "rods." These rods resemble the phage tail in structure and dimension and, 

 unlike the doughnuts, can be fixed to the phage receptor sites of bacterial ceU 



