1 92 BACTERIOPHAGES 



lated as determined by plaque counts. The quantitative rela- 

 tionship between the number of highly refractile particles seen 

 in the dark field microscope and the number of infectious par- 

 ticles determined by plaque count was confirmed much later by 

 Schlesinger (1933b). 



Dark field observations of phage lysis of E. coli in hanging drop 

 cultures, made by Merling-Eisenberg (1938), may be summa- 

 rized as follows. He found that the motile organisms became 

 motionless following infection. Then fine internal granules be- 

 gan to appear, which were in motion in some cases, while the 

 bacterium increased in size. After a variable period of time 

 lysis occurred. The disruption might occur without any fur- 

 ther change in the size or shape of the bacillus as if the whole cell 

 membrane suddenly became permeable. On other occasions 

 an opening appeared at one end or at the side of the bacterium, 

 through which the granules were spewed. The explosion re- 

 leased a very distinct cloud of particles with a blue diffraction 

 color showing marked Brownian movement and soon leaving 

 their "birthplace." They could be readily distinguished from 

 bacterial debris and unspecific particles by this blue color and 

 their fairly uniform size. The phage bodies (1 50 to 350 per cell) 

 were expelled by the force of the explosion to a distance esti- 

 mated at 5 to 6 times the length of the bacterium. During lysis the 

 number of phage bodies free in the medium soon reached astro- 

 nomical figures. This very interesting description of lysis is illus- 

 trated with photographs which clearly demonstrate the refractile 

 granules both inside the bacteria and in the medium. Merling- 

 Eisenberg (1941) made similar observations of phage lysis of 

 staphylococci, in which 1 to 4 granules per cell were released. 

 Very nice motion pictures of phage lysis were made with dark 

 field illumination by Pijper (1945), which are in agreement with 

 the earlier observations described above. Similar dark field 

 observations were reported again bv Weigle (see Benzer, Del- 

 briick, Dulbecco, Hudson, Stent, Watson, Weidel, Weigle, and 

 Wollman, 1950). 



The possible use of the dark field ultraviolet microscope for 



