CYTOLOGIGAL CHANGES IN THE INFECTED HOST CELL 193 



Studying phage multiplication was suggested by the work of Barn- 

 ard who published very fine pictures of some of the larger viruses. 

 Pictures of phage CI 6 before and after agglutination by anti- 

 phage serum were taken by Barnard and published by Burnet 

 (1933c). The phage particles were very easily visible, could be 

 counted, and were barely below the limits of resolution of the 

 microscope. This beautiful optical system does not seem to have 

 been applied to the study of phage-infected bacteria. 



The use of the light field ultraviolet microscope for pho- 

 tography of phage infected bacteria by F. L. Gates was described 

 by Bronfenbrenner (1928). He wrote: "In unstained cells 

 photographed at this stage (swollen but not lysed) by means of 

 ultraviolet light, the cytoplasm appears to be of uneven density, 

 quite unlike that of normal bacteria." 



Studies of E. coli infected with phage T2 were made by ultra- 

 violet microscopy by Heden (1951) and compared with observa- 

 tions by phase contrast microscopy, electron microscopy, and 

 light microscopy of stained preparations. This permitted a 

 direct comparison of developmental changes in unfixed and living 

 cells (ultraviolet and phase contrast observations) with corre- 

 sponding stages in fixed cells (electron microscope and stained 

 preparations). Uninfected cultures of £". coli in the logarithmic 

 growth phase were uniformly opaque in the ultraviolet micro- 

 scope, presumably because of the cytoplasmic RNA which ob- 

 scured the DNA-containing bodies, which are so prominent in 

 Feulgen stained cells. After infection with T2 and incubation 

 the bacterial poles gradually became much more light-absorbent 

 at 2,570 A than the equatorial region. After several hours of in- 

 cubation under conditions of lysis inhibition the polar bodies con- 

 tained several times more light-absorbing material than did the 

 rest of the cytoplasm. This contrast between polar and equato- 

 rial regions was not evident at a wave length of 3,300 A, suggesting 

 that nucleic acid is in reality the light-absorbing material. 



In phase contrast studies of living bacteria, the cells appeared 

 uniform until infected. Within 15 minutes after infection the 

 bipolar appearance became evident, and the contrast between 



