1 94 BACTERIOPHAGES 



polar and equatorial regions increased until lysis. In dried 

 preparations fixed with osmium the bipolar arrangement had 

 disappeared. Polar bodies were not seen in fixed and stained 

 preparations, which suggests that the procedures of fixation and 

 dehydration may create artefacts having little resemblance to the 

 situation in living infected cells. 



Similar phase contrast observations of living, phage-infected 

 cultures were made with Salmonella typhimurium (Boyd, 1949a) and 

 E. coli (Boyd, 1949b). Again the uniformly dense appearance 

 of the uninfected bacteria quickly changed after infection to a bi- 

 polar distribution of dense areas. For reasons that are not clear, 

 Boyd assumed that the less dense areas are due to bacteriophage 

 and the darker polar areas are clue to bacterial cytoplasm dis- 

 placed toward the poles by the developing phage. The obser- 

 vations of Heden with the ultraviolet microscope, referred to 

 above, suggest that it is the DNA and, presumably, bacteriophage 

 synthesis which is concentrated at the poles rather than the bac- 

 terial cytoplasm. This suggestion of Heden's is further sup- 

 ported by the dark field photographs of phage-infected bacteria 

 taken by Merling-Eisenberg (1938) which clearly show a polar 

 distribution of the phage particles. 



Boyd (1949b) studied all seven T phages by phase contrast 

 microscopy using strain B of E. coli as host. He found charac- 

 teristic changes in cellular appearance which were similar for 

 serologically related phages but differed from one serological 

 group to another. These observations are significant in demon- 

 strating that the physiological response of the bacterium to in- 

 fection depends on the genetic constitution of the phage and sug- 

 gest a rather remarkable control by the phage of enzymatic 

 processes in the bacterium. Such characteristic phage-specific 

 changes in morphology have been defined with greater clarity 

 in stained preparations as we will see. Other observations of 

 phage action using phase contrast have been reported by Hofer 

 (1947), Delaporte (1949), Rice, McCoy, and Knight (1954), and 

 Murray and Whitfield (1953). 



