CYTOLOGICAL CHANGES IN THE INFECTED HOST CELL 197 



duced bv several different phage strains on a number of different 

 host bacteria. They found that in general the bacterial nucleus 

 was distorted or destroyed and replaced with a mass of chro- 

 ma tinic material, and the bacterium swelled. The type of 

 lesion produced was characteristic of the phage and not of the 

 host cell because the same phage produced essentially the same 

 sequence of changes in different bacteria, whereas different 

 phages acting on the same bacterial strain produced characteris- 

 tically different lesions. Delaporte (1949), using similar meth- 

 ods, studied the action of phage T4 on normal E. coll cells and 

 on ultraviolet-treated bacteria. Lysing cells liberated large 

 numbers of tiny, stained particles which may have been the 

 phage particles. The action of a phage on B. cereus was also de- 

 scribed briefly. Rita and Silvestri (1 950) gave a brief description 

 of modifications in the host cell nucleus resulting from infection 

 of £". coli with phages of the T series, including T5. 



P'an, Tchan, and Pochon (1949) studied the action of phage 

 on Pasteur ellapestis. The cell and its nuclei increased in size after 

 infection, and the nuclei fused into a convoluted axial filament. 

 At lysis the cell outline became less distinct and the axial filament 

 disappeared. 



A rather detailed description of progressive cytological changes 

 produced in E. coli following infection with phages Tl, T2, T4, 

 T6, and T7 was published by Luria and Human (1950). Sam- 

 ples of the infected bacteria were taken at intervals during the 

 latent period and added to a formalin-dichromate fixative. Af- 

 ter fixing the bacteria were washed, and spread on agar, and im- 

 pression films were stained by the Robinow technique. Again 

 the sequence of cytological changes was found to be characteris- 

 tic of the phage type ; the effect of the closely related phages T2, 

 T4, and T6 being very similar but quite distinct from the changes 

 caused by Tl and T7. With ultraviolet-inactivated T2 phage, 

 infection was followed by the same initial destruction of the bac- 

 terial nucleus as occurs with active T2 phage. However, these 

 cells did not subsequently fill up with granular chromatin as do 

 cells infected with active phage. With inactivated Tl phage 



