CYTOLOGICAL CHANGES IN THE INFECTED HOST CELL 199 



Murray and Whitfield (1953) made a detailed study of the 

 effects of phage strains of the T5 species on various host cells. 

 Again it was found that the cytological changes were characteris- 

 tic of the phage and independent of the particular host strain, 

 whether Escherichia, Shigella, or Salmonella. The sequence of 

 changes was similar to that observed with the T-even phages: 

 cessation of cell division, loss of stainable nuclear chromatin and 

 disruption of nuclear sites, followed during the second half of the 

 latent period by a new synthesis of finely granular chromatin. 

 However, unlike the T-even phage infections, the loss of stain- 

 able chromatin was virtually complete and was not accompanied 

 by migration to the periphery of the cell. In the case of three of 

 the phages (T5, BG3, and 29 alpha) lysis occurred at this stage; 

 with three other strains (PB, PBl, and poona) there was an ac- 

 cumulation of spherical masses of chromatin during" the last 10 

 minutes of the latent period preceding lysis. Five hybrids of 

 T5 with PB isolated and characterized by Adams (1951b) were 

 studied by Murray and Whitfield. Three were found to behave 

 like the T5 parent and two like the PB parent. The cytological 

 pattern segregated independently of host range, latent period, 

 and heat resistance, and thus behaved as an additional genetic 

 marker for this group of phages. Adsorption of ultraviolet- 

 inactivated phage to the host cell caused the initial nuclear disin- 

 tegration, but the secondary synthesis of granular chromatin did 

 not occur. Adams (1949b) had previously demonstrated that 

 calcium ion was essential for some step in the reproduction of 

 phage T5 subsequent to adsorption. Murray and Whitfield 

 found that when infection occurred in the absence of calcium ion 

 there was nuclear disintegration as usual, but the new synthesis 

 of granular chromatin did not follow. Later Luria and Steiner 

 (1 954) demonstrated that calcium was essential for the penetration 

 of phage T5 nucleic acid into the host cell. These results show 

 that adsorption of phage T5 to the host cell is of itself enough to 

 cause nuclear disintegration, even though the phage nucleic acid 

 does not penetrate to initiate phage replication. Adsorption of 

 phage T5 in the absence of calcium may be analogous in its ef- 

 fects to the adsorption of ghosts of phage T2 (Herriott, 1951a). 



