CYTOLOGIGAL CHANGES IN THE INFECTED HOST CELL 201 



Study. Observations of cultures of lysogenic B. megaterium 899 

 after induction by ultraviolet light have been reported by Dela- 

 porte and Siminovitch (1952). For the first 17 minutes after 

 irradiation the bacterial cells continue to grow and divide in a 

 manner that is not distinguishable from nonirradiated cultures. 

 After this time the nucleus is disposed as an axial filament nearly 

 as long as the cell. This then begins to swell at some points and 

 the chromatin seems to move to form the surface of a cylinder, 

 considerably larger in diameter than the original filament. Dur- 

 ing this time the bacterial cell increases in both length and diam- 

 eter. The central nuclear cylinder is about Vs the diameter of 

 the bacterium at the end of the latent period. At the time of 

 lysis the bacterial membrane loses its rigidity and the cytoplasm 

 disperses, followed immediately by the disappearance of the 

 axial cylinder. This sequence of events diflfers from that seen 

 with previously studied lytic systems. The authors suggest that 

 the phage particles are synthesized at the surface of the hyper- 

 trophied nuclear filament. 



Whitfield and Murray (1954) studied the sequence of cytolog- 

 ical events during the lysogenization of Shigella dysenteriae with 

 the temperate phages PI and P2. The first stage of infection 

 with both phages consisted of the condensation of the chromatin 

 into annular aggregates which tended to fuse and form into an 

 axial filament. In PI infections this stage occupied about half 

 of the latent period. The axial filament then expanded within 

 the cell to form a complex reticulum which, in the case of lyso- 

 genization, separated into discrete normal nuclear elements. 

 The resulting cells were indistinguishable from those of the sensi- 

 tive strain. The same stages were observed when a virulent 

 mutant of PI was used, but lysis occurred while the chromatin 

 was in the stage of expansion. In P2 infections, on the other 

 hand, the axial filament of chromatin persisted up to lysis or, in 

 the event of lysogenization, the chromatinic filament showed ex- 

 pansion and separation into discrete normal elements, starting 

 from the poles of the growing elongating cells. The first normal- 

 appearing cells were nipped off" from the terminal portion of the 

 cell and division was not equatorial. 



